Self-administration of Δ<Superscript>9</Superscript>-tetrahydrocannabinol (THC) by drug naive squirrel monkeys

Rationale Interest in therapeutic activities of cannabinoids has been restrained by the fact that they are most often mediated through activation of cannabinoid CB₁ receptors, the same receptors that mediate the effects of ⁹-tetrahydrocannabinol (THC) and are responsible for the abuse liability of marijuana. Persistent intravenous self-administration of THC by animals was first demonstrated in squirrel monkeys and shown to be mediated by CB₁ receptors, but monkeys in the study had a history of cocaine self-administration, raising the possibility that persistent neurobiological adaptations might subsequently predispose animals to self-administer THC.Objectives To demonstrate persistent intravenous self-administration of THC in drug-naive squirrel monkeys.Methods Monkeys with no history of exposure to other drugs learned to press a lever for intravenous injections (0.2 ml in 0.2 s) of THC under a 10-response, fixed-ratio schedule with a 60-s time-out after each injection. Acquisition of THC self-administration was rapid and the final schedule was reached in 11–34 sessions. Dose of THC was then varied from 1 to 16 µg/kg per injection with vehicle extinction following each dose of THC.Results THC maintained significantly higher numbers of self-administered injections per session and higher rates of responding than vehicle at doses of 2, 4 and 8 µg/kg per injection, with maximal rates of responding at 4 µg/kg per injection. Response rates, injections per session and total THC intake per session were two- to three-fold greater in monkeys with no history of exposure to other drugs compared to previous findings in monkeys with a history of cocaine self-administration.Conclusions THC can act as an effective reinforcer of drug-taking behavior in monkeys with no history of exposure to other drugs, suggesting that self-administration of THC by monkeys provides a reliable animal model of human marijuana abuse.     

The opioid antagonist naltrexone reduces the reinforcing effects of Δ9-tetrahydrocannabinol (THC) in squirrel monkeys

Rationale Experimental evidence from animal studies suggests reciprocal functional interactions between endogenous brain cannabinoid and opioid systems. There is recent evidence for a role of the opioid system in the modulation of the reinforcing effects of synthetic cannabinoid CB1 receptor agonists in rodents. Since ⁹–tetrahydrocannabinol (THC), the natural psychoactive ingredient in marijuana, is actively and persistently self-administered by squirrel monkeys, this provides an opportunity to directly study involvement of opioid systems in the reinforcing effects of THC in non-human primates.Objectives To study the effects of naltrexone, an opioid antagonist, on THC self-administration behavior in squirrel monkeys.Methods Monkeys pressed a lever for intravenous injections of THC under a ten-response, fixed-ratio (FR) schedule with a 60-s time-out after each injection. Effects of pre-session treatment with naltrexone (0.03–0.3 mg/kg intramuscularly, 15 min before session) for 5 consecutive days on self-administration of different doses of THC (2–8 µg/kg per injection) were studied.Results Self-administration responding for THC was significantly reduced by pretreatment with 0.1 mg/kg naltrexone for five consecutive daily sessions. Naltrexone pretreatment had no significant effect on cocaine self-administration responding under identical conditions.Conclusions Self-administration behavior under a fixed-ratio schedule of intravenous THC injection was markedly reduced by daily pre-session treatment with naltrexone, but remained above saline self-administration levels. These findings demonstrate for the first time the modulation of the reinforcing effects of THC by an opioid antagonist in a non-human primate model of marijuana abuse.     

Biphasic effects of δ9-tetrahydrocannabinol on variable interval schedule performance in rats

Four rats were trained to barpress for water reinforcement under a variable interval 60 sec schedule. Nine acute administrations of (–) ⁹-trans-tetrahydrocannabinol, in amounts ranging from 0.25 to 16.0 mg/kg, produced dose-related effects on responding; overall response rate increased at lower doses, while higher doses produced ataxia and a complete suppression of responding. Increased response rates reflected changes both in response spacing and in the lengths of post-reinforcement pauses. It was concluded that marihuana has a biphasic effect on variable interval water-reinforced behavior in rats.The authors thank Reid Vandell for his assistance in collecting the data. Research supported by National Institute of Mental Health Grant MH20363-01 to D. P. Ferraro. Synthetic ⁹-THC obtained by approval of the FDA-NIMH Psychotomimetic Agents Advisory Committee. The animals involved in this study were maintained in accordance with Guide for Laboratory Animal Facilities and Care as published by the National Academy of Sciences-National Research Council.     

Effects of Δ9-Tetrahydrocannabinol (THC) and chlordiazepoxide (CDP) on state-dependent learning: evidence for asymmetrical dissociation

⁹-Tetrahydrocannabinol (THC) and chlordiazepoxide (CDP) were studied for their effects upon acquisition, performance, state-dependent learning and reciprocal substitution in mice. Over a wide dose range, CDP had no effect while THC had a biphasic effect (depression at low doses and facilitation at high doses) on avoidance acquisition. Both agents elicited evidence for state-dependent learning; mice trained under drugged conditions failed to transfer learning to the non-drugged state. In contrast, performance decrements occurred only after high doses (40 mg/kg) of each were given to avoidance trained mice. Administration of CDP facilitated avoidance performance in drug naive mice after doses of 1.25 mg/kg and above. THC failed to prevent learning deficits in CDP-trained subjects. In contrast, CDP prevented avoidance deficits after doses of 5 mg/kg and above in THC-trained subjects. These results suggest that an asymmetrical dissociation exists between THC and CDP or between either agent and saline.Presented in part at 5th International Congress on Pharmacology, July 23–28, 1972, San Francisco, California.     

The effect of δ 9-tetrahydrocannabinol and LSD on the acquisition of an active avoidance response in the rat

The course of active avoidance learning of rats in a symmetrical Y-maze under the influence of 1, 3, and 9 mg/kg of ⁹-THC i.p., and 5, 20, and 100 g/kg of LSD was investigated. ⁹-THC in a dosage of 1 mg/kg had no effect on avoidance learning. Three and to a lesser extent 9 mg/kg produced more rapid learning with a significantly better performance. Learning under ⁹-THC proved to be statedependent. The withdrawal of ⁹-THC caused a decrease in the avoidance rate, which was dependent on the dosage. Upon renewal of the THC doses, the animals reattained their earlier performance. In the course of the experiment there was rapid tolerance development, especially of the sedative properties of THC.LSD retarded the rate of acquisition of the active avoidance response. Whereas the control animals displayed over 80% successful active avoidance from the 14th session onwards, this was achieved by the LSD groups only after the 20th session. However, in contrast to the control group the LSD animals were able to increase their avoidance rate to over 90%, and this was maintained to the end of the experiment (a total of 24 sessions with LSD). The sudden withdrawal of LSD produced a fall in avoidance rate, which was dependent on the previous training dosage; as with ⁹-THC state-dependent learning can also be assumed for LSD.     

Acute and chronic effects of Δ9-Tetrahydrocannabinol on complex behavior of squirrel monkeys

Squirrel monkeys were, trained to press either two (phase one) or five (phase two) differently colored keys sequentially. Food presentation resulted if colors were pressed in a specific order, and high levels or accuracy were generated. Acutely, ⁹-tetrahydrocannabinol reduced accuracy and rate of responding in a dose-related fashion under both the two-key and fivekey conditions. Responding, however, was more sensitive to the drug under the five-key procedure. Accuracy of responding at the beginning of a sequence tended to be more sensitive to drug effects than responding near the end. Daily (chronic) administration resulted in the development of tolerance to both the rate-and accuracy-reducing effects of the drug, although tolerance developed more rapidly to the accuracy-reducing effects. Tolerance developed more slowly under the five-key procedure than under the two-key procedure. Details of tolerance development were related to aspects of acute effects, suggesting that some facets of tolerance development may be predictable from acute drug effects.     

Cocaine,d-amphetamine,and pentobarbital effects on responding maintained by food or cocaine in rhesus monkeys

The effects of IM injections of cocaine, d-amphetamine, and pentobarbital were studied in rhesus monkeys whose lever-press responding was maintained under a second-order fixed-interval, fixed ratio schedule of reinforcement. Within each session, fixed-interval components, ending with the IV injection of 30 g/kg cocaine (one group of monkeys) or the delivery of a 300 mg food pellet (second group of monkeys), alternated with fixed-interval components ending without an injection of cocaine or the delivery of food (extinction). Drug pretreatments generally caused comparable dose-related decreases in the overall rates of responding reinforced either by cocaine or by food. Response rates during extinction usually increased and then decreased as the dose of each drug increased. An analysis of the drug effects on response rates in different temporal segments of the fixed intervals showed that in both the reinforcement and extinction components, the normally low control rates of responding which occurred earlier in the intervals were usually increased, while higher control rates which occurred later in the intervals were increased less or decreased. Thus, the effects of these drugs were relatively independent of the reinforcing event (food or cocaine) and tended to depend more on the ongoing rate of responding under these conditions.     

Modulation of avoidance behavior in squirrel monkeys after chronic administration and withdrawal of d-amphetamine or α-methyl-p-tyrosine

Two squirrel monkeys were trained on a nondiscriminated (Sidman) avoidance schedule that presented a conditional aversive stimulus (CAS) whenever the animals failed to respond within 20 sec. Shock was paired with the CAS 20% of the time. A 3 min tone followed by unavoidable shock was superimposed upon this avoidance schedule. Amphetamine (1.0, 2.0 mg/kg) increased responding without consistently affecting shock or CAS rate, while -methyl-p-tyrosine (150, 225 mg/kg) decreased response rate and led to more CAS presentations and shocks. Withdrawal of amphetamine produced behavioral effects similar in direction but not intensity to those seen after the administration of -methyl-p-tyrosine. Neither drug reliably altered the facilitation of avoidance response rate normally noted during the 3-min tone. These results were interpreted to reflect the role of the catecholamines in modulating the performance of an avoidance task. Furthermore, an attempt was made to speculate on the mechanism that may be responsible for the behavioral effects noted after the withdrawal of chronic amphetamine administration.     

Effects of opiate antagonists and putative κ agonists on unpunished and punished operant behavior in the rat

The effects of naloxone and diprenorphine, opiate antagonists with different receptor-binding properties, and the putative -receptor agonists, ketocyclazocine and ethyl-ketocyclazocine (EKC), were studied on food-reinforced responding in rats. Behavior was maintained under a multiple-component 1-min variable-interval schedule in which 12-min periods of unpunished responding alternated with 4-min periods in which each response was punished by a brief electric footshock. Daily sessions were 1 h. Naloxone (0.01–10 mg/kg) decreased unpunished responding only slightly; punished responding was decreased significantly to 66% of control by 10 mg/kg. Diprenorphine (0.01–10 mg/kg) did not affect unpublished responding and increased punished responding dose-dependently to as much as 190% of control. EKC (0.01–1.0 mg/kg) decreased unpunished and punished responding dose-dependently and comparably, whereas ketocyclazocine (0.01–1.0 mg/kg)decreased unpublished responding but did not significantly affect punished responding. Diprenorphine was more potent than naloxone in blocking the decreases in responding produced by the agonists. Differences in the behavioral effects of naloxone and diprenorphine appear to reflect the different receptor-binding properties of the two opiate antagonists.     

Interaction between Δ 9-tetrahydrocannabinol and morphine on the motor activity of mice

The present experiments dealt the effects of ⁹-tetrahydrocannabinol (THC) on the locomotor activity stimulating action of morphine in mice. In the first series of experiments, the pretreatments of mice by THC in doses up to 20 mg/kg have been found to potentiate the morphine-induced hyperactivity in dose-dependent manner, but higher doses of THC did not produce such an action. In the second series of experiments the dose-response curve of morphine for the motor activity has been found to shift to the left by the pretreatment of mice with 10 mg/kg of THC. These results show a synergism between morphine and THC and suggest that both drugs may share some common site of action.     

