Clozapine: Agonistic and Antagonistic Salivary Secretory Actions

Polymorphisms in ERBB4 and TACR1 associated with dry mouth in clozapine-treated patients

BACKGROUND

Sialorrhea is a common and uncomfortable adverse effect of clozapine, and its severity varies between patients. The aim of the study was to select broadly genes related to the regulation of salivation and study associations between sialorrhea and dry mouth and polymorphisms in the selected genes.

METHODS

The study population consists of 237 clozapine-treated patients, of which 172 were genotyped. Associations between sialorrhea and dry mouth with age, sex, BMI, smoking, clozapine dose, clozapine and norclozapine serum levels, and other comedication were studied. Genetic associations were analyzed with linear and logistic regression models explaining sialorrhea and dry mouth with each SNP added separately to the model as coefficients.

RESULTS

Clozapine dose, clozapine or norclozapine concentration and their ratio were not associated with sialorrhea or dryness of mouth. Valproate use (p = 0.013) and use of other antipsychotics (p = 0.015) combined with clozapine were associated with excessive salivation. No associations were found between studied polymorphisms and sialorrhea. In analyses explaining dry mouth with logistic regression with age and sex as coefficients, two proxy-SNPs were associated with dry mouth: epidermal growth factor receptor 4 (ERBB4) rs3942465 (adjusted p = 0.025) and tachykinin receptor 1 (TACR1) rs58933792 (adjusted p = 0.029).

CONCLUSION

Use of valproate or antipsychotic polypharmacy may increase the risk of sialorrhea. Genetic variations in ERBB4 and TACR1 might contribute to experienced dryness of mouth among patients treated with clozapine.

Pharmacological interventions for antipsychotic-related sialorrhea: a systematic review and network meta-analysis of randomized trials

Antipsychotic-induced sialorrhea carries a significant burden, but evidence-based treatment guidance is incomplete, warranting network meta-analysis (NMA) of pharmacological interventions for antipsychotic-related sialorrhea. PubMed Central/PsycInfo/Cochrane Central database/Clinicaltrials.gov/WHO-ICTRP and the Chinese Electronic Journal Database (Qikan.cqvip.com) were searched for published/unpublished RCTs of antipsychotic-induced sialorrhea (any definition) in adults, up to 06/12/2023. We assessed global/local inconsistencies, publication bias, risk of bias (RoB2), and confidence in the evidence, conducting subgroup/sensitivity analyses. Co-primary efficacy outcomes were changes in saliva production (standardized mean difference/SMD) and study-defined response (risk ratios/RRs). The acceptability outcome was all-cause discontinuation (RR). Primary nodes were molecules; the mechanism of action (MoA) was secondary. Thirty-four RCTs entered a systematic review, 33 NMA (n = 1958). All interventions were for clozapine-induced sialorrhea in subjects with mental disorders. Regarding individual agents and response, metoclopramide (RR = 3.11, 95% C.I. = 1.39-6.98), cyproheptadine, (RR = 2.76, 95% C.I. = 2.00-3.82), sulpiride (RR = 2.49, 95% C.I. = 1.65-3.77), propantheline (RR = 2.39, 95% C.I. = 1.97-2.90), diphenhydramine (RR = 2.32, 95% C.I. = 1.88-2.86), benzhexol (RR = 2.32, 95% C.I. = 1.59-3.38), doxepin (RR = 2.30, 95% C.I. = 1.85-2.88), amisulpride (RR = 2.23, 95% C.I. = 1.30-3.81), chlorpheniramine (RR = 2.20, 95% C.I. = 1.67-2.89), amitriptyline (RR = 2.09, 95% C.I. = 1.34-3.26), atropine, (RR = 2.03, 95% C.I. = 1.22-3.38), and astemizole, (RR = 1.70, 95% C.I. = 1.28-2.26) outperformed placebo, but not glycopyrrolate or ipratropium. Across secondary nodes (k = 28, n = 1821), antimuscarinics (RR = 2.26, 95% C.I. = 1.91-2.68), benzamides (RR = 2.23, 95% C.I. = 1.75-3.10), TCAs (RR = 2.23, 95% C.I. = 1.83-2.72), and antihistamines (RR = 2.18, 95% C.I. = 1.83-2.59) outperformed placebo. In head-to-head comparisons, astemizole and ipratropium were outperformed by several interventions. All secondary nodes, except benzamides, outperformed the placebo on the continuous efficacy outcome. For nocturnal sialorrhea, neither benzamides nor atropine outperformed the placebo. Active interventions did not differ significantly from placebo regarding constipation or sleepiness/drowsiness. Low-confidence findings prompt caution in the interpretation of the results. Considering primary nodes' co-primary efficacy outcomes and head-to-head comparisons, efficacy for sialorrhea is most consistent for the following agents, decreasing from metoclopramide through cyproheptadine, sulpiride, propantheline, diphenhydramine, benzhexol, doxepin, amisulpride, chlorpheniramine, to amitriptyline, and atropine (the latter not for nocturnal sialorrhea). Shared decision-making with the patient should guide treatment decisions regarding clozapine-related sialorrhea.

