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The practices associated with preclinical assessment of abused drugs have generally relied upon a characterization of their physicochemical structure and physiological activity in relationship to known standards of pharmacological equivalence (Martin, 1977). The increasingly prominent role of behavioral methodologies has extended the range of such evaluations and provided a more comprehensive basis for analysis of a drug’s functional properties. The resulting advances in knowledge about drug actions, and particularly in research technology, have made possible an operational approach to pharmacological assessment of abused drugs and have called attention to the need for reappraisal of traditional concepts and definitions in the field. The distinction between physical and psychic or psychological dependence, for example, has long since outlived its usefulness, and even the dichotomy between physical and behavioral factors has not provided a completely satisfactory framework for analyzing the essential dimensions of drug-related problems. And while the terms dependence and abuse are generally considered preferable to addiction as a basis for operational analysis, terminological ambiguities persist.

The word dependence, for example, continues to be widely used in at least two quite different ways. In the scientific and technical literature, the term dependence or, more precisely, physical dependence is used with specific reference to the chemical and biological effects which follow repeated exposure to a drug resulting in tolerance and an abstinence syndrome when the drug is withdrawn (e.g., Clouet & Iwatsubo, 1975; Cochin, 1970; Eddy, 1973; Kalant, LeBlanc, & Gibbins, 1971). Less technically, however, dependence or, more commonly, drugdependence is often used synonymously with the term abuse to refer to a range of complex phenomena frequently characterized as "loss of voluntary control over drug-taking," "compulsive drug use," and "reduced range of behavioral options." In contemporary research literature, however, the term abuse is used more operationally to refer to the behavioral changes (e.g., drug-seeking and drug discriminating) which precede or accompany self-administration of a pharmacologic agent (e.g., Brady, 1981; Brady & Griffiths, 1977; Griffiths, Bigelow, & Henningfield, 1980; Woods, Young, & Herling, 1982). In a broader context, drug abuse references generally incorporate the adverse physiological, behavioral, and/or social effects of such drug self-administration as well (Schuster & Fischman, 1975).

The relevance and importance of maintaining an operational distinction between the terms dependence (i.e., physical dependence or, perhaps more appropriately, physiological dependence) and abuse reside in the fact that, from the perspective of drug evaluation, their defining properties are not coextensive, they do not invariably occur together, and the methods for their assessment differ. Determinations of dependence potential, based primarily upon measures of tolerance and withdrawal, do not necessarily predict a drug’s abuse liability as measured by the generation and maintenance of drug-seeking and drug-taking behaviors. There are compounds (e.g., propranolol) which produce tolerance and an abstinence syndrome after drug withdrawal but which do not give rise to drug-seeking or drug self-administration (Ambrus, Ambrus, & Harrison, 1951; Crandall, Leake, Loevenhart, & Muehlberger, 1931; Jaffe, 1980; Myers & Austin, 1929; Rector, Seldin, & Copenhaver, 1955). On the other hand, drug-seeking and drug self-injection performances can be maintained in strength by use patterns and doses of drugs (e.g., cocaine) which produce no significant degree of tolerance or withdrawal (Johanson, Balster, & Bonese, 1976; Jones & Prada, 1977; Schuster & Woods, 1967).

Interactions between physiological dependence and drug abuse are, of course, commonplace. Changes in drug-seeking and drug-taking often occur as sequelae to both the acute effects of a pharmacological agent and to the tolerance and withdrawal effects which follow more chronic drug exposure (Musto, 1973). Conversely, the chemical and biological changes which define physical dependence can as well be sequelae to the self-administration of abused drugs (Jaffe, 1980). But the relative contributions of these distinguishable processes to drug-related problems can vary widely with different pharmacologic agents as a function of dose, environmental circumstances, and previous drug history (Mendelson & Mello, 1982). Moreover, the methods used to assess a pharmacologic agent’s dependence potential and abuse liability, both in laboratory animals and humans, are quite distinct.

The temporal ordering of physiological and behavioral changes in relationship to the drug intake event provides an operational basis for characterizing the range of a pharmacologic agent’s functional properties and for differentiating its dependence potential and abuse liability as well. A convenient and useful framework for such drug evaluation focuses upon the changes or events antecedent to repeated drug-taking, on the one hand, and those consequent to it, on the other. Operationally, measures of the reactive biochemical, physiological, and behavioral changes which follow as consequences of repeated drug intake provide an effective means of assessing dependence potential, while measures of the proactive drug-seeking and drug-discriminating behaviors which occur as antecedents to habitual drug use can reliably index a pharmacologic agent’s abuse liability. Significantly, the conceptual and methodological bases for developing laboratory procedures for pharmacological assessment within the framework of this operational approach derive from the data base provided by systematic analysis of the stimulus properties of drugs.

Over the past two decades, an extensive research literature focusing on the reinforcing, discriminative, and eliciting stimulus properties of drugs has provided an effective technology for evaluating the abuse liability and dependence potential of a broad range of pharmacological agents. Most notably, experimental procedures for the generation and maintenance of drug self-administration performances based upon the reinforcing properties of chemical substances have now become the mainstays of abuse liability assessment in both animals and human research volunteers. More recently, techniques for differentiating between drugs based upon their discriminative properties have added an important dimension to abuse liability evaluation by expanding the scope of drug discrimination methodologies for the operational measurement of self-report (i.e., "subjective") effects in laboratory animals as well as in humans. And, finally, procedures based upon the eliciting properties of drugs continue to provide for the assessment of a pharmacological agent’s dependence potential as measured by the biochemical, physiological, and behavioral sequelae, both acute and chronic, to drug exposure. Indeed, the relevance and importance of a drug’s eliciting properties have also been emphasized by the increasingly prominent role of behavioral toxicity assessment procedures in the comprehensive pharmacological evaluation of both dependence potential and abuse liability.

Research over the past decade has demonstrated that there is a good correspondence between the range of chemical compounds self-injected by laboratory animals and those abused by humans (Brady & Griffiths, 1976a, 1976b, 1977; Brady, Griffiths, & Winger, 1975; Griffiths, Brady, & Snell, 1978a, 1978b; Griffiths, Lukas, Bradford, Brady, & Snell, 1981; Griffiths, Winger, Brady, & Snell, 1976; Griffiths et al., 1980). Moreover, the variables of which such drug administration are a function (e.g., dose, response requirement, schedule of availability, environmental conditions, past history) have been found to exert their influence in a similar fashion independently of the type of substance maintaining the performance or the species of organism involved (Griffiths et al., 1980). This cross-species and cross-substance generality has provided the basis for assessing the reinforcing properties of a range of drugs and the procedures developed with laboratory primates for this purpose have been previously described in detail (Brady & Griffiths, 1976a, 1976b, 1977, Brady, Griffiths, & Hienz, 1983; Brady et al., 1975; Griffiths, Ator, Lukas, & Brady, 1983; Griffiths, Bradford, & Brady, 1979; Griffiths, Brady, & Bradford, 1979; Griffiths et al., 1978a).

Briefly, mature male baboons (Papio anubis) are adapted to a standard harness/tether system (Lukas, Griffiths, Bradford, Brady, Daley, & DeLorenzo, 1982) and are individually housed in sound-attenuated chambers with water and the opportunity to respond for food continuously available. After initial behavioral training on a progressive-ratio paradigm with food reinforcement, each subject is surgically prepared with a chronic silastic catheter in the internal jugular, femoral, or axillary vein. The catheter is passed subcutaneously, exited at the middle of the back, and attached to a valve system which allows for the slow continuous administration of heparinized saline via a peristaltic pump to maintain catheter patency. Drugs are injected into the valve system near the animal by means of a second peristaltic pump and then flushed into the animal with saline from a third pump. This system necessitates a delay of approximately 20 seconds between the onset of drug delivery and actual injection into the vein. All drugs are delivered within a 2-minute period, however, and the volume of drug solution per injection as well as the injection duration remain constant throughout an experiment.

