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.     

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.     

Drug effects on response duration differentiation I: differential effects of drugs of abuse

Rats were trained to respond under a response duration differentiation schedule in which responses on a lever were reinforced if lever press durations were greater than or equal to 1.00 s but were also less than 1.30 s. Dose-effect curves were generated for cocaine, methamphetamine, pentobarbital, phencyclidine, delta-9-tetrahydrocanninabol (⁹-THC), and morphine. All drugs produced dose-dependent decreases in accuracy (the percentage of total response durations that were reinforced); however, the degree to which changes in accuracy were accompanied by changes in response rates varied among drugs. Pentobarbital and morphine affected primarily longer (> 1.3 s) response durations, phencyclidine and ⁹-THC affected primarily shorter response durations, whereas cocaine and methamphetamine affected both shorter and longer response durations. High doses of methamphetamine and cocaine increased the dispersion of response duration distributions with increasing dose, whereas higher doses of pentobarbital, ⁹-THC and morphine did not increase dispersion of response duration distributions as much. These data show that behavior under this novel schedule is differentially sensitive to a number of pharmacologic manipulations, and that the schedule can provide a useful addition to the analysis of drug effects upon behavior.     

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.     

Stimulus effects of Δ9-THC and its interaction with naltrexone and catecholamine blockers in rats

Rats trained in a T-shaped maze to discriminate the effects of i.p. injections of ⁹-tetrahydrocannabinol (⁹-THC, 4mg/kg) and the effects of the vehicle were tested for antagonism and generalization to the ⁹-THC stimulus by naltrexone (4 mg/kg), haloperidol (0.32 mg/kg), propranolol (20 mg/kg), and phenoxybenzamine (10 mg/kg). None of these drugs blocked the ⁹-THC stimulus, nor were they found to generalize to ⁹-THC.     

Acute effects of two tetrahydrocannabinols (Δ9-THC and Δ8-THC) on water intake in water deprived rats: Implications for behavioral studies on marijuana compounds

Water intake was studied in water deprived albino rats at various time intervals after injections of two tetrahydrocannabinols ( ⁹-THC and ⁸-THC) and solvents. The dose levels used were: 1.25, 2.5, and 5.0 mg/kg of ⁹-THC and 2.5, 5.0, and 10.0 mg/kg of ⁸-THC. The results show a clear, dose dependent inhibitory effect on water intake as compared to the controls.Reduced intake of food was seen at 1 day post injection. This effect was, however, significant only for the groups treated with 5.0 and 10.0 mg/kg of ⁸-THC. A decreased body weight was also recorded after the drug treatment, especially with ⁸-THC. With respect to cannabis-induced vocalization the data suggest an increased possibility of its appearance with increasing dosages of THC.     

A repeated tests procedure to assess onset and duration of the cue properties of (−)Δ 9-THC, (−)Δ 8-THC-DMH and (+)Δ 8-THC

Rats were trained in a two-lever operant box in a drug discrimination procedure to respond differentially to the effects induced by 3 mg/kg of (-) ⁹-tetrahydrocannabinol and the drug vehicle. Tests with (-) ⁸-THC and the dimethyl-heptyl (DMH) homologue of (-) ⁸-THC indicated that (-) ⁸-THC-DMH was more potent but had a slower onset of action than (-) ⁸-THC. Two ways of testing the onset and duration of action were compared. In one procedure (separate tests) the time course of the drug action was established by testing each time interval on separate days with a new injection each test day, whereas in the other procedure (repeated tests) all intervals were evaluated after a single injection. The results were similar for both procedures. The median time intervals for the decay of the (-) ⁸-THC stimulus were 122 and 127 min for the separate and repeated tests procedures respectively. The median time intervals for the onset of action of the (-) ⁹-THC effects of (-) ⁸-THC were 65 and 62 min for the separate and repeated tests procedures respectively. The median time intervals for the decay of (-) ⁸-THC-DMH (0.30 and 0.56 mg/kg) was between 8 and 24 h after injection. Furthermore, a stereoselective action is indicated, as (+) ⁸-THC (5.6 and 10 mg/kg) did not substitute for (-) ⁹-THC.     

