Discriminative stimulus properties of a small dose of cocaine

Discriminative stimulus properties of a small dose of cocaine

Neuropharmacology Vol. 22, pp. 97-101, 1983 Printed in Great Britain 0028-3908/83/01009%05503.00/0 Pergamon Press Ltd DISCRIMINATIVE STIMULUS PROPER...

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Neuropharmacology Vol. 22, pp. 97-101, 1983 Printed in Great Britain

0028-3908/83/01009%05503.00/0 Pergamon Press Ltd

DISCRIMINATIVE STIMULUS PROPERTIES OF A SMALL DOSE OF COCAINE M. W. EMMETT-OGLESBY*,M. WURST and H. LAL Department of Pharmacology, Texas College of Osteopathic Medicine, Fort Worth, TX 76107, U.S.A.

(Accepted 7 July 1982) Summary--This study characterized the interoceptive discriminative stimulus (IDS) produced by a small dose of cocaine. Rats were trained to use a dose of cocaine of 1.25 mg/kg vs saline as the basis for choosing one of two levers for food reinforcement on a fixed ratio 10 schedule. The discrimination was acquired over approx. 60 training sessions, d-Amphetamine generalized to cocaine with approximately equal potency (EDs0's for cocaine and d-amphetamine were 0.07 and 0.06 mg/kg, respectively); 20 mg/kg cocaine and 10mg/kg methylphenidate also generalized to the cocaine lever. Pentylenetetrazol, 20 mg/kg, did not generalize to the cocaine lever, and diazepam, lC mg/kg, did not block the 1.25 mg/kg cocaine discrimination. These data indicate that when a small dose of cocaine is used as the basis of discrimination training, the discriminative stimulus that it produces is qualitatively and quantitatively similar to that produced by small doses of amphetamine, is still discriminated with a large dose of cocaine, and is dissimilar to the discriminative stimulus produced by pentylenetetrazol.

the discriminative stimuli for a particular drug are dose-dependent, and cross-generalize to drugs that humans report as having comparable properties; these discriminative stimuli are also blocked by drugs that block the subjective effects of the drug in man. Indeed, one of the striking properties of drug discrimination procedures is that where direct comparison is possible, results obtained in animals agree precisely with subjective effects reported by man (Glennon and Rosecrans, 1981). Using rats as subjects, the discriminative stimulus produced by 4-10 mg/kg of cocaine has been found to generalize to those produced by amphetamine-type compounds (D'Mello and Stolerman, 1977; Huang and Ho, 1974) and to be blocked by prior administration of dopamine antagonists (Colpaert, Niemegeer and Jannsen, 1978b; McKenna and Ho, 1980). Recently, however, Shearman and Lal (1981) found that in rats trained to discriminate pentylenetetrazol, a large dose of cocaine (20 mg/kg) generalized to the pentylenetetrazol-induced discriminative stimulus, whereas smaller doses (2.5 and 10mg/kg) did not. Furthermore, generalization of cocaine to the pentylenetetrazol discriminative-stimulus was blocked by diazepam and not by haloperidol, suggesting that cocaine in large doses exerts a non-dopaminergic discriminative stimulus. Pentylenetetrazol-induced discriminative stimuli have been proposed (Shearman and Lal, 1981; Lal and Shearman, 1980) to relate to the feelings of intense anxiety this compound induces in man (Rodin and Calhoun, 1970). Among other ob*Address all correspondence to: M. W. Emmettservations, although pentylenetetrazol is a convulsant, Oglesby, Ph.D., Department of Pharmacology, Texas Col- typical non-benzodiazepine anticonvulsants, which lege of Osteopathic Medicine, Fort Worth, TX 76107, block the convulsant effects of pentylenetetrazol but U.S.A. have no anxiolytic or sedative-hypnotic effects in Key words: cocaine, drug discrimination, amphetamine, man, do not block the pentylenetetrazol-induced dismethylphenidate, pentylenetetrazol.

The most frequently reported effect of small doses of cocaine in humans, evaluated under controlled laboratory conditions, is elevation of mood (Byck, Jatlon, Barash and Van Dyke, 1977; Resnick, Kestenbaum and Schwartz, 1977; Fischman and Schuster, 1982). However, although less well known, there are reports of cocaine producing unpleasant subjective effects, such as anxiety (Cohen, 1975; Siegel, 1977; Wesson and Smith, 1977). This anxiogenic effect has not been examined systematically in clinical laboratory investigations, primarily because of the large doses necessary to elicit it. The anxiogenic effect of cocaine has been difficult to evaluate in animals, in part because of the pitfalls inherent in attributing human cognitive and affective states to animals (Honig, 1978) and in part because most animal models of anxiety are intended to detect anxiolytic rather than anxiogenic effects of drugs (e.g. shockbased conflict procedures--for discussion see Lal and Shearman, 1982). One approach to this problem involves the use of drug discrimination paradigms. Over the past decade, research with drugs as discriminative stimuli has demonstrated that this technique is useful in characterizing subjective drug effects in animals (Lal, 1977; Schuster, Fischman and Johanson, 1981). In this procedure, animals are forced to use only the effects of the drug (interoceptive discriminative stimuli, IDS) in order to select one of two or more responses as correct to emit. Results from this procedure show that

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criminative stimulus; on the other hand, diazepam and other anxiolytics do block this response. Based upon those and other considerations, Shearman and Lal (1981) proposed that cocaine in large doses may have discriminative stimulus properties distinctly different from small doses, and these large dose effects may be related to the anxiogenic effect of cocaine in humans. Because of the indication of two different subjective effects produced by small and large doses of cocaine, the present authors tested whether rats could be trained to discriminate a small dose of cocaine. Furthermore, a test was made as to whether the discriminative stimuli produced by a small dose of cocaine differed from those of large doses of cocaine and whether they could also be blocked by administration of diazepam. METHODS

Subjects Male hooded rats of Long-Evans strain (Charles Rivers Breeding Laboratories, Wilmington, MA) weighing between 250-300 g at the beginning of the investigation were used as subjects. The animals were housed in single cages in a large colony room thermostatically maintained at 21 + I°C. Room lights were turned off from 8:00p.m. to 8:00a.m. Water was continuously available in home cages, but food was restricted to 20 g a day, made available 1-4 hr following each operant session. All injections were given intraperitoneally.

Apparatus The behavioral apparatus consisted of conventional behavioral chambers (Coulbourn Instruments) housed in lightproof, sound-attenuating and fan-ventilated shells. Each chamber contained two levers, one on either side of a food cup, which was equidistant from the levers. Scheduling of reinforcement contingencies and recording of data was done through TRS-80 microcomputers (Radio Shack) and printers connected to the chambers through LVB interfaces (Med Associates, Inc.) using a program described by EmmettOglesby, Spencer and Arnoult (1982).

Discrimination training The rats were first magazine-trained and shaped to lever press for food reinforcement (for training methodology see Lal, Gianutsos and Miksic, 1977). The subjects were then trained to press one of the levers 15rain after an injection of cocaine and the other lever 15 rain after an injection of 1 ml/kg of saline. Twelve rats were given a 1.25 mg/kg dose of cocaine and another 10 rats were given 5 mg/kg of cocaine for training. In the latter group, after 15 sessions with cocaine, the dose was reduced to 1.25 mg/kg. Every tenth press (FR 10) on the appropriate lever resulted in the delivery of a 45 mg food pellet (Noyes Co.). Responses on the incorrect lever were recorded but

did not result in the delivery of food. The lever on which responding was first acquired was designated as the "cocaine-correct" lever. Once responding was stable on the fixed ratio 10, saline was injected before each session until the subject obtained at least 30 reinforcements in 20 rain by responding on the "salinecorrect" lever. Subsequently, cocaine was injected before each session until the subjects again obtained 30 reinforcements in 30 rain, this time by responding on the "cocaine-correct" lever. From this point on, the sequence of cocaine/saline injections was alternated irregularly, sessions lasted 10 min per day, and they were conducted 7 days per week. During training sessions, the responses emitted on each lever prior to the first reinforcement were recorded as well as the total session responses on the correct and incorrect levers and numbers of reinforcements received.

Discrimination testin9 When the subjects had attained a stable response rate and made not more than four responses on the incorrect lever (i.e. "saline" lever following injection of cocaine and "cocaine" lever following injection of saline) prior to the first reinforcement (10 responses on the correct lever) on each of ten consecutive sessions, testing of generalization and antagonism was begun. These tests consisted of 10 min sessions separated by at least four consecutive practice sessions in which saline and cocaine were correctly discriminated. Test sessions were always followed by a practice session in which saline was injected. For generalization testing, the doses of drug were administered to all subjects in a randomized order. On the test day, 15 rain after an injection of saline, cocaine or another drug, each rat was placed in its assigned behavioral chamber and allowed to respond until ten responses were completed on one of the levers. The lever on which ten responses were emitted first was considered the selected lever. For antagonism testing, the same procedure was followed except that diazepam, 10mg/kg, was injected 15rain prior to cocaine, 1.25 mg/kg.