The availability of a drug for self-injection by the animal is indicated by the illumination of a jewel light directly over a standard Lindsley lever mounted on the baboon’s work panel which compromises the back wall of the cage housing. The light remains illuminated until completion of a fixed-ratio requirement (e.g., FR-160), on the Lindsley manipulandum, at which time the drug injection begins. A time-out period of 3 hours follows each injection. Thus, a maximum of eight injections is possible each day.
 

Figure 1: Daily pattern of self-injection maintained by various doses of diethylpropion HCl and saline in the same subject. The initial 3-day period of each determination shows the self-injection performance maintained by cocaine HCl (0.4 mg/kg) prior to substitution of saline or the indicated dose of diethylpropion. Reproduced with permission from Brady and Griffiths, 1983.

A substitution procedure is utilized to determine whether an unknown compound will maintain self-injection behavior. Self-injection performance is first established with cocaine at a dose of 0.32 mg/kg/injection. After 3 consecutive days of cocaine availability, during which six or more injections are taken each day, a specified dose of the test drug or saline is substituted for the cocaine. At least 12 days of access to each dose of drug or saline is permitted. Cocaine is then reinstated, and when the criterion of 3 consecutive days of six or more injections per day has been met, another dose or another drug is substituted. The order of exposure to drugs, saline, and different doses is mixed; all animals are given at least 12 days of access to saline during the examination of different drugs at different doses. This basic approach thus provides a standardized procedure for evaluating the reinforcing properties of drugs.

A progressive-ratio procedure is used to measure a drug’s relative reinforcing efficacy by determining the maximum amount of responding (i.e., work) that can be maintained by the compound. Progressive-ratio experiments involve first introducing a drug on a low response requirement (e.g., FR-160) to obtain a high and stable drug self-injection performance. The response requirement is then systematically increased until the rate of drug self-injection falls below a criterion level which defines the breaking point. The sequence of fixed-ratio values typically includes: 160, 320, 640, 1280, 2400, 3600, 4800, 6000, and 7200. After the breaking point is determined, the response requirement on the Lindsley lever is lowered, and the original performance recovers prior to replication of the experiment or substitution of another dose of drug.

Figure 1 presents illustrative data obtained with the substitution procedure in examining a range of doses of the substituted phenylethylamine, diethylpropion, in one baboon. The figure shows that when saline or a low dose (0.1 mg/kg) of diethylpropion was substituted for cocaine, the self-injection performance decreased during the 12-day substitution period until the subject self-administered only two or three injections per day. At a dose of 0.5 mg/kg diethylpropion, self-administration performance was maintained at an average rate of about five injections per day, which was higher than saline but lower than the preceding cocaine control periods. Finally, the figure shows that at the highest doses tested (1.0 and 2.0 mg/kg), the number of injections per day was stable and was comparable to that during the cocaine control period (seven or eight injections per day).

Figure 2 also presents illustrative data from the substitution procedure showing daily patterns of self-injection performance maintained by saline and several doses of phentermine in three baboons. The figure shows that when saline was substituted for cocaine, the number of injections per day progressively decreased. At a dose of 0.5 mg/kg, phentermine self-injection performance was maintained at levels similar to cocaine control levels. At a dose of 1.0 mg/kg, self-injection performance was also maintained in all three animals; however, drug intake was characterized by a cyclic pattern in which a number of consecutive days of self-injection at a high rate (six or more injections per day) was followed by several consecutive days at a lower rate and then by a return to the higher rate. Previous experiments (Griffiths et al., 1976; Pickens & Thompson, 1971) have documented a virtually identical cyclic pattern of self-injection performance with d-amphetamine.
 

Figure 2: Daily pattern of self-injection maintained by saline or phentermine in three baboons. Ordinates: number of injections. Abscissae: experimental days. Intravenous injections were delivered upon completion of 160 lever presses; a 3-hour time-out followed each injection, permitting a maximum of eight injections per day. The initial 3-day period of each determination shows the number of injections maintained by cocaine prior to substitution of saline or the indicated dose (mg/kg/injection) of phentermine. Reproduced with permission from Griffiths, Brady, and Bradford, 1979. Copyright 1979 by Academic Press.

Figure 3 presents the chemical structures, and Figure 4 the mean levels of self-injection for the 14 phenylethylamines evaluated. As shown in Figure 4, of all the drugs examined d-amphetamine was the most potent, maintaining levels of self-administration above saline at doses of 0.05 and 1.0 mg/kg. Phentermine, diethylpropion, phenmetrazine, phendimetrazine, benzphetamine, and MDA all maintained levels of self-administration above saline at doses of 0.5 and 1.0 mg/kg. l-Ephedrine, clortermine, and chlorphentermine were the least potent of the drugs which maintained performance, supporting self-injection rates above saline control levels at doses of 3.0 and 10.0 mg/kg (l-ephedrine), 3.0 and 5.0 mg/kg (clortermine), and 2.5 and 5.0 mg/kg (chlorphentermine). In contrast to most of the other phenylethylamines that maintained self-administration behavior, the pattern of self-administration with l-ephedrine was particularly unstable and was characterized by either an erratic or cyclic pattern over days. Finally, fenfluramine, PMA, DOM, and DOET were not self-administered at a rate higher than saline at any of the doses studied (means of the determinations at each dose did not exceed the range of saline values).
 

Figure 3: Chemical structures of the 14 phenylethylamines tested to determine whether they maintained drug self-administration. Reproduced with permission from Griffiths, Brady, and Bradford, 1979. Copyright 1979 by Academic Press.

A comparison of Figures 3 and 4 provides some information about structure-activity relationships of phenylethylamines. Other laboratory studies (Tessel, Woods, Counsell, & Basmadjian, 1975a; Tessel, Woods, Counsell, & Lu, 1975b) have demonstrated that the ability of a series of N-ethylamphetamines substituted at the meta position of the phenyl ring either to increase locomotor activity in mice or to increase isolated guinea-pig atrial rate is inversely related to the size of the meta-substituted groups. In a subsequent study (Tessel & Woods, 1975), it was demonstrated that N-ethylamphetamine maintained self-injection performance in rhesus monkeys, whereas fenfluramine (meta-trifluoromethyl-N-ethylamphetamine) failed to maintain self-injection performance. These results indicate that the failure of fenfluramine to maintain self-injection behavior is attributable to its meta-trifluoromethyl group. The results obtained with the present series of phenylethylamines extend these findings and suggest that ring substitutions in general may decrease the potency of the phenylethylamines in maintaining self-injection behavior. The seven compounds shown in the right columns of Figures 3 and 4 had substitutions on the phenyl ring; generally, these compounds were less potent (on a mg/kg basis) in maintaining self-injection than the compounds in the left columns of Figures 3 and 4, which did not have ring substitutions.
 

Figure 4: Average number of injections per day with 14 phenylethylamines. Intravenous injections were delivered upon completion of 160 lever presses: a 3-hour time-out followed each injection permitting a maximum of eight injections per day. C indicates mean of all the 3-day periods with cocaine which immediately preceded every substitution of a phenylethylamine or saline. S indicates mean of Days 8 to 12 after substitution of saline (two saline substitutions in each of 15 animals). Brackets indicate ranges of individual animal’s means. Drug data points indicate mean of Days 8 to 12 after substitution of a drug for individual animals. Lines connect means at indicated doses of drug. Reproduced with permission from Griffiths, Brady, and Bradford, 1979. Copyright 1979 by Academic Press.

Figures 5, 6, 7, 8, and 9 present additional results from assessing the reinforcing properties of a range of other centrally active drugs using the substitution procedure described above.

The reinforcing efficacy of a selected series of CNS stimulants was systematically exhausted using the progressive-ratio procedure described above. Figure 10 shows the results of the progressive-ratio breaking point determinations over a range of doses of fenfluramine, chlorphentermine, diethylpropion, and cocaine. Doses of fenfluramine (0.02, 0.1, 0.5 and 2.5 mg/kg) did not maintain criterion-level self-injection performance in the two baboons tested and therefore were assigned breaking point scores of zero. As shown in Figure 10, chlorphentermine maintained self-injection performance at some of the intermediate doses tested (1.0, 3.0, 5.6 mg/kg) in three baboons. In all three animals, lower and higher doses failed to maintain criterion-level self-injection performance. In the fourth baboon tested (SA), chlorphentermine did not maintain self-injection performance at the doses tested (1.0, 3.0, 5.6 mg/kg). As shown in Figure 10, 0.1 mg/kg diethylpropion did not maintain self-injection performance, while at doses ranging from 1.0 to 10.0 mg/kg, the drug maintained self-administration in all five baboons tested. Therefore, the dose-breaking point function obtained for diethylpropion was an inverted U-shaped curve with a peak at 1.0 or 3.0 mg/kg. Finally, Figure 10 shows that the 0.01 mg/kg cocaine dose did not maintain performance; the 0.03 mg/kg dose maintained performance in three of the four baboons tested; and doses of 0.1, 0.4, and 1.0 mg/kg maintained self-injection behavior in all five baboons. Examination of the figure reveals that, for the four baboons that were exposed to a number of intermediate doses of cocaine, the breaking point values generally increased as doses increased up to 0.1 or 0.4 mg/kg.
 

Figure 5: Average number of injections per day as a function of dose of cocaine, caffeine, and nicotine. Details of the experiments are presented in the legend for Figure 4. Reproduced with permission from Griffiths, Brady, and Bradford, 1979. Copyright 1979 by Academic Press.

Figure 6: Average number of injections per day as a function of dose of three barbiturates. Details of the experiments are presented in the legend of Figure 4. Reproduced with permission from Griffiths, Lukas, Bradford, Brady, and Snell, 1981. Copyright 1981 by Springer-Verlag.

Figure 7: Average number of injections per day as a function of dose of six benzodiazepines. Details of the experiment are presented in the legend of Figure 4. Reproduced with permission from Griffiths, Lukas, Bradford, Brady, and Snell, 1981. Copyright 1981 by Springer-Verlag.

Comparisons of the maximum breaking points maintained by the different drugs indicate that for a given baboon cocaine generally maintained the highest breaking points, followed in order by diethylpropion, chlorphentermine, and fenfluramine. More specifically, data presented in Figure 10 show that some doses of cocaine maintained higher average breaking points in a given baboon than all the doses of diethylpropion, chlorphentermine and fenfluramine tested. Similar within animal comparisons reveal that some doses of diethylpropion maintained higher breaking points than all doses of chlorphentermine and fenfluramine and, finally, that some doses of chlorphentermine maintained higher breaking points than all doses of fenfluramine.

Although the progressive-ratio procedure provides a useful measure of the relative reinforcing efficacy of drugs, the procedure is quite time consuming. An alternative procedure was therefore examined to determine whether it might provide a more efficient alternative to the progressive-ratio procedure. Specifically, the alternative procedure involved measuring stable response rates on a drug-maintained fixed-ratio schedule. A study was undertaken to determine whether fixed-ratio schedules and progressive-ratio schedules would provide similar information about the relative reinforcing efficacy of different cocaine doses. The progressive-ratio procedure was identical to that previously described. On the fixed-ratio schedule, 160 responses were required for each injection. Each injection was followed by a time-out of either 3 or 12 hours. Each dose of cocaine was available for at least 15 days and until response rates showed no trends. Figure 11 shows that with the 3-hour time-out, the dose-breaking point function on the progressive ratio schedule (left-hand column) was similar to the dose-response rate function on the fixed-ratio schedule (center column). As shown, these dose-effect functions were inverted U-shaped curves characterized by a graded ascending limb (0.01 to 0.32 mg/kg) and by a downturn at the highest dose (3.0 to 4.0 mg/kg). On the fixed-ratio schedule, the downturn in the dose response rate function was probably attributable to a cumulative drug effect, as revealed by manipulation of the time-out duration (i.e., 3 or 12 hours), by analysis of sequential interresponse time distribution, and by cumulative response records. Overall, the study showed that these fixed-ratio and progressive-ratio schedules provide similar information about the relative reinforcing efficacy of different cocaine doses and that both schedules may be useful in the assessment of drug reinforcing efficacy.
 

Figure 8: Average number of injections per day as a function of dose of five phencyclidine-analogue compounds. Details of the experiment are described in the legend of Figure 4. Reproduced with permission from Lukas, Griffiths, Brady, and Wurster, 1984. Copyright 1984 by Springer-Verlag.

Utilizing information collected during the initial screening of drugs, it has been possible to develop another potentially valuable dimension for ranking the abuse liability of anorectic drugs. The rationale for this analysis is similar to that for the Therapeutic Index, which provides a measure of the toxicity of a drug in terms of a ratio expressing the relationship between the therapeutic or effective dose and a dose which produces a given toxic effect. In the present case, the anorectic-reinforcement ratio (Griffiths et al., 1978b) provides a means of rank ordering drugs in terms of a ratio between two doses—a dose which produces a specified anorectic effect ("therapeutic" effect) and a dose which produces a specified reinforcing effect ("toxic" effect). Clearly, the most desirable anorectic drug would have potent anorectic properties but minimal reinforcing properties. An undesirable anorectic drug would be a weak anorectic but a powerful reinforcer. Undoubtedly, existing anorectic drugs fall on a continuum between these extremes; a quantitative measure of this continuum is provided by the anorectic-reinforcement ratio. A number of substituted phenylethylamines and cocaine have been evaluated using this measure.
 

Figure 9: Average number of injections per day as a function of dose of eight opioid agonist and antagonist compounds. Details of the experiment are presented in the legend of Figure 4.

The analysis of the reinforcing properties of a drug requires examination of a substantial range of doses. The lowest reinforcing dose maintaining self-injection performance at FR-160 provided the denominator of the anorectic-reinforcement ratio (Table 1, Column B). Concurrent assessments were made of the anorectic effects of these compounds, and the dose which suppressed food intake to 50% of saline control levels was calculated individually for all compounds. These calculated values are shown in Table 1, Column C and provided the numerator for the anorectic-reinforcement ratio.

Table 1, Column D and the filled bars of Figure 12 show the resulting anorectic-reinforcement ratios (based upon adjustment to an arbitrarily assigned d-amphetamine value of 1.0) derived from the relationship between food suppression dose (i.e., Column C, numerator) and criterion reinforcing dose (i.e., Column B, denominator) for each of the drugs studies. The ratio values range from a low of zero for fenfluramine and phenylpropanolamine to a high of 14.81 for cocaine and reflect the fact that compounds with high ratio values are more potent reinforcers (relative to their anorectic potency) than compounds with lower ratio values. To provide more information about anorectic potency of the drugs, an alternative set of values was derived by utilizing the lowest recommended daily human anorectic doses. These doses appear in Column E of Table 1 and provide the numerator for computing a comparative set of ratio values (Column F). Since cocaine is not used clinically as an anorectic, no entry appears in Column E. Comparisons of the values in Columns D and F (also the striped bars vs. the filled bars of Figure 10) show the correspondence between the ratios based upon these two independent measures of anorectic potency.
 

Figure 10: Breaking point values for doses of fenfluramine, chlorphentermine, diethylpropion, and cocaine in five baboons. Ordinates: breaking points. Abscissae: dose (mg/kg/injection). Each point represents a single breaking point observation. Lines connect the means of the breaking point observations at different doses of drug. Filled circles indicate data obtained during the first exposure to a drug dose. Unfilled circles indicate data obtained during a second exposure to a drug dose. Reproduced with permission from Griffiths, Brady, and Snell, 1978a. Copyright 1978 by Springer-Verlag.

