Activity of SR141716 on post-reinforcement pauses in operant responding for sucrose reward in rats.

Cannabinoids increase food intake, via CB1 receptors. The CB1 antagonist, SR141716, has been reported to decrease palatable food consumption in both operant and non-operant procedures. Similarly, CB1 receptor blockade diminished responding for normal food pellets under a fixed-ratio 15 (FR-15) schedule of reinforcement. The present experiment investigated whether the control of a continuous schedule of reinforcement (CRF) for sucrose pellets would be sensitive to the CB1 antagonist in mildly deprived rats. SR141716 dose-dependently reduced responding in a CRF procedure, by increasing post-reinforcement pauses. Together with formerly published conclusions, the data suggest that CB1 blockade reduces the rewarding efficacy of both palatable and non-palatable food.

Improvement of memory in rodents by the selective CB1 cannabinoid receptor antagonist, SR 141716.

Social short-term memory in rodents is based on the recognition of a juvenile by an adult conspecific when the juvenile is presented on two successive occasions. Cannabimimetics are claimed to induce memory deficits in both humans and animals. In the brain, they mainly bind to CB1 receptors for which anandamide is a purported endogenous ligand. SR 141716, a specific antagonist of CB1 receptors, dose-dependently reverses biochemical and pharmacological effects of cannabimimetics. More particularly, it antagonizes the inhibition of hippocampal long-term potentiation induced by WIN 55,212-2 and anandamide, and it increases arousal when given alone. The present experiments study the ability of SR 141716 (from 0.03 to 3 mg/kg SC) to facilitate short-term olfactory memory in the social recognition test in rodents. SR 141716 improved social recognition in a long intertrial paradigm with a threshold dose of 0.1 mg/kg SC. At 1 mg/kg, it antagonized the memory disturbance elicited by retroactive inhibition. Scopolamine (0.06 mg/kg IP) partially reversed its memory-enhancing effect. Moreover, SR 141716 reduced memory deficit in aged rats (0.03-0.1 mg/kg) and mice (0.3-1 mg/kg). As SR 141716 is not known to exhibit any pharmacological activity which is not mediated by CB1 receptors, the results strongly support the concept that blockade of CB1 receptors plays an important role in consolidation of short-term memory in rodents and suggest there may be a role for an endogenous cannabinoid agonist tone (anandaminergic) in forgetting.

Central mediation of the cannabinoid cue: activity of a selective CB1 antagonist, SR 141716A.

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.

Antidepressant-like effects of SR 57227A, a 5-HT3 receptor agonist, in rodents.

We have investigated the effect of SR 57227A, a selective 5-HT3 receptor agonist which crosses the blood brain barrier, on three rodent models in which antidepressants are active. In the forced swimming test, SR 57227A dose-dependently reduced the duration of immobility in mice and rats after i.p. administration. (ED50 = 14.2 mg/kg i.p. in mice, and 7.6 mg/kg i.p. in rats.) The compound was also active in both species after oral administration. In a time-course study in mice, SR 57227A (20 mg/kg p.o.) produced a significant effect lasting 6 h. SR 57227A (1 and 3 mg/kg i.p.) reduced the elevation of the escape failures in the learned helplessness model in rats by 50-60% on the last two days of the avoidance task, and reduced isolation-induced aggressivity in mice by 50 to 85%, an effect which was antagonised by zacopride (1 mg/kg i.p.). These results suggest that the stimulation of 5-HT3 receptors can produce antidepressant-like effects in behavioral tests in rodents.

Social olfactory recognition in rodents: deterioration with age, cerebral ischaemia and septal lesion.

