PACAP Increases the Cytosolic Ca2−/ Concentration and Stimulates Somatodendritic Vasopresson Release in Rat Supraoptic Neurons

Pituitary adenylate cyclase activating polypeptide (PACAP)-like immunoreactivity and its receptor mRNA have been reported in the supraoptic and the paraventricular nucleus (SON and PVN, respectively) and PACAP has been implicated in the regulation of magnocellular neurosecretory cell function. To examine the site and the mechanism of the action of PACAP in the neurosecretory cells, we measured AVP release from SON slice preparations and the cytosolic Ca2+ concentration ([Ca2+]i) from single dissociated SON neurons. PACAP at concentrations from 10(-12) to 10(-7) M increased [Ca2+]i in dissociated SON neurons in a dose-dependent manner. The patterns of the PACAP-induced [Ca2+]i increase were either sustained increase or cytosolic Ca2+ oscillations. PACAP (10[-7] M) increased [Ca2+]i in 27 of 27 neurons and glutamate (10[-4] M) increased [Ca2+]i in 19 of 19 SON neurons examined, whereas angiotensin II (10[-7] M) increased [Ca2+]i in only 15 of 60 SON neurons examined. PACAP at lower concentrations (10[-10] to 10[-8] M) increased [Ca2+]i in 70-80% of neurons examined. Although the onset and recovery of the PACAP-induced [Ca2+]i increase were slower than those observed with glutamate, the spatial distribution of the [Ca2+]i increases in response to the two ligands were similar: [Ca2+]i increase at the proximal dendrites was larger and faster and that at the center of the soma was smaller and slower. The PACAP-induced [Ca2+]i responses were abolished by extracellular Ca2+ removal, the L-type Ca2+-channel blocker, nicardipine, or by replacement of extracellular Na+ with N-methyl D-glucamine, and were partially inhibited by the Na+-channel blocker, tetrodotoxin. The N-type Ca2+-channel blocker, omega-conotoxin GVIA did not significantly inhibit the PACAP-induced [Ca2+]i responses. Furthermore, PACAP (10[-7] M) as well as glutamate (10[-4] M) increased AVP release from SON slice preparations, and extracellular Ca2+ removal or nicardipine inhibited the AVP release in response to PACAP. These results indicate that PACAP enhances Ca2+ entry via voltage-gated Ca2+ channels and increases [Ca2+]i, which, in turn, stimulates somatodendritic vasopressin release by directly activating PACAP receptors on SON neurons. The results also suggest that PACAP in the SON may play a pivotal role in the control of the neurohypophyseal function at the level of the soma or the dendrites.