There is now abundant evidence that both laboratory animals and humans can be trained to respond differentially in the presence of different stimulus conditions, whether the stimuli are presented exteroceptively (e.g., vision, audition) or interoceptively as in the case of drugs (Colpaert & Rosecrans, 1978). In the typical drug discrimination experiment, for example, differential reinforcement procedures are used to strengthen one response (e.g., pressing the left lever) after a certain drug dose is administered and a different response (e.g., pressing the right lever) after saline or the drug vehicle alone is administered. The presence or absence of the training drug comes to set the occasion for making one of two (or more) responses, only one of which will be reinforced (e.g., produce food) in any given training session (see reviews by Colpaert, this volume; Overton, this volume; Schuster & Balster, 1977; Winter, 1978). This procedure permits study not only of the discriminability of individual drugs (Overton, 1982a) but also of the range of drugs that will occasion the same response as the training drug in tests of drug stimulus generalization. Drugs that are pharmacologically similar to the training drug generally occasion responding similar to that under the training drug condition at some doses, while drugs from different classes do not (Colpaert & Rosecrans, 1978; Schuster & Balster, 1977). Such cross-drug test procedures may be operationally analogous to the assessment of self-reported comparisons of drug effects in experienced human drug users (Haertzen & Hickey, this volume; Hill, Haertzen, Wolbach, & Miner, 1963).
 

Figure 11: Left-Hand Column: Breaking point values for doses of cocaine in five baboons; Y-Axis = cocaine dose (mg/kg/injection), log scale. Each point represents a single breaking point observation. Lines connect at means at different doses of drug. Right-Hand Column: Response rates maintained by saline and various cocaine doses in four baboons on an FR-160 schedule with either a 3-hour or 12-hour time-out following each injection; Y-Axis = responses per second; X-Axis = cocaine dose (mg/kg/injection), log scale; S = saline. Data points and brackets indicate mean response rate ± SEM for the last 25 injections, except for the data points for SA at 0.32 and 0.1 mg/kg with the 12-hour time-out, which are based on the last 12 injections. Unfilled circles = data obtained during first exposure to a drug dose; filled circles = data obtained during second exposure to a drug dose. Reproduced with permission from Griffiths, Bradford, and Brady, 1979. Copyright 1979 by Springer-Verlag.

Figure 12: Anorectic-reinforcement ratios for cocaine and eight anorectic drugs. Filled bars show data derived entirely from baboon experiments. Striped bars show data derived from both human clinical information and baboon experiments. Compounds with high ratio values are more potent reinforcers (relative to their anorectic potency) than compounds with lower ratio values. Reproduced with permission from Griffiths, Brady, and Snell, 1978b. Copyright 1978 by the Society of Biological Psychiatry.

Procedures for assessing sedative-hypnotics as discriminative stimuli have been developed for baboons and used as part of an overall assessment of novel psychoactive compounds (Ator & Griffiths, 1983). Some baboons were trained to discriminate the benzodiazepine lorazepam, and others were trained to discriminate the barbiturate pentobarbital in a two-lever drug discrimination procedure. Sessions were 20 minutes long with a 60-minute presession time-out that served as the pretreatment time. Under training conditions food delivery depended on 20 consecutive responses on one lever when the session was preceded by the training drug or on the other lever under no-drug or vehicle conditions. Test sessions were interspersed among training sessions after criterion performance had been maintained for the last 4 of at least 10 sessions of alternation of drug and no-drug conditions. Criterion performance was defined as 96 to 100% of the total session responses on the reinforced lever in the absence of 20 or more consecutive responses on the nonreinforced lever before the first pellet delivery of the session. Test sessions were also 20 minutes long and 20 consecutive responses on either lever produced food. Under test conditions, then, the baboon was always "right." Alternative procedures have been reported in which responding under test conditions was never reinforced (e.g., Barry & Krimmer, 1978) or in which the first completed response sequence determined the lever that would remain the reinforced lever for the entire session (Colpaert, Niemegeers, & Janssen, 1976b).
 

When tested with other doses of the training drug, the baboons responded predominantly on the drug lever following all but the lowest doses tested. Equivalent cross-drug generalization between the lorazepam-and pentobarbital-trained baboons did not occur, however. Pentobarbital-trained baboons responded predominantly on the drug lever after both pentobarbital and lorazepam, but the lorazepam-trained baboons responded predominantly on the drug lever only after lorazepam and not after pentobarbital (Ator & Griffiths, 1983). Similar asymmetries have been found with other training drugs (fentanyl/apomorphine, Colpaert, Niemegeers, & Janssen, 1976a; nalorphine/cyclazocine, Hirschhorn, 1977; ethanol/diazepam, Jarbe & McMillan, 1983; ethanol/pentobarbital, York, 1978). Further work is needed to establish the determinants and limits of such asymmetries, but it has been shown that other benzodiazepines (e.g., alprazolam, diazepam, triazolam) occasioned drug lever responding in the lorazepam-trained baboons but, like pentobarbital, phenobarbital did not. Indeed, there is considerable interest in the possibility of manipulating training conditions in order to produce greater or lesser specificity in drug stimulus generalization (Barry & Krimmer, 1978; Overton, 1978; 1982b). To the extent that greater specificity can be attained as the result of training with certain drugs, it may be possible to gain a better understanding of those structural variables that determine the discriminative stimulus properties of individual compounds.

Figure 13 shows the results of an experiment in which two compounds, CPDD 0001 and CPDD 0002, were studied under "blind" conditions in lorazepam-trained baboons. The drugs had been received for assessment from the Committee on Problems of Drug Dependence without any accompanying information about their chemical structure or pharmacological activity. The results summarized in Figure 13 include drug discrimination data previously obtained with lorazepam, diazepam, and pentobarbital for comparison and show that both CPDD compounds occasioned 100% drug lever responding in all baboons. When the "blind" was broken, the drugs were revealed to be the benzodiazepines diazepam (CPDD 0001) and bromazepam (CPDD 0002). It was of interest that the results with diazepam when studied "blind" were completely consistent with diazepam results obtained earlier under "nonblind" conditions.
 

Figure 13: Mean percentage of drug-lever responding (upper panel) and response rate expressed as percentage of mean response rate in no drug training sessions (lower panel) for three baboons trained to discriminate lorazepam (1.0 mg/kg). Connected points represent mean data for test sessions with lorazepam, diazepam, pentobarbital, and two compounds studied under blind conditions (CPDD 0001 and 0002). CPDD 0001 and 0002 were revealed to be the benzodiazepines, diazepam and bromazepam, respectively. Points over ND and D represent responding in the no drug and drug training sessions, respectively, that preceded test sessions; V indicates test sessions conducted with drug vehicle. Vertical bars around D and ND points indicate ranges.

That the discriminative stimulus function of drugs may be mediated by specific populations of receptors has been suggested by studies in which discriminative stimulus effects were antagonized by compounds known to be competitive antagonists in a variety of other preparations (e.g., morphine, Shannon & Holtzman, 1976; nicotine, Meltzer, Rosecrans, Aceto, & Harris, 1980). It also has been shown that the discriminative stimulus effects of diazepam and lorazepam, but not pentobarbital, were antagonized by the imidazodiazepine derivative Ro 15-1788 (Ator & Griffiths, 1983; Herling & Shannon, 1982); and diazepam and lorazepam also were antagonized by the pyrazoloquinolone CGS 8216 (Ator & Griffiths, 1985; Shannon & Herling, 1983). The results with CGS 8216 and lorazepam in the baboon, illustrated in Figure 14, show that CGS 8216 produced a complete shift to the no-drug lever in a dose-related manner, and this antagonism was surmountable with higher doses of lorazepam.
 

Figure 14: Interactions of CGS 8216 with lorazepam in a baboon trained to discriminate lorazepam 1.0 mg/kg (i.m.) from the no-drug (ND) condition. Drug lever responding (upper panel) is shown as a percentage of total session responding in test sessions after administration of lorazepam alone (filled symbols) and in combination with doses of CGS 8216 (p.o.). Pretreatment time was 60 minutes for both drugs. Overall response rates (lower panel) in those test sessions are presented as percentages of the mean rate in the ND training sessions immediately preceding test sessions. Points at ND and D represent mean responding in the no-drug and drug training sessions, respectively, that immediately preceded test sessions; V indicates administration of the lorazepam vehicle. Vertical bars around ND and D points indicate ranges unless the mean was encompassed by the data point.

