Hypophysiotropic Role and Hypothalamic Gene Expression of Corticotropin-Releasing Factor and Vasopressin in Rats Injected with Interleukin-1β Systemically or into the Brain Ventricles

Intact adult male rats were injected intravenously (i.v., 400 ng/kg), intraperitoneally (i.p., 400 ng/kg) or intracerebroventricularly (i.c.v., 100 ng/kg) with interleukin-1β (IL-1β) or its vehicle. In comparison with vehicle-treated animals, IL-1β induced significant (P<0.01) increases in plasma ACTH levels measured 30 min later regardless of the route of cytokine administration. These changes were markedly blunted in rats administered specific antibodies directed against corticotropin-releasing factor (CRF). In contrast, vasopressin (VP) antibodies significantly blunted ACTH released by the i.c.v. injection of IL-1β, but only modestly altered the effect of the systemic injection of the cytokine. We then used semi-quantitative in situ hybridization analysis to measure changes in steady-state mRNA levels, as they might occur in response to these same doses of IL-1β. Following administration of the vehicle, measurement of gene expression in the paraventricular (PVN) portion of the hypothalamus indicated a measurable amount of hybridization signals for both CRF and VP. No detectable changes in either CRF or VP gene expression were observed in rats injected with IL-1β i.v. or i.p. 5 h earlier. In contrast, the i.c.v. administration of the cytokine significantly (P<0.01) increased both CRF and VP mRNA levels measured 5 h later. These results suggest that while endogenous CRF modulates the response of the corticotrophs to this cytokine regardless of the route of administration, the role of VP is more important in rats injected centrally than in those injected peripherally. The observation that at the dose tested and over the time-course studied, systemic injection of IL-1β failed to alter CRF or VP gene expression, supports our earlier hypothesis that blood-born IL-1β acts primarily at the level of nerve terminals in the median eminence.