Role of cyclo-oxygenase pathways in the stimulatory influence of immune challenge on the transcription of a specific CRF receptor subtype in the rat brain

Abstract

The aim of this study was to investigate the role of prostaglandins (PGs) on the expression of corticotropin-releasing factor (CRF) receptors in the brains of immune-challenged rats. Intravenous (i.v.) administration 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 μg/100 g b.w.), moderate (25 μg/100 g b.w.), or low (2.5 μg/100 g b.w.) dose of the immune activator lipopolysaccharide (LPS). Three and six hours after the i.p. treatment with the endotoxin LPS, male Sprague-Dawley rats (230–260 g) were sacrified. Frozen brains were mounted on a microtome and cut from the olfactory bulb to the medulla in 30-μm coronal sections. mRNAs encoding CRF receptors (types 1, 2α, and 2β) were assayed by in situ hybridization using 35S-labeled riborpobes. Strong basal levels of CRF 1 receptor transcript were detected in multiple regions of the brain, whereas CRF 2α receptor message was highly localized in few structures of the limbic system and positive signal for CRF 2β receptor mRNA was observed only in the choroid plexus. The transcription of the gene encoding the CRF type 1 (but not types 2) receptor was highly stimulated by LPS administration in selective hypothalamic nuclei. Indeed, a high dose of LPS caused strong expression of CRF 1 receptor mRNA in both parvocellular and magnocellular paraventricular nucleus (PVN) and in the supraoptic nucleus (SON), although low and moderate doses of endotoxin induced a more specific expression of this transcript in the parvocellular division of the PVN. Pretreatment with indomethacin did not prevent the induction of CRF 1 receptor transcription in the PVN of rats injected with a high dose of LPS. In contrast, inhibition of cyclo-oxygenase pathways significantly inhibited the expression of CRF 1 receptor in the PVN and SON of rats sacrificed 6 h after being injected with a moderate or a low dose of LPS; the CRF 1 receptor mRNA levels were approximately three (moderate dose) and two (low dose) times higher in rats receiving the endotoxin alone than in those submitted to a treatment combining both i.v. indomethacin and i.p. LPS. These results indicate that the mRNA encoding the type 1 but not the type 2 CRF receptor is specifically regulated in endocrine hypothalamus of immune-challenged rats, whereas the role of PGs in mediating the stimulatory influence of immune challenge on the transcription of CRF 1 receptor in the PVN and SON seems to depend on the severity of this systemic stressful situation.