Subjects with human immunodeficiency virus type 1 (HIV-1) infection display increased activity of the hypothalamo-pituitary-adrenal (HPA) axis, which may play a role in both HIV-related neurodegenerative processes and disease progression. It has been speculated that the HIV coat protein gp120 may be responsible for these changes, and previous experimental evidence in both transgenic and nontransgenic mice supports this view. We speculated that one of the effects of gp120 in the CNS is to act within the hypothalamus to affect both corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), the principal regulators of HPA axis. We therefore administered i.p. gp120 (100 ng/rat) or vehicle to male Wistar rats and then detected Fos protein (an index of neuronal activation), CRH, and AVP immunoreactivity in the cellular compartments of the hypothalamic paraventricular nucleus (PVN). In addition, we tested the direct effect of various concentrations of gp120 on the release of CRH and AVP from rat hypothalamic explants maintained in vitro. Any modulation of gp120 effects by nitric oxide (NO) pathways was also sought by coadministering i.p. to rats or adding to the hypothalamic preparations the NO synthase inhibitor N(G)-methyl-l-arginine (l-NMMA). Gp120 induced the expression of Fos protein in both the parvo- and the magnocellular PVN, which was significantly attenuated by l-NMMA 10(-6) nM/L (P < 0.001 vs gp120 alone). Double immunochemistry showed costaining for Fos protein and CRH or AVP in the PVN following gp120; the number of double-labeled CRH and AVP cells for Fos protein was markedly reduced (P < 0.001) by coadministration of l-NMMA 10(-6) nM/L. In the in vitro studies, addition of gp120 to the hypothalamic explants in the dose range of 10 pM-1 nM resulted in a clear stimulation of both CRH and AVP release (P < 0.05-0.001 compared to control); in the presence of l-NMMA at 10-fold higher concentrations the stimulatory effect of gp120 on the release of both peptides was completely lost. It would therefore appear that gp120 activates CRH and AVP-producing neurons in the hypothalamic PVN and stimulates the release of both peptides in vitro via NO-dependent mechanisms. These findings, in line with previous evidence, further suggest that the increased activity of the HPA axis associated with HIV infection may be of central origin, due to the effects of gp120 on hypothalamic CRH and AVP release.