Effects of (−) δ9-trans-tetrahydrocannabinol on social behavior of squirrel monkey dyads in a water competition situation

δ⁹-THC in doses of 0.25, 0.5 or 1.0 mg/kg was administered to pairs of squirrel monkeys in a water competition test. After stable baseline measures for the pairs were obtained, repeated testing of pairs followed the sequence of orally treating the dominant member, the submissive member, and both members of the pairs. Total competition behaviors decreased when both members received 1 mg/kg, but increased when either the submissive member or both members of low competitive pairs received 0.25 mg/kg. Noncompetitive social behaviors increased when both animals were drugged, an effect which was maximal at 1.0 mg/kg. δ ⁹-THC produced the most salient effects when both animals were drugged and demonstrated a biphasic dose effect on competition.     

Discriminative effects of combinations of Δ9-Tetrahydrocannabinol and pentobarbital in pigeons

The purpose of the present study is to examine potentially additive effects of pentobarbital and ⁹-Tetrahydrocannabinol ⁹-THC using a drug discrimination paradigm. Three groups of pigeons were trained to discriminate between the effects induced by i.m. administrations of either (a) 0.25 mg/kg ⁹-THC and vehicle, (b) 4 mg/kg pentobarbital and saline, and (c) ⁹-THC and pentobarbital. Test probes under extinction conditions produced orderly dose generalization gradients among the drug-vs nondrug-trained animals. ED₅₀ for pentobarbital was 1.60 mg/kg and ED₅₀ for ⁹-THC was 0.10 mg/kg. Tests in birds discriminating between pentobarbital and ⁹-THC suggested a sharpening of the drug cue effects; tests with the vehicles resulted in approximately a random key selection (33%–66%) while tests with combinations of the two training drugs suggested that ⁹-THC was the more salient cue in this group. Tests with combinations of various doses of pentobarbital and ⁹-THC in the drug-vs nondrug-trained birds did not increase responding on the respective drug-training associated key. Thus the cue effects of pentobarbital and ⁹-THC were not summational under these experimental conditions. In conclusion, rather low doses of pentobarbital and ⁹-THC are effective as discriminative cues in pigeons and the cues thus induced are different. Combinations of the two drugs are not necessarily summational, and the pentobarbital vs the ⁹-THC discriminations augmented the discriminable effects of the two drugs. In addition, the analeptic drug, bemegride, antagonizes the pentobarbital (4 mg/kg) stimulus in the group trained to discriminate between this barbiturate and saline, which agrees with earlier drug antagonism data obtained among mammals (gerbils and rats), required to discriminate between barbiturates and the nondrug condition.     

Effects of propranolol and chlordiazepoxide on conflict behavior in rats

Clinical reports have suggested that propranolol, a -adrenergic blocker, has antianxiety properties. In the present laboratory experiments, this suggestion was explored by testing propranolol and chlordiazepoxide in rat lever-pressing conflict procedures that selectively identify antianxiety compounds. Chlordiazepoxide produced anticonflict effects in such a procedure at doses from 1.25 to 40 mg/kg. Propranolol, tested at doses of 2.5 to 80 mg/kg, did not produce the anticonflict pattern typical of standard antianxiety agents, which is characterized by progressive dose-related increases in punished responding that frequently are several 100% above control levels. In contrast, the largest increases produced by propranolol, which occurred in the 10–40 mg/kg dose range, were 18–26% above control; only the 26% increase observed at the 20 mg/kg dose was statistically significant. In a different conflict test, a 40 mg/kg dose of propranolol was found to have a slight, but not significant, anticonflict effect. When this dose level was combined with doses of 2.5, 5, or 10 mg/kg of chlordiazepoxide p.o., the combined treatments produced greater anticonflict effects than did the corresponding individual doses of chlordiazepoxide.     

Effects of morphine alone and in combination with naloxone or d-amphetamine on shock-maintained behavior in the squirrel monkey