Systematic study of the time course of the drug discrimination stimulus effects has been approached either by varying the pretreatment time before the test session (e.g., Winger & Herling, 1982) or by reinstating the test procedure at intervals after the initial test session. This latter procedure is illustrated in Figure 15 which presents further data from the study with the benzodiazepine antagonist CGS 8216 in combination with lorazepam (Ator & Griffiths, 1985). Following a test session with the usual pretreatment time of 1 hour, the experimental procedure was reinstated under test conditions at 3, 5, 7, 9, 11, 13, and 15 hours after drug or vehicle administration. Figure 15 shows consistent responding on the no-drug lever across these test sessions after drug vehicle. After lorazepam (1.0 mg/kg), however, responding shifted from the drug to the no-drug lever by 11 hours post administration. When CGS 8216 (10.0 mg/kg) was administered in combination with this dose of lorazepam, there was complete antagonism of lorazepam (i.e., all responding was on the no-drug lever after the usual pretreatment time of 1 hour and across the entire 15-hour period). With a higher dose of lorazepam (3.2 mg/kg), however, the CGS 8216 antagonism was surmountable (i.e., all responding occurred on the drug lever at the 1-hour test time); but the time course procedure revealed a late onset of CGS 8216 antagonism at this dose combination (i.e., responding shifted from the lorazepam lever to the no-drug lever during subsequent test sessions). Because such a shift never occurred by 3 hours after administration of this dose of lorazepam alone, the results of the time course procedure suggest an increasing effect of CGS 8216 across time.
 

Figure 15: Cumulative records of responding in 10-minute test sessions conducted at 1, 3, 5, 7, 9, 11, 13, and 15 hours after drug or vehicle administration in a baboon trained to discriminate lorazepam (1.0 mg/kg). Lorazepam was administered intramuscularly and CGS 8216 was administered orally. In these test sessions, pellet delivery depended on 20 consecutive responses on either lever. The upper pen in each record stepped upward with each drug-lever response and the lower pen deflected with each response on the no-drug lever; the upper pen deflected with all pellet deliveries, regardless of the lever on which responding occurred. The recorder did not run during the 2-hour intervals between test sessions; the upper pen reset to baseline after either 175 responses or at the end of the test session.

The results of the time course procedure shown in Figure 15 also serve to illustrate that, contrary to the oft-stated hypothesis that once pellet delivery occurs it precludes switching levers (i.e., the "win-stay, lose shift" strategy, Barry & Krimmer, 1978; Colpaert et al., 1976b; Goudie, 1977), baboons trained and tested under the conditions described do exhibit responding on both levers under some test conditions. This has been shown at lower and intermediate doses of drugs as well as with time-course procedures for a number of drugs (Ator & Griffiths, 1983; unpublished observations). For example, in the study with Ro 15-1788 (Ator & Griffiths, 1983), it was found that over time the effects of the antagonist apparently decreased and drug lever responding emerged. The time course of this effect of RO 15-1788 and of lorazepam was consistent with other data for the time course of these compounds in humans (Darragh, Lambe, Kenny, Brick, Taaffe, & O’Boyle, 1982; Greenblatt, Shader, Divoll, & Harmatz, 1981) and emphasizes the impressive degree to which a drug can exert discriminative control of behavior.

In addition to evaluating a pharmacologic agent’s reinforcing and discriminative stimulus properties, the assessment of abuse liability and dependence potential requires analysis of the physiological and behavioral changes, both acute and chronic, which are elicited following drug administration. Substances with only minimal, if any, disruptive physiological/behavioral effects are not generally regarded as having significant dependence potential or abuse liability even though their use may be widespread (e.g., caffeine in tea or coffee). In contrast, compounds self-administered even sparingly which are associated with disruptive physiological/behavioral changes are considered to have high abuse liability (e.g., lysergic acid diethylamide: LSD). Drugs may fall anywhere on the continua defined by these assessment parameters, and a comparative evaluation of drug dependence potential and/or abuse liability is greatly enhanced by an interactive analysis of reinforcing, discriminative, and eliciting functions.

Physiological Dependence

The methods used for assessment of physiological dependence of the opioid-type have been extensively described, and their effectiveness as predictors of physiological dependence potential has been well documented (e.g., Deneau, 1956; Seevers, 1936; Seevers & Deneau, 1963; Villarreal, 1973). Substitution tests are often conducted first to assess the suppression of withdrawal signs in opiate-dependent animals denied their usual dose of morphine. If such procedures indicate only equivocal dependence potential, primary (or direct) dependence and precipitated withdrawal tests are conducted in which animals are treated chronically with drug and observed for withdrawal signs after drug administration is stopped or after administration of an opioid-antagonist (e.g., naloxone).

Physiological dependence on sedative-hypnotics, anxiolytics, and alcohols is potentially more dangerous than dependence on opioids, because sedative withdrawal can be life threatening. Withdrawal signs from sedatives, in particular from the benzodiazepines, can occur in patients taking therapeutic doses for prolonged periods of time when drug treatment is suddenly stopped (Petursson & Lader, 1981; Pevnick, Jasinski, & Haertzen, 1978; Rifkin, Quitkin, & Klein, 1976; Tyrer, Rutherford, & Hugget, 1981; Winokur, Rickels, Greenblatt, Snyder, & Schatz, 1980). In contrast to the opioids, however, sedatives have received relatively less systematic study from the perspective of dependence potential. Phenobarbital, pentobarbital, barbital, alcohol, chloroform, meprobamate, diazepam, chlordiazepoxide, oxazolam, and lorazepam have been shown to suppress barbital withdrawal signs and/or to produce direct physiological dependence in rhesus monkeys (Yanagita & Miyasato, 1976; Yanagita & Takahashi, 1970, 1973; Yanagita, Takahashi, & Oinuma, 1975). In contrast, benzoctamine, perlapine, and methaqualone have been reported by these same investigators neither to suppress barbital withdrawal nor to produce direct physiological dependence, and the results of these laboratory animal studies (with the possible exception of the methaqualone findings) are in general agreement with what is known about the dependence potential of these compounds in man.

The physiological dependence-producing properties of benzodiazepines and benzodiazepine-like compounds have been studied using baboons (Papio anubis) implanted with intragastric catheters, protected by a harness-tether system that allows the animal virtually unrestricted movement (Lukas et al., 1982). The baboons are housed in modified primate cages and have ad libitum access to food and water. Two methods are currently employed to evaluate the dependence potential of benzodiazepines and benzodiazepine-like compounds. The first, precipitated-withdrawal, involves administration of the test compound via a continual intragastric injection using a peristaltic pump. After daily administration (e.g., 7 to 10 days), the benzodiazepine antagonist Ro 15-1788 (5 mg/kg) is administered. The baboons are then evaluated for the presence of precipitated-withdrawal signs for 1 to 2 hours after the administration of the antagonist. The second method, a direct dependence test, involves continued daily administration of the test compound for a period of weeks (e.g., 35 to 40 days) followed by abrupt termination of drug administration and observation for withdrawal signs.

When diazepam (20 mg/kg/day) was administered for 7 days followed by Ro 15-1788, withdrawal signs were observed (Lukas & Griffiths, 1982). These signs included bruxism, retching, vomiting, abnormal posturing, and occasional limb tremor. After 35 days of diazepam administration, the antagonist-precipitated signs were more intense and additional withdrawal signs were observed including lip-licking, nose-rubbing, head and body tremors, preconvulsive posturing, and, on one occasion, convulsions. Table 2 summarizes the results obtained when diazepam administration was stopped after 45 days and the baboons were observed for spontaneously occurring withdrawal signs. The indicated signs first appeared by Day 6 or 7, peaked on Days 9 and 10, and had essentially disappeared by the 15th day after cessation of diazepam administration. Signs of spontaneous withdrawal included nose-rubbing, abnormal postures, limb-tremor, and head and body tremor, but no convulsions were seen. Figure 16 shows that starting 8 days after diazepam administration was discontinued and until the end of the 15-day observation period, food-intake was reduced to 25% of normal.