Association Between N-Desmethylclozapine and Clozapine-Induced Sialorrhea: Involvement of Increased Nocturnal Salivary Secretion via Muscarinic Receptors by N-Desmethylclozapine

Clozapine-induced sialorrhea (CIS) is a common side effect of clozapine. There is no established standard treatment of CIS since the underlying mechanism remains unknown. This study aimed to elucidate the mechanisms involved in CIS. In our clinical study, a prospective observational study evaluated the association between serum and saliva concentrations of clozapine or its metabolites and Drooling Severity and Frequency Scale (DSFS) score. In our in vivo study, we first developed a new CIS animal model; subsequently, we measured salivary secretion and concentrations of clozapine or its metabolites in the animal model. In our in vitro study, we measured the calcium ion (Ca²⁺) response to evaluate the effect of clozapine or its metabolites on human salivary gland cell line (HSY cells) and then examined whether their effect was inhibited by atropine. In our clinical study, serum and saliva N-desmethylclozapine concentrations were significantly correlated with nocturnal DSFS score. In our in vivo study, daily single oral administration of 100 mg/kg clozapine for 7 days significantly increased salivary secretion in rats. Furthermore, N-desmethylclozapine concentrations in serum and submandibular glands of the rats were higher than clozapine concentrations. In our in vitro study, N-desmethylclozapine only elicited an increase in the intracellular Ca²⁺ in HSY cells. N-desmethylclozapine-induced Ca²⁺ responses were inhibited by atropine. These results suggest that N-desmethylclozapine is implicated in CIS by increasing nocturnal salivation via the muscarinic receptors. Moreover, our developed animal model that reflects CIS in clinical condition plays a key role as a bridge between basic and clinical research. SIGNIFICANCE STATEMENT: Clozapine-induced sialorrhea (CIS) is a severe and frequent adverse reaction, but the mechanism underlying CIS is less well understood. This paper reports that N-desmethylclozapine, a metabolite of clozapine, is implicated in CIS by increasing nocturnal salivation via the muscarinic receptors and that oral administration of clozapine at 100 mg/kg once daily for 7 days to rat is the optimum method for establishing the new animal model reflecting the clinical scenario of CIS.

Treatment Strategies for Clozapine-Induced Sialorrhea: A Systematic Review and Meta-analysis

BACKGROUND

Clozapine is the most effective medication for treatment-refractory schizophrenia. However, it has a high burden of adverse events, including common adverse events such as sialorrhea. Sialorrhea can lead to severe physical complications such as aspiration pneumonia, as well as psychological complications including embarrassment and low self-esteem. Compromised adherence and treatment discontinuation can occur due to intolerability. There have been no meta-analyses examining strategies to mitigate clozapine-induced sialorrhea.

METHODS

We systematically searched Chinese and Western research databases for randomised controlled trials examining agents for clozapine-induced sialorrhea. No limit to language or date were applied to the search. Where sufficient data for individual agents was available, pairwise meta-analyses were conducted. Results were provided as risk ratios and number needed to treat. Sensitivity analysis was conducted by study quality. Adverse events were provided as number needed to harm.