Key-pressing behavior in the squirrel monkey was maintained under an 8-min fixed-interval (FI) schedule of electric-shock delivery. The acute i.m. administration of morphine prior to a daily session decreased response rates at doses of 1.0–3.0 mg/kg but had little systematic effect on rate at doses of 0.03–0.3 mg/kg. When naloxone was administered concomitantly with morphine prior to a session, 0.01 mg/kg naloxone required a three-fold increase in the dose of morphine necessary to obtain decreased response rates, 0.1 mg/kg naloxone required a 30-fold increase in morphine, and 1.0 mg/kg required more than a 30-fold increase in morphine. Moreover, the administration of naloxone with morphine resulted in increased rates of responding at certain combinations of doses of the two drugs. The administration of d-amphetamine (0.03 or 0.1 mg/kg) alone increased mean response rates under the FI schedule; when combined with 0.03–0.3 mg/kg morphine the increases in responding were greater than obtained with d-amphetamine alone. The negative slope of the linear regression lines relating the effects of morphine to control rates of responding engendered under the FI schedule was decreased when morphine was combined with naloxone, but not with d-amphetamine. These results show that naloxone, but not d-amphetamine, can antagonize the response-rate decreasing effect of morphine when responding in the squirrel monkey is maintained by response-produced electric shock.     

Facilitory effect of Δ9-tetrahydrocannabinol on hypothalamically induced feeding

Six male Lewis rats were tested for the effect of ⁹-tetrahydrocannabinol (⁹-THC) on feeding evoked by electrical stimulation of the lateral hypothalamus. Treatment with ⁹-THC (0.4 mg/kg IP) decreased frequency threshold for feeding by 20.5% (±4.3), causing a leftward shift in the function relating stimulation frequency to the latency to begin eating 45-mg food pellets upon stimulation onset; there was no change in the asymptotic performance that was approached with sufficiently high stimulation frequencies. Naloxone (1 and 2 mg/kg) reduced the facilitory effect of ⁹-THC, but did so at doses that can inhibit feeding in the no-drug condition. These data are consistent with evidence implicating endogenous opioids in feeding, and suggest (but do not confirm) that the facilitation of feeding by ⁹-THC may be mediated by endogenous opioids. The facilitation of stimulation-induced feeding by doses of ⁹-THC that have been found to facilitate brain stimulation reward is consistent with evidence suggesting common elements in the brain mechanisms of these two behavioral effects of medial forebrain bundle stimulation.     

Behavioural effects of selective μ-, κ-, and δ-opioid agonists in neonatal rats

The behavioural effects of selective -, - and -opioid agonists in 5-, 10- and 20-day-old rats were investigated by observational analysis. The predominant response to -agonists was behavioural depression. High doses (10 mg/kg IP) of morphine and DAGO (d-Ala², NMe-Phe⁴, Glyol⁵-enkephalin) produced overt sedation in all the age groups and also induced catalepsy which was particularly apparent in the 5- and 10-day-old animals. These compounds did not produce any signs of behavioural activation in the neonatal rats. In contrast, rat pups treated with the -agonists U50,488H and PD 117,302 (1,10 mg/kg IP) exhibited marked hyperactivity with increases in wall-climbing and locomotion. Sedative effects of the highest dose of the -agonists began to emerge, however, as the animals grew older, resulting in significant decreases in behaviours such as gnawing and grooming at 20 days of age. The -agonist (+)-tifluadom (0.1–10 mg/kg), but not its corresponding (-)-isomer, produced an increase in activity in 5-day-old rats, thus extending the observations made with U50,488H and PD 117,302 and establishing the stereoselective nature of the response. The involvement of -receptors in opioid-induced hyperactivity was further substantiated by using a variety of opioid antagonists. In this context, the increase in activity induced by U50,488H (10 mg/kg) in 5-day-old neonates was attenuated by naltrexone (1 mg/kg IP) but not by larger doses (10 mg/kg) of either M8008 (which has low affinity for -receptors) or the selective -receptor antagonist ICI 174,864. Finally, DPDPE (d-Pen², d-Pen⁵-enkephalin) which acts selectively at -opioid receptors, did not exert any behavioural effects in either the 5-, 10- or 20-day-old rat pups at doses of up to 10 mg/kg. These results demonstrate behavioural effects of - and -but not -agonists in neonatal rats. There is a clear differentiation between - and -receptor effects and both - and -mediated behaviours show dissimilarities from the adult profile.     