Social olfactory recognition in rodents has been shown to assess short-term memory and to be sensitive to cholinergic drugs. It is based on the investigation of a juvenile by an adult rat and is measured by a reduction in duration of exploration during the second of two successive exposures lasting 5min. The present experiments further characterize rodent social recognition in pathophysiological models known to impair memory. Social recognition was distrupted by ageing in both rats and mice, by vincristine-induced septal lesion and by damaging the CA1 hippocampal layer after cerebral ischaemia in rats. These memory deficits could be compensated by reducing the time interval between the two presentations of the juvenile and/or by prolonging the juvenile encounter. Similarly, muscarinic agonists (arecoline, SR 46559A) counterbalanced the memory impairment in the three models. The present results indicate that the hippocampus plays a key role in social recognition. They suggest that in the three pathophysiological models, memory ability is still present although it is of very short duration; however, it can still be improved by pharmacological treatments.

Cholinomimetic activities of minaprine.

The cholinomimetic activities of the antidepressant drug minaprine have been investigated, in vitro and in vivo, in rodents. Minaprine, and its metabolite SR 95070B [3-(2-morpholinoethylamino)-4-methyl-6-(2-hydroxyphenyl) pyridazine hydrochloride] selectively displaced [3H]-pirenzepine from its cortical and hippocampal binding sites, and only weakly inhibited the binding of [3H]-N-methylscopolamine in either the rat cerebellum, heart and salivary glands, or the guinea-pig ileum. In mice, none of these drugs induced the typical cholinergic side-effects up to lethal doses. Minaprine and SR 95070B antagonized rotations induced by an intrastriatal injection of pirenzepine in mice, after intraperitoneal and/or oral administration. Minaprine also antagonized atropine-induced mydriasis in mice. Both minaprine and SR 95070B potentiated the tremorigenic effect of oxotremorine without inducing tremor when injected alone. Finally, minaprine and SR 95070B, after parenteral and/or oral injection, antagonized the scopolamine-induced deficit in passive avoidance learning, and enhanced short-term retention in the social memory test, in rats. The muscarinic agonists arecoline, oxotremorine and RS 86 [2-ethyl-8-methyl-2,8 diazaspiro-4,5 decan-1,3 dion hydrobromide], as well as the acetylcholine esterase inhibitors physostigmine and tacrine were active in most of these models. These results indicate that minaprine, and its metabolite SR 95070B, have cholinomimetic activities which could be, at least in part, mediated by their selective affinity for M1 muscarinic receptors. Thus minaprine could represent a potential useful drug for the treatment of senile dementias and cognitive impairments occurring in elderly people.

Systemic injection of pirenzepine induces a deficit in passive avoidance learning in rats.

When injected IP, the M1 muscarinic receptor antagonist pirenzepine dose-dependently induced a deficit in passive avoidance learning in rats. This activity was optimal at 75 mg/kg injected 1 h before the acquisition session. The deficit induced by pirenzepine was antagonized by oxotremorine (0.03-0.3 mg/kg SC) and physostigmine (0.1 mg/kg SC), but not neostigmine. By comparison, under the same experimental conditions, physostigmine and oxotremorine also antagonized the deficit induced by an equipotent dose of scopolamine (0.5 mg/kg IP), although the activity of physostigmine appeared stronger against scopolamine than against pirenzepine. These results suggest that pirenzepine could produce a centrally-mediated behavioural disruption when injected systemically.

Specific modulation of social memory in rats by cholinomimetic and nootropic drugs, by benzodiazepine inverse agonists, but not by psychostimulants.

The recognition of an unfamiliar juvenile rat by an adult rat has been shown to imply short-term memory processes. In this study the effect of various psychotropic drugs on this investigatory behaviour was examined. The procedure was as follows: an unfamiliar juvenile rat was placed in the home cage of an adult rat for 5 min. The time spent by the adult rat in investigating the juvenile was recorded. The adult rat was then immediately treated with vehicle or test compounds, and was again exposed for 5 min to the same juvenile 2 h later. At this time point vehicle-treated rats no longer recognized the juvenile rat, i.e. the time of investigation was similar to that observed during the first presentation. Arecoline (1 and 3 mg/kg IP), physostigmine (0.05 and 0.1 mg/kg SC), RS86 (0.5 mg/IP) and nicotine (0.125 and 0.5 mg/kg IP) reduced in a dose-dependent fashion the time spent in investigating the juvenile during the second exposure. This result cannot be attributed to nonspecific effects, since it was not observed when a different juvenile was used for the second exposure. The effect of arecoline was reversed by scopolamine, but not by methylscopolamine. Aniracetam reduced investigatory behaviour at the dose of 50 mg/kg IP. FG 7142 (5 mg/kg IP) and beta-CCM (0.4 mg/kg IP) were also active and their effect was reversed by Ro 15-1788. DL-Amphetamine (0.5 and 1 mg/kg IP), nomifensine (1.25-10 mg/kg IP) and strychnine (0.25 and 0.5 mg/kg IP) were ineffective or reduced this behaviour unspecifically.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization of the physostigmine stimulus in rats.