RESULTS

19 studies provided data for use in the meta-analysis. Improvement in clozapine-induced sialorrhea was seen in meta-analyses of propantheline (studies = 6, risk ratio [RR] 2.38, 95% confidence interval [CI] 1.52-3.73; number needed to treat [NNT] 3, 95% CI 1.9-2.7), diphenhydramine (studies = 5, RR 3.09, 95% CI 2.36-4.03; NNT 2, 95% CI 1.5-2.0), chlorpheniramine (studies = 2, RR 2.37, 95% CI 1.59-3.55; NNT 3, 95% CI 1.6-3.5), and benzamide derivatives (odds ratio [OR] 6.93, 95% CI 3.03-15.86). When meta-analyses were limited to high-quality studies, all these results remained significant. Single studies of benzhexol, cyproheptadine, doxepin and Kongyan Tang showed promise. Propantheline increased rates of constipation with a number needed to harm (NNH) of 9 (95% CI 4.2-204.1).

CONCLUSION

Clozapine-induced sialorrhea is a potentially serious adverse event. Included studies in this meta-analysis were limited by poor study quality. Diphenhydramine, chlorpheniramine and benzamide derivatives appear to have the best supporting evidence and lowest reported adverse events. Caution should be exercised when using propantheline given its increased risk of constipation.

A comprehensive review of swallowing difficulties and dysphagia associated with antipsychotics in adults

INTRODUCTION

This is a comprehensive review of antipsychotic (AP)-induced dysphagia and its complications: choking and pneumonia. Areas covered: Four PubMed searches were completed in 2018. The limited literature includes: 1) 45 case reports of AP-induced dysphagia with pharmacological mechanisms, 2) a systematic review of APs as a risk factor for dysphagia, 3) reviews suggesting adult patients with intellectual disability (ID) and dementia are prone to dysphagia (APs are a risk factor among multiple others), 4) studies of the increased risk of choking in patients with mental illness (APs are a contributing factor), 5) naturalistic pneumonia studies suggesting that pneumonia may contribute to AP-increased death in dementia, and 6) naturalistic studies suggesting that pneumonia may be a major cause of morbidity and mortality in clozapine patients. Expert commentary: The 2005 Food and Drug Administration requirement that package inserts warn of AP-induced dysphagia jumpstarted this area, but current studies are limited by: 1) its naturalistic nature, 2) the lack of dysphagia studies of patients with IDs and dementia on APs, and 3) the assumed indirect association between dysphagia with choking and pneumonia. Future clozapine studies on pneumonia, if they lead to a package insert warning, may have high potential to save lives.

Salivary secretion in health and disease

Saliva is a complex fluid produced by 3 pairs of major salivary glands and by hundreds of minor salivary glands. It comprises a large variety of constituents and physicochemical properties, which are important for the maintenance of oral health. Saliva not only protects the teeth and the oropharyngeal mucosa, it also facilitates articulation of speech, and is imperative for mastication and swallowing. Furthermore, saliva plays an important role in maintaining a balanced microbiota. Thus, the multiple functions provided by saliva are essential for proper protection and functioning of the body as a whole and for the general health. A large number of diseases and medications can affect salivary secretion through different mechanisms, leading to salivary gland dysfunction and associated oral problems, including xerostomia, dental caries and fungal infections. The first part of this review article provides an updated insight into our understanding of salivary gland structure, the neural regulation of salivary gland secretion, the mechanisms underlying the formation of saliva, the various functions of saliva and factors that influence salivary secretion under normal physiological conditions. The second part focuses on how various diseases and medical treatment including commonly prescribed medications and cancer therapies can affect salivary gland structure and function. We also provide a brief insight into how to diagnose salivary gland dysfunction.

A Guide to Medications Inducing Salivary Gland Dysfunction, Xerostomia, and Subjective Sialorrhea: A Systematic Review Sponsored by the World Workshop on Oral Medicine VI

BACKGROUND

Medication-induced salivary gland dysfunction (MISGD), xerostomia (sensation of oral dryness), and subjective sialorrhea cause significant morbidity and impair quality of life. However, no evidence-based lists of the medications that cause these disorders exist.

