Psychopharmacology (2005) 178: 451–460 DOI 10.1007/s00213-004-2017-1 ORIGINAL INVESTIGATION Marilyn A. Davies . Beth Ann Compton-Toth . Sandra J. Hufeisen . Herbert Y. Meltzer . Bryan L. Roth The highly efficacious actions of N-desmethylclozapine at muscarinic receptors are unique and not a common property of either typical or atypical antipsychotic drugs: is M 1 agonism a pre-requisite for mimicking clozapine’s actions? Received: 9 April 2004 / Accepted: 27 August 2004 / Published online: 13 October 2004 # Springer-Verlag 2004 Abstract Rationale: Recent studies have suggested that the salutary actions of clozapine in schizophrenia may be due to selective activation of M 1 muscarinic receptors by clozapine and/or its major active metabolite N-desmethyl- clozapine. Objective: We systematically tested this hy- pothesis by screening a large number of psychoactive compounds, including many atypical antipsychotic drugs, for agonist activity at cloned, human M 1 ,M 3 and M 5 mus- carinic receptors. Results: Only three of the 14 atypical antipsychotic drugs we tested were found to possess partial agonist actions at M 1 muscarinic receptors (fluperlapine, JL13, clozapine). A few additional miscellaneous com- pounds had a modest degree of M 1 agonist actions. Only carbachol and N-desmethylclozapine had appreciable M 3 muscarinic agonism at M 3 muscarinic receptors, although several were M 5 partial agonists including MK-212, N- desmethylclozapine and xanomeline. Conclusion: Although M 1 muscarinic receptor-selective partial agonists have shown promise in some preclinical antipsychotic drug models, these studies indicate that it is unlikely that the salutary actions of clozapine and similar atypical antipsy- chotic drugs are mediated solely by M 1 muscarinic receptor activation. It is possible, however, that the M 1 agonism of N-desmethylclozapine contributes to the uniquely bene- ficial actions of clozapine. Thus, these results are consistent with the notion that a balanced degree of activity at mul- tiple biogenic amine receptors, including M 1 muscarinic agonism, is responsible for the uniquely beneficial actions of clozapine. Keywords Clozapine . Atypical antipsychotic . N-desmethylclozapine . Muscarinic Introduction The muscarinic-cholinergic receptor system was first implicated in the pathogenesis and treatment of schizo- phrenia more than 45 years ago (Pfeiffer and Jenney 1957) and has undergone a renaissance over the past two decades (Tandon and Greden 1987, 1989; Dean et al. 2003). Indeed, a recent in vivo imaging study showed a region- selective reduction of muscarinic-cholinergic receptors in unmedicated schizophrenics (Raedler et al. 2003b) while post-mortem studies have documented decreased numbers of M 1 muscarinic receptors in prefrontal cortices of schizophrenics (Crook et al. 2001; Dean et al. 2002). Additionally, there is evidence from genetic association studies that M 1 muscarinic receptor polymorphisms may be associated with certain psychological symptoms of schizophrenia (Liao et al. 2003). It has also been known for nearly 30 years that many typical antipsychotic medications possess potent anticho- M. A. Davies . B. L. Roth Department of Psychiatry, Case Western Reserve University Medical School, Cleveland, OH 44106, USA B. A. Compton-Toth . S. J. Hufeisen . B. L. Roth (*) Department of Biochemistry, Case Western Reserve University Medical School, Cleveland, OH 44106, USA e-mail: bryan.roth@case.edu Tel.: +1-216-3682730 Fax: +1-216-3683419 B. L. Roth Department of Neurosciences, Case Western Reserve University Medical School, Cleveland, OH 44106, USA H. Y. Meltzer Department of Psychiatry, Vanderbilt University Medical School, Nashville, Tenn., USA H. Y. Meltzer Department of Pharmacology, Vanderbilt University Medical School, Nashville, Tenn., USA B. L. Roth NIMH Psychoactive Drug Screening Program, Case Western Reserve University Medical School, Cleveland, OH 44106, USA