ROLE OF CYCLOOXYGENASE PATHWAYS IN THE STIMULATORY INFLUENCE OF IMMUNE CHALLENGE ON THE TRANSCRIPTION OF A SPECIFIC CRF RECEPTOR SUBTYPE INTHE RAT-BRAIN

The aim of this study was to investigate the role of prostaglandins (P Gs) on the expression of corticotropin-releasing factor (CRF) receptor s in the brains of immune-challenged rats. Intravenous (i.v.) administ ration of indomethacin (0.8 mg/100 g body weight (b.w.)), an inhibitor of PG synthesis, was performed 15 min before the intraperitoneal (i.p .) injection of a high (250 mu g/100 g b.w.), moderate (25 mu g/100 g b.w.), or low (2.5 mu g/100 g b.w.) dose of the immune activator lipop olysaccharide (LPS). Three and six hours after the i.p. treatment with the endotoxin LPS, male Sprague-Dawley rats (230-260 g) were sacrific ed. Frozen brains were mounted on a microtome and cut from the olfacto ry bulb to the medulla in 30-mu m coronal sections. mRNAs encoding CRF receptors (types 1, 2 alpha, and 2 beta) were assayed by in situ hybr idization using S-35-labeled riboprobes. Strong basal levels of CRF(1) receptor transcript were detected in multiple regions of the brain, w hereas CRF(2 alpha) receptor message was highly localized in few struc tures of the limbic system and positive signal for CRF(2 beta) recepto r mRNA was observed only in the choroid plexus. The transcription of t he gene encoding the CRF type 1 (but not types 2) receptor was highly stimulated by LPS administration in selective hypothalamic nuclei. Ind eed, a high dose of LPS caused strong expression of CRF(1) receptor mR NA in both parvocellular and magnocellular paraventricular nucleus (PV N) and in the supraoptic nucleus (SON), although low and moderate dose s of endotoxin induced a more specific expression of this transcript i n the parvocellular division of the PVN. Pretreatment with indomethaci n did not prevent the induction of CRF(1) receptor transcription in th e PVN of rats injected with a high dose of LPS. In contrast, inhibitio n of cyclo-oxygenase pathways significantly inhibited the expression o f CRF(1) receptor in the PVN and SON of rats sacrificed 6 h after bein g injected with a moderate or a low dose of LPS; the CRF(1) receptor m RNA levels were approximately three (moderate dose) and two (low dose) times higher in rats receiving the endotoxin alone than in those subm itted to a treatment combining both i.v. indomethacin and i.p. LPS. Th ese results indicate that the mRNA encoding the type 1 but not the typ e 2 CRF receptor is specifically regulated in endocrine hypothalamus o f immune-challenged rats, whereas the role of PGs in mediating the sti mulatory influence of immune challenge on the transcription of CRF(1) receptor in the PVN and SON seems to depend on the severity of this sy stemic stressful situation.