These observations have been extended in a systematic investigation of intensity of the Ro 15-1788 precipitated-withdrawal syndrome as a function of the diazepam dose and duration of treatment (Lukas & Griffiths, 1983). Baboons were exposed to 20 mg/kg/day diazepam for different lengths of time ranging from 1 hour to 35 days. No precipitated withdrawal signs were seen after 1 hour of diazepam exposure. After 1 day of diazepam exposure, however, a significant number of precipitated withdrawal signs were observed, and these were most intense in baboons which previously had been exposed to benzodiazepines. In general, as the duration of diazepam exposure increased, the number and severity of withdrawal signs increased. In a second series of studies, various doses of diazepam ranging from 0.125 to 20 mg/kg/day were administered for a fixed period of 7 days. At doses of diazepam as low as 0.25 mg/kg/day, Ro 15-1788 administration precipitated withdrawal signs at the end of this 7-day treatment period. As the diazepam dose increased, the number and severity of the precipitated withdrawal signs also increased. These results suggest that even doses of diazepam within the therapeutic range may produce physiological dependence after only brief periods of treatment.
 

Table 2 Time Course of Spontaneous Withdrawal from Diazepam*

Symbols indicate that the withdrawal sign occurred one or more times in the time block except for yawns for which 4 or more were required. Data shown are for 10 minute observation periods at noon. Body postures are described in Lukas and Griffiths (1982). Individual subjects are indicated by different symbols: PH, open circles; AL, filled circles; SA, filled squares; and JE, open squares.

Behavioral Toxicity

Procedures for assessing the behavioral toxicity of a range of abused drugs in terms of their sensory/motor effects have been previously described in detail (Brady, Bradford, & Hienz, 1979; Hienz & Brady, 1981; Hienz, Lukas, & Brady, 1981). The basic methodology focuses on a reaction time (RT) technique that has been used extensively with primates as an index of sensory function (Moody, 1970; Pfingst, Hienz, Kimm, & Miller, 1975a; Pfingst, Hienz, & Miller, 1975b). Briefly, laboratory baboons are required to press and hold down a lever for a variable time, following which a signal is presented (e.g., a white light flash or tone burst). Release of the lever within 1.5 seconds following the onset of the signal is then reinforced with food presentation (190 mg banana pellets). The reaction time is the time elapsed between the onset of the signal and the release of the lever and has been found to be inversely related to stimulus intensity, decreasing as stimulus intensity increases. This relationship between reaction time and stimulus intensity is a naturally occurring one, needs no prior training, and is distinctly advantageous as a measure of sensory function. In addition, the procedure is easily employed with either auditory or visual stimuli, can be used to obtain stimulus detection thresholds, and can be easily modified, when required, to extend research into other areas of possible drug effects (e.g., auditory frequency and intensity discrimination). There is also an already existing large data base on auditory reaction time with a number of primate species, including the baboon.
 

Figure 16: Effects of spontaneous withdrawal from diazepam on food intake. Average daily food intake in grams is shown for the four baboons for which withdrawal effects are summarized in Table 2.

The use of reaction time has allowed for the separation of sensory deficits from extreme motor deficits by requiring that an animal hold a lever down for a considerable period of time prior to stimulus presentation. Thus, for example, an ataxic subject would produce a number of premature lever releases as compared to a normal subject. Further, since variability of reaction times in primates is generally quite small, changes resulting from administration of specific compounds are readily detected. The measurement of the threshold levels for pure tones in primates, however, has been demonstrated not to change greatly when different reaction time criteria are used for estimating detection (Pfingst et al., 1975b). Thus, a compound that produced a motor deficit resulting in longer reaction times would not necessarily cause an increase in sensory threshold estimates unless the deficit was debilitating enough to be detectable by the monitoring of premature releases. Both small and large motor effects of specific compounds can thus be separated from purely sensory effects.

The use of both auditory and visual stimuli in the reaction time procedure has provided for the assessment of drug-related changes in specific sensory systems. If drug-related changes occur in auditory reaction times but not in visual reaction times, for example, the observed effects can be attributed specifically to changes in the auditory system. Further, drug-related changes that produce consistently longer reaction times to near-threshold auditory stimuli, and not to high intensity auditory stimuli, provide a laboratory primate analogue of the clinical phenomenon of loudness recruitment, which typically occurs as a result of a brief exposure to intense sounds (temporary threshold shift; Moody, 1973).

The sequence of events during each reaction time trial was as follows. In the presence of a flashing cue light (5/second), a lever press changed the flashing red light to a steady red light that remained steady as long as the animal held the response lever in the down position. At varying intervals (ranging 1.0 to 7.3 seconds) following initiation of this maintained holding response, a test stimulus (white light on the circular patch or tone burst through the speaker) was presented for 1.5 seconds. Release of the lever within the 1.5 second test stimulus interval delivered a single banana pellet and initiated a 1 second intertrial interval (ITI) during which no stimuli were presented and lever responses re-initiated the ITI. Incorrect responses (i.e., lever presses prior to test stimulus onset or after the 1.5 second test stimulus interval) were punished with a 2 to 5 second time-out, followed by a return to the ITI without reinforcement. Following the ITI, the flashing red cue light signaled initiation of the next trial in the series of several hundred which comprised each daily 2 to 3 hour experimental session. Asymptotic levels of performance on this procedure typically required 2 to 3 months of such daily training sessions. Auditory and visual testing sessions were conducted separately.

Auditory and visual thresholds were determined by randomly varying the intensity of the test stimuli from trial to trial (method of constant stimuli) and examining detection frequencies (i.e., percent correct lever releases) at each intensity. For the auditory modality (where the baboon detects tones across a range of frequencies extending to 40 kHz), four intensity levels (10 dB apart) of a 16.0 kHz pure tone were used, with the lowest level set just below the animal’s estimated threshold. Interspersed among the "test" trials were a series of "catch" trials during which no tone was presented in order to measure the false alarm or "guessing" rate. For the visual modality, four intensity levels (0.5 log density units apart) of the white light were used with the lowest level again set just below the animal’s estimated threshold. Again, catch trials with no light were programmed to occur intermittently. In addition, sessions involving visual and auditory threshold determinations were preceded by a 15-minute "warm-up" with the various stimulus intensities to be used in the session.

For both the auditory and visual threshold determinations, each test session was divided into four blocks of 140 trials with each of the four intensity levels plus catch trials presented randomly approximately 28 times during each block. This provided four independent within-session estimates of the sensory thresholds and functions relating reaction time to intensity. Sensory thresholds were determined from percent correct detections at each intensity by interpolating to that intensity producing a detection score halfway between the false alarm rate and 100%. Stable auditory thresholds were based on determinations from three successive test sessions with estimates which varied by no more than 4 dB. Stable visual thresholds were based on determinations from three successive test sessions with estimates which varied by no more than 0.2 log density units. In both cases threshold stability required a false alarm rate below 30% and no systematic changes in the data. Since reaction time distributions are typically skewed due to the physiological limits on lever release time, the measure of central tendency employed for such distributions was the median, with variability reported in terms of the interquartile range. All drugs were administered intramuscularly at the beginning of each experimental session, followed by a 30-minute delay (dark adaptation and warm-up) before formal threshold determinations were begun. Saline control sessions were conducted between drug sessions, and a return to baseline performance was required during these intervening saline control sessions before further drug administrations.

Figure 17 shows the orderly reaction time-lengthening effects of increasing doses of amobarbital sodium (i.e., 3.2, 10.0, 17.0 mg/kg, i.m.) as a function of light intensity in the baboon. The functions relating reaction time to stimulus intensity (latency-intensity functions) were recorded at the peak action time of 1 to 2 hours after drug administration and show the systematic relationship between drug dose and reaction time at all but the lowest (i.e., threshold) intensity. Even at the highest intensities where response variability was minimal, the orderly progression of longer reaction times with increasing doses is apparent.
 

