Behavioural Pharmacology

Published by Lippincott, Williams & Wilkins
Print ISSN: 0955-8810
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The emergence of high-efficacy synthetic cannabinoids as drugs of abuse in readily available K2/'Spice' smoking blends has exposed users to much more potent and effective substances than the phytocannabinoids present in cannabis. Increasing reports of adverse reactions, including dependence and withdrawal, are appearing in the clinical literature. Here we investigated whether the effects of one such synthetic cannabinoid, 1-pentyl-3-(1-naphthoyl)indole (JWH-018), would be altered by a prior history of Δ-tetrahydrocannabinol (Δ-THC) exposure, in assays of conditioned taste aversion and conditioned place preference. In the conditioned taste aversion procedure, JWH-018 induced marked and persistent aversive effects in mice with no previous cannabinoid history, but the magnitude and duration of these aversive effects were significantly blunted in mice previously treated with an ascending dose regimen of Δ-THC. Similarly, in the conditioned place preference procedure, JWH-018 induced dose-dependent aversive effects in mice with no previous drug history, but mice exposed to Δ-THC before place conditioning showed reduced aversions at a high JWH-018 dose and apparent rewarding effects at a low dose of JWH-018. These findings suggest that a history of Δ-THC exposure 'protects' against aversive effects and 'unmasks' appetitive effects of the high-efficacy synthetic cannabinoid JWH-018 in mice. This pattern of results implies that cannabinoid-naive individuals administering K2/'Spice' products for the first-time may be at an increased risk for adverse reactions, whereas those with a history of marijuana use may be particularly sensitive to the reinforcing effects of high-efficacy cannabinoids present in these commercial smoking blends.
 
Nicotine withdrawal produces cognitive deficits that can predict relapse. Amelioration of these cognitive deficits emerges as a target in current smoking cessation therapies. In rodents, withdrawal from chronic nicotine disrupts contextual fear conditioning (CFC), whereas acute nicotine enhances this hippocampus-specific learning and memory. These modifications are mediated by β2-subunit-containing (β2*) nicotinic acetylcholine receptors in the hippocampus. We aimed to test ABT-089, a partial agonist of α4β2*, and ABT-107, an α7 nicotinic acetylcholine receptor agonist, for amelioration of cognitive deficits induced by withdrawal from chronic nicotine in mice. Mice underwent chronic nicotine administration (12.6 mg/kg/day or saline for 12 days), followed by 24 h of withdrawal. At the end of withdrawal, mice received 0.3 or 0.6 mg/kg ABT-089 or 0.3 mg/kg ABT-107 (doses were determined through initial dose-response experiments and prior studies) and were trained and tested for CFC. Nicotine withdrawal produced deficits in CFC that were reversed by acute ABT-089, but not ABT-107. Cued conditioning was not affected. Taken together, our results suggest that modulation of hippocampal learning and memory using ABT-089 may be an effective component of novel therapeutic strategies for nicotine addiction.
 
Increased oxidative stress is implicated in the pathogenesis of Parkinson's disease in which dopaminergic neurons are intrinsically susceptible to oxidative damage. Swiss albino mice were pretreated with Pycnogenol (PYC), an extract of Pinus maritime bark [20 mg/kg body weight, intraperitoneally (i.p.)] once daily for 15 days. Thereafter, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (20 mg/kg body weight, intraperitoneally) was given four times at 2-hour intervals on 1 day only. Behaviours were altered in the MPTP group as compared with the vehicle-treated group and were restored in the PYC-pretreated MPTP group. The activity of antioxidant enzymes and the content of glutathione were significantly depleted in the MPTP-induced Parkinsonian group. The MPTP group pretreated with PYC showed significant protection of the activity of antioxidant enzymes and glutathione content when compared with the vehicle-treated MPTP group. A significantly elevated level of thiobarbituric acid reactive substances in the MPTP group was decreased significantly in the animals pretreated with PYC. An increase in the number of dopaminergic D2 receptors and decrease in the level of dopamine and its metabolite 3,4-dihydroxyphenyl acetic acid in the striatum were observed after MPTP injection, and restored significantly after PYC pretreatment. Thus, PYC may be used to prevent or reduce the deterioration caused by free radicals, thereby preventing subsequent behavioural and biochemical changes that occur in Parkinsonian mice.
 
The atypical antidepressant, bupropion, causes a partial reversal of motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primates. However, its monoamine uptake blocking actions are believed to be mediated by the major metabolites, racemic (-)-(2R,3R)-2-(3-chlorophenyl-3,5,5-trimethyl-2-morphinol) (R,R-hydroxybupropion) and (+)-(2S,3S)-2-(3-chlorophenyl-3,5,5-trimethyl-2-morphinol) (S,S-hydroxybupropion). Therefore, we have evaluated the ability of enantiomers to improve locomotor activity and motor disability in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated common marmosets. Bupropion produced a little increase in locomotor activity and a more pronounced improvement in motor disability. The S,S-hydroxybupropion, but not the R,R-hydroxybupropion, enantiomer dose-dependently increased both locomotor activity and reversed motor disability. Combined administration of S,S-hydroxybupropion and R,R-hydroxybupropion at the same dose (analogous to the racemate) again improved motor function and to the same extent as produced by S,S-hydroxybupropion alone. The data suggest that the S,S-enantiomer of hydroxybupropion may possess potential antiparkinsonian activity.
 
Pigeons trained to discriminate 0.1 mg/kg flumazenil, proposed as an in-vivo model to study interactions with diazepam-insensitive gamma-aminobutyric acid (GABA)A receptors, were tested with various GABAergic and non-GABAergic compounds. As a result of its pharmacological selectivity, the model was suitable for further examining previously reported flumazenil-like effects of gamma-hydroxybutyrate (GHB). Flumazenil and the GABAA negative modulator Ro 15-4513 produced 82-100% flumazenil-appropriate responding. Diazepam and the direct-acting GABAA agonists muscimol and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol (THIP) produced 38-64% flumazenil-appropriate responding. GHB, its precursors 1,4-butanediol (1,4-BD) and gamma-butyrolactone (GBL), and the GABAB agonists baclofen and SKF97541 produced 0-24% flumazenil-appropriate responding. Baclofen shifted the flumazenil dose-response curve to the right and down, possibly involving perceptual masking of the discriminative stimulus effects of flumazenil by agonist activity at GABAB receptors. These masking effects of baclofen were blocked by the GABAB antagonist CGP35348. When CGP35348 was given together with GHB to block its GABAB agonist effects, GHB did not produce flumazenil-appropriate responding. Conceivably, effects of GHB at non-GABAB receptors (e.g. diazepam-sensitive GABAA receptors and GHB receptors) may interfere with the expression of its flumazenil-like discriminative stimulus effects. The asymmetric substitution between GHB and flumazenil is consistent with the hypothesis that the discriminative stimulus effects of GHB consist of several components, not all of which are mimicked by flumazenil.
 
Gamma-hydroxybutyric acid has been proposed as a pharmacotherapy for alcoholism in part based on similar discriminative stimulus effects as ethanol. To date, drug discrimination studies with gamma-hydroxybutyric acid and ethanol have exclusively used rodents or pigeons as subjects. To evaluate possible differences between species, sex, and route of administration, this study investigated the substitution of gamma-hydroxybutyric acid (intragastrically or intramuscularly) for ethanol 30 or 60 min after administration in male (n=6) and female (n=7) cynomolgus monkeys trained to discriminate 1.0 and 2.0 g/kg ethanol. At least one dose of gamma-hydroxybutyric acid completely or partially substituted for ethanol in three of the 13 monkeys tested, with each case occurring in female monkeys. Ethanol-appropriate responding did not increase with gamma-hydroxybutyric acid dose. Monkeys were more sensitive to the response rate decreasing effects of gamma-hydroxybutyric acid administered intramuscularly compared with intragastrically. The lack of gamma-hydroxybutyric acid substitution for ethanol suggests that these drugs have different receptor bases for discrimination. Furthermore, the data do not strongly support shared discriminative stimulus effects as the rationale for gamma-hydroxybutyric acid pharmacotherapy for alcoholism.
 