Generalization of the discriminative stimulus properties of x0394; 9 in rats

Rats were trained in a water maze to discriminate between IP injections of 3 mg/kg ⁹- (⁹⁽¹¹⁾-THC) and its vehicle. Both ⁸- and ⁹⁽¹¹⁾ were generalized to the training drug. In contrast to our observations in rhesus monkeys, where ⁹-THC is at least 100 times less potent than ⁹-THC, ⁹⁽¹¹⁾ was found to be only seven times less potent in the rat. Relative potencies, expressed as the dosage at which 50% of the animals gave drug responses (ED₅₀) were 1.8 mg/kg and 12.2 mg/kg for ⁹-THC and ⁹⁽¹¹⁾ respectively. Twenty-four hours after receiving 7×ED₅₀=12 mg/kg ⁹ the tests showed intermediate results when conducted with the training dosage; 4×ED₅₀=50 mg/kg ⁹-THC 48 h prior to the training dosage of 3 mg/kg ⁹-THC completely blocked drug-appropriate responses. Coinjection of ED₅₀ dosages of ⁹- and ⁹⁽¹¹⁾-THC led to 90% drug responses, demonstrating the additivity of the cannabis-like effect of both cannabinoids. Differences in the individual sensitivity of the rats to the tested cannabinoids were observed. Findings are interpreted in terms of the receptor mechanism for cannabis-like activity.This paper is dedicated to the memory of Dr. M. Binder who died on February 15, 1984     

Morphine and δ9-tetrahydrocannabinol: Tolerance to the stimulus effects

Morphine and saline served as discriminative Stimuli for one group of rats in a 2-lever discrimination task. ⁹-THC and vehicle were discriminative stimuli for a second group. Depression of one lever resulted in reinforcement following the administration of morphine or ⁹-THC and the opposite lever was reinforced after vehicle. A high degree of discriminated responding occurred with both drugs. During daily supplemental injections of morphine or ⁹-THC up to several times the training dose for a period of 2 months, subjects still discriminated morphine or ⁹-THC from vehicle. However, the degree of discrimination was reduced indicating a limited tolerance to either drug. Naloxone precipitated narcoticlike withdrawal symptoms in rats chronically treated with either morphine or ⁹-THC, indicating that there is an interaction between chronic administration of ⁹-THC and the narcotic antagonist, naloxone.     

Effects of Δ8-THC,and Δ9-THC on the acquisition of a discriminative positional habit in rats

Using a reversal learning paradigm the dissociative effects of two tetrahydrocannabinols (THC) on the acquisition and reversal of a discriminative positional habit in rats were studied. A T-shaped water maze was used. From these experiments it is concluded that learning under the influence of ⁸-THC (10 and 20 mg/kg), and ⁹-THC (5 mg/kg) is state-dependent (StD) in the rat.Numbering system according to IUPAC rules.Parts of the results were presented at the Symposium on the chemistry and biological activity of cannabis, Stockholm, October 26–28, 1971 and at the Symposium on medical plants of Brazil, Sao Paulo, April 17–20, 1972.     

EEG and behavioral effects of Δ9-Tetrahydrocannabinol in combination with stimulant drugs in rabbits

In rabbits, ⁹-tetrahydrocannabinol (THC) at 0.5 mg/kg, i.v., was challenged with i.v. methamphetamine (0.1 mg/kg), cocaine (1 mg/kg), apomorphine (1 mg/kg), or caffeine (12.5 mg/kg), and observations were made on quantified EEG and behavior. Cortical and hippocampal alterations produced by THC were antagonized by methamphetamine, cocaine, and caffeine and only briefly by apomorphine. Postural and activity behaviors were reversed by methamphetamine and caffeine but only briefly by cocaine and apomorphine. Additionally, stereotypy resulted from the combination of THC with methamphetamine, cocaine, or apomorphine. These data indicate that the effects of THC were antagonized by stimulant drugs of which caffeine was the most effective. However, novel toxicity also resulted from the interaction of THC with catecholaminergic drugs.     

Morphine and Δ9-tetrahydrocannabinol: Two-way cross tolerance for antinociceptive and heart-rate responses in the rat

Tail-flick analgesic responses and heart-rate changes were measured in male Sprague-Dawley rats challenged with an acute IP morphine sulfate (MS) or ⁹-THC injection after receiving daily injections of ⁹-THC or morphine, respectively. Degree of tolerance development to each agent was determined before the cross-tolerance challenge was administered. Cross tolerance occurred to analgesic and bradycardic effects of a 10 mg/kg THC challenge in rats receiving 50 mg/kg MS injections over a 23-day period. Cross tolerance to the bradycardic effects of a 20 mg/kg MS challenge occurred in rats receiving seven daily 10 mg/kg ⁹-THC injections and to MS tail-flick analgesia after 14 days. Although rapid tolerance occurred during administration of both agents, cross tolerance to THC bradycardia occurred only in groups exhibiting complete tolerance to MS injections; cross tolerance to MS bradycardia was observed in animals that were only partially tolerant to THC injections. The data extend earlier cross tolerance data in the mouse to the rat, and provide new information using heart rate, a response that may mirror aversive internal states induced by drugs.     

Analytical Method for the Quantification of 2′,3′-Didehydro-3′-Deoxythymidine,a New Anti-Human Immunodeficiency Virus (HIV) Agent,by High-Performance Liquid Chromatography (HPLC) and Ultraviolet (UV) Detection in Rat and Monkey Plasma

Pharmaceutical Research -     

Pharmacokinetic Interaction Between Intravenous 2′,3′-Dideoxyinosine and Pentamidine in Rats

Purpose. This study examined the pharmacokinetic interaction between 2',3'-dideoxyinosine (ddl) and pentamidine. Background, ddl and pentamidine are often coadministered to patients with acquired immunodeficiency syndrome, and are both associated with pancreatic toxicity. Information on potential interaction would be useful to assess the need for dose modification and the basis of the higher incidence of pancreatic toxicity associated with coadministration of the two drugs. Methods. ddl (200 mg/kg) and pentamidine (10 mg/kg) were administered by continuous infusion to rats over 3 hr, either alone or concomitantly. Drug analysis was by high pressure liquid chromatography with UV or fluorescence detection, or by radioimmunoassay. Results. Pentamidine coadministration significantly increased the apparent volume of distribution at steady state of ddl from 1.4 to 3.4 l/kg (p = 0.004), and increased the mean residence time from 36.3 to 50.0 min (p = 0.015). Pentamidine enhanced the distribution of ddl from plasma into pancreas (p = 0.001) and muscle (p = 0.026). ddl distribution into spleen and liver was also increased, with differences approaching statistical significance (p = 0.08 and 0.06, respectively). In contrast, ddl coadministration did not affect the total body clearance but increased the urinary excretion and the renal clearance of pentamidine by about 5-fold (p = 0.0003). Conclusions. These data indicate that pentamidine increased the distribution of ddl into pancreas and muscle, whereas ddl increased the renal elimination of pentamidine.     

Δ9-Tetrahydrocannabinol impairs spatial memory through a cannabinoid receptor mechanism

The purpose of the present study was to investigate whether the cannabinoid and cholinergic systems impair working memory through a common mechanism. This hypothesis was tested by examining whether the cannabinoid antagonist SR141716A would ameliorate radial-arm performance deficits caused by either the naturally occurring cannabinoid, ⁹-THC, or scopolamine, a muscarinic antagonist. In addition, we evaluated whether the cholinesterase inhibitor, physostigmine, would prevent ⁹-THC-induced impairment of spatial memory. Finally, because the locomotor suppressive effects of cannabinoids may decrease radial arm choice accuracy independent of a direct effect on memory, we examined the impact of increasing the intertrial error on radial arm choice accuracy. As previously reported, ⁹-THC impaired maze performance (ED₅₀=3.0 mg/kg). Increasing the intertrial interval from 5 s to 30 s resulted in a three-fold increase in the amount of time required to complete the maze without affecting choice accuracy. Importantly, SR141716A prevented ⁹-THC-induced deficits in radial-arm choice accuracy in a dose-dependent manner (AD₅₀=2.4 mg/kg); however, the cannabinoid antagonist failed to improve the disruptive effects of scopolamine. Conversely, physostigmine failed to improve performance deficits produced by ⁹-THC. These data provide strong evidence that ⁹-THC impairs working memory through direct action at cannabinoid receptors. Moreover, these results suggest that scopolamine and ⁹-THC do not impair spatial memory in a common serial pathway, though they may converge on a third neurochemical system.     

Metabolism and autoradiographic distribution of Δ8- and Δ9 tetrahydrocannabinol in some organs of the monkey callithrix jacchus

Summary Metabolism and autoradiographic distribution of the two isomeric tetrahydrocannabinols, (2,4-¹⁴C)-⁸-THC and (2,4-¹⁴C)-⁹-THC), were studied in the marmoset Callithrix jacchus. Of the two cannabinoids, ⁸-THC had a slower initial rate of biotransformation to the psychopharmacologically more potent 11-hydroxylated metabolite. This may explain the minor psychopharmacological activity of the ⁸-isomer. In glandular tissues an accumulation of unchanged ⁹-THC was observed.Autoradiography revealed characteristic label distributions in some organs 30 min after the administration of the drugs. This labelling pattern was found to be changed after a 6-hr incorporation period. The autoradiographic distribution of ⁸- and ⁹-THC appeared to be identical.     

Δ 1-tetrahydrocannabinol but not cannabidiol reduces contact and aggressive behavior of rats tested in dyadic encounters

A low and a high dose of ¹-Tetrahydrocannabinol ( ¹-THC) and of cannabidiol (CBD) were IP injected in rats that had been isolated for 7 days. Forty-five minutes after injection, the rats were tested for social interactions with non-isolated, untreated test partners in dyadic encounters under standardized conditions. Different aspects of social behavior were analyzed. The high dose of ¹-THC (10 mg/kg) prevented nearly all social interactions. The low dose of ¹-THC (1 mg/kg) exerted selective and specific effects on social interactions. Social contact behavior, including crawl over/mounting, and social grooming, and aggressive behavior, including fighting, kicking, and biting, were markedly decreased, whereas social exploratory behavior (exploration of the partner and anogeniaal investigation) and the behavioral item, approach/follow, were hardly affected by ¹-THC treatment. Both doses of CBD (2 and 20 mg/kg) failed to change the various aspects of social interaction. It is postulated that the effects of ¹-THC on close and intimate contact behavior of rats may contribute to the understanding of marihuana taking in humans.     

The mechanism of action of Δ 9 -tetrahydrocannabinol on body temperature in mice

The effect of ⁹-tetrahydrocannabinol ( ⁹-THC) on body temperature of the mouse was studied. A dose-response relationship (5–100 mg/kg) for the hypothermic effect of ⁹-THC was seen. The investigation as to the mechanism underlying the hypothermic action of ⁹-THC was also investigated. The relatively specific dopamine antagonist haloperidol potentiated ⁹-THC-induced hypothermia as did the -adrenoceptor antagonist phenoxybenzamine. However, depletion of serotonin with P-chlorophenylalanine reduced the hypothermic response to ⁹-THC as did pretreatment with the serotonin antagonist methysergide. Inhibition of re-uptake of serotonin with clomipramine potentiated the hypothermia following ⁹-THC. It is suggested that the hypothermic effect of ⁹-THC in the mouse is mediated to a large extent via serotonergic mechanisms.     

Effect of some cannabinoids on naloxone-precipitated abstinence in morphine-dependent mice

Mice were rendered morphine-dependent by the subcutaneous implantation of a pellet containing 75 mg of morphine base; 72 h after the implantation, the animals were injected intraperitoneally either with vehicle or with various doses of ₉-tetrahydrocannabinol, ₈-tetrahydrocannabinol, cannabidiol, cannabinol, or 11-hydroxy-₈-tetrahydrocannabinol. Thirty minutes after injection of the cannabinoids, the antagonist, naloxone HCl, was administered to induce the stereotyped withdrawal jumping syndrome. The dose of naloxone needed to induce withdrawal jumping in 50% of the animals (ED₅₀) was determined for each dose of the cannabinoids. All of the cannabinoids inhibited the naloxone-precipitated morphine abstinence as evidenced by an increase in the naloxone ED₅₀. Two additional signs of morphine abstinence, defecation and rearing behavior, were also suppressed by the cannabinoids. The relative effectiveness of the cannabinoids in inhibiting morphine abstinence appeared to be in the following order: ₉-tetrahydrocannabinol > ₈-tetrahydrocannabinol > 11-hydroxy-₈-tetrahydrocannabinol > cannabidiol > cannabinol.These data suggest that cannabinoids may be useful in facilitating narcotic detoxification.     

Influence of (−) Δ9-trans-tetrahydrocannabinol and mescaline on the behavior of rats submitted to food competition situations

The effects of laggy-tetrahydrocannabinol (Lagman-THC) on rat interaction were studied using two different food competition situations. With one of the methods the drug increased winning behavior while with the other an opposite result was obtained. Mescaline increased winning behavior although it has been previously reported as able to inhibit dominance in rats. These data suggest that methodology is an important variable when the effects of drugs on rat interaction is being studied. The results obtained with ⁹-THC are tentatively explained in terms of a drug-induced distortion in perception.This research was submitted by J. M. in partial fulfillment of the requirements for the doctor's degree at Escola Paulista de Medicina. The suggestions and criticisms of Dr. E. A. Carlini are gratefully acknowledged.With a fellowship from Fundação de Amparo à Pesquisa do Estado de São Paulo.