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 [³⁵S]-GTPγS-G_q/11α 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 [³⁵S]-GTPγS-G_q/11α binding studies. In [³⁵S]-GTPγS-G_q/11α 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 [³⁵S]-GTPγS-G_q/11α binding to human cortical membranes but antagonised the response to oxo-M (2μM) showing a pK_B of 6.8, comparable to its human recombinant M₁ mAChR affinity (pK_i=6.9) derived from [³H]-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.