The selective serotonin (5-HT) reuptake inhibitor, citalopram, is a racemic mixture of the stereoisomers, S-(+)-citalopram (escitalopram) and R-(-)-citalopram (R-citalopram). R-citalopram has been shown to counteract the 5-HT enhancing properties of escitalopram in acute studies in animals. In the present study we report, for the first time, on an interaction between R-citalopram and escitalopram after repeated dosing in a rat chronic mild stress (CMS) model of depression. The effect of escitalopram (2.0, 3.9 and 7.8 mg/kg per day), R-citalopram (7.8 mg/kg per day) and escitalopram 3.9 mg/kg per day plus R-citalopram 7.8 mg/kg per day were studied and compared to the effect of citalopram (8.0 mg/kg per day), imipramine and R-fluoxetine (8.9 mg/kg per day). Significant effects relative to a vehicle-treated group were achieved from week 1 for escitalopram (3.9 and 7.8 mg/kg per day), from week 2 for citalopram (8.0 mg/kg per day), from week 3 for R-fluoxetine (8.9 mg/kg per day) and from week 4 for escitalopram (2.0 mg/kg per day) and imipramine (8.9 mg/kg per day). R-citalopram (7.8 mg/kg per day) and escitalopram (3.9 mg/kg per day) plus R-citalopram (7.8 mg/kg per day) did not differ significantly from vehicle. There were no drug-induced effects in non-stressed control groups. In conclusion, escitalopram showed a shorter time to response in the rat CMS model of depression than citalopram, which was faster acting than R-fluoxetine and imipramine. R-citalopram counteracted the effect of escitalopram. The mechanism of action of R-citalopram is, at the moment unclear, but may be relevant to the improved clinical antidepressant activity seen with escitalopram in comparison with citalopram, and may also indicate an earlier response to escitalopram compared to other selective serotonin reuptake inhibitors (SSRIs).
 
Clozapine, the prototype for atypical antipsychotic drugs, is used in the drug discrimination paradigm as a model for screening atypical from typical antipsychotic drugs. Previous drug discrimination studies in rats have shown that a 1.25 mg/kg clozapine training dose provides full stimulus generalization (i.e.) >or=80% condition-appropriate responding) to most atypical antipsychotic drugs, although a 5.0 mg/kg clozapine training dose appears necessary to provide stimulus generalization to other atypical antipsychotic drugs. The present study sought to characterize the pharmacological mechanisms that mediate these clozapine training doses. In rats trained to discriminate 1.25 vs. 5.0 mg/kg clozapine vs. vehicle in a three-choice drug discrimination task, various receptor-selective compounds were tested for stimulus generalization. The antidepressant mianserin was also tested. Full stimulus generalization from the 1.25 mg/kg clozapine training dose occurred only to mianserin (98.8%). Partial substitution (i.e. >or=60% and <80% condition-appropriate responding) to the 5.0 mg/kg clozapine training dose occurred for the muscarinic receptor antagonist scopolamine. The combined total percentage of responding on the 1.25 and 5.0 mg/kg clozapine levers, however, was well above the full substitution criteria at the 0.25, 0.5, and 1.0 mg/kg scopolamine doses. The M1 agonist N-desmethylclozapine, the nicotinic antagonist mecamylamine, the D1 antagonist SCH 23390, the D4 antagonist LU 38-012, the 5-HT1A agonist (+)-8-OH-DPAT, the 5-HT1A antagonist WAY 100 635, the 5-HT2A/2B/2C antagonist ritanserin, the 5-HT6 antagonist RO4368554, the alpha1 antagonist prazosin, the alpha2 antagonist yohimbine, and the histamine H1 antagonist pyrilamine all failed to substitute for either the 1.25 or the 5.0 mg/kg clozapine training doses. These results are consistent with previous evidence that antidepressant drugs have a tendency to substitute for clozapine and that muscarinic receptor antagonism may mediate the discriminative stimulus properties of 5.0 mg/kg clozapine. The lack of stimulus generalization from either clozapine training dose to other receptor-selective compounds, however, fails to explain how this model screens atypical from typical antipsychotic drugs and suggests that the discriminative stimulus properties of clozapine consist of a compound cue.
 
The prototypical atypical antipsychotic drug (APD) clozapine (CLZ) elicits a discriminative cue that appears to be similar to the stimulus properties elicited by atypical, but not typical, antipsychotic drugs in two-choice drug discrimination procedures. However, the ability of CLZ to generalize to atypical APDs depends on the training dose, since several atypical APDs (e.g. sertindole, risperidone) do not substitute for a 5.0 mg/kg CLZ training dose in rats, but do so for a 1.25 mg/kg CLZ training dose. Yet, a 1.25 mg/kg CLZ discriminative stimulus has not generalized to all atypical APDs either (e.g. quetiapine); thus, both 1.25 mg/kg and 5.0 mg/kg CLZ discriminative stimuli may be necessary to provide a better screen for atypical APDs. The present study sought to determine whether a three-choice 1.25 mg/kg CLZ versus 5.0 mg/kg CLZ versus vehicle drug discrimination task in rats might better distinguish atypical from typical APDs. Adult male Sprague-Dawley rats were trained in this three-choice drug discrimination task with a fixed ratio 30 reinforcement schedule for food. Clozapine produced full substitution (>or=80% condition-appropriate responding) for both the 1.25 mg/kg CLZ dose (ED50=0.09 mg/kg) and the 5.0 mg/kg CLZ dose (ED50=2.71 mg/kg). The atypical APD olanzapine produced full substitution for the 5.0 mg/kg CLZ dose, but not for the 1.25 mg/kg CLZ dose (ED50=1.55 mg/kg). In contrast, the atypical APD quetiapine produced full substitution for the 1.25 mg/kg CLZ dose (ED50=0.13 mg/kg), but not for the 5.0 mg/kg CLZ dose. Similarly, the atypical APD sertindole produced full substitution for only the 1.25 mg/kg CLZ dose (ED50=0.94 mg/kg). Risperidone, another atypical APD, produced partial substitution (>or=60% and <or=80% condition-appropriate responding) for the 1.25 mg/kg CLZ dose, and failed to substitute for the 5.0 mg/kg CLZ dose. The atypical APD ziprasidone and the typical APDs haloperidol and chlorpromazine failed to substitute for either CLZ training dose. These results demonstrated that the 1.25 mg/kg CLZ training dose provides partial or full stimulus generalization to more atypical APDs than does the 5.0 mg/kg CLZ training dose. Full substitution by olanzapine for only the 5.0 mg/kg CLZ dose suggests that this higher training dose is also important for screening atypical APDs.
 
The selective 5-HT(1A) antagonist WAY-100635 was employed to further clarify the respective contributions of 5-HT(1A) receptors to the effects of the 5-HT(1A) agonist 8-OH-DPAT, the 5-HT(2) agonist DOI, and the mixed 5-HT(1A/2) agonist LSD on exploratory locomotion in rats. In nocturnal studies of well-handled rats during their first exposure to the Behavioral Pattern Monitor, which enables analyses of quantitative and qualitative changes in locomotor activity, locomotor and investigatory responses were reduced by treatment with either 8-OH-DPAT, DOI, or LSD. The hypoactivity produced by 8-OH-DPAT, but not that produced by DOI, was antagonized by pretreatment with WAY-100635. These results substantiate the effectiveness and functional specificity of WAY-100635 as a 5-HT(1A) antagonist. Furthermore, these results are inconsistent with a functional interaction between 5-HT(1A) and 5-HT(2) receptors in the control of locomotor behavior. The decreases in locomotion produced by LSD were attenuated by pretreatment with WAY-100635, indicating that the effects of LSD in this paradigm are due partly to agonist actions at 5-HT(1A) receptors. Therefore, 5-HT(1A) receptors appear to play a direct role in mediating the effects of LSD on rodent locomotion.
 
Acute and chronic antidepressant drug treatments respectively decrease and increase the aggressive behaviour of resident rats during encounters with unfamiliar conspecifics. We have now examined the effect of the 5-hydroxytryptamine1A receptor antagonist, WAY-100635, on fluoxetine-, paroxetine- or venlafaxine-induced changes in aggression. WAY-100635 (0.1 mg/kg), which did not modify behaviour when given alone, potentiated the venlafaxine (5.54 mg/kg)-induced reduction in aggression after acute treatment and, during chronic treatment, accelerated the fluoxetine (0.34 mg/kg/day)-induced increase in aggression, from day 5 to day 2. A similar change in time course was seen with paroxetine (0.33 mg/kg/day), although the increase in aggression was smaller. Venlafaxine (5.54 mg/kg/day, alone or co-administered with WAY-100635) increased aggression by day 2. During chronic treatment, therefore, venlafaxine, at the dose used, had a more rapid onset of action than either fluoxetine or paroxetine, whereas the fluoxetine- and paroxetine-, but not the venlafaxine-, induced increase in aggression was accelerated by WAY-100635. These studies further support the hypothesis that selective blockade of the 5-hydroxytryptamine1A receptor augments the effects of antidepressant drugs in an animal model predictive of antidepressant activity, presumably by concomitant blockade of the somatodendritic 5-hydroxytryptamine1A autoreceptor-mediated negative feedback system of serotonergic neurones.
 
In this experiment we examined the effect of a serotonin receptor (5-HT1A) agonist and antagonist WAY-100635 (N-[2-(4-[2-methoxy-phenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexane-carboxamide) on temporal differentiation, in intact rats and rats whose serotonergic (5-HTergic) pathways had been destroyed by 5,7-dihydroxytryptamine (5,7-DHT). Thirteen rats received 5,7-DHT-induced lesions of the median and dorsal raphe nuclei; 14 rats received sham lesions. They were trained to press two levers (A and B) in 50-s trials, in which reinforcement was contingent upon responding on A in the first half, and B in the second half, of the trial. Logistic psychophysical curves were fitted to the relative response rate data (percent responding on B, %B), for derivation of timing indices [T50 (time corresponding to %B=50%), slope, Weber fraction] following WAY-100635, 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin], combinations of WAY-100635+8-OH-DPAT, and vehicle alone. WAY-100635 (30, 100 and 300 microg/kg, s.c.) did not affect the timing indices. 8-OH-DPAT (100, 200 microg/kg, s.c.) reduced T50 without affecting the Weber fraction. WAY-100635 (300 microg/kg) abolished the effect of 8-OH-DPAT on T50 in both the lesioned and sham-lesioned groups. 5-HT levels in the neocortex, hippocampus, amygdala, nucleus accumbens and hypothalamus of the lesioned group were <20% of those in the sham-lesioned group; catecholamine levels were unaffected. The results confirm that 8-OH-DPAT disrupts temporal differentiation in a free-operant psychophysical schedule, reducing T50, and indicate that this effect of 8-OH-DPAT is mediated by postsynaptic 5-HT1A receptors.
 
The serotonergic system has a broad influence on behavior, but its specific contribution to novel object exploration remains to be examined. Toward this end, we assessed the impact of the 5-HT1A agonist, 8-OHDPAT (0.01-0.05 mg/kg) and the 5-HT1A antagonist, WAY-100635 (0.01-0.05 mg/kg) on novel object exploration in a familiar open-field environment. 8-OHDPAT produced a dose-related inhibition of responding to the novel object, whereas, WAY-100635 treatment induced a dose-related increase in the investigatory response to the novel object. Combined, the effects of WAY and 8-OHDPAT treatments were statistically indistinguishable from saline. In terms of locomotor activity, only the highest dose of 8-OHDPAT (0.05 mg/kg) altered locomotor activity and the effect was inhibitory. These findings provide evidence for an involvement of the serotonergic system in the response to novel stimuli and indicate that this effect can be dissociated from effects on overall activity including locomotor, rearing and grooming behaviors.
 
In-vitro studies have shown that WAY 100635 is not only a potent 5-HT1A antagonist, but also has high affinity and efficacy at the dopamine D(4) receptor. Nevertheless, the behavioral effects of this compound have not been investigated. This study sought to characterize the discriminative stimulus effects produced by WAY 100635. Male Sprague-Dawley rats were trained in a two-lever, fixed ratio 50, food-reinforced task with WAY 100635 (10 micromol/kg) as a discriminative stimulus. Substitution tests with different doses of WAY 100635 and (-)-pindolol, and combination tests with two 5-HT1A agonists, 8-OH-DPAT and LY 293284; and two dopamine D(4) antagonists, sonepiprazole and A-381393, were performed. Rats trained with a low dose (0.74 micromol/kg) of WAY 100635 could not learn the discrimination task after more than 3 months of sessions. Rats trained to discriminate 10 micromol/kg of WAY 100635 from saline achieved the criterion of accuracy after approximately 35 training sessions. WAY 100635 (2.5-10 micromol/kg) produced a dose-dependent increase in WAY 100635-appropriate responding, with a mean effective dose of 3.44 micromol/kg, whereas saline or pindolol (5-25 micromol/kg) administration resulted in 0% drug lever responding. Pretreatment with the 5-HT(1A) agonists 8-OH-DPAT or LY 293284 did not modify the WAY 100635 curve, but pretreatment with the selective dopamine D(4) antagonists sonepiprazole or A-381393 completely blocked the cue. These results indicate that the discriminative stimulus effect produced by WAY 100635 is mediated by activation of dopamine D(4) receptors.
 
Development of N-methyl-D-aspartate (NMDA) antagonists for a variety of disorders has been hindered by their production of phencyclidine (PCP)-like psychological effects and abuse potential. There is, however, evidence to suggest that this problem might be mitigated by targeting NMDA receptors subtypes, in particular, those containing the NR2B subunit. To further test this hypothesis, the NR2B selective antagonist CP-101 606 (traxoprodil) was evaluated in two animal models: drug discrimination, a model of the subjective effects of drugs in humans, and self-administration, which evaluates the reinforcing properties of the drug. In the first study, CP-101 606(3-300 microg/kg/infusion) was tested for intravenous self-administration in rhesus monkeys experienced in PCP (5.6 microg/kg/infusion, intravenously) self-administration. In the second study, CP-101 606 was tested for production of PCP-like discriminative stimulus effects in rats (3-56 mg/kg, intraperitoneally) and rhesus monkeys (0.3-5.6 mg/kg intravenously). Evidence was obtained for reinforcing effects of at least one dose of CP-101 606 in all four monkeys. In rats, CP-101 606 produced more than 80% mean PCP-lever selection (2.0 mg/kg, intraperitoneally) but, unlike PCP itself, the dose producing the highest level of substitution was accompanied by more than 50% suppression of response rates. In monkeys, CP-101 606 produced more than 90% PCP-lever selection (0.1 mg/kg intramuscularly) in three of four animals at doses that did not significantly decrease rates of responding. The data show that CP-101 606 has some PCP-like discriminative stimulus effects in rats and monkeys and functions as a positive reinforcer in monkeys. These results suggest that inhibition of NR2B subunit containing NMDA receptors plays a role in the production of the subjective effects and abuse potential associated with many subtype-nonselective NMDA receptor antagonists such as PCP.
 
The present study investigated the hypothesis that behavioural sensitization to psychomotor stimulants is expressed only if the internal state of the animal is the same as it was at the time of sensitization development. This state-dependency hypothesis has previously been put forward to explain the apparent blockade of sensitization by N-methyl-D-aspartate (NMDA) receptor antagonists. The present study used amphetamine (0.375 mg/kg) as the stimulant and chlordiazepoxide (CDP) as a secondary stimulus. Mice were given double injections of either amphetamine + CDP, amphetamine + saline, CDP + saline, or saline + saline daily over 8 days. On the ninth and tenth days, all mice were challenged with amphetamine + saline and with CDP + saline, in a counterbalanced design. CDP does not show evidence of locomotor sensitization and does not act at NMDA receptors, but in other studies it has been shown to give rise to state-dependent-like effects. Thus any progressive augmentation of the response to repeated amphetamine + CDP treatment would be attributable to amphetamine sensitization, and any blockade of sensitization would not be due to NMDA receptor antagonism. Both groups receiving amphetamine became sensitized over the first 8 days (shown by progressive increases in locomotion). When challenged with amphetamine alone, the amphetamine + CDP group failed to show a sensitized response. This observation supports the state-dependency hypothesis and emphasizes the importance of considering the context provided by drug-induced internal states in studies of sensitization.
 
Mice lacking the dopamine transporter (DAT-/-) are characterized by high extracellular dopamine levels and spontaneous hyperlocomotion. We performed a detailed analysis of the behavioural phenotype of DAT-/- mice in order to identify other behavioural impairments associated with the hyperdopaminergic tone of these mutant mice. In particular, we investigated locomotor activity, exploration, and social and maternal behaviours, which are known to be regulated by dopamine. DAT-/- mice were easily aroused by novelty and always responded with hyperlocomotion, which interfered with habituation to the testing environment, exploratory behaviour in an open field and the coping response to forced swimming stress. Social behaviours such as interaction with an unknown congener or aggressiveness were not modified in DAT-/- mice compared with DAT+/- and DAT+/+ mice, although the maternal behaviour of mutant females was severely disturbed. Haloperidol and clozapine reversed the hyperactivity in DAT-/- mice, with a rightward shift of the dose-response curve compared with control animals, suggesting a dopamine-mediated effect. These results emphasize the role of dopamine regulation in locomotion, exploration and maternal behaviours and suggest that mice with a genetic deletion of DAT may represent a useful model to elucidate the altered behavioural processes accompanying pathological conditions associated with hyperdopaminergic function.
 
Negative or defect symptoms refer to a reduction in normal functioning. In schizophrenia, negative symptoms encompass, among others, anhedonia, flat affect, avolition and social withdrawal. These symptoms have been found to be particularly prominent in the more chronic phase of the illness and seem to be virtually insensitive to current antipsychotic treatment. This review focuses on the possibilities and limitations of animal models for the negative symptoms of schizophrenia. Following a review of the negative symptoms in schizophrenia, attention is focused on the two symptoms most often modelled in animals - anhedonia and social withdrawal. We then look at the important question of how to model schizophrenic pathology in animals. Since the exact pathology is still far from clear, most efforts have in the past concentrated on using psychotomimetic drugs such as amphetamine or phencyclidine. The recently accumulated knowledge that schizophrenia probably results from disturbances in the normal development of the brain has led to a surge of new animal models in which the long-term consequences of early manipulations are investigated. However, so far these models have predominantly concentrated on the positive rather than the negative symptoms of schizophrenia. The last part of this review is dedicated to the question of validation of animal models for anhedonia and social withdrawal. The general conclusion is that very few models have so far been adequately tested. The lack of currently effective treatment further hampers the study of such validation.
 
The present study investigated the potential benefit of the ethyl ester of N-phenylacetylprolylglycine (GVS-111) on the model of bilateral frontal lobectomy (BFL) in rats. The animals in Experiment 1 were trained in an active avoidance task and subsequently underwent BFL. The animals in Experiment 2 were first assessed in an open field and in a passive avoidance test before the BFL was performed. BFL dramatically decreased performance in the active avoidance test, disturbed habituation of horizontal activity in the open field and diminished the latency to enter the dark compartment in the passive avoidance test. GVS-111, administered in a dose of 0.5 mg/kg/day i.p. for 9 days following the operation, was found to improve performance in both active avoidance and passive avoidance and restored habituation of horizontal activity in the lobectomized animals.
 
Local thrombosis of the frontal cortex (Fr1 and Fr3 fields), caused by combination of the intravenous photosensitive dye Rose Bengal administration with focused high-intensity illumination of the frontal bone, was shown to provoke a pronounced deficit in step-through passive avoidance performance in rats without concomitant motor disturbances. N-Phenylacetyl-L-prolylglycine ethyl ester (GVS-111) administered intravenously at a dose of 0.5 mg/kg/day, for the first time 1 h after ischaemic lesion and then for 9 post-operative days, with the last administration 15 min before testing, attenuated the deficit. This treatment significantly diminished the volume of the infarcted area. Thus, post-ischaemic injection of GVS-111 demonstrated both cognition-restoring and neuroprotective properties. The cognition-restoring effect is probably based on an increase in neocortical and hippocampal neuronal plasticity. Neuroprotective effects of GVS-111 combine antioxidant activity with the ability to attenuate glutamate-provoked neurotoxicity and block voltage-gated ionic channels, i.e. the compound mitigates the main metabolic shifts involved in pathogenesis of brain ischaemia.
 
Three experiments were performed to investigate the effects of combining the active D-stereoisomer of CGP 37 849, i.e. the glutamatergic antagonist, CGP 40 116, with l-dopa, in mice that had undergone treatment with the neurotoxin, MPTP. In the first experiment, the decreased motor activity in MPTP-treated mice was alleviated by the administration of a low dose of l-dopa (5mg/kg, s.c.) together with a low dose of CGP 40 116 (30µg/kg). This dose was inactive in the control (saline-treated) mice. The highest dose of CGP 40 116 used (3000µg/kg) stimulated activity in the control mice. In Experiment 2, the inactive L-stereoisomer, i.e., CGP 40 117, was found to be inactive at doses (3 and 30µg/kg) effective with CGP 40 116. The effects of CGP 40 116 and l-dopa on the 24-h activity of mice tested under either day-night or night-day conditions, were more marked and longer lasting in the night-day condition. Taken together, the results from all three experiments show that CGP 40 116 in a dose range of 1-30µg/kg in combination with l-dopa (5mg/kg, s.c.) alleviated the reduced motor activity in MPTP-treated mice whereas higher doses of CGP 40 116 (100, 300, or 3000µg/kg) or lower doses (0.1 and 0.3µg/kg) were without effect. These experiments are interpreted as support for current views on glutamatergic-dopaminergic interactions in Parkinsonism and offer further evidence for the MPTP mouse model of the disease.
 
Obsessive-compulsive disorder (OCD) is a disorder characterized by unwanted and intrusive thoughts, images, or impulses and/or repetitive behavior. OCD is a major cause of disability; however, the genetic factors and pathophysiological mechanisms underlying this complex, heterogeneous disorder remain largely unknown. During the past decade, a number of putative mouse genetic models of OCD have been developed for the purpose of studying the neural mechanisms underlying this disorder and developing novel treatments. This review presents and evaluates these experimental preparations to date. Models using knockout or transgenic approaches, as well as those examining variation in genetically diverse populations, are evaluated and discussed.
 
It has recently been proposed that the "serenic" (antiaggressive) agents, fluprazine and eltoprazine, may enhance fear/anxiety reactions in laboratory rodents. In the present study, the influence of these compounds (1.25-10.0 mg/kg) on anxiety-related behaviour in male mice was examined in the elevated plus-maze test. For comparative purposes, the effects of 8-OH-DPAT (0.01-1.0 mg/kg) CGS 12066B (1.25-10 mg/kg), TFMPP (0.63-5.0 mg/kg) and mCPP (0.5-4.0 mg/kg) were also assessed. Behavioural analysis incorporated not only traditional parameters but also several novel measures of defensive behaviour (i.e. "risk assessment"). The selective 5-HT1A agonist 8-OH-DPAT produced effects only at 1.0 mg/kg, with evidence of an anxio-lytic/sedative action at this dose. In the absence of other behavioural changes, CGS 12066B (a selective 5-HT1B agonist) caused a preferential and dose-dependent (2.5-10.0 mg/kg) stimulation of closed arm entries, an effect also seen with low doses of TFMPP (0.63 mg/kg) and the serenics (1.25-2.5 mg/kg). In addition, both TFMPP and mCPP (5-HT1C/1B agonists) induced dose dependent anxiogenic-like effects over the dose ranges tested, with the most pronounced changes observed on measures of risk assessment. The profiles of fluprazine and eltoprazine on plus-maze behaviour were not only similar to one another but, on most parameters, were also remarkably like those observed with TFMPP and mCPP. These data question the behavioural selectivity of the serenics and further support the proposal that these compounds may potentiate anxiety. Findings are discussed in relation to underlying receptor mechanisms, and the utility of a more ethological approach to the analysis of behaviour on the elevated plus-maze. (C) Lippincott-Raven Publishers.
 
The effects of CGS 12066B (3-14mg/kg), a putative 5-HT(1B) agonist, on 5-HT behavioral syndrome, motor activity and body temperature, were investigated in rats. The animals were well adapted to the experimental conditions before testing, and data sampling started at the same hour for each rat. The highest dose of CGS 12066B clearly reduced body temperature and induced flat body posture and hindlimb abduction. No significant change was seen in motor activity. The CGS 12066B-induced changes were not antagonized by 5-HT(1B) receptor antagonists, or antagonists at other 5-HT receptor. In fact, the 5-HT(1A) antagonist NAN-190 and especially the non-selective 5-HT antagonist methiothepin, with definite 5-HT(1B) receptor blocking properties, both potentiated the decrease in body temperature. The findings suggest that the behavioral and body temperature effects of CGS 12066B are not easily explained by 5-HT(1B) receptor stimulation, but may be mediated by activation of non-serotonergic mechanisms. Similar conclusions in studies with other 5-HT(1B) agonists suggest a common problem with such drugs.
 
