Neuropeptide-Y potentiates the secretion of vasopressin from the neurointermediate lobe of the rat pituitary gland

Neuropeptide-Y (NPY) is colocalized with vasopressin and oxytocin in magnocellular neurons of the hypothalamo-neurohypophysial system, and a very high density of NPY-binding sites is present within the neurohypophysis. To investigate the possibility that NPY exerts a modulatory role on the release of neurohypophysial hormones, we have studied the actions of NPY on potassium-evoked release of vasopressin and oxytocin from the rat neurointermediate lobe in vitro. NPY dose-dependently potentiated vasopressin release evoked by high extracellular potassium (56 mM), with a maximal enhancement of 223% (10(-7) M NPY). A similar effect was obtained with the Y2-selective agonist NPY-(13-36). In contrast, no effect on the potassium-evoked release of oxytocin was observed at this concentration. In the absence of Ca2+ in the incubation medium, NPY did not potentiate vasopressin secretion, indicating that the effect of NPY on potassium-evoked secretion of neurohypophysial vasopressin is critically dependent on extracellular calcium ions. The number of neurohypophysial NPY-binding sites is drastically down-regulated in animals subjected to chronic osmotic stimulation. In the present study, it was observed that the potentiating effect of NPY on vasopressin secretion was completely abolished in neurointermediate lobes recovered from animals that had been drinking 2% NaCl for 12 days, reflecting the concomitant down-regulation of neurohypophysial NPY-binding sites observed during this state. Finally, it was confirmed that stimulation by high K+ significantly evoked the release of endogenous NPY from neurointermediate lobes of the pituitary gland. The present results provide evidence that NPY selectively and potently enhances evoked vasopressin secretion. Considering the coexistence of the two neuropeptides in magnocellular hypothalamo-neurohypophysial neurons, this action is likely to be part of an autostimulatory feedforward loop. NPY may be an important component in the mechanisms associated with the control of body fluid homeostasis.