Rats were trained to discriminate 0.10 mg/kg SC physostigmine from saline in a two-lever food-reinforced task. There was generalization to the acetylcholine esterase inhibitor THA as well as to the muscarinic receptor agonists arecoline, oxotremorine and RS 86, but not to neostigmine or nicotine. The physostigmine cue was blocked by SC scopolamine hydrobromide and by ICV pirenzepine, but not by scopolamine methylbromide or by mecamylamine. These antagonism studies suggest that the discriminative cue elicited by physostigmine might be mainly mediated by central M1 receptors.

Characterization of the scopolamine stimulus in rats.

The discriminative stimulus properties of scopolamine, a potent antagonist at muscarinic receptors, were used for testing the discriminative effects of drugs known to act on cholinergic transmission. Rats were trained in a standard two-bar operant conditioning procedure with food as the reinforcer, according to a FR10 schedule. The training dose of scopolamine was progressively reduced from 0.25 mg/kg SC to the low dose of 0.062 mg/kg SC. Scopolamine yielded an accurate discrimination in all the six rats tested. The generalization gradient resulted in an ED50 of 0.027 mg/kg. The scopolamine cue lasted for 1 h and was of central origin, since it was not mimicked by scopolamine methylbromide. The scopolamine stimulus generalized to atropine and trihexyphenidyl (respective ED50 values 2.20 and 0.21 mg/kg SC). Atropine depressed rate of responding, while trihexyphenidyl did not. Antagonism experiments with both direct agonists at the muscarinic receptor (arecoline and oxotremorine) and indirect agonists, i.e., inhibitors of the acetylcholine esterase [physostigmine and tetrahydroaminoacridine (THA)], led to inconsistent results. Increasing the doses of the agonists in order to block the scopolamine cue may be limited by their rate suppressant effect on responding. Based upon previously published results, it is suggested that the muscarinic agonist cue is more useful than the antagonist cue for investigating muscarinic transmission.

Effects of intracerebroventricular pirenzepine on muscarinic discriminations in rats.

In rats, mixed M1/M2 muscarinic ligands induce a discrimination which is of central origin and selectively mediated by either one or both muscarinic receptor subtypes. In the present study we examined the effects of intracerebroventricular (i.c.v.) pirenzepine, a relatively selective M1 receptor antagonist which does not cross the blood-brain barrier, on muscarinic discriminations. Groups of six rats were trained to discriminate, in a two-lever operant task, either 0.062 mg/kg subcutaneous (s.c.) scopolamine or 0.075 mg/kg s.c. oxotremorine. When the rats had been well trained in the procedure, the discriminative effects of various i.c.v. muscarinic ligands were examined. Scopolamine (1.5-12 micrograms i.c.v.), but not pirenzepine (20-40 micrograms i.c.v.), generalized to s.c. scopolamine. Oxotremorine (0.75-6 micrograms i.c.v.) generalized to s.c. oxotremorine. Scopolamine (12 micrograms i.c.v.), but not pirenzepine (20-40 micrograms i.c.v.), antagonized the oxotremorine cue. These results suggest that activation of the M1 receptor is not the prominent component of muscarinic stimulus control.

A 7-phenyl substituted triazolopyridazine has inverse agonist activity at the benzodiazepine receptor site.