Dose-dependent effects of 7-OH-PIPAT and PD-128,907 on motor behaviors and place conditioning were examined in rats. Four 2-day conditioning trials were conducted over 8 consecutive days. On one day of each trial, animals received an injection of either saline, one of six doses of 7-OH-PIPAT (0.01-10.0 mg/kg), or one of five doses of PD-128,907 (0.01-1.0 mg/kg), and were placed into a distinct compartment for 40 min. On the other day, animals received an injection of saline and were placed into a different compartment for 40 min. Locomotion, sniffing, and yawning were measured following the first and last drug injections. Place conditioning was assessed the day following the last conditioning trial. None of the doses of 7-OH-PIPAT or 0-0.3 mg/kg PD-128,907 produced place conditioning. However, 1 mg/kg PD-128,907 produced conditioned place preference (CPP). Across doses, both 7-OH-PIPAT and PD-128,907 produced a U-shaped change in sniffing and locomotion and an inverted U-shaped change in yawning. Across time, lower doses produced a decrease in sniffing and locomotion and an increase in yawning that were evident immediately, whereas higher doses produced a biphasic change in that there was an initial decrease followed by an increase in sniffing and locomotion. Behaviors produced by both low and high doses were sensitized following repeated administration. PD-128,907 produced CPP and was more potent than 7-OH-PIPAT in altering motor behaviors, possibly due to its greater selectivity for the D3 receptor.
 
In humans, the progression to cocaine addiction presumably involves increases in the effectiveness of cocaine to function as a reinforcer. Here we use breakpoints assessed using the progressive ratio (PR) schedule as an index of the efficacy of cocaine as a reinforcer. To date, no preclinical studies have demonstrated an increase in breakpoint as a consequence of self-administration history. In the current study, baseline performances on fixed ratio (FR) and PR schedules were determined. Rats were then exposed to different self-administration histories and deprivation periods, and responding under FR and PR schedules was reassessed. Exposure to a discrete-trials procedure (access to cocaine 4 times/hour, 24 hours/day; DT4) for 7 or 10 days, coupled with a deprivation period of 7 days, resulted in increases in breakpoint on a PR schedule, with no change in FR1 schedule responding. Exposure to an FR1 schedule for 72 consecutive hours followed by 7 days of deprivation, failed to change breakpoints, but increased rates of intake assessed with an FR1 schedule. Thus, the type of self-administration history and the length of deprivation experienced contribute to changes in the reinforcing efficacy of cocaine as measured by a PR schedule.
 
It has been known for years that systemic administration of the stress hormones, adrenocorticotrophin (ACTH), lysine-vasopressin, adrenaline, or beta-endorphin, enhances retrieval of aversive behaviours acquired one or a few days before. Here we show that the pre-test i.p. injection of the hormones in rats can also enhance retrieval when given months after the original training. The effectiveness of the treatments changed with time. When animals were tested 3 months after training the hormones enhanced retrieval only at doses five times higher than those needed 1 day after training. Between 6 and 9 months from training the hormones either lost their effect (vasopressin, beta-endorphin) or actually inhibited retrieval (ACTH, adrenaline). The effects of the hormones cannot be explained by a decrease in locomotor activity: none of the treatments had such an effect, as measured in an open field. However, when the animals were tested between 12 and 19 months after training, the hormones once again became as effective as they had been 1 day after training. This was so in spite of the fact that control retention levels became very low with age, probably as a result of extinction. The oscillation of the sensitivity of retrieval to the hormones does not appear to depend on changes in anxiety levels with ageing or to effects of the hormones on locomotor activity.
 
Some animal models of depression, including the majority of the more recently introduced models, are better characterized as models of predisposition to depression. In the first part of this paper, we show that the basis for such a model could be either a procedure that increases the ease with which an analogue of major depression may be evoked, or a presentation analogous to dysthymia (chronic mild depression). We then consider how the concepts of predictive, face, and construct validity apply to such models. Next, we review the validity of the available models of predisposition to depression, which derive from genetics, genomics, developmental manipulations, and brain lesioning. Finally, we compare the performance of the different models, using a novel scoring system that formalizes the evaluation of animal models against each of the three sets of validation criteria.
 
The involvement of dopamine (DA) in drug reinforcement is well established, but much less in known about its contribution to addiction. We have used positron emission tomography to investigate in humans the role of DA in drug reinforcement, addiction and drug vulnerability. We have shown that during drug intoxication increases in striatal DA are associated with the drug's reinforcing effects only if the DA changes occur rapidly. These results corroborate the relevance of drug-induced DA increases and of pharmacokinetics in the rewarding effects of drugs in humans. During withdrawal, we have shown significant reductions in DA D(2) receptors and in DA release in drug abusers, which is likely to result in decreased sensitivity to non-drug-related reinforcing stimuli. The DA D(2) reductions were associated with decreased activity in the orbitofrontal cortex, which we postulate is one of the mechanisms underlying compulsive drug administration in the addict. In fact, during craving the orbitofrontal cortex becomes hyperactive in proportion to the desire for the drug. In non-drug-abusing subjects striatal DA D(2) receptors levels predicted the reinforcing responses to stimulant drugs, providing evidence that striatal DA D(2) receptors modulate reinforcing responses to stimulants in humans and may contribute to the predisposition for drug self-administration.
 
Low-affinity channel-blocking -methyl-D-aspartate (NMDA) antagonists have been of interest for clinical development because they are purported to produce few phencyclidine (PCP)-like side-effects, particularly at therapeutic doses. In the current study, two low-affinity NMDA channel blockers, AR-R 13950AA and AR-R 16283AA, were evaluated for NMDA antagonist-associated behavioral effects. The drugs were tested in rats and rhesus monkeys trained to discriminate PCP from saline, using a standard two-lever drug discrimination paradigm, under a fixed-ratio (FR) schedule of food reinforcement. Both drugs were also tested in rats trained to discriminate NPC 17742, a competitive NMDA antagonist, from saline in a similar experimental procedure. In rats, both AR-R 13950AA and AR-R 16283AA resulted in intermediate levels of PCP-lever selection (up to 60%). Testing in NPC 17742-trained rats produced at most 30% NPC 17742-lever responding. In rhesus monkeys, AR-R 13950AA produced virtually no PCP-lever responding at any dose, while AR-R 16283AA produced a dose-dependent substitution for PCP in all four subjects. The results with AR-R 16283AA in monkeys suggest that, at doses above therapeutic levels, it may produce PCP-like intoxication in humans. Overall, the results suggest that, while there is some overlap of the discriminative stimulus effects produced by the AR-R compounds with those of PCP, there are also important differences.
 
Muscarinic cholinergic activation is a critical component of basolateral amygdala (BLA)-mediated modulation of memory consolidation. The receptor(s) mediating this activation during consolidation have not been elucidated. This study investigated the roles of muscarinic subtype 1 (m1) and subtype 2 (m2) receptors in memory enhancement, by post-training intra-BLA infusions of the non-selective muscarinic agonist oxotremorine. Rats received intra-BLA infusions of either oxotremorine alone (10 microg in 0.2 microl per side), oxotremorine together with the selective m1 antagonist telenzipine (1.7, 5.0, 17 or 50 nmol/side), oxotremorine with the selective m2 antagonist methoctramine (1.7, 5.0, 17 or 50 nmol/side), oxotremorine with a combination of the above doses of telenzipine and methoctramine, or only vehicle, immediately after inhibitory avoidance training. Performance on a 48-hour retention test was significantly enhanced in oxotremorine-treated rats relative to vehicle-infused controls. Intra-BLA co-infusion of oxotremorine with either telenzipine (5, 17 or 50 nmol/side) or methoctramine (17 or 50 nmol/side) blocked the oxotremorine-induced enhancement. Combinations of these antagonists did not act additively to block memory enhancement by oxotremorine. These findings indicate that modulation of memory consolidation induced by cholinergic influences within the BLA requires activation of both m1 and m2 receptor synapses. Plausible mechanisms for m1- and m2-mediated influences on BLA circuitry are discussed.
 