Drugs Cocaine.HC1 (Merck Pharmaceuticals), d-amphetamine, sulfate (Sigma), methylphenidate. HCI (CIBAGEIGY) and pentylenetetrazol (Aldridge Chroal) were dissolved in 0.9% saline. All doses refer to the weight of the salt. Diazepam (Roche Pharmaceuticals) was dissolved in propyleneglycol and diluted with saline (1 : 1, v/v) just before injection. RESULTS Both groups of rats, one given a small dose (1.25 mg/kg) from the beginning and the other started with a larger dose (5 mg/kg) and then reduced gradually to the maintenance dose of 1.25 mg/kg, acquired the cocaine-saline discrimination. Eight of the ten rats at 5 mg/kg were able to attain the criterion of

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Fig. 2. Dose-dependent generalization of cocaine stimulus in the rats trained to discriminate cocaine, at a dose of 1.25 mg/kg, from saline. Each point is based upon 8 rats.

TRAmmaSESmON Fig. 1. Acquisition curve of cocaine-saline discrimination in rats. Numbers of training sessions in which either cocaine (1.25mg/kg) or saline was injected are plotted against the percentage of rats selecting the cocaine-appropriate lever. selecting the correct lever on 10 consecutive sessions; the other rats were discarded from the study. Out of 12 subjects trained throughout with 1.25mg/kg, 8 reached the discrimination criterion (Fig. 1). In both groups, EDs0 values of discrimination of cocaine were comparable; therefore, they were pooled for generalization and antagonism testing. The interoceptive stimuli produced by cocaine were dose-dependent. Whereas only 20% of the rats selected the drug-appropriate lever at a dose of 0.02 mg/kg, 92% of the rats selected the drug-lever at 0.64mg/kg (Fig. 2). The calculated EDso (Litchfield and Wilcoxon, 1949) was 0.07 mg/kg with 95% confidence limits of 0.03~3.15 mg/kg. The doses of cocaine that were clearly discriminated did not produce any overt behavioral change. At the time of generalizatior~ testing, the mean response rate was 75 (standard error, 3) responses per minute after saline and 70 (standard error, 3) responses per minute after cocaine. As shown in Table 1, d-amphetamine generalized to cocaine in a dose-dependent manner with comparable potency (EDso = 0.06 mg/kg); methylphenidate,

10mg/kg, and a much larger dose of cocaine [20mg/kg) also generalized to the cocaine stimulus. Pentylenetetrazol, 20 mg/kg, did not generalize to the cocaine discriminative stimulus, nor did diazepam, 10mg/kg, block the cocaine discriminative stimulus when given prior to the injection of cocaine. DISCUSSION The data reported here demonstrate that a small dose of cocaine, 1.25 mg/kg, can produce interoceptive stimuli that rats can discriminate reliably. The discriminative stimuli thus produced are similar to those of d-amphetamine, which generalized to cocaine discriminative stimuli equally with respect to potency and efficacy. These findings differ in two ways from previous studies in which larger doses (4-10 mg/kg) of cocaine were used to establish discriminated responding: first, the EDs0 values for cocaine and amphetamine are significantly lower than found in previous studies (eg, Colpaert, Niemegeer and Jannsen, 1978a); second, in contrast to the present data, d-amphetamine was found to be 4-10 times more potent than cocaine in other studies (Huang and Ha, 1974; Ha and McKenna, 1978). These differences may have arisen in part because of the small dose of cocaine used in the present experiment to train the discrimination;

Table 1. Generalization and antagonism of drugs to the cocaine discriminative stimulus in rats trained to discriminate 1.25 mg/kg cocaine from saline Drug Saline Cocaine Cocaine + diazepam d-Amphetamine Methylphenidate Pentylenetetrazol

Dose (mg/kg)* N 20 1.25 10 0.02 0.16 0.64 l0 20

13 7 1 8 3 9 7 13 13 7

% Selecting cocaine lever 0 86 Behavioral toxicityt 88 Behavioral toxicityt 22 58 92 92 0

* Diazepam was injected 30min before testing. All other drugs were injected 15 rain before testing. t Behavioral toxicity indicates numbers of subjects that did not emit 10 responses on a lever under the test candition.

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smaller training doses typically produce lower EDs0 REFERENCES values (Overton, 1971). Ando, K. and Yanagita, F. (1978). The discriminative The present finding that cocaine and d-amphestimulus properties of intravenously administered tamine produced discriminative stimuli of equal cocaine in rhesus monkeys. In: Stimulus Properties of strength, thought not in agreement with previous data Drugs: Ten Years of Progress (Colpaert, F. C. and Rosecrans, J. A., Eds), pp. 281-299. Elsevier Press, obtained in rats with large training doses, does agree Amsterdam, with results from monkeys (Ando and Yanagita, 1978) Byck, R., Jatlon, P., Barash, P. and Van Dyke, C. (1977). and man (Fischman and Schuster, 1982) where typiCocaine: Blood concentration and physiological effect cally smaller doses are used experimentally. In the after intranasal application in man. In: Cocaine and Fischman and Schuster experiment, human subjects Other Stimulants (Kilby, M. M. and Ellingwood, E. H., Eds), pp. 629-646. 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