Figure 17: Median reaction time as a function of visual stimulus intensity after saline and the indicated doses of amobarbital for one animal. Reproduced with permission from Brady, Griffiths, and Hienz, 1983.

Figure 18 illustrates the differential dose-dependent effects of pentobarbital upon absolute auditory and visual thresholds, on the one hand, and the similar effects upon auditory and visual reaction times, on the other hand, for three animals. All data points are the average of at least two determinations with each animal at each dose, and represent the difference between those values at peak drug action time and the corresponding saline values during the preceding day’s control session. Reaction time values are for auditory stimuli presented at approximately 25 dB above the auditory thresholds and for visual stimuli presented at 1.25 log relative density units above the visual thresholds. The 95% confidence limits of the variability for all saline sessions preceding a drug session are shown to the left in each graph for each animal. Consistent elevations in visual thresholds and in both visual and auditory reaction times were observed following doses of 10.0 and 17.0 mg/kg with no change in auditory thresholds over this same dose range for two of three baboons.

Figure 19 illustrates the nature of these differential threshold changes in more detail and shows individual auditory and visual psychometric functions and their corresponding reaction time functions at peak drug action time for animal IK after pentobarbital administration over the dose range 1.0 to 17.0 mg/kg. Percent correct lever releases and reaction times for correct releases are plotted as a function of stimulus intensity in dB sound pressure level (SPL) for the auditory functions (top) and in log relative density units for the visual functions (bottom). The saline points were similarly derived from the control sessions conducted on days preceding each drug session. At the highest dose (17.0 mg/kg), pentobarbital produced different effects upon auditory and visual thresholds, though similar increases occurred in both auditory and visual reaction times. The clear shift in the visual threshold function shown in Figure 19 (bottom) at the 17.0 mg/kg dose occurred in the absence of any change in the auditory threshold function (Figure 19, top)—under identical drug conditions. Also, the drug-induced reaction time increases were approximately parallel shifts for both the auditory and the visual curves.
 

Figure 18: Changes in absolute auditory and visual thresholds and their respective median reaction times for three animals as a function of pentobarbital dose. The 95% confidence limits in variability for all saline sessions preceding a drug session are shown to the left in each graph for each animal. Values obtained following vehicle administration are marked "V." Reproduced with permission from Hienz, Lukas, and Brady, 1981. Copyright 1981 by Ankho International, Inc.

Similar changes in visual psychometric functions have been observed following doses of d-methylamphetamine, as shown in Figure 20 which presents percent correct lever releases as a function of stimulus intensity for both the previous day’s saline session (dashed lines) and the 0.32 mg/kg d-methylamphetamine drug session (solid lines) for one animal. As with pentobarbital, the d-methylamphetamine-induced visual threshold elevations occurred in the absence of any change in auditory sensitivity. Changes in reaction time were, however, quite dissimilar to those produced by pentobarbital, with d-methylamphetamine frequently having reaction time-shortening effects, as shown in Figure 21. Auditory reaction times for 0.1 (see Figure 21, left) and 0.32 (see Figure 21, right) mg/kg doses of d-methylamphetamine (solid lines) are compared to the total range of saline control session reaction times (shaded areas) for a single animal. Similar reaction time-decreasing effects of the drug were observed for visual reaction times as well.
 

Figure 19: Changes in individual auditory and visual psychometric functions and their corresponding reaction time functions during the time of peak drug effect for animal IK over the dose range of 1.0 to 17.0 mg/kg pentobarbital. Filled circles represent data from drug sessions; open circles represent data similarly derived from the preceding day’s saline control session. Reproduced with permission from Hienz, Lukas, and Brady, 1981. Copyright 1981 by Ankho International, Inc.

Figure 22 further compares these progressive decreases in visual reaction time over time (lower panel) with the time course of changes in visual thresholds (upper panel) following this animal’s first and second exposures at 0.1 mg/kg d-methylamphetamine. Reaction time became progressively shorter from 60 to 100 minutes after drug injection, while threshold elevations occurred within the first 40 minutes following drug injection—before significant changes in reaction times occurred. And recovery to control threshold values occurred by 100 minutes post-drug, a time when the greatest shortening in reaction times was observed. Clearly, d-methylamphetamine has a differential effect on the sensory and motor components of the psychophysical performance.

The psychophysical profile of the prototypical benzodiazepine, diazepam, is shown in Figure 23. Auditory and visual thresholds, as well as reaction times, were measured in three baboons as a function of dose (0.1, 0.32, 1.0, 3.2, 10.0 mg/kg) at peak drug action time 1 to 2 hours following intramuscular administration. The range of saline control values is shown by the vertical lines on each point. Clear effects upon reaction time and visual thresholds as a function of dose, and auditory thresholds were similarly affected. For the most part, all three effects—lengthened reaction times, visual threshold elevations, and auditory threshold elevations—appeared in a dose-dependent manner with progressively greater decrements through the range from 1.0 to 10.0 mg/kg. In contract, observations with chlordiazepoxide over approximately the same dose range (1.0 to 32.0 mg/kg) used in the pentobarbital (Figure 18) and diazepam (Figure 23) experiments revealed only modest elevations in auditory thresholds and reaction times at the highest dose (32.0 mg/kg) and no elevations of visual threshold at any of the indicated doses.
 

Figure 20: Percent correct lever releases as a function of visual stimulus intensity following a 0.32 mg/kg dose of d-methylamphetamine in animal PE, with time after drug as a parameter (solid lines). Similar data for the immediately preceding saline control day are shown for comparison (dashed lines).

Figure 21: Latency-intensity functions showing median reaction time as a function of auditory stimulus intensity for animal PE following the second exposures to 0.1 mg/kg (left) and 0.32 mg/kg (right) d-methylamphetamine, with time after drug administration as the parameter. Shaded areas encompass total range in median reaction times for each drug day’s immediately preceding saline control day.

To examine the consequences of long-term benzodiazepine administration, repeated diazepam injections were given on a daily schedule followed immediately by daily determinations of visual thresholds and visual reaction times. Figure 24 presents changes in visual thresholds (top) and median reaction times (bottom) for visual stimulation over 21 consecutive days of daily intramuscular administration. Successive within-session estimates of visual thresholds and median reaction times are plotted for each session, and these within-session points are connected with solid lines for drug days and with dotted lines for saline days. Thus each series of two to five connected data points represents one day’s data for changes in visual threshold (top graph) and changes in visual reaction time (bottom graph). Day 1 shows the acute effects of 0.32 mg/kg diazepam on visual reaction times, a small but consistent increase in reaction time, with no apparent changes in visual thresholds. Tolerance the drug’s effects on reaction time developed gradually, and by Day 6 reaction times were back to normal values. Starting at Day 15 an anomalous decrease in visual reaction times occurred, possibly related to the appetite-inducing properties of the drug. (We have previously shown that large changes in deprivation can shift baseline reaction times.) Diazepam was withheld from the animal starting on Day 22, and reaction times then approximated normal values, although the within-session variability increased considerably during the withdrawal period. Visual thresholds continued to show little change.

This general picture of reaction time effects, the development of tolerance to these effects, and highly variable changes in reaction times during withdrawal was magnified somewhat at the daily dose of 1.0 mg/kg diazepam shown in Figure 25. Again, visual thresholds showed little or no change during either drug administration or withdrawal periods, while the initial effect of this dose of diazepam on reaction time was slightly greater, with tolerance to this reaction-time-increasing effect developing over 10 days as compared to 6 days for the 0.32 mg/kg dose. When the drug was stopped, reaction times showed a progressive and pronounced increase that peaked at Day 10 following cessation of the drug, with a gradual recovery following. Again, during this withdrawal period the variability of within-session estimates of reaction times was extreme.
 