Gerbils are a highly social species and extremely sensitive to social manipulations. In this laboratory, separating male/female pairs has been found to produce significant effects on these animal's subsequent social behaviour. The present studies were conducted in order to examine the effects of a short period of individual housing in females of this species, as this may also be predicted to produce alterations in social responding. It was found that 21 days' individual housing induced a marked reduction in social behaviour directed towards an untreated male placed in the same arena. This was indicated by a highly significant increase in immobile-in-contact, a behaviour that involves females freezing while, and only while, they are being socially investigated. This represents the declining of an invitation to socially interact and so high levels of immobile-in-contact indicate low levels of social motivation. There was also an increase in evading, upon another animal's approach, and a decrease in social investigation of other animals. The effects of 15 days of fluoxetine were found to be highly dependent on housing condition. In individually housed females, 10 mg/kg increased their social investigation of other animals and markedly reduced the duration of immobile-in-contact. Twenty mg/kg also reduced levels of immobile-in-contact and increased the frequency of active approaches towards other animals. Fluoxetine therefore acts to increase social motivation in individually housed animals. By direct contrast, in group-housed female gerbils, fluoxetine had no effects on social behaviour and produced clear indications of sedation. While housing condition had no influence on levels of corticosterone, fluoxetine produced dose-related increases in corticosteroid levels in both group- and individually housed animals. These findings show that: (1) a short period of individual housing induces a significant reduction in these animals' motivation towards social behaviour; (2) the effects of fluoxetine on behaviour are greatly influenced by housing condition--prosocial effects are seen in individually housed animals but only sedative effects are seen in animals maintained in groups; and (3) while housing condition has no effects on levels of corticosterone, fluoxetine dose-dependently stimulates corticosteroid release. It can be concluded that the effects of fluoxetine on gerbil behaviour are independent of its stimulatory influence on HPA axis functioning, and that the prosocial effects of this selective serotonin reuptake inhibitor (SSRI) can only be seen in animals with a pre-existing social deficit.
 
Though drug discrimination techniques have proven invaluable in characterizing the interoceptive properties of drugs of abuse, antipsychotics and anxiolytics, with the exception of some fragmentary data with tricyclic agents, surprisingly few studies have been undertaken with antidepressants. Nevertheless, the preferential dopamine (DA) reuptake inhibitor, bupropion, elicits a robust discriminative stimulus in rodents. Moreover, in rats trained on a two-lever FR-10 schedule for food reward, the selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram, and the noradrenaline (NA) reuptake inhibitor (NARI), reboxetine, elicit discriminative stimuli at doses that selectively elevate extracellular levels of 5-HT and NA, respectively. In generalization tests, mixed inhibitors of 5-HT and NA reuptake, such as venlafaxine, substitute for both citalopram and reboxetine, while SSRIs substitute for citalopram but not for reboxetine. Intriguingly, selective NARIs appear to substitute both for reboxetine and for citalopram though, owing to long-term instability of the citalopram cue, the latter observation will require confirmation. Bupropion and the atypical antidepressant, mirtazapine - a 5-HT2/alpha2-adrenoceptor (AR) antagonist devoid of affinity for 5-HT and NA reuptake sites - substitute for neither citalopram nor reboxetine, indicating that 'antidepressant' effects per se do not account for their interoceptive properties. Moreover, mirtazapine abolishes the citalopram cue, an action mimicked by the selective 5-HT2C antagonist, SB242,084. The discriminative stimulus elicited by reboxetine is blocked by the alpha1-AR antagonist, prazosin. In contrast, it is not significantly attenuated by the alpha2-AR antagonist, RX821,002, nor by betaxolol or ICI118,551, antagonists at alpha1- and alpha2-ARs, respectively. These observations indicate that 5-HT2C receptors and alpha1-ARs contribute to the discriminative stimulus properties of SSRIs and NARIs, respectively. The present article reviews the literature devoted to the discriminative stimulus properties of antidepressant agents as training drugs, focusing in particular upon novel data with citalopram and reboxetine. In addition, several open questions and future research directions are evoked. It would be of considerable interest to extend such drug discrimination studies to other classes of antidepressants or potential antidepressants, including venlafaxine, mirtazapine and antagonists at neuropeptide (corticotropin releasing factor1 and neurokinin1) receptors.
 
Based on previous animal work, the present study investigated whether individual differences in motor activity in a novel environment predicted reinforcing and behaviorally activating effects of D-amphetamine (0, 5, 10 and 20 mg p.o.) in healthy adults. When exposed to a novel environment, 18 participants had high levels of motor activity (high responders; HR) and six had low levels (low responders; LR). These group differences were used to predict effects of D-amphetamine on drug reinforcement, salivary cortisol, motor activity, subjective effects, and acoustic startle reflex in subsequent sessions. Unlike observations in rodents, (1). dose-dependent reinforcing effects of D-amphetamine were evident but without group differences; (2). motor activity was greater in HR but did not vary with D-amphetamine dose; and (3). cortisol levels were not related to the reinforcing effects of D-amphetamine. Startle reflex amplitudes were greater in HR following placebo, but D-amphetamine 20 mg equalized this group difference. There was a trend towards less prepulse inhibition of the acoustic startle reflex in HR compared to LR. LR reported greater overall negative effect following amphetamine administration, but this was not consistently related to dose. Finally, participants with high sensation-seeking personality scores exhibited less prepulse inhibition. The results are discussed in terms of the extant literature.
 
(-)-Adamantyl-Delta8-tetrahydrocannabinol (AM-411) is a 'classical' tricyclic cannabinoid CB1 receptor agonist in which the C-3 alkyl side-chain has been replaced with an adamantyl group. The compound is cannabinoid CB1 receptor subtype selective (CB1 Ki=6.86 nmol/l, CB2 Ki=52.0 nmol/l). We examined the effects of AM-411 alone and in combination with the cannabinoid CB1 receptor antagonist/inverse agonist, SR-141716, on open-field behaviors of rats. The lowest effective dose of AM-411, 3 mg/kg, suppressed ambulation (horizontal activity) and rearing (vertical activity) and increased circling frequency compared to vehicle control levels. Co-administration of SR-141716 normalized these changes. SR-141716 (3 and 5.6 mg/kg) also produced significant increases in scratching and grooming (both frequency and duration), effects that were not eliminated in the presence of AM-411. Coupled with previous drug discrimination data, the open-field profile of AM-411 suggests that this high-affinity CB1 cannabinoid receptor agonist induces behavioral effects similar to the natural cannabinoid Delta9-tetrahydrocannabinol and different from (R)-methanandamide, a chiral analog of the endogenous ligand anandamide.
 
SR 141716 (1 and 3 mg/kg p.o.), a selective central (CB1) cannabinoid receptor antagonist, selectively reduced feeding of a very highly palatable cane-sugar mixture in marmosets. In contrast, standard primate pellet intake was not modified at the lower dose, but was slightly increased (+29%; p<0.01) by the higher dose of SR 141716. These results are in agreement with the hypothesis that endogenous cannabinoid systems are involved in the modulation of the appetitive value of food. (C) 1998 Rapid Science Ltd.
 
This study examined effects of the CB1 receptor antagonist/inverse agonist SR-141716 and the CB1 receptor agonist delta9-tetrahydrocannabinol (delta9-THC) on feeding behavior in male Sprague-Dawley rats. Rats were housed individually with free access to regular pelletized laboratory chow [after a 2 weeks handling phase, animals had access to regular chow for 21 h (Study 1) or 22 h (Study 2); high-fat powder food for 3 h in Study 1 and 2 h in Study 2, respectively], and free access to water. Animals were maintained on a reversed 12-h light/dark cycle (dark beginning at noon). Rats were habituated to this type of feeding and light/dark schedule for 3 weeks until a stable baseline for food intake was achieved. In Study 1, animals were examined after administration of delta9-THC alone (dose range 0.1-1.8 mg/kg), SR-141716 alone (dose range 0.03-0.3 mg/kg), and the two drugs combined; injections were given i.p. at the beginning of the second hour after presenting the high-fat diet and drugs were given twice weekly. There was a dose-related increase in high-fat diet intake, peaking at 0.56-1 mg/kg delta9-THC. SR-141716 alone suppressed the high-fat diet intake below control levels. A combination of 0.3 mg/kg SR-141716 and 0.56 mg/kg delta9-THC counteracted the effects on consumption of either drug alone. In Study 2, experimental rats were treated initially with 0.56 mg/kg delta9-THC for six consecutive days; controls received vehicle. Attenuation of the hyperphagia (high-fat diet) was evident after the second injection. Increasing doses of delta9-THC (1 and 1.8 mg/kg, for two and three consecutive days, respectively) did not reinstate the initial hyperphagia. In conclusion, low-to-moderate doses of delta9-THC produced hyperphagia (to a high-fat food source), which was antagonized by SR-141716. SR-141716 singly suppressed intake of the high-fat diet. Delta9-THC-induced hyperphagia dissipated rapidly upon chronic treatment; however, it is unclear whether this reflects pharmacological tolerance or the emergence of a conditioned taste aversion in Study 2.
 
Active cannabimimetic drugs are known to bind to two receptor subtypes: one, called CB1, is mainly localised in the central nervous system while the other (CB2) is expressed preferentially in the immune system. SR 141716A has been demonstrated to have a nanomolar affinity for CB1 receptor subtypes and a micromolar affinity for CB2 receptors. Moreover, it is an effective antagonist at these receptors both in vitro (antagonism of cannabinoid activity in vas deferens) and in vivo (suppression of the hypothermia elicited by WIN 55,212-2). The present experiments were thus undertaken to investigate the role of CB1 receptors in cannabinoid discrimination. Rats were trained to discriminate WIN 55,212-2 (0.3mg/kg s.c.) from saline in a standard operant (FR10) food rewarded discrimination procedure. Acquisition of the discrimination required 16 days on average and the ED(50) of WIN 55,212-2 was 0.032mg/kg s.c. CP55,940 and delta-9-tetrahydrocannabinol (Delta(9)-THC) generalised to the WIN 55,212-2 stimulus with the respective ED(50)s of 0.007mg/kg (s.c.) and 0.64mg/kg (p.o.). Pretreatment with SR 141716A antagonised the cue elicited by WIN 55,212-2 (ED(50) = 1.6mg/kg) as well as the generalisation to CP 55,940 (ED(50) = 0.08mg/kg) and to Delta(9)-THC (ED(50) = 0.15mg/kg). SR 140098 is a CB1 antagonist as potent as SR 141716A in vitro. This compound is unlikely to pass into the brain since it failed to displace [(3)H]-CP55, 940 from rat brain membranes ex vivo, and to reverse WIN 55,212-2-induced hypothermia. SR 140098, in contrast to SR 141716A, did not antagonise the WIN 55,212-2 stimulus. Taken together, the present results demonstrate that the brain CB1 receptor subtype mediates the cannabinoid cue.
 
Cannabinoid CB1 receptor agonists, including delta-9-tetrahydrocannabinol (Delta 9-THC) (the main psychoactive ingredient in marijuana) have been shown to increase feeding in rats and humans. Conversely, it has been reported that acute administration of the CB1 receptor antagonist SR 141716A reduces food intake in rats. Based upon this observation, it has been suggested that CB1 antagonists could be useful as appetite suppressant drugs. The present studies were designed to provide a detailed examination of the effects of CB1 antagonists on food intake across a range of paradigms. Two CB1 antagonists (SR 141716A and AM 251) were administered to rats trained on fixed-ratio schedules with two different ratio requirements (fixed-ratio 1 and fixed-ratio 5). Both drugs produced a dose-dependent decrease in lever pressing, and had a relatively long duration of action (T1/2: SR 141716A, 15.1 h; AM 251, 22.0 h). Furthermore, intake of three diets with differing macronutrient composition (lab chow, high fat, high carbohydrate) was studied. Both drugs significantly suppressed intake of all three foods, and there were no significant interactions between drug dose and diet type. These findings support the hypothesis that CB1 receptor antagonists could be useful pharmacological tools for the suppression of appetite.
 
To investigate the effects of the cannabinoids on learning and on scopolamine-induced disruptions in learning, delta9-tetrahydrocannabinol (delta9-THC), SR 141716A (an antagonist at CB1 receptors) and scopolamine were administered to squirrel monkeys responding in a repeated-acquisition task. In this task, monkeys acquired a different three-response sequence each session and responding was maintained by food presentation under a second-order fixed-ratio 5 schedule. When either delta9-THC (0.1-0.56 mg/kg, i.m.) or SR 141716A (1-10 mg/kg, i.m.) was administered alone, 60 and 75 min before the session, respectively, both cannabinoid ligands dose-dependently decreased the overall rate of responding and increased the overall percentage of errors. However, at a dose that had little or no effect alone (i.e. 1 mg/kg), SR 141716A antagonized the disruptive effects of delta9-THC (0.18-1.8 mg/kg) on acquisition, shifting the dose-effect curves for rate of responding and percentage of errors at least 1/2 log unit to the right. Finally, when either delta9-THC (0.001-1 mg/kg) or SR 141716A (0.32-10 mg/kg) was administered with scopolamine (0.01 or 0.032 mg/kg, 15 min before the session), greater rate-decreasing and error-increasing effects were obtained than with scopolamine alone. These results suggest that while low doses of SR 141716A can antagonize the effects of delta9-THC in squirrel monkeys, high doses can also disrupt acquisition when administered alone and potentiate the disruptive effects of scopolamine on acquisition.
 
The intraperitoneal (i.p.) injection of apomorphine or d-amphetamine significantly increased locomotor activity in Sprague-Dawley rats. Prior administration of the cannabinoid receptor antagonist, SR 141716A, significantly enhanced the stimulant effect of both d-amphetamine and apomorphine in a dose-dependent manner. Administration of SR 141716A alone had no effect on locomotor activity. These data indicate that endogenous cannabinoids exert an inhibitory action on the increase in locomotor activity produced by amphetamine and apomorphine.
 
The prototypic cannabinoid CB1 antagonist SR 141716A is one important pharmacologic tool for examining CB1 receptors that mediate the behavioral and physiologic effects of delta9-tetrahydrocannabinol (delta9-THC). This study examined the effects of SR 141716A on the rate-decreasing, hypothermic and discriminative stimulus effects of delta9-THC in rhesus monkeys. In monkeys (n=4) responding under a multiple fixed ratio (FR-10:FR-10) schedule of food presentation and stimulus-shock termination, the potency of i.m. delta9-THC to decrease responding in the food component (ED50=0.64 mg/kg) was threefold greater than its potency in the stimulus-shock termination component (ED50=2.14 mg/kg). In the same monkeys, hypothermia was induced by delta9-THC at a dose (e.g. 0.32 mg/kg) that did not alter responding in either schedule component; the maximum decrease was 2.1 degrees C at a dose of 3.2 mg/kg. A dose of 0.32 mg/kg of SR 141716A, significantly attenuated delta9-THC-induced hypothermia without attenuating the rate-decreasing effects of delta9-THC in either component of the multiple schedule. The largest dose of i.m. SR 141716A that was studied, 1.0 mg/kg, significantly decreased rectal temperature and responding in the food component but did not significantly decrease responding in the stimulus-shock termination component of the multiple schedule. In a separate group of monkeys (n=3) that discriminated i.v. delta9-THC (0.1 mg/kg) while responding under an FR-5 schedule of stimulus-shock termination, SR 141716A (0.32 and 1 mg/kg) significantly increased the ED50 of the delta9-THC by 2.3- and 3.7-fold, respectively. Collectively, these results demonstrate that the behavioral effects of delta9-THC are not equally attenuated by SR 141716A.
 
We employed the CB1 cannabinoid receptor antagonist SR 141716A (3 mg/kg, i.p.) to investigate whether behavioural effects induced in rats by anandamide, an endogenous cannabinoid (20 mg/kg, i.p.), were mediated by the cannabinoid CB1 receptor. Anandamide reduced ambulatory (67%) and non-ambulatory activities (rearing and grooming, 84% and 90% respectively), with a strong cataleptic effect, produced hypothermia (about -1 degree C) and hindlimb splaying, and reduced defecation (79%). It did not significantly increase either the tail-flick or hot-plate latencies. Except for the decreased defecation, these responses were all blocked by SR 141716A. Although only single doses of the agonist and antagonist were used, the findings indicate that these behavioural effects are probably mediated by an interaction with cannabinoid CB1 receptors.
 
