A double dissociation between serial reaction time and radial maze performance in rats subjected to 192 IgG-saporin lesions of the nucleus basalis and/or the septal region.

The cholinergic basal forebrain has been implicated in aspects of cognitive function including memory and attention, but the precise contribution of its major components, the basalocortical and the septohippocampal systems, remains unclear. Rats were subjected to lesions of either the nucleus basalis magnocellularis (Basalis), the medial septum/vertical limb of the diagonal band of Broca (Septum), or both nuclei (Basalis + Septum), using the selective cholinotoxin 192 IgG-saporin. Cognitive performance was evaluated in tasks taxing attention (the five-choice serial reaction time task, 5-CSRTT) and spatial working memory (radial arm maze, RAM). Nucleus basalis lesions disrupted performance of the 5-CSRTT, as demonstrated by decreased choice accuracy, increased incidence of missed trials, increased latencies to respond correctly, and a disrupted pattern of response control. Combined lesions of the Basalis and Septum resulted in qualitatively similar deficits to Basalis lesions alone, although interestingly, these rats were unimpaired on measures of response speed, and showed weaker deficits on accuracy and omissions. Decreasing the attentional load by lengthening stimulus duration reversed some of the deficits in Basalis and Basalis + Septum rats, suggesting an attentional deficit rather than motivation or motor perturbations. Performance in rats with septal lesions was only affected when task difficulty was increased. In the RAM an opposing pattern of effects was observed, with Septum and Basalis + Septum rats showing dramatic impairments, and Basalis rats performing normally. Taken together, these data provide clear evidence for a functional dissociation between septohippocampal and basalocortical cholinergic systems in aspects of cognitive function.

Alpha2-adrenoceptor antagonism is neither sufficient nor necessary for the distinctive action of atypical neuroleptics on intracranial self-stimulation in the rat.

Response rates in variable-interval intracranial self-stimulation (ICSS) in rats can provide a continuous record of drug-induced changes in brain function. Use of this procedure has been found to distinguish between typical and atypical neuroleptics, with the latter producing a similarly intense but much briefer depression of responding. This difference has been ascribed to the alpha2-adrenoceptor antagonist properties of atypical neuroleptics, but the evidence is ambiguous. The role of alpha2-adrenoceptors was examined in the present study using ventral tegmental ICSS to track the depressant effects of the typical and atypical neuroleptics, haloperidol (0.075 mg/kg) and olanzapine (0.9 mg/kg), injected alone or in combination with an alpha2-adrenoceptor agonist or antagonist. Neither haloperidol nor olanzapine (despite its atypical features) shows appreciable affinity for the alpha2-adrenoceptor. In the present study, olanzapine was found to depress self-stimulation responding dose-dependently, but with considerable recovery after 4 h. Simultaneous administration of alpha2-adrenoceptor receptor agonists or antagonists (respectively clonidine 0.015 mg/kg or idazoxan 3.0 mg/kg) failed significantly to increase or decrease the action of either haloperidol or olanzapine. These results indicate that alpha2-adrenoceptor antagonism does not necessarily promote recovery from neuroleptic-induced depression, and that 'atypical' features do not necessarily depend on alpha2-adrenoceptor antagonism. Various other explanations remain possible, but the accelerated time course of the atypical agents appears to support more recent explanations, based on their rapid dissociation from the D2 receptor.

Donepezil reverses a mnemonic deficit produced by scopolamine but not by perforant path lesion or transient cerebral ischaemia.

The purpose of these studies were threefold. Firstly, to further characterize the effect of perforant path transection on a test of short-term memory: delayed matching (or nonmatching)-to-position [D(N)MTP]. Secondly, to evaluate the effect of a transient cerebral ischaemia in the same task. Both surgical procedures were chosen as they produce a CNS lesion similar to that described in Alzheimer's Disease (AD). Thirdly, the effect of the acetylcholinesterase inhibitor, donepezil (Aricept(R), E2020), on the resulting cognitive impairment was studied. Perforant path transection produced a robust, delay-dependent impairment of choice accuracy in rats performing either a delayed matching- or nonmatching-to-position task. Sample latency was also reduced following lesion, yet the lesion-induced impairment was not affected by increasing the response requirement at the sample stage. An 11-min period of transient ischaemia (two-vessel occlusion model) resulted in almost complete loss of hippocampal CA1 pyramidal cells and a delay-dependent impairment in DMTP performance. However, unlike perforant path lesions, this deficit was unstable and declined in magnitude over the experimental period. Increasing the delay interval restored this deficit. Donepezil, at doses that robustly attenuated a scopolamine (0.06 mg/kg s.c.)-induced DMTP accuracy impairment in naïve, unoperated rats, had no effect against either lesion-induced impairment. The results are considered in terms of the effectiveness of acetylcholinesterase inhibitors in noncholinergic-based preclinical cognitive models.

A combined pharmacological and genetic approach to investigate the role of orphanin FQ in learning and memory.

Using a combination of the selective opioid receptor-like1 (ORL1) receptor agonist, Ro 64-6198, and orphanin FQ/nociceptin (OFQ/N) peptide knockout (KO) mice, the influence of OFQ/N on cognition has been studied in the rodent. In wild type, C57BL/6J mice, Ro 64-6198 (0.3-1 mg/kg i.p.) impaired the acquisition of spatial learning in the Morris water maze, although a mild neurological impairment was evident which complicated precise interpretation. In Lister hooded rats, Ro 64-6198 (6 mg/kg i.p.) produced delay dependent impairments in rats performing either a delayed matching or a delayed nonmatching to position task with only a modest (< 20%) effect on omissions - an effect consistent with a short-term memory impairment. Electrophysiological studies demonstrated an inhibitory effect of OFQ/N on LTP recorded from the CA1 region of wild type mice, but not in ORL1 receptor knockout mice. In contrast to the ORL1 agonist, mice deficient in the OFQ/N peptide showed some evidence of improved spatial learning, fear conditioning and passive avoidance retention. However, CA1 LTP was similar between OFQ/N peptide KO mice and wild type controls. Subsequent receptor radioautography studies demonstrated the presence of ORL1 receptors within various regions of the medial temporal lobe system: i.e. CA1, dentate gyrus molecular layer, subiculum, perirhinal cortex. Taken together, these results suggest a bi-directional effect of OFQ/N containing systems on aspects of cognitive behaviour, particularly those elements associated with hippocampal function. This is consistent with a likely modulatory role of OFQ/N on hippocampal and associated cortical circuitry.

Activation of 5-HT(2C) receptors reduces the locomotor and rewarding effects of nicotine.

RATIONALE: Various compounds believed to selectively interact with the 5-HT(2C) receptor have been demonstrated to alter the functioning of ascending dopamine systems. We postulated that this functional interaction may extend to the behavioural effects of drugs of abuse whose rewarding properties are critically dependent upon mesolimbic DA activity. OBJECTIVES: The present studies focussed on interactions between 5-HT(2C) receptor function and behaviours either supported or induced by nicotine. METHODS: The effect of Ro 60-0175, a 5-HT(2C) agonist, was assessed for its ability to modify 1) nicotine-induced locomotor activity in nicotine-treated rats, 2) lever pressing maintained by either food or IV administration of nicotine, and 3) the development of nicotine-induced hyperactivity. The specificity of this effect was further measured in locomotor activity studies by additional administration of the selective 5-HT(2C) antagonist SB 242,084. RESULTS: Ro 60-0175 (0.3-3 mg/kg SC) dose-dependently reduced nicotine-induced activity, an effect which was reversed by SB 242,084 (0.5 mg/kg IP), thus confirming receptor selectivity of the response. Responding both for food and nicotine on an FR5TO1 min schedule of reinforcement was reduced by Ro 60-0175 (0.1-1 mg/kg) with proportionally similar effects on responses for both types of reinforcer. Co-administration of Ro 60-0175 (1 mg/kg SC) and nicotine (0.4 mg/kg SC) for 10 days blocked the sensitised response that developed in subjects treated with nicotine alone. CONCLUSIONS: The present data support an involvement for the 5-HT(2C) receptor in mediating mesolimbic DA functioning as assessed by changes in behaviours indicative of nicotine reward.

Effect of LSD on prepulse inhibition and spontaneous behavior in the rat. A pharmacological analysis and comparison between two rat strains.

The goal of the present study was to better delineate the mechanisms of action of the prototypical hallucinogen LSD. LSD (0.03, 0.1 and 0.3 mg/kg, s.c.) produced locomotor hyperactivity, disruption of PPI and a number of behaviors indicative of 5-HT activation such as wet-dog shakes, back muscle contractions and forepaw treading. These various behavioral effects of LSD were studied in both Sprague-Dawley and Wistar rats, although with the exception of back muscle contractions which were more prominent in Sprague-Dawley rats, no major strain differences were detected. The PPI disruption induced by LSD (0.1 mg/kg) in Sprague-Dawley rats was completely reversed by pretreatment with the selective 5-HT(2A) antagonist MDL 100907 (0.5 and 1 mg/kg, s.c.). In contrast, pretreatment with antagonists at 5-HT(2C), (SB 242084 (0.5 mg/kg, i.p.)); 5-HT(2B/2C) (SDZ SER 082 (1 mg/kg, s.c.)); 5-HT(1A), ((+)-WAY 100135 (1 and 20 mg/kg, s.c.)) and 5-HT(6) receptors, (RO 04-6790 (30 mg/kg, i.p.)), all failed to influence LSD-induced disruption of PPI. The dopamine DA(2like) receptor antagonist, haloperidol (0.1 and 0.2 mg/kg, s.c.), was without effect against an LSD-induced disruption of PPI. Finally, selective blockade of 5-HT(2A) but not 5-HT(2C) receptors completely abolished the locomotor hyperactivity induced by LSD. These findings provide empirical evidence to support the view that the hallucinogenic effects of LSD are mediated by a direct agonist effect at 5-HT(2A) receptors.

Influence of the selective ORL1 receptor agonist, Ro64-6198, on rodent neurological function.

Identification of synthetic agonists and antagonists at orphan receptors represents an important step for understanding their physiological function and therapeutic potential. Accordingly, we have recently described a non-peptide agonist at the opioid receptor like (ORL1) receptor (1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triazaspiro[4.5]decan-4-one (Ro64-6198; Jenck et al., PNAS 94 (2000) 4938; Wichmann et al., Eur. J. Med. Chem. 35 (2000) 839). We have investigated the effects of this compound in various tests of rodent neurological function, utilising ORL1 knockout mice to examine the pharmacological specificity of Ro64-6198. In male C57BL/6J mice, effects on balance and motor co-ordination were detected following low doses (0.3-1mg/kg IP) of Ro64-6198. At higher doses (1-3mg/kg IP), effects on swim behaviour and hypothermia was observed. At 10mg/kg, each effect became more profound and a severe neurological disturbance appeared, including loss of righting reflex. These effects of Ro64-6198 (10mg/kg IP) were absent in ORL1 receptor knockout mice. In male, hooded Lister rats, Ro64-6198 (6-10mg/kg IP), produced some disturbance of neurological function, including hypoactivity, rotarod performance, grip strength and mild hypothermia. An impairment of food responding under a variable interval (VI) 20s schedule of reinforcement was noted at 3mg/kg. These results confirm Ro64-6198 to be a highly selective pharmacological tool to investigate ORL1 receptor function in vivo and, furthermore, that activation of this receptor is accompanied by a variety of effects on neurological function.

Influence of the 5-HT(2C) receptor antagonist SB242,084 on behaviour produced by the 5-HT(2) agonist Ro60-0175 and the indirect 5-HT agonist dexfenfluramine.

Ro60-0175 has been described as a selective agonist at the 5-HT(2C) receptor, yet it has only 10- fold higher affinity at the 5-HT(2C) compared to the 5-HT(2A) subtype, and equivalent affinity for the 5-HT(2B) receptor. The selective 5-HT(2C) receptor antagonist SB242,084 (0.5 mg kg(-1) i.p.), blocked the hypoactivity and penile grooming induced by Ro60-0175 (1 mg kg(-1) s.c.). The combination of SB242,084 (0.5 mg kg(-1) i.p.) and Ro60-0175 (3 - 10 mg kg(-1)) produced a completely different pattern of behaviours including wet-dog shakes, hyperactivity and back muscle contractions. These latter effects were blocked by the selective 5-HT(2A) receptor antagonist MDL100,907 (0.5 mg kg(-1) i.p.), but not the 5-HT(2B) receptor antagonist SB215,505 (3 mg kg(-1) p.o.). The indirect 5-HT releaser/reuptake inhibitor dexfenfluramine (1 - 10 mg kg(-1) i.p.) produced a mild increase in locomotor activity, penile grooming, and occasional back muscle contractions and wet-dog shakes. Pre-treatment with SB242,084 (0.5 mg kg(-1)), blocked the incidence of penile grooming, and markedly potentiated both the dexfenfluramine-induced hyperactivity, the incidence of back muscle contractions, and to a lesser extent wet-dog shakes. Some toxicity was also evident in animals treated with dexfenfluramine (10 mg kg(-1))/SB242,084 (0.5 mg kg(-1)), but not in any other treatment groups. The hyperactivity and toxicity produced by the dexfenfluramine (10 mg kg(-1))/SB242,084 (0.5 mg kg(-1)) combination was replicated in a further study, and hyperthermia was also recorded. Both hyperthermia and toxicity were blocked by MDL100,907 (0.5 mg kg(-1)) but not SB215,505 (3 mg kg(-1)). An attenuation of the hyperlocomotor response was also observed following MDL100,907. These findings suggest that 5-HT(2C) receptor activation can inhibit the expression of behaviours mediated through other 5-HT receptor subtypes.

A study of the nicotinic agonist SIB-1553A on locomotion and attention as measured by the five-choice serial reaction time task.

SIB-1553A is a novel ligand with reputed agonist selectivity at nicotinic receptors containing the beta(4) subunit. As such, it represents an interesting pharmacological tool with which to probe the function of nicotine receptor subtypes. In the present studies, we compared SIB-1553A with nicotine in its ability to stimulate locomotion and to enhance attention in rats as assessed using the five-choice serial reaction time task (5-CSRTT). In nicotine-naive rats, SIB-1553A (10-40 mg/kg) induced a comparable increase in locomotion to nicotine (0.4 mg/kg), whereas in nicotine-sensitised rats, an enhanced locomotor response was seen to nicotine (0.4 mg/kg) but not to SIB-1553A (10-80 mg/kg). Similarly, chronic treatment with either SIB-1553A or nicotine did not lead to a cross-sensitised locomotor response. Unlike nicotine, SIB-1553A-induced locomotion was insensitive to antagonism by either mecamylamine (1 mg/kg) or DH beta E (3 mg/kg), suggesting a non-nicotinic mechanism. In young and aged rats, nicotine (0.4 mg/kg) enhanced attention as demonstrated by an increase in response accuracy and speed. SIB-1553A (3-10 mg/kg) did not mimic any of these changes and at the highest dose tended to disrupt performance. These results lend further support to the involvement of a high affinity site, possibly alpha(4)beta(2), in the locomotor and attentional-enhancing properties of nicotine.

Effect of subtype selective nicotinic compounds on attention as assessed by the five-choice serial reaction time task.

Nicotine can improve attentional functioning in humans, and a number of studies have recently demonstrated that under specific task conditions, nicotine can also improve attention in the rat. Neuronal nicotinic receptors comprise combinations of alpha(2-9) and beta(2-4) subunits, arranged to form a pentameric receptor, with the principal CNS subtypes currently believed to be alpha(4)beta(2) and a homomeric alpha(7) receptor. In the present studies, we attempted to delineate the particular nicotinic receptor subtype(s) contributing to the effects of nicotine on attention by assessing various nicotinic ligands on performance in the five-choice serial reaction time task (5-CSRTT). In rats performing below criterion (<80% correct, >20% omissions to a 1-s visual stimulus), subchronic dosing with nicotine (0.2 mg/kg sc) and the alpha(4)beta(2) agonist SIB 1765F (5 mg/kg sc) increased correct responding and decreased response latencies across the treatment week; whereas the alpha(7) agonist AR-R 17779 (20 mg/kg sc) was without effect. In subjects meeting the criterion, the competitive high affinity (including alpha(4)beta(2)) nicotine receptor antagonist DHbetaE (1-10 mg/kg sc) and the alpha(7) antagonist methyllycaconitine (MLA: 5-10 mg/kg i.p.) did not disrupt performance, whereas at the highest dose, the non-competitive antagonist mecamylamine (0.3-3 mg/kg sc) decreased accuracy and increased response latencies. These changes bore some similarities to those of pre-feeding and the non-competitive NMDA antagonist dizocilpine (0.03-0.06 mg/kg sc), suggesting that mecamylamine-induced performance disruption may relate to non-nicotinic receptor effects. In subjects chronically treated with nicotine, acute nicotine challenge (0.4 mg/kg sc) significantly increased accuracy whilst having no effect on any other performance measures. Finally, in these same nicotine pre-treated rats, the decrease in latency and increase in premature responses induced by nicotine (0.2 mg/kg sc) to a target stimulus of 150 ms was fully antagonised by DHbetaE (3 mg/kg sc) but not MLA (5 mg/kg i.p.). These results suggest that alpha(7) receptors do not play a role in any of the behavioural effects of nicotine observed in the 5-CSRTT, whereas a high affinity site, perhaps alpha(4)beta(2), is more likely involved.

Spatial and associative learning deficits induced by neonatal excitotoxic hippocampal damage in rats: further evaluation of an animal model of schizophrenia.

Neonatal ventral hippocampal lesions in the rat result in post-pubertal onset of behavioural abnormalities, modelling some aspects of schizophrenia. We further assessed the behavioural effects of neonatal lesions in rats in a variety of cognitive tasks and in the prepulse inhibition (PPI) of startle response paradigm. Prepubescent, lesioned rats exhibited startle responses and PPI similar to controls whereas, at adulthood, they showed a deficit in PPI. Lesioned rats acquired both passive and active avoidance responses. However, compared to controls, they showed a deficit in passive avoidance retention and in acquisition of active avoidance responses. In a cued Morris water-maze task, lesioned rats demonstrated adequate sensorimotor functions and appropriate motivation to escape from water. However, they were impaired in place learning and in remembering the location of a submerged platform. In conclusion, neonatal ventral hippocampal lesions result in the post-pubertal emergence of long-lasting deficits in sensorimotor gating and in the capacity to acquire and retain information in tests of spatial and avoidance learning. Therefore, this neurodevelopmental model of schizophrenia seems to exhibit an interesting degree of validity in possibly simulating some cognitive impairments and sensorimotor gating deficits frequently observed in psychotic patients.

Studies to investigate the role of 5-HT(2C) receptors on cocaine- and food-maintained behavior.

The present series of studies were designed to investigate the 5-HT(2C) receptor agonist Ro 60-0175 on cocaine- and food-maintained behavior in the rat. Ro 60-0175 (0.1-3 mg/kg, s.c.) reduced cocaine (15 mg/kg, i.p.)-induced hyperactivity. This inhibitory effect of Ro 60-0175 (1 mg/kg, s.c.) was completely blocked by pretreatment with the selective 5-HT(2C) antagonist SB 242,084 (0.5 mg/kg, i.p.). In further studies, Ro 60-0175 (1-3 mg/kg, s.c.) reduced responding for both food (45-mg Noyes pellet) and cocaine (0.25 mg/infusion) maintained under identical schedules of reinforcement (fixed ratio (5), time out 1 min, 60-min duration). The effect on food-maintained responding was blocked by SB 242,084 (0.5 mg/kg, i.p.). Ro 60-0175 (0.3-3 mg/kg, s.c.) also reduced the breakpoint for cocaine self-administration under a progressive ratio schedule of reinforcement. After a period of extinction training, where cocaine solution was substituted with saline, an acute priming injection of cocaine (15 mg/kg, i.p.) but not Ro 60-0175 (1 mg/kg, s.c.) reinstated cocaine responding. In this model of relapse, Ro 60-0175 (1-3 mg/kg, s.c.) pretreatment attenuated the priming effect of acute cocaine injection. In a final series of studies to examine the cataleptogenic properties of Ro 60-0175, very mild indices of catalepsy were observed at the 3 mg/kg dose only. These catalepsy scores were significantly lower than that produced by haloperidol (0. 5 mg/kg, s.c.). In further tests of motor function using the Rotarod, deficits were again seen at the 3 mg/kg dose, but not at lower doses. Taken together, these studies suggest that, in addition to reducing food intake, 5-HT(2C) receptor agonists reduce cocaine-reinforced behavior. This would be consistent with electrophysiological and biochemical evidence suggesting an important modulatory influence of 5-HT(2C) receptor activation on mesolimbic dopamine function.

Evidence that nicotinic alpha(7) receptors are not involved in the hyperlocomotor and rewarding effects of nicotine.

Neuronal nicotinic receptors are comprised of combinations of alpha(2-9) and beta(2-4) subunits arranged to form a pentameric receptor. Currently, the principal central nervous system (CNS) subtypes are believed to be alpha(4)beta(2) and a homomeric alpha(7) receptor, although other combinations almost certainly exist. The identity of the nicotinic receptor subtype(s) involved in the rewarding effects of nicotine are unknown. In the present study, using some recently described subtype selective nicotinic agonists and antagonists, we investigated the role of the alpha(7) nicotinic receptor in the mediation of nicotine-induced hyperactivity and self-administration in rats. The alpha(7) receptor agonists AR-R 17779 and DMAC failed to stimulate locomotor activity in both nicotine-nontolerant and -sensitized rats. In contrast, nicotine and the putative alpha(4)beta(2) subtype selective agonist SIB1765F increased activity in both experimental conditions. In nicotine-sensitized rats, the high affinity (including the alpha(4)beta(2) subtype) nicotinic antagonist dihydro-beta-erythroidine (DHbetaE), but not the selective alpha(7) antagonist methyllycaconitine (MLA), antagonized a nicotine-induced hyperactivity. Similarly, DHbetaE, but not MLA, pretreatment reduced nicotine self-administration. Electrophysiology experiments using Xenopus oocytes expressing the human alpha(7) receptor confirmed AR-R 17779 and DMAC to be potent agonists at this site, and further studies demonstrated the ability of systemically administered AR-R 17779 to penetrate into the CNS. Taken together, these results indicate a negligible role of alpha(7) receptors in nicotine-induced hyperlocomotion and reward in the rat, and support the view for an involvement of a member from the high-affinity nicotinic receptor subclass, possibly alpha(4)beta(2). Issues such as drug potency, CNS penetration, and desensitization of the alpha(7) receptor are discussed.

The alpha4beta2 agonist SIB 1765F, but not the alpha7 agonist AR-R 17779, cross-sensitises to the psychostimulant effects of nicotine.

RATIONALE: Repeated administration of nicotine leads to an augmentation of its locomotor activating effects. Although studies have begun to identify the nicotinic receptor subtype(s) mediating the psychostimulant properties of nicotine, none as yet have investigated the subtypes which contribute to the process of sensitisation. OBJECTIVES: We therefore investigated cross-sensitisation to nicotine using subjects chronically treated with two nicotine subtype-selective agonists in an attempt to identify the relative contribution of each to the sensitisation process. METHODS: Rats received ten daily injections of either vehicle, nicotine (0.4 mg/kg), the alpha7-agonist AR-R 17779 (20 mg/kg), or the alpha4beta2-agonist SIB 1765F (3 mg/kg), and their subsequent locomotor response to acute challenge with each of these compounds was assessed. RESULTS: Chronic administration of both nicotine and SIB 1765F, but not AR-R 17779, resulted in an enhanced locomotor response to acute challenge with either nicotine or SIB 1765F but not AR-R 17779. CONCLUSIONS: These data support a role for the alpha4beta2 receptor in both the initiation and expression of sensitisation to the psychomotor stimulant effects of nicotine.

Neurotransmitter system interactions revealed by drug-induced changes in motivated behavior.

The present article reviews studies conducted either in collaboration with Jac Herberg, or in parallel with those studies that used consummatory behavior and responding for intracranial self-stimulation (ICSS) to investigate interactions between neurotransmitter systems. The studies reviewed include investigations of the role of dopamine in 8-OH-DPAT-induced feeding; the role of 5-HT3 receptors in the stimulant and depressant effects of nicotine on responding for ICSS; the interaction of D2 and 5-HT2 antagonists in sucrose consumption, and the differential contributions of alpha2-adrenoceptor and 5-HT2 antagonism to the rapid recovery of ICSS responding from depression produced by atypical neuroleptics. Further studies of the role of alpha2-adrenoceptor antagonism in the pattern of response decrements produced by neuroleptics on schedule-controlled responding for food confirm that the behavioral effects of monoamine interactions vary, depending on the specific receptor subtypes targeted and the behavioral paradigm employed. Consequently, the clinical relevance of findings will crucially depend on the choice of appropriate behavioral measures.

Rapid recovery of self-stimulation responding from depression by clozapine is prevented by the alpha 2-adrenoceptor agonist, clonidine.

Typical and atypical antipsychotic agents were tested on rats responding for variable-interval electrical stimulation of the ventral tegmental area (VTA). The typical neuroleptics, chlorpromazine and haloperidol, led to prolonged depression of responding lasting at least 4 h, whereas response rates after similarly effective doses of the atypical agents, clozapine and risperidone, recovered to control levels in the same period. The role of alpha 2-adrenoceptors in producing these differences was investigated by administering clozapine together with an alpha 2-adrenoceptor agonist, clonidine, and chlorpromazine together with an alpha 2-adrenoceptor antagonist, idazoxan. The addition of clonidine extended the response-depressant activity of clozapine, resulting in prolonged depression comparable to that produced by chlorpromazine or haloperidol. Conversely, the addition of idazoxan to chlorpromazine shortened the duration of chlorpromazine's suppressant action to a level comparable to that of clozapine or risperidone. These results suggest that the brevity of clozapine's effects on operant behaviour (a feature which may be related to its reduced liability to extrapyramidal side-effects) may be a consequence of its alpha 2-adrenoceptor antagonist properties.

Rapid recovery of self-stimulation from depression produced by the atypical neuroleptic risperidone is not prevented by 5-HT2 receptor stimulation.

Behavioral effects of the antipsychotic drug risperidone were tested in rats responding for variable-interval stimulation of the ventral tegmental area (VTA). Risperidone (0-0.9 mg/kg) produced a dose-dependent depression of responding in the 60 min after injection. Self-stimulation tests delayed for 30 or 120 min after injection showed that inhibition of responding by risperidone was limited in duration, with response rates recovering to pre-injection levels in a time-dependent manner. Recovery occurred regardless of opportunity to engage in self-stimulation, and was virtually complete at a time when receptor occupancy has been shown to be almost undiminished. The atypical properties of risperidone have been ascribed to its potent antagonist activity at 5-HT2 receptors; however, spontaneous recovery from the effects of risperidone was not prevented by simultaneous administration of a selective 5-HT2 agonist (DOI), even though DOI when given alone produced a 50-70% reduction in response rates. These results show that the inhibitory effect of risperidone on operant performance may be self-limiting in a manner that is not accounted for by its pharmacokinetic properties nor by its antagonist activity at central 5-HT2 receptors.

α 2-Adrenoreceptor antagonism may contribute to the atypical properties of risperidone: experimental support for the Nutt case.

The therapeutic efficacy and reduction in side effects claimed for new antischizophrenic drugs such as clozapine and risperidone have been ascribed to their heightened affinity for serotonin 5-HT(2) receptors rather than D-2 receptors. A case for α(2)-adrenoreceptor antagonism has recently been argued. We have confirmed that at least one atypical property of risperidone (a rapid decrement in its ability to depress self-stimulation) can be partly prevented by an α(2)-adrenoreceptor agonist (clonidine) but not by a 5-HT( 2) receptor agonist (DOI). This result supports the suggested role of α( 2)-adrenoreceptor antagonism in counteracting extrapyramidal effects during treatment with risperidone.