No support for any inhibitory influence of amisulpride was found. Conversely, amisulpride universally enhanced secretion, suggesting that amisulpride is a potential drug for dry-mouth treatment. The mechanism behind the potentiation is currently unknown.

Individuals receiving clozapine treatment for schizophrenia complain of drooling. Reports on salivary flow measurements are contradictory in humans and lacking in animals. Clozapine has affinity for several different receptor types and may, hypothetically, both stimulate and inhibit salivary secretion. In rats, intravenous clozapine evoked a long-lasting secretion, being more prominent from submandibular than from parotid glands. Chronic denervation enhanced the responses. Clozapine acted on muscarinic (M1-) receptors of acinar cells, independent of central nervous mechanisms, pre-synaptic intraglandular events, or circulating catecholamines. A fraction of the methacholine- and parasympathetic-nerve-evoked secretion was abolished by clozapine at doses below those evoking secretion. Sympathetic-nerve-evoked secretion was partially reduced by clozapine, due to antagonistic action on alpha-adrenoceptors; the beta-adrenoceptor-mediated response persisted. Subsecretory doses of clozapine enhanced secretion induced by the beta-adrenoceptor agonist isoprenaline. The overall actions of clozapine suggest that, in clozapine-treated humans, salivation is increased during sleep and at rest, but is decreased during meals.

N-Desmethylclozapine is a major metabolite of the atypical antipsychotic drug clozapine, used in the treatment of therapy-resistant schizophrenia. Patients under clozapine treatment report a troublesome sialorrhea. Recent experiments show clozapine to exert mixed agonist/antagonist actions on salivary secretion in rats. The present study was performed to define the secretory role of N-desmethylclozapine and to compare it with that of its parent compound. N-Desmethylclozapine evoked secretion by acting directly on the muscarinic acetylcholine M1-receptors of 'silent' duct-cannulated parotid and submandibular glands of the anaesthetized rat. In chronic surgically denervated glands, the secretory response was enlarged. The methacholine-evoked secretion, as well as the parasympathetic nerve-evoked secretion, were reduced by N-desmethylclozapine and involved blockade of M3-receptors, while the sympathetic nerve-evoked response was reduced, involving blockade of alpha(1)-adrenergic receptors. Synergistic interactions between N-desmethylclozapine and the beta-adrenergic receptor agonist, isoprenaline, occurred. Compared with clozapine, the excitatory efficacy of N-desmethylclozapine was higher and the inhibitory efficacy was lower (parasympathetic activity) or about the same (sympathetic activity). Theoretically, in humans treated with clozapine, an increase in the N-desmethylclozapine : clozapine ratio would contribute to salivation during the night and at rest, and, furthermore, the magnitude of the reduction in the reflexly elicited secretion is likely to diminish.

Many drugs (e.g. amisulpride) have been used to treat troublesome clozapine-induced salivation; however, varying success has been achieved in this respect, probably because, until recently, the salivatory action of clozapine has been largely unexplained. In the rat, clozapine and its main metabolite, N-desmethylclozapine, were found to exert mixed secretory actions: excitatory, through muscarinic acetylcholine M1-receptors giving rise to a long-lasting, low-level flow of saliva; and inhibitory, through muscarinic M3-receptors and α(1) -adrenoceptors reducing the parasympathetically and sympathetically nerve-evoked flow of saliva. The aim of the present study was to define the interactions between clozapine and N-desmethylclozapine, and clozapine and amisulpride, with respect to the excitatory response. Submandibular glands, sensitized by chronic parasympathetic preganglionic denervation, were studied in pentobarbitone-anaesthetized rats. To prevent clozapine from being metabolized to N-desmethylclozapine by hepatic enzymes, the liver was, under terminal anaesthesia, excluded from the circulation. The weak receptor-stimulating clozapine prevented the strong receptor-stimulating N-desmethylclozapine, at specific ratios in humans and in rats, from exerting its full agonistic action. In conclusion, the contribution of N-desmethylclozapine to the clozapine-induced sialorrhoea was, at most, only partly additive. Furthermore, the present experimental set-up failed to demonstrate any anti-salivatory action of amisulpride on the clozapine-induced flow of saliva.

Amisulpride induced ultrastructural signs of secretory activity but to varying extent; in submandibular glands, in contrast to parotid glands, changes were not brought beyond the granular anchoring stage. Amisulpride may provide an overall readiness for secretion that will result in augmented responses to agonists, a phenomenon of potential interest in dry-mouth treatment.