OBJECTIVE

Our objective was to compile a list of medications affecting salivary gland function and inducing xerostomia or subjective sialorrhea.

DATA SOURCES

Electronic databases were searched for relevant articles published until June 2013. Of 3867 screened records, 269 had an acceptable degree of relevance, quality of methodology, and strength of evidence. We found 56 chemical substances with a higher level of evidence and 50 with a moderate level of evidence of causing the above-mentioned disorders. At the first level of the Anatomical Therapeutic Chemical (ATC) classification system, 9 of 14 anatomical groups were represented, mainly the alimentary, cardiovascular, genitourinary, nervous, and respiratory systems. Management strategies include substitution or discontinuation of medications whenever possible, oral or systemic therapy with sialogogues, administration of saliva substitutes, and use of electro-stimulating devices.

LIMITATIONS

While xerostomia was a commonly reported outcome, objectively measured salivary flow rate was rarely reported. Moreover, xerostomia was mostly assessed as an adverse effect rather than the primary outcome of medication use. This study may not include some medications that could cause xerostomia when administered in conjunction with others or for which xerostomia as an adverse reaction has not been reported in the literature or was not detected in our search.

CONCLUSIONS

We compiled a comprehensive list of medications with documented effects on salivary gland function or symptoms that may assist practitioners in assessing patients who complain of dry mouth while taking medications. The list may also prove useful in helping practitioners anticipate adverse effects and consider alternative medications.

Acetylcholinesterase Inhibitors and Drugs Acting on Muscarinic Receptors- Potential Crosstalk of Cholinergic Mechanisms During Pharmacological Treatment

BACKGROUND

Pharmaceuticals with targets in the cholinergic transmission have been used for decades and are still fundamental treatments in many diseases and conditions today. Both the transmission and the effects of the somatomotoric and the parasympathetic nervous systems may be targeted by such treatments. Irrespective of the knowledge that the effects of neuronal signalling in the nervous systems may include a number of different receptor subtypes of both the nicotinic and the muscarinic receptors, this complexity is generally overlooked when assessing the mechanisms of action of pharmaceuticals.

METHODS

We have search of bibliographic databases for peer-reviewed research literature focused on the cholinergic system. Also, we have taken advantage of our expertise in this field to deduce the conclusions of this study.

RESULTS

Presently, the life cycle of acetylcholine, muscarinic receptors and their effects are reviewed in the major organ systems of the body. Neuronal and non-neuronal sources of acetylcholine are elucidated. Examples of pharmaceuticals, in particular cholinesterase inhibitors, affecting these systems are discussed. The review focuses on salivary glands, the respiratory tract and the lower urinary tract, since the complexity of the interplay of different muscarinic receptor subtypes is of significance for physiological, pharmacological and toxicological effects in these organs.

CONCLUSION

Most pharmaceuticals targeting muscarinic receptors are employed at such large doses that no selectivity can be expected. However, some differences in the adverse effect profile of muscarinic antagonists may still be explained by the variation of expression of muscarinic receptor subtypes in different organs. However, a complex pattern of interactions between muscarinic receptor subtypes occurs and needs to be considered when searching for selective pharmaceuticals. In the development of new entities for the treatment of for instance pesticide intoxication, the muscarinic receptor selectivity needs to be considered. Reactivators generally have a muscarinic M2 receptor acting profile. Such a blockade may engrave the situation since it may enlarge the effect of the muscarinic M3 receptor effect. This may explain why respiratory arrest is the major cause for deaths by esterase blocking.

World Workshop on Oral Medicine VI: a systematic review of medication-induced salivary gland dysfunction

The aim of this paper was to perform a systematic review of the pathogenesis of medication-induced salivary gland dysfunction (MISGD). Review of the identified papers was based on the standards regarding the methodology for systematic reviews set forth by the World Workshop on Oral Medicine IV and the PRISMA statement. Eligible papers were assessed for both the degree and strength of relevance to the pathogenesis of MISGD as well as on the appropriateness of the study design and sample size. A total of 99 papers were retained for the final analysis. MISGD in human studies was generally reported as xerostomia (the sensation of oral dryness) without measurements of salivary secretion rate. Medications may act on the central nervous system (CNS) and/or at the neuroglandular junction on muscarinic, α-and β-adrenergic receptors and certain peptidergic receptors. The types of medications that were most commonly implicated for inducing salivary gland dysfunction were those acting on the nervous, cardiovascular, genitourinary, musculoskeletal, respiratory, and alimentary systems. Although many medications may affect the salivary flow rate and composition, most of the studies considered only xerostomia. Thus, further human studies are necessary to improve our understanding of the association between MISGD and the underlying pathophysiology.

Low-dose Amisulpride for Debilitating Clozapine-induced Sialorrhea: Case Series and Review of Literature

Clozapine-induced sialorrhea (CIS) affects about one-third of patients treated with clozapine, at times can be stigmatizing, socially embarrassing, disabling, affect quality-of-life, cause poor compliance and can be potentially life-threatening adverse effect. Prompt and effective treatment of CIS may assist treatment tolerability, adherence, and better outcomes in patients with treatment nonresponsive schizophrenia. The beneficial effect of amisulpride augmentation of clozapine therapy for such patients may be enhanced by its anti-salivatory effect on CIS. Current series of five subjects who developed CIS that responded poorly to anticholinergic drugs found drastic improvement in daytime and nocturnal CIS with very low-dose (50-100 mg/day) of amisulpride. Low-dose amisulpride augmentation may also provide strong ameliorating effect on CIS. Nevertheless, a long-term, large-scale study with a broader dose range is warranted to evaluate the stability of this effect across time.

Polymorphism in alpha 2A adrenergic receptor gene is associated with sialorrhea in schizophrenia patients on clozapine treatment

OBJECTIVE

Clozapine-induced sialorrhea (CIS) is a common, inconvenient and socially stigmatizing adverse effect. The pathophysiology of CIS may be related to the effect of clozapine on the muscarinic and adrenergic receptors as well as the disruption of the circadian rhythms. The aim of this study was to find out if polymorphisms in muscarinic M1 and M3 receptor genes (CHRM1 and CHRM3), adrenoceptor alpha 2A gene (ADRA2A) or clock circadian regulator gene (CLOCK) are associated with CIS.

METHODS

Two hundred and thirty-seven clozapine-treated Finnish schizophrenia patients were genotyped for CHRM1, CHRM3, CLOCK and ADRA2A polymorphisms, and their salivary dysfunction was assessed with two questions. Twenty-six of these patients had previously been on medication to treat CIS. Comparisons of the genotypes between patients with excessive versus non-excessive salivation were analysed. Genotype distributions between patients and control group and haplotypes were also studied.

RESULTS

CHRM1, CHRM3 and CLOCK polymorphisms and haplotypes were not associated with CIS. ADRA2A (rs1800544) genotype was associated with CIS (p = 0.029). In patients with CIS, CC genotype (n = 103) was more common than in G-allele carriers (n = 79) (p = 0.013, OR 2.13, 95% CI: 1.17-3.88). No differences were found in the distributions of genotypes between patients and controls.

CONCLUSIONS

ADRA2A genotype was associated with CIS.

The antipsychotic amisulpride: ultrastructural evidence of its secretory activity in salivary glands

OBJECTIVE

Amisulpride is reported to inhibit clozapine-induced sialorrhea. Preclinically, clozapine evokes muscarinic-M1-type-mediated secretion that, however, amisulpride does not reduce. Instead, amisulpride, without causing any overt secretion per se, enhances both nerve- and autonomimetic-evoked salivation by unknown mechanism(s). Hypothesizing that amisulpride prepares the gland for secretion, we looked for ultrastructural events indicating secretory activity in intercellular canaliculi of serous/seromucous cells, that is, density increase in protrusions (reflecting anchored granules) and in microbuds (reflecting recycling membranes and/or vesicle secretion) and decrease in microvilli (reflecting the cytoskeletal re-arrangement related to exocytosis).

MATERIAL AND METHODS

Rat parotid and submandibular glands were exposed to amisulpride in vivo or in vitro. Glands were processed for transmission electron and scanning electron microscopy and then morphometrically assessed.

RESULTS

Cells were packed with secretory granules. The density of protrusions increased in both glands, whereas significant and parallel changes in microvilli and microbuds occurred only in parotid glands, and in vitro.

CONCLUSIONS

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.

Behavioral interventions for antipsychotic induced appetite changes

Weight gain remains a well recognized yet difficult to treat adverse effect of many anti-psychotic drugs including agents of the first and second generation. The weight gain liabilities of antipsychotic drugs are partly associated with their ability to increase appetite. Most behavioral interventions for weight control remain of limited efficacy, possibly because they do not specifically target the neuroendocrine factors regulating appetite. Identifying new weight management interventions directly acting on the biochemical and neuroendocrine mechanisms of anti-psychotic induced weight gain may help to improve the efficacy of behavioral weight management programs. Such potentially specific strategies include (1) using diets which do not increase appetite despite calorie restriction; (2) countering thirst as an anticholinergic side-effect; (3) discouraging cannabis use and (4) adding metformin to a behavioral intervention. In view of our currently rather limited treatment repertoire it seems timely systematically to explore such novel options.

Salivary secretion effects of the antipsychotic drug olanzapine in an animal model

OBJECTIVE

Olanzapine, introduced as an alternative to clozapine in schizophrenia therapy, is thought to display a receptor affinity similar to that of clozapine. Antipsychotics are well-known xerogenic drugs. However, clozapine exerts both antagonistic and agonistic salivary effects ('clozapine-induced sialorrhea'), the latter probably via muscarinic M1 type of receptor. We hypothesise that olanzapine also has dual salivary effects.

MATERIAL AND METHODS

Effects of intravenous olanzapine were examined in rats, including those subjected to chronic preganglionic parasympathetic denervation (submandibular glands) or combined postganglionic parasympathetic and sympathetic denervation (parotid glands). Secretion was evoked reflexly, and by intravenous methacholine and the tachykinin substance P.

RESULTS

At 0.01-1 mg kg(-1), olanzapine dose dependently reduced secretion in response to methacholine or reflex stimulus but not that to substance P. At 10 mg kg(-1), olanzapine evoked a long-lasting secretion, independent of the autonomic innervation as well as of α- and β-adrenergic receptors and muscarinic receptors. The secretion was reduced, but not abolished, by a substance P receptor antagonist.

CONCLUSIONS

Like clozapine, olanzapine evoked secretion. The response to olanzapine was greater and, in contrast to clozapine, involved non-traditional gland receptors (such as substance P receptors). The findings imply that olanzapine plays an excitatory role via tachykinin receptors in humans.

Atypical antipsychotics--effects of amisulpride on salivary secretion and on clozapine-induced sialorrhea

OBJECTIVE

Amisulpride is suggested for treatment of clozapine-induced sialorrhea. However, objective measurements of its effectiveness are lacking and, preclinically, amisulpride has no effect. We currently hypothesise that amisulpride acts by reducing the nervous- rather than the clozapine-driven salivary secretion.

MATERIAL AND METHODS

Effects of intravenous amisulpride (as well as of clozapine and raclopride, a dopamine D2/D3 antagonist) were investigated in rats, including those subjected to chronic preganglionic parasympathetic denervation (submandibular glands) or combined postganglionic parasympathetic and sympathetic denervation (parotid glands). In duct-cannulated glands, secretion was evoked reflexly, at low and maximum flow rates, and by electrical stimulation of the parasympathetic and sympathetic innervations, and administration of autonomimetics (including substance P).

RESULTS

Unlike clozapine, amisulpride had no effect on the reflexly evoked secretion at maximum rate. With respect to reflex secretion at low rate and to the secretion evoked by muscarinic, α-adrenergic, β-adrenergic and substance P receptors, amisulpride (in contrast to raclopride) dose dependently potentiated the responses. Amisulpride had no effect on gland blood flow.

CONCLUSIONS

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.

Clozapine-induced salivation: interaction with N-desmethylclozapine and amisulpride in an experimental rat model

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.