Inhibition of apomorphine-induced climbing in mice by cholinergic drugs and neuroleptics

Muscarinic m4 receptor activation by some atypical antipsychotic drugs

To clarify the findings that clozapine is both a muscarinic receptor agonist and antagonist, we examined the effects of neuroleptics on forskolin-stimulated cAMP accumulation in Chinese hamster ovary cells expressing human muscarinic m4 receptors (CHO-hm4) and in rat striatum. With CHO-hm4 cells, clozapine induced a concentration-dependent and atropine-sensitive inhibition on cAMP formation, with EC50 = 60 nM and Emax = 74% of carbachol maximum. Other atypical neuroleptics, fluperlapine, tenilapine and olanzapine, were similar but less potent, while risperidone, rilapine, quetiapine (ICI 204,636), sertindole, and ziprasidone had almost no effect. Typical neuroleptics, haloperidol, chlorpromazine, fluphenazine, thiothixene, thioridazine, and molindone, showed either no effect or an atropine-resistant inhibition of cAMP formation. However, in rat striatal tissues, clozapine, up to 10 microM, did not show a significant inhibition of cAMP formation, probably due to a relatively low abundance of muscarinic m4 receptors and the presence of multiple types of muscarinic and other receptors, with which clozapine interacts. Nevertheless, muscarinic m4 receptor agonism, to some extent, may be a relevant mechanism for the therapeutic efficacy and side effects of clozapine and some atypical neuroleptics.

Sigma receptor binding affinity and inhibition of apomorphine-induced climbing

Several relatively selective compounds with affinity for the sigma binding site were assessed for their ability to inhibit apomorphine-induced climbing in the mouse. Although, the majority of compounds inhibited apomorphine-induced climbing, there was no correlation between the ability to inhibit climbing and potency in sigma binding assays using [3H]1,3-di-o-tolylguanidine (DTG) or [3H](+)-pentazocine as ligands. The potency of the compounds to inhibit binding to muscarinic M1 or M2 receptors correlated with the potency to inhibit apomorphine-induced climbing. However, several of the compounds that inhibit climbing had microM affinity at muscarinic receptors. Whether these concentrations were achieved in vivo is unclear. Our data suggest that sigma activity per se is not responsible for inhibition of apomorphine-induced climbing.

A comparison of motor behaviours in groups of rats distinguished by their climbing response to apomorphine

Administration of apomorphine hydrochloride (0.5 mg kg-1 s.c.) to adult male or female Wistar rats previously acclimatized to the test environment induced climbing behaviour in approximately 50% of animals examined. The proportion of animals climbing was related to age, being maximal at 8-9 weeks. Those animals showing an initial climbing response to apomorphine (0.5 mg kg-1 s.c.), climbed when challenged with this dose of apomorphine on subsequent occasions. In 'climbing' animals the intensity of response was related to the dose of apomorphine administered; no dose-response relationship was observed in 'non-climbing' animals. No overall differences in the spontaneous motor behaviour of 'climbing' and 'non-climbing' animals were apparent as assessed by measurement of spontaneous climbing behaviour, by holeboard activity, and by locomotor activity measured in either photocell cages or in a treadwheel. There was no overall difference in the ability of apomorphine to induce locomotor activity or stereotyped behaviour in 'climbing' and 'non-climbing' animals. However, the administration of apomorphine induced rearing and treadwheel activity only in those animals classified as 'climbers'. There was no difference between the number (Bmax) of specific [3H]-spiperone binding sites or the dissociation constant (KD) in striatal or mesolimbic tissue preparations for 'climbing' and 'non-climbing' rats. The ability of an animal to climb in response to apomorphine appears to be dependent on an ability to orient vertically, since this is a component of behaviour common to climbing, rearing, and treadwheel activity. The ability to climb does not appear to be related to differences in dopamine receptor numbers in brain or to the penetration of apomorphine into brain.

Spontaneous climbing behaviour of mice, its measurement and dopaminergic involvement

Measurement of spontaneous climbing behaviour of mice, made in Mouse Climbing Monitors which used photocell detection of vertical movements, was most reliable and intense when made over a 40 min period using two mice per cage and taking readings before 2:00 p.m. The climbing response was antagonised by low doses of the neuroleptics (-)-sulpiride and spiroperidol, and by low doses of the dopamine agonists apomorphine, N-n-propylbenzo(f)quinoline, N-n-propylbenzo(g)quinoline and 2-di-n-propylamino-5,6-dihydroxytetralin; N,N-dipropyldopamine was less effective. Higher doses of the dopamine agonists induced climbing. The inhibition afforded by apomorphine was partially antagonised by (-)-sulpiride and spiroperidol (at doses which do not in themselves modify spontaneous climbing) but not by prazosin or yohimbine. Bilateral electrolesions of the caudate-putamen, nucleus accumbens or tuberculum olfactorium attenuated spontaneous climbing for the first 5-9 postoperative days, the accumbens lesions being most effective. Thus, spontaneous climbing behaviour of mice can be precisely quantified, dopamine-containing forebrain areas are involved with its expression and it can be potently modified by dopamine agonists and antagonists.

Effect of L-histidine and chlorcyclizine on apomorphine-induced climbing behaviour and methamphetamine stereotypy in mice

Pretreatment with L-histidine (850 and 1000 mh/kg, i.p.), a precursor of brain histamine, and chlorcyclizine (25 and 50 mg/kg, i.p.), a H1-receptor blocker, failed to modify apomorphine-induced climbing behaviour in mice. In contrast, pretreatment with L-histidine (850 and 1000 mg/kg) was found to antagonise methamphetamine stereotypy while pretreatment with chlorcyclizine (25 mg/kg) was found to potentiate methamphetamine-induced stereotypy in mice. The results suggest that drugs which influence central histaminergic mechanisms are effective only in modifying the behaviour induced by the indirectly acting DA agonist methamphetamine and fail to modify the behaviour induced by apomorphine, a directly acting DA agonist.