Efficient coupling of m5 muscarinic acetylcholine receptors to activation of nitric oxide synthase

The coupling of m5 muscarinic acetylcholine receptors to the generation and release of nitric oxide (NO) was investigated. Chinese hamster ovary cells, which stably express m5 receptors, were transiently transfected with the gene encoding neuronal NO synthase and used as a model system. Increased generation of NO upon stimulation of cells by muscarinic agonists was detected by an increase in cyclic GMP in admixed mouse neuroblastoma N1E-115 cells or more directly by measuring the conversion of L-arginine into L- citrulline. Carbachol increased cyclic GMP formation in the mixture of cells in a time- and concentration-dependent manner, with a half-maximal response occurring in the nanomolar range. This response was significantly attenuated by scavengers of NO or inhibitors of NO synthase. This high potency of carbachol was also observed in measurements of L-citrulline formation. A series of muscarinic agonists were as efficacious as carbachol in stimulating NO synthase, whereas McN-A-343 and pilocarpine were partial agonists in this regard. Evidence for an exceptionally high efficiency of coupling of m5 receptors to this response and its possible implication in the interaction between cholinergic and dopaminergic neurotransmission is discussed.