Enhancement of maximal activation of neuronal nitric oxide synthase at muscarinic M₁ receptors following prolonged agonist treatment

It was previously believed that the neuronal type of nitric oxide (NO) synthase was constitutive in nature, and that changes in the concentration of intracellular Ca²⁺ represent the sole input that regulates its activity. Recent reports, however, suggested that this enzyme could also be induced under certain conditions. We report here that prolonged stimulation of M₁ muscarinic acetylcholine receptors results in potentiation of maximal receptor-mediated activation of neuronal NO synthase in Chinese hamster ovary cells. This effect was dependent on the concentration of agonist during the treatment and was abolished by a muscarinic receptor antagonist. These findings are important for understanding the sequelae of prolonged administration of muscarinic agonists in vivo.