Comparative analysis of pharmacological properties of xanomeline and N-desmethylclozapine in rat brain membranes

Background: 3(3-Hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine (xanomeline) and N-desmethylclozapine are of special interest as promising antipsychotics with better efficacy, especially for negative symptoms and/or cognitive/affective impairment.

Methods: The guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding experiments were performed using (1) conventional filtration technique, (2) antibody-capture scintillation proximity assay, and (3) immunoprecipitation method, in brain membranes prepared from rat cerebral cortex, hippocampus, and striatum.

Results: Xanomeline had agonistic activity at the M1 Muscarinic Acetylcholine Receptor (mAChR) in all brain regions, as well as at the 5-HT1A receptor in the cerebral cortex and hippocampus. On the other hand, N-desmethylclozapine exhibited slight agonistic effects on the M1 mAChR, and agonistic properties at the 5-HT1A receptor in the cerebral cortex and hippocampus. This compound also behaved as an agonist at the δ-opioid receptor in the cerebral cortex and striatum. In addition, the stimulatory effects of N-desmethylclozapine on [(35)S]GTPγS binding to Gαi/o were partially mediated through mAChRs (most likely M4 mAChR subtype), at least in striatum.

Conclusions: The agonistic effects on the mAChRs (particularly M1 subtype, and also probably M4 subtype), the 5-HT1A receptor and the δ-opioid receptor expressed in native brain tissues, some of which are common to both compounds and Others specific to either, likely shape the unique beneficial effectiveness of both compounds in the treatment for schizophrenic patients. These characteristics provide us with a clue to develop newer antipsychotics, beyond the framework of dopamine D2 receptor antagonism, that are effective not only on positive symptoms but also on negative symptoms and/or cognitive/affective impairment.