The clozapine metabolite N-desmethylclozapine displays variable activity in diverse functional assays at human dopamine D2 and serotonin 5-HT1A receptors

N-desmethylclozapine (NDMC or norclozapine) is the major active metabolite of the antipsychotic clozapine in humans. The activity of NDMC differs from clozapine at a number of neurotransmitter receptors, probably influencing the pharmacological effects of clozapine treatment. Here, we tested the properties of NDMC in comparison with clozapine at recombinant human dopamine D2 and serotonin 5-HT1A receptors, using a panel of functional assays implicating diverse signalling pathways. At dopamine D2 receptors, NDMC as well as clozapine did not display agonist activity in measures of G protein activation by [35S]GTPγS binding and in the sensitive Extracellular Signal-Regulated Kinase 1/2 (ERK1/2) phosphorylation assay. In contrast, there were weak partial agonist actions of NDMC (but not of clozapine) for dopamine D2-dependent activation of Ca2+ liberation via coexpressed chimeric Gαq/o proteins and for G protein-coupled inward rectifier potassium channel (GIRK) current induction in Xenopus oocytes. Intriguingly, GIRK currents induced by NDMC via dopamine D2 receptors showed a rapid and transient time course, strikingly different from currents recorded with other receptor agonists. At serotonin 5-HT1A receptors, NDMC was a more efficacious partial agonist than clozapine for [35S]GTPγS binding, ERK1/2 phosphorylation and GIRK activation. Respective low and moderate partial agonist properties of NDMC at dopamine D2 and serotonin 5-HT1A receptors thus differentiate the metabolite from its parent drug and may contribute to the overall effects of clozapine pharmacotherapy.