PTHrP is detected in the supraoptic nucleus (SON) and paraventricular nucleus. We have recently demonstrated that PTHrP(1-34) is involved in AVP release and synthesis in the SON in vivo and in vitro. PTHrP and AVP, which act on blood vessels, may interact by autocrine and paracrine mechanisms in the central nervous system. The present study was undertaken to determine the mutual regulation of AVP and PTHrP secretion in dissociated magnocellular neurons of the SON. Both AVP and PTHrP existed in the dissociated SON neurons by immunohistochemistry. PTHrP(1-34) stimulated AVP secretion from the cells dose dependently, but PTHrP(7-34) and PTH(1-34) did not. PTHrP(1-34)-stimulated AVP secretion was associated with cAMP generation. PTHrP(1-34)-induced cAMP generation was inhibited by a 100-fold molar excess of PTHrP(7-34) but not by that of PTH(1-34). PTHrP(1-34) also stimulated AVP mRNA expression in the cells. These results are consistent with our previous observations that PTHrP(1-34) is involved in AVP secretion through a receptor distinct from type I PTH/PTHrP receptor. Next, AVP stimulated dose-dependent PTHrP release from the dissociated SON neurons. The AVP-induced PTHrP release was suppressed by both OPC-21268 (V(1a) receptor antagonist) and dP[Thy(Me)(2)]AVP (V(1a)/V(1b) receptor antagonist) but not by OPC-31260 (V(2) receptor antagonist). AVP increased PKC activity dose dependently but not cAMP generation in the SON neurons. The AVP-stimulated PTHrP release was blocked by staurosporine (PKC inhibitor), nicardipine (L-type calcium channel blocker) or omega-agatoxin IVA (N type). Furthermore, AVP stimulated PTHrP mRNA expression for 12 h in the SON neurons. These results indicate that AVP caused increases in PTHrP secretion and its mRNA levels through V(1a) and/or V(1b) receptors in the SON neurons. Our observations, taken together, suggest that PTHrP stimulates AVP secretion into the extracellular space of the SON, which in turn leads to further secretion of AVP and PTHrP by an autocrine/paracrine mechanism.