Intracellular calcium increase and somatodendritic vasopressin release by vasopressin receptor agonists in the rat supraoptic nucleus: involvement of multiple intracellular transduction signals.

Vasopressin neurones of the supraoptic nucleus are autoregulated by vasopressin released from their soma and dendrites. Vasopressin binds to specific autoreceptors to trigger an influx of Ca(2+), and this response involves both phospholipase C (PLC) and adenylate cyclase (AC) pathways that, in the periphery, are activated by V(1) (V(1a) and V(1b))- and V(2)-type receptors. To investigate the pathways involved in the [Ca(2+)](i) response, [Ca(2+)](i) measurements were made on freshly dissociated neurones using Fura-2 microspectrofluorimetry, and vasopressin release was measured from isolated supraoptic nuclei. The [Ca(2+)](i) increase and vasopressin release induced by the V(1a) agonist were strongly inhibited by a PLC blocker, an IP(3) receptor antagonist, and a PKC blocker. An AC inhibitor did not affect the V(1a) response, while PKA inhibitors significantly reduced the V(1a)-induced [Ca(2+)](i) and release responses. The [Ca(2+)](i) increase and vasopressin release elicited by the V(2) agonist were attenuated not only by AC pathway blockers, but also by PLC inhibitors. Surprisingly, the V(1b) agonist showed no [Ca(2+)](i) or vasopressin release response. In conclusion, the V(1a) agonist activates both PLC and AC pathway, confirming the functional expression of a V(1a) vasopressin receptor on vasopressin neurones. The V(2) agonist activation of both PLC and AC pathways could result from an action on the PLC-linked unknown receptor, and/or the AC-linked dual angiotensin II-vasopressin receptor.