LPS-induced Fos expression in oxytocin and vasopressin neurons of the rat hypothalamus

The aim of this study was to examine the involvement of the hypothalamic ox ytocin (OXT) and vasopressin (AVP) neurons in acute phase reaction using qu antitative dual-labeled immunostaining with Fos and either OXT and AVP in s everal hypothalamic regions. Administration of low dose (5 mu g/kg) and hig h dose (125 mu g/kg) of LPS induced intense nuclear Fos immunoreactivity in many OXT and AVP neurons in all the observed hypothalamic regions. The per centage of Fos-positive nuclei in OXT magnocellular neurons was higher than that of AVP magnocellular neurons in the supraoptic nucleus (SON), the mag nocellular neurons in the paraventricular nucleus (magPVN), rostral SON (rS ON), and nucleus circularis (NC), whose axons terminate at the posterior pi tuitary for peripheral release. The percentage of Fos-positive nuclei in AV P parvocellular neurons in the paraventricular nucleus (parPVN) was higher than that of OXT parvocellular neurons, whose axons terminate within the br ain for central release. Moreover, the percentage of Fos-positive nuclei in AVP magnocellular neurons of the SON and rSON was significantly higher tha n that of the magPVN and NC when animals were given LPS via intraperitoneal (i.p.)-injection. This regional heterogeneity was not observed in OXT magn ocellular neurons of i.p.-injected rats or in either OXT or AVP magnocellul ar neurons of intravenous (i.v.)-injected rats. The present data suggest th at LPS-induced peripheral release of AVP and OXT is due to the activation o f the magnocellular neurons in the SON, magPVN, NC, and rSON, and the centr al release of those hormones is in part derived from the activation of parv ocellular neurons in the PVN. It is also suggested that the activation of A VP magnocellular neurons is heterogeneous among the four hypothalamic regio ns, but that of OXT magnocellular neurons is homogenous among these brain r egions in response to LPS administration. (C) 2000 Elsevier Science B.V. Al l rights reserved.