N-desmethylclozapine (NDMC) is an antagonist at the human native muscarinic M1 receptor

N-desmethylclozapine (NDMC) has been reported to display partial agonism at the human recombinant and rat native M1 mAChR, a property suggested to contribute to the clinical efficacy of clozapine. However, the profile of action of NDMC at the human native M1 mAChR has not been reported. The effect of NDMC on M1 mAChR function was investigated in human native tissues by assessing its effect on (1) M1 mAChR-mediated stimulation of ( 35 S)-GTPgS-Gq/11a binding to human post mortem cortical membranes and (2) the M1 mAChR-mediated increase in neuronal firing in human neocortical slices. NDMC displayed intrinsic activities of 46 � 9%, compared to oxo-M, at the human recombinant M1 receptor, in FLIPR studies and 35 � 4% at rat native M1 receptors in ( 35 S)-GTPgS-Gq/11a binding studies. In ( 35 S)-GTPgS-Gq/11a binding studies in human cortex, oxo-M stimulated binding by 240 � 26% above basal with a pEC50 of 6.56 � 0.05. In contrast, NDMC did not stimulate ( 35 S)-GTPgS-Gq/11a binding to human cortical membranes but antagonised the response to oxo-M (2 mM) showing a pKB of 6.8, comparable to its human recombinant M1 mAChR affinity (pKi ¼6.9) derived from ( 3 H)-NMS binding studies. In human, contrary to the rat neocortical slices, NDMC did not elicit a significant increase in M1 mAChR-mediated neuronal firing, and attenuated a carbachol-induced increase in neuronal firing when pre-applied. These data indicate that, whereas NDMC displays moderate to low levels of partial agonism at the human recombinant and rat native M1 mAChR, respectively, it acts as an antagonist at the M1 mAChR in human cortex.