EFFECT OF DEXFENFLURAMINE ON THE TRANSCRIPTIONAL ACTIVATION OF CRF AND ITS TYPE-1 RECEPTOR WITHIN THE PARAVENTRICULAR NUCLEUS OF THE RAT HYPOTHALAMUS

1 The present study investigated the effect of intraperitoneal (i.p.) administration of the indirect 5-hydroxytryptamine (5-HT) receptor ago nist, dexfenfluramine, on the transcriptional activity of corticotropi n-releasing factor (CRF) and its type 1 receptor in the brains of cons cious male Sprague-Dawley rats via in situ hybridization histochemistr y (ISHH) using both intronic and exonic probe technology. 2 The immedi ate early gene (IEG) c-fos mRNA was also used as index of cellular act ivity, whereas localization between CRF-immunoreactive (ir) perikarya and the IEG was accomplished to determine the site of CRF neuronal act ivation in the brain of dexfenfluramine-treated rats. 3 Thirty minutes , 1, 3, and 6 h after a single injection of either dexfenfluramine (10 mg kg(-1)) or the vehicle solution, adult male rats (230-260 g) were deeply anaesthetized and rapidly perfused with a 4% paraformaldehyde-b orax solution (PF). The brains were removed from the skull, postfixed, and placed in a solution of 4% PF-10% sucrose overnight al 4 degrees C. Frozen brains were mounted on a microtome and cut from the olfactor y bulb to the medulla in 30-mu m coronal sections. 4 Dexfenfluramine i nduced a general neuronal activation as indicated by the strong signal of c-fos mRNA in several structures of the brain, including the parie tal cortex, caudate putamen, circumventricular organs, medial preoptic area, bed nucleus of the stria terminalis, choroid plexus, choroidal fissure, supraoptic nucleus, paraventricular nucleus of the hypothalam us (PVN), paraventricular nucleus of the thalamus, central nucleus of the amygdala, dorsomedial nucleus of the hypothalamus, laterodorsal te gmental nucleus, locus coeruleus, and several subdivisions of the dors al vagal complex. In most of these structures, the signal was maximal at 30 min, still strong and positive at 60 min, largely decreased at 3 h, and had completely disappeared 6 h after injection. 5 In the parvo cellular division of the PVN, the large majority of CRF-ir perikarya d isplayed a positive signal for the mRNA encoding c-fos, indicating a p rofound CRFergic activation within this neuroendocrine nucleus after d exfenfluramine administration. 6 Colocalization between CRF-ir neurone s and c-fos positive cells was not detected in any other regions. This selective activation of PVN CRF neurones was also confirmed by the pr esence of CRF primary transcript; 30 min after i.p. injection of the i ndirect 5-HT agonist, a positive signal for CRF hnRNA was observed, sp ecifically in the parvocellular PVN. 7 Transcription of the gene encod ing the type 1 receptor for CRF was highly stimulated in the PVN follo wing 5-HT activation. Although this hypothalamic nucleus exhibited a b arely detectable signal under basal conditions, dexfenfluramine induce d a strong signal of CRF(1) receptor mRNA in the parvocellular PVN. In terestingly, CRF-ir neurones displayed a positive signal for the mRNA encoding the CRF(1) receptor, 3 and 6 h after systemic treatment with dexfenfluramine. 8 These results indicate that although dexfenfluramin e can generate a wide neuronal activation throughout the brain, this 5 -HT agonist triggers the activity of CRF neurones selectively in the p arvocellular division of the PVN, a mechanism possibly related to the activity of hypothalamic-pituitary-adrenal axis. Induction of CRF(1) r eceptor mRNA in CRF cells of the PVN indicates that neuroendocrine CRF neurones can be targeted by CNS CRF under 5-HT stimulation.