INVOLVEMENT OF SEROTONERGIC PATHWAYS IN MEDIATING THE NEURONAL-ACTIVITY AND GENETIC TRANSCRIPTION OF NEUROENDOCRINE CORTICOTROPIN-RELEASINGFACTOR IN THE BRAIN OF SYSTEMICALLY ENDOTOXIN-CHALLENGED RATS

The present study investigated the effect of serotonin depletion on th e neuronal activity and transcription of corticotropin-releasing facto r in the rat brain during the acute-phase response. Conscious male rat s received an intraperitoneal (i.p.) injection with the immune activat or lipopolysaccaride (25 mu g/100 g body wt) after being treated for t hree consecutive days with para-chlorophenylalanine (30mg/100g/day). T his irreversible inhibitor of tryptophane-5-hydroxylase decreased hypo thalamic serotonin levels by 96%. One, 3 and 6h after a single i.p, in jection of lipopolysaccharide or vehicle solution, rats were killed an d their brains cut in 30-mu m coronal sections. Messenger RNAs encodin g c-fos, nerve-growth factor inducible-B gene, corticotropin-releasing factor and the heteronuclear RNA encoding corticotropin-releasing fac tor primary transcript were assayed by in situ hybridization using S-3 5-labeled riboprobes, whereas Fos-immunoreactive nuclei were labeled b y immunocytochemistry. Lipopolysaccharide induced a wide neuronal acti vation indicated by the expression of both immediate-early gene transc ripