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 oxytocin (OXT) and vasopressin (AVP) neurons in acute phase reaction using quantitative dual-labeled immunostaining with Fos and either OXT and AVP in several hypothalamic regions. Administration of low dose (5 μg/kg) and high dose (125 μg/kg) of LPS induced intense nuclear Fos immunoreactivity in many OXT and AVP neurons in all the observed hypothalamic regions. The percentage of Fos-positive nuclei in OXT magnocellular neurons was higher than that of AVP magnocellular neurons in the supraoptic nucleus (SON), the magnocellular neurons in the paraventricular nucleus (magPVN), rostral SON (rSON), and nucleus circularis (NC), whose axons terminate at the posterior pituitary for peripheral release. The percentage of Fos-positive nuclei in AVP parvocellular neurons in the paraventricular nucleus (parPVN) was higher than that of OXT parvocellular neurons, whose axons terminate within the brain for central release. Moreover, the percentage of Fos-positive nuclei in AVP magnocellular neurons of the SON and rSON was significantly higher than that of the magPVN and NC when animals were given LPS via intraperitoneal (i.p.)-injection. This regional heterogeneity was not observed in OXT magnocellular neurons of i.p.-injected rats or in either OXT or AVP magnocellular neurons of intravenous (i.v.)-injected rats. The present data suggest that LPS-induced peripheral release of AVP and OXT is due to the activation of the magnocellular neurons in the SON, magPVN, NC, and rSON, and the central release of those hormones is in part derived from the activation of parvocellular neurons in the PVN. It is also suggested that the activation of AVP magnocellular neurons is heterogeneous among the four hypothalamic regions, but that of OXT magnocellular neurons is homogenous among these brain regions in response to LPS administration.