Effects of marihuana extract on the operant behavior of chimpanzees

Six chimpanzees were trained to panel push under a food reinforcement baseline in which three operant schedules, each associated with a different stimulus, were presented successively. The fixed ratio (FR) reinforcement schedule required the emission of 40 responses for reinforcement. Reinforcement under the differential reinforcement of low rate (DRL) schedule was delivered only when successive responses were spaced by at least 10 sec. During the extinction or time out from positive reinforcement schedule (TO), no responses were reinforced. In Experiment 1, amounts of marihuana extract containing from 0.2 to 4.0 mg/kg (?)-Δ ⁹-trans-tetrahydrocannabinol (Δ ⁹-THC) were orally administered 1 h prior to experimentation. In Experiment 2, 1.0 mg/kg Δ ⁹-THC was orally administered between 1 and 23 h prior to experimental sessions. No disruption of stimulus control or drug effects during TO were observed. Both DRL and FR response suppression occurred at the highest drug dose. Lower Δ ⁹-THC doses produced facilitation of DRL responding up to 12 h following drug administration. Although FR responding was less sensitive, Δ ⁹-THC stimulated FR behavior from 2 to 5 h following drug administration. It was concluded that marihuana has a biphasic effect on food reinforcement schedule controlled operant behavior.     

Effects of apomorphine and haloperidol on “spontaneous” stereotyped licking behaviour in the Cebus monkey

Three recently arrived drug naive Cebus apella monkeys with spontaneous stereotyped oral movements were treated with apomorphine and haloperidol using a wide dose range. Low doses of apomorphine (0.05–0.1 mg/kg) suppressed the oral stereotypies without affecting normal behaviour such as grooming and scratching. Higher doses of apomorphine (0.25–1.0 mg/kg) and haloperidol (0.01–0.1 mg/kg) also decreased or abolished the oral stereotypies, but induced generalized stereotypies (apomorphine) or dystonia/parkinsonism (haloperidol), suppressing normal behaviour. The findings indicate that dopamine is involved in these presumably stress-induced (not drug-induced) stereotypies.     

Pharmacological specificity of Δ9-tetrahydrocannabinol discrimination in rats

While many previous studies have shown that a variety of cannabinoids substitute and cross-substitute for ⁹-tetrahydrocannabinol (THC) in drug discrimination procedures, few have systematically examined potential THC-like effects of non-cannabinoid compounds. The purpose of the present study was to delineate further the pharmacological specificity of THC discrimination. Rats were trained to discriminate THC (3.0 mg/kg) from vehicle. Following determination of a dose-effect curve with THC, substitution tests with selected compounds from a variety of pharmacological classes, includingl-phenylisopropyl adenosine, dizocilpine, dextromethorphan, clozapine, buspirone, MDL 72222, muscimol, midazolam and chlordiazepoxide, were performed. Whereas THC produced full dose-dependent substitution, substitution tests with non-cannabinoid drugs resulted in less than chance (50%) levels of responding on the THC-appropriate lever, with the exception of (+)-MDMA (2.5 mg/kg, 50%) and diazepam (3.0 mg/kg, 67%). These results are consistent with those of previous studies and suggest that the discriminative stimulus effects of THC exhibit pharmacological specificity.     

The interaction between prostaglandin E1 andΔ 9-Tetrahydrocannabinol on intestinal motility and on the abdominal constriction response in the mouse

The interaction between ⁹-tetrahydrocannabinol (THC) and PGE₁ was studied using two pharmacological parameters-the rate of passage of a charcoal meal through mouse small intestine and the abdominal constriction response in the mouse. PGE₁ administered intraperitoneally produced a dose-dependent decrease in intestinal motility, and this effect was antagonized by low (0.25 mg/kg) doses of THC and potentiated by higher doses of THC (1 mg/kg). Kinetic analysis suggested that the interaction was of a mixed but predominantly competitive type. PGF₂ produced an increase in intestinal motility but this was not dose-dependent. THC antagonized the effect of PGF₂ in a dose-dependent manner suggestive of a physiological antagonism.THC (0.25–2 mg/kg) antagonized the dosedependent PGE₁ abdominal constriction response in a fashion which suggested a mixed (though mainly competitive) antagonism.It would seem, therefore, that on the two pharmacological parameters studied THC appears to be interacting with PGE₁ at the same receptor site.Although the doses of THC used are within the range of those used in man, it is not implied that these results are necessarily implicated in the psychoactivity of the drug.