To investigate further the structural requirements for benzodiazepine (BZD) receptor ligands, we synthesized SR 95195, [7-phenyl-3-methyl-1,2,4-triazolo-(4,3-b) pyridazine], a positional isomer of the 6-phenyl-triazolo-pyridazines, which were the first non-BZD derivatives to exhibit high affinity for the BZD receptor and BZD-like activity in vivo. In vitro, SR 95195 displaced specifically bound [3H]-flunitrazepam from rat cerebellar and hippocampal membranes with respective IC50 values of 4 and 8 microM. In vivo, SR 95195 lacked BZD-like activity. At high doses SR 95195 induced clonic seizures in mice (threshold convulsant dose: 150 mg kg-1; CD50: 160 mg kg-1 i.p.) which were antagonized by Ro 15-1788. At non-convulsant doses (25 mg kg-1 i.p. and 100 mg kg-1 i.p.) SR 95195 significantly decreased punished responding in an operant conflict procedure in the rat, suggesting SR 95195 has intrinsic anxiogenic activity. SR 95195, in mice, reversed the anticonvulsant and myorelaxant actions of diazepam 3 mg kg-1, orally (respective ED50 values: 45 mg kg-1 i.p. and 44 mg kg-1 i.p.). In an operant-conflict test in rats, SR 95195 at non-anxiogenic doses, antagonized the disinhibitory action of diazepam 4 mg kg-1, i.p. (ED50: 8.6 mg kg-1, i.p.), but not that of pentobarbitone 15 mg kg-1, i.p. It is concluded that SR 95195 has the pharmacological profile of an inverse BZD agonist and that displacing the phenyl from the 6- to the 7-position in the triazolopyridazine series causes a shift from agonist to inverse agonist type activity at the BZD receptor site.

Evaluation of two anticonvulsant amino-pyridazine derivatives in the conflict test in rats.

Two amino-phenyl-pyridazine derivatives, SR 41378 and CM 40907, have been reported to antagonize seizures in mice, rats and Papio papio baboons with comparable potencies. Structurally, SR 41378 differs from CM 40907 by an additional chlorine in position 6 of the phenyl ring. In the present study the activity of these two compounds in the operant approach-avoidance conflict test in rats was examined and compared with that of diazepam, pentobarbital, meprobamate and valproate. SR 41378 increased punished responding, a measure of anticonflict activity (ED50 = 5.2 mg/kg), and decreased nonpunished responding, a measure of sedative activity, with a threshold active dose of 20 mg/kg i.p. The overall potency of SR 41378 was comparable to that of pentobarbital. CM 40907 (10-40 mg/kg i.p.) did not affect punished responding and decreased nonpunished responding at the dose of 40 mg/kg i.p. The duration of the anticonflict activity of SR 41378 increased with the dose and lasted over 4 h at the 20-mg/kg i.p. dose. At this dose, sedation lasted 1 h. An increase in anticonflict potency and tolerance to sedation were observed after a 5-day treatment with SR 41378 (20 mg/kg i.p.). The anticonflict and sedative activities of SR 41378 were not antagonized by Ro 15-1788 or CGS 8216. In vitro SR 41378 did not interact with benzodiazepine receptor sites. In conclusion, although CM 40907 and SR 41378 exhibit similar anticonvulsant activities, the present study reveals a major pharmacological difference between the two compounds because SR 41378 also possesses anticonflict properties.

[Pharmacologic profile of an original psychotropic drug. Minaprine: comparison with six reference antidepressives]. / Profil pharmacologique d'un psychotrope original, la minaprine: comparaison avec six antidépresseurs de référence.

Minaprine (MIN) is a 3-amino-pyridazine derivative which exhibits a profile of psychotropic activities which resembles that of antidepressant drugs as well as that of several dopaminomimetic drugs. This spectrum of activity differs from those observed in the same conditions for tricyclic (imipramine, clomipramine) and atypical (indalpine, nomifensine, amineptine, mianserin) antidepressant drugs. It must be noted that MIN is devoid of anticholinergic and motor stimulant effects. In addition, MIN induces behavioural effects predictive of a dopaminergic stimulation; the profile of this activity differs from that of apomorphine, as well as from those of amphetamine and nomifensine, but somewhat resembles that of bromocryptine. MIN does not induce neuroleptic, anxiolytic or anticonvulsant activities in rodents. These data suggest that MIN is an atypical antidepressant drug which activates both serotonergic and dopaminergic neurotransmissions, by as yet not clearly identified mechanisms.

Ethyl loflazepate: a prodrug from the benzodiazepine series designed to dissociate anxiolytic and sedative activities.

Ethyl loflazepate (Victan) is an original 1,4-benzodiazepine (BZD) being used as a potent, nonsedative, minor tranquillizer. Ethyl loflazepate was designed to be a prodrug that would gradually release within the organism an active 1,4-BZD. It was hypothesized that this type of pharmacokinetic profile would dissociate anxiolytic from sedative activities. Previously published pharmacokinetic studies performed in rats, baboons and humans have confirmed that ethyl loflazepate gradually releases in the plasma an active 1,4-BZD, descarboxyloflazepate. In the present study, the activity of ethyl loflazepate and its main plasmatic metabolites were examined in rodent models predictive of anxiolytic, anticonvulsant, myorelaxant and sedative properties. The activities of ethyl loflazepate and its metabolites were compared to those of reference BZDs: diazepam, bromazepam, nitrazepam, flunitrazepam, lorazepam, clorazepate. Ethyl loflazepate and its metabolites revealed the typical profile of activity of minor tranquillizers. The activity of ethyl loflazepate was long-lasting. The overall anxiolytic potency of ethyl loflazepate was similar to that of diazepam but ethyl loflazepate appeared to be less sedative than diazepam. Thus in the approach-avoidance conflict procedure in rats the threshold anxiolytic doses were 1 and 2 mg/kg i.p. for ethyl loflazepate and diazepam, respectively, whereas the threshold sedative doses were 16 and 8 mg/kg i.p., respectively. In vitro ethyl loflazepate exhibited a weak affinity for the BZD receptor site. In vivo ethyl loflazepate displaced [3H]-flunitrazepam from mice brain membranes with a potency comparable to that of lorazepam. In vitro, descarboxyloflazepate, the main plasmatic metabolites of ethyl loflazepate, revealed a 4-fold greater affinity for the BZD receptor than diazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of the effects of the benzodiazepine derivative clorazepate on corticosterone secretion with its behavioural actions. Antagonism by Ro 15-1788.

The effectiveness of Ro 15-1788, a benzodiazepine receptor antagonist, in modifying the effects of the benzodiazepine derivative clorazepate on schedule-controlled behaviour and pituitary-adrenal activity of rats was investigated. At low doses (5 mg/kg) clorazepate increased punished responding and slightly decreased basal plasma corticosterone levels. At high doses (40 mg/kg), clorazepate suppressed fixed ratio responding, raised basal plasma levels of corticosterone and reduced the stress response produced by exposure to a novel environment. Pretreatment with Ro 15-1788 blocked the behavioural and neuroendocrine effects of the high dose of clorazepate. These results suggest that both types of effects of clorazepate share a common mechanism which involves brain benzodiazepine receptors.

Behavioural evidence for partial agonist properties of RO 15-1788, a benzodiazepine receptor antagonist.

Rats trained to discriminate pentylenetetrazol from saline had this cue antagonized by the benzodiazepine, clorazepate. Ro 15-1788 reversed the antagonism of the pentylenetetrazol cue produced by clorazepate. Similarly, Ro 15-1788 blocked the anti-conflict effect of clorazepate. Rats trained to discriminate clorazepate from saline, however, generalized this cue to Ro 15-1788. These results demonstrate that Ro 15-1788 is not a pure benzodiazepine antagonist, but has partial agonist properties.