Figure 22: Changes in visual threshold (top) and reaction times to visual stimuli (bottom) as a function of time after injection following first and second exposures to 0.1 mg/kg d-methylamphetamine in animal PE. Vertical bars indicate ± 2 standard deviations of variability for each preceding saline control day.

Increasing the dose again to 3.2 mg/kg diazepam produced even more pronounced effects on visual reaction times. The cumulative effects of the drug on reaction time peaked at Day 2, while the first clear effects on visual thresholds occurred on Days 4 and 5. Initial recovery from the reaction time effects was evident by Day 6 but not completed until Day 21. Stopping drug administration again produced an immediate effect upon reaction time, with complete recovery from this effect not occurring for 19 days after drug administration was stopped.

A procedure has been developed for an interactive analysis of the disruptive eliciting effects of abused drugs upon sensory/motor functions, on the one hand, and for their reinforcing effects in maintaining self-administration, on the other. The reinforcement/toxicity ratio (Brady & Griffiths, 1983) compares the relative potency of a drug as a reinforcer with its relative potency in eliciting disruptive sensory/motor effects. A drug with potent reinforcing properties (i.e., maintains self-injection at relatively low doses) but which produces disruptive effects only at relatively high doses would have a low reinforcement/toxicity ratio, whereas a drug that maintains self-injection only at relatively high doses but produces disruptive sensory/motor effects at relatively low doses would have a high reinforcement/toxicity ratio. The measure thus provides a potentially useful preclinical assessment of the extent that self-administration of a compound may disrupt basic sensory/motor processes as well.
 

Figure 23: Changes in absolute auditory and visual thresholds and their respective median reaction times for three animals as a function of diazepam dose. The 95% confidence limits for all saline sessions are shown to the left in each graph for each animal. Values obtained following vehicle administration are marked "V."

The reinforcing properties of a series of barbiturates, benzodiazepines, stimulants, and dissociative anesthetics were evaluated with laboratory baboons to determine the dose of each compound that maintained criterion self-administration. The procedure for determining reinforcing properties has been previously described in detail (Griffiths et al., 1979; 1981) and involved the initial establishment of drug self-administration with cocaine followed by

substitution of saline or another drug for cocaine as described above. A 3-hour time-out period followed each injection, permitting a maximum of eight injections per day. When saline or low doses of drugs were substituted, the number of injections taken decreased over successive days. When higher doses of the drugs were substituted for cocaine, however, the self-injection rate was reliably maintained above saline control levels. Figure 26, for example, shows the effects of dose on the number of pentobarbital injections per day self-administered by two baboons and illustrates the procedure used for determining the criterion reinforcing dose values.
 

Figure 24: Changes in visual thresholds (top graph) and visual reaction times (bottom graph) over 21 consecutive days of daily intramuscular administration of 0.32 mg/kg diazepam and for subsequent days of saline administration. Threshold and reaction time changes from baseline are plotted for each successive block of trials within each session, and within-session points are connected by solid lines on drug days and by dashed lines on saline days. Reproduced with permission from Brady et al., 1984.

All drugs were also evaluated to determine the criterion dose which produced a 50% change in auditory and/or visual thresholds and/or a 10% change in motor reaction time. The procedures for determining such sensory/motor toxicity have been previously described in detail (Brady et al., 1979; Hienz & Brady, 1981; Hienz et al., 1981) and involved training baboons to perform the reaction time procedure described above. Figure 27 shows an example of the effects of pentobarbital dose on visual thresholds in three baboons and illustrates the determination of toxic doses.

Table 3 and Figure 28 summarize the relationships revealed by the reinforcement/toxicity ratios for amobarbital, pentobarbital, secobarbital, d-methylamphetamine, triazolam, diazepam, phencyclidine (PCP), and ketamine. With a ratio value of 1.0 representing equality between reinforcing and toxic drug doses, Table 3 and Figure 28 show that all three barbiturates, d-methylamphetamine, and the short-acting benzodiazepine triazolam are characterized by both sensory and motor ratio values below 1.0, indicating that the disruptive sensory/motor changes occurred at doses well above the criterion reinforcing dose. A consistent relationship between the sensory and motor effects of these compounds is also apparent because the motor effects appear at lower doses than the sensory effects (i.e., motor ratios higher than sensory ratios). A similar relationship between motor and sensory effects also characterizes the long-acting benzodiazepine diazepam, but disruptive motor effects appear at doses well below those required to maintain self-administration (i.e., motor ratio greater than 1.0). In the case of the dissociative anesthetics, PCP and ketamine are readily differentiated from the other compounds by a reversal of the relationship between sensory and motor effects seen with the other compounds. For PCP and ketamine sensory changes appear at doses below those which produce motor effects (i.e., sensory ratios higher than motor ratios). And PCP appears unique among the compounds thus far studied because both sensory and motor ratio values are greater than 1.0, indicating that the criterion self-administration dose is higher than the doses which produce significant behavioral toxicity.
 

Figure 25: Changes in visual thresholds (top graph) and visual reaction times (bottom graph) over 21 consecutive days of daily intramuscular administration of 1.0 mg/kg diazepam and for subsequent days of saline administration. Further description as in Figure 24. Reproduced with permission from Brady et al., 1984.

The studies with laboratory primates reviewed in this chapter have documented the efficacy of a range of experimental methods and procedures for assessing the reinforcing, discriminative, and eliciting properties of drugs. The good correspondence between the drugs which generate and maintain self-injection in the baboon and those that are abused by man argues convincingly for the validity and utility of abuse liability assessment procedures based upon the reinforcing functions of drugs. In addition, the rapidly developing experimental methodologies for differentiating between drugs on the basis of their discriminative functions enhance the laboratory animal testing procedures by operationalizing the subjective self-report domain which has long been the mainstay of abuse liability assessment armamentaria with human test subjects.
 

Figure 26: Daily pentobarbital self-injections as a function of dose. The criterion dose for self-administration (dotted line) was halfway between the performance maintained by saline (i.e., 1 to 2 injections/day) and the maximal performance on cocaine (i.e., 7 to 9 injections/day). Reproduced with permission from Brady, Lukas, and Hienz, 1983.

Figure 27: Visual threshold shift as a function of pentobarbital dose. Values are the means of three baboons. The criterion sensory threshold toxicity dose (dotted line) was halfway between central and maximal change. Reproduced with permission from Brady, Lukas, and Hienz, 1983.

Figure 28: Relationship between criterion sensory and motor toxicity doses and criterion self-administration doses for three barbiturates, two benzodiazepines, two dissociative anesthetics, and a stimulant. The broken diagonal line represents equality between the reinforcing and toxic doses. Reproduced with permission from Brady et al., 1984.

Standards in the area of dependence potential evaluation on the other hand have, of course, been set for many decades on the basis of the eliciting functions associated with the biochemical, physiological, and behavioral sequelae of repeated drug administration to laboratory animals. The recent pharmacological development of an extended range of agonist, antagonist, and mixed agonist-antagonist compounds, however, has greatly increased the sensitivity and precision (not to mention the speed) with which animal experimental methods for evaluating the eliciting properties of drugs have provided valid and reliable assessments of a pharmacologic agent’s dependence potential in humans.

These studies also show the relevance and importance of evaluating a drug’s eliciting functions as they are expressed in the form of behavioral toxicity now recognized as an essential defining feature of comprehensive assessment approaches involving both abuse liability and dependence potential. The increasingly prominent role and scope of these behavioral methodologies in virtually all aspects of drug assessment have extended the range of dependence potential and abuse liability evaluations and provided a more comprehensive and coherent framework for analysis of drug action in relationship to known standards of pharmacological equivalence.

Research reported in this chapter was supported in part by the National Institute on Drug Abuse Contract 271-83-4023, NIDA grants DA01147, DA02490 and DA00018. Many people have contributed to the studies reported here. We would like especially to thank R. Wurster, J. Snell, R. Atkinson, D. Bowers, B. Campbell, E. Cook, L. Daley, D. Spicer, and P. Thiess. We thank D. Sabotka and J. Nierenberg for their help in preparation of this manuscript.

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