Latent inhibition (LI) is a measure of retarded conditioning to a previously-presented nonreinforced stimulus, that is impaired in schizophrenic patients and in rats treated with amphetamine. Neuroleptic drugs are known to produce two effects in this test paradigm: to antagonise amphetamine-induced disruption of LI, and to enhance LI when administered on their own. The present experiments tested the effects on LI of a potential antipsychotic, sigma ligand BMY-14802. The experiments used a conditioned emotional response (CER) procedure in rats licking for water, consisting of three stages: preexposure, in which the to-be-conditioned stimulus (a tone) was repeatedly presented without being followed by reinforcement; conditioning, in which the preexposed stimulus was paired with reinforcement (a foot shock); and test, in which LI was indexed by animals' degree of suppression of licking during tone presentation. In Experiment 1, 20 tone preexposures and two conditioning trials were given and the effects of 5, 15, and 30mg/kg BMY-14802 were assessed. Experiment 2 tested the effects of 15 and 30mg/kg on LI using ten preexposures and two conditioning trials. Experiment 3 investigated the effects of 15 and 30mg/kg on LI using 40 preexposures and extended conditioning consisting of five tone-shock pairings. Experiments 4 and 5 investigated antagonism of amphetamine-induced disruption of LI by 15 and 30mg/kg BMY-14802, respectively. BMY-14802 was found to antagonise amphetamine-induced disruption of LI and enhance LI when low numbers of preexposures and two conditioning trials were given, but not following extended conditioning. These results provide partial support for the suggestion that BMY-14802 may possess antipsychotic properties.
 
A history of responding maintained by a shock-avoidance procedure can alter the effect of psychomotor stimulant drugs on punished responding. This study was designed to evaluate whether another type of historical intervention could also alter the effects of cocaine on punished responding and, as a result, clarify certain variables contributing to this effect. Lever pressing of four squirrel monkeys was suppressed by a punishment procedure consisting of a fixed-interval 5-min schedule of food presentation in which every 30th response also produced a 200-ms 5-mA electric shock. Cocaine (0.03-1.0mg/kg) had no effect on or reduced punished responding. Conditions were then changed and responding was maintained for several sessions by a differential reinforcement of low rate (DRL) schedule in which food was delivered only when one response followed another by at least 25s. The punishment schedule was then reinstated and the effects of cocaine redetermined; the dose-response curve was similar to that initially obtained. The monkeys then responded on a shock-avoidance schedule in which each response postponed the next scheduled shock for 25s; shocks occurred every 5s in the absence of responding. Subsequently, the shock-avoidance schedule was replaced by the punishment schedule and the effects of cocaine were redetermined. In contrast to the initial determination of the effects of cocaine on punished responding, and the effects obtained following training on the DRL schedule, cocaine then produced response rate increases or no change in rate at several doses that formerly reduced responding. These results demonstrate that the rate-decreasing effects of cocaine on punished responding may be reversed by a history of responding on a shock-avoidance schedule and also indicate that a history of responding on a DRL schedule is not sufficient to reverse the effects of cocaine. These data suggest that a history of responding under schedules, such as the DRL, that engender responding that is typically increased by psychomotor stimulant drugs, is not sufficient to reverse the effects of these drugs on punished responding. The reversal of the effects of cocaine on punished responding resulting from a history of avoidance responding appears to be attributable to factors other than the rate-increasing effect of cocaine on responding maintained by avoidance.
 
The effects of chlordiazepoxide (2.5-15.0 mg/kg), a full benzodiazepine receptor agonist, and bretazenil (5.0-30.0 mg/kg), a partial benzodiazepine receptor agonist, were examined in the murine elevated plus-maze paradigm. Behaviours recorded comprised the traditional indices of anxiety as well as a number of ethologically derived measures. Results show that chlordiazepoxide (10-15 mg/kg) and bretazenil (5-30 mg/kg) not only decreased traditional indices of anxiety but also reduced risk assessment behaviours such as head-dipping and stretch attend postures from secure areas of the maze. Both compounds produced these effects without adversely affecting general activity levels. While traditional indices of anxiety did not clearly discriminate between the two compounds, some differences were apparent on the ethological measures. The dose-response curves for bretazenil were generally shallower than those for chlordiazepoxide, confirming its partial agonist profile. Together, these data support the view that benzodiazepine receptor partial agonists may have utility in the management of human anxiety disorders (C) Lippincott-Raven Publishers.
 
Chronic stress is broadly used to model anxiety and depression. However, in chronic stress models, anxiety- and depression-like behaviors might be masked by unspecific effects of stress. We tested whether chronic stress in mice can induce unspecific changes in locomotion, and whether these changes interfere with the measurement of anxiety and forced-swimming behaviors. Also, we studied these latter behaviors in relation to the duration of stress, the lighting conditions during testing, and after the injection of diazepam. We employed a 4-week chronic stress paradigm, adopted from a model of stress-induced anhedonia and a 1-week subchronic stress, both consisting of rat exposure, restraint stress and tail suspension. Chronically stressed mice, tested under bright and moderate illumination, exhibited 'anxiolytic-like' behavior along with prolonged swimming and hyperactivity. These behaviors were not detectable under weak illumination or after the injection of diazepam (0.25 mg/kg). Instead, normal locomotion, increased anxiety and inhibited swimming were revealed under these conditions. Thus, chronic stress can induce hyperlocomotion in mice, which is triggered by acute stressors such as light, and interferes with the evaluation of anxiety and forced swimming. One week of stress did not change locomotion and forced swimming, and increased anxiety irrespective of illumination applied during testing. Our data can possibly explain previously reported contradictions in the behavioral testing of mice with chronic stress models of anxiety and depression.
 
Monoaminergic ligands modified a naltrexone discriminative stimulus in rhesus monkeys dependent on 2 mg/kg per day of the mu opioid L-alpha-acetylmethadol (LAAM). This study examined a role for monoamines in the directly observable and physiologic manifestations of LAAM withdrawal induced by naltrexone in the same monkeys. The effects of saline, clonidine (0.032 mg/kg), haloperidol (0.032 mg/kg), cocaine (1.0 mg/kg), amphetamine (1.0 mg/kg) and imipramine (10.0 mg/kg) were examined in LAAM-dependent monkeys that subsequently received saline or naltrexone (0.0001-1.0 mg/kg). Naltrexone dose-dependently increased respiration, abdominal rigidity and salivation. Clonidine attenuated each of these withdrawal signs, whereas haloperidol increased some (i.e. respiration) and decreased others (i.e. salivation). When administered alone, cocaine and amphetamine increased respiration and also increased the respiratory stimulant effects of naltrexone; cocaine and amphetamine did not attenuate any measure of withdrawal. With the exception of a decrease in naltrexone-induced salivation, imipramine was without effect. These results are strikingly different from results in these same LAAM-dependent monkeys showing that cocaine and amphetamine, but not clonidine, markedly attenuated a naltrexone discriminative stimulus. That monoaminergic ligands differentially alter the directly observable and discriminative stimulus effects of naltrexone in LAAM-dependent monkeys supports the view that monoamines differentially mediate the physical manifestations (norepinephrine) and subjective experience (dopamine) of opioid withdrawal.
 
The present article focuses on psychoneuroendocrine effects of cannabinoids in developing animals, with special emphasis on the perinatal, periweanling and periadolescent periods. We describe and discuss published data dealing with acute and long-term effects of exposure to cannabinoid agonists in such critical periods. Human studies have demonstrated that the consumption of marijuana by women during pregnancy affects the neurobehavioural development of their children. Investigations using animal models provide useful information for a better understanding of the long-lasting deleterious consequences of cannabis exposure during pregnancy and lactation. The increasing use of cannabis among adolescents and its associated public health problems have led to a parallel increase in basic research on appropriate animal models. Chronic administration of cannabinoid agonists during the periadolescent period causes persistent behavioural alterations in adult animals. Some of these alterations may be related to a possible increased risk of psychosis and other neuropsychiatric disorders in early onset cannabis users.
 
A previous experiment had shown that the benzodiazepine receptor (BZR) agonist, clonazepam, selectively increased the consumption of a 0.05% sodium saccharin solution without change in water intake, in a two-choice preference test. The first aim of this study was to investigate the effects of two benzodiazepine receptor partial agonists, bretazenil (Ro 16-6028) and Ro 17-1812, in the same test. The results showed that both drugs produced effects similar to those observed earlier with clonazepam. The second aim was to investigate the effects of the two compounds on consumption of a 0.005% quinine solution in a two-choice test. Both drugs increased the consumption of the quinine solution without alteration in water intake. In addition, in separate single-choice acceptance tests, both drugs significantly increased the consumption of a familiar, highly palatable 3% sucrose solution. These data are considered in relation to alternative hypotheses for BZR-mediated effects in choice tests.
 
Top-cited authors
Ivan Izquierdo
  • Pontifícia Universidade Católica do Rio Grande do Sul
Paul Willner
  • Swansea University
Irwin Lucki
  • University of Pennsylvania
Joao Quevedo
  • University of Texas Health Science Center at Houston
Gene Jack Wang
  • National Institutes of Health