Intrahypothalamic vasopressin release. An inhibitor of systemic vasopressin secretion?

Vasopressin and oxytocin are released into the extracellular space of the supraoptic (SON) and paraventricular nuclei (PVN). The dendrites of these neurones contain a high density of neurosecretory granules, and exocytotic profiles have been visualised by electron microscopy. Release within the SON has been measured using microperfusion techniques; release is tetrodotoxin-independent, calcium-dependent, and is activated by a range of physiological stimuli, including suckling, dehydration, haemorrhage and stress. Release of vasopressin into the SON is regulated by a number of forebrain and brainstem areas. Dendritic release does not necessarily parallel neurohypophyseal release, and may occur semi-independently of spike activity in the soma and axons. The physiological consequences of dendritic vasopressin release are not clear, vasopressin and oxytocin appear to induce further vasopressin and oxytocin release from the dendrites. In contrast, by combining retrodialysis and electrophysiology we have shown that, unlike oxytocin which excites oxytocin neurones, vasopressin inhibits the electrical activity of vasopressin neurones, and hence suppresses vasopressin release from the pituitary. Thus, vasopressin released from dendrites may act on vasopressin neurones to regulate their phasic activity by an auto-inhibitory action within the SON. Since dendritic vasopressin release is increased and prolonged after various stimuli, this mechanism may act to restrain excitation of vasopressin neurones (and hence vasopressin secretion from the neurohypophysis) during continuing stimulation.