The inhibitory effect of anandamide on oxytocin and vasopressin secretion from neurohypophysis is mediated by nitric oxide

The neurohypophyseal hormones oxytocin (OT) and vasopressin (VP) are involved in behavioral, autonomic and neuroendocrine functions. Both peptides are synthesized in magnocellular neurons of paraventricular and supraoptic nuclei at hypothalamic level whose axons terminate in the neurohypophysis (NH), from where OT and VP are released into the systemic circulation. NH contains abundant nitric oxide (NO) synthase suggesting that NO plays a role in the release of these neuropeptides. The endocannabinoid system is present in magnocellular neurons of the hypothalamic neurohypophyseal system, and we have previously demonstrated that endocannabinoids modulate OT secretion at hypothalamic level.

In the present work, we investigated the in vitro effect of the endocannabinoid anandamide (AEA) on OT and VP release from NH of untreated adult male rats and the involvement of NO in this action.

Our results showed that AEA decreased OT and VP secretion from NH. AEA action was mediated by NO, since the inhibition of NO synthesis completely blocked this inhibitory effect. We found that cannabinoid receptor type 2 (CB₂) and transient receptor potential cation channel subfamily V member 1 (TRPV1) are involved in the inhibitory effect of AEA because AM630 and capsazepine, CB₂ and TRPV1 antagonists respectively, but not AM251, a CB₁ antagonist, blocked AEA effect at neurohypophyseal level.

These findings revealed an interaction between endocannabinoid, nitric oxide and oxytocin/vasopressin systems that could be involved in the modulation of homeostatic, behavioral and reproductive processes.