Cyclic nucleotide dynamics in the rat hypothalamus during osmotic stimulation: in vivo and in vitro studies

Abstract

The molecular mechanisms which regulate expression of vasopressin (AVP)- and oxytocin (OT)-encoding genes are unknown. We have investigated the regulatory role of one class of second messenger, the cyclic nucleosides, by examining levels of both adenosine 3′,5′-monophosphate (cAMP) and guanosine 3′,5′-monophosphate (cGMP) in hypothalamic nuclei of rats during osmotic stimulation. In vivo studies, in which rats were given 2% saline to drink for different periods (salt loading), demonstrated elevated levels of cAMP in the supraoptic nucleus (SON) after 2 days. Raised levels were also evident at 3 and 7 days. A similar (less marked) pattern was observed in the paraventricular nucleus (PVN) but not in the suprachiasmatic nucleus (SCN). cGMP was present at much lower levels than cAMP and did not exhibit parallel dynamics during salt loading; however, significant changes in cGMP levels were found in the SON and PVN. In vitro studies, in which explant cultures of punched hypothalamic nuclei were challenged with hypertonic media, demonstrated that increasing medium osmolality from 290 to 310 mOsm/kg doubled the level of cAMP in the SON but did not change levels in the PVN or SCN. A greater stimulus, 325 mOsm/kg, caused a 4-fold increase in SON cAMP, and small cAMP responses in the PVN and SCN. Marked cGMP responses were also observed in the SON following stimulation at 310 and 325 mOsm/kg, smaller responses being found in the PVN and SCN. These results are consistent with previous demonstrations of SON neuron osmosensitivity. The present study has provided evidence to support the hypothesis that cyclic nucleotides mediate osmotic effects on hypothalamic gene expression, and describes an in vitro model system whereby this hypothesis may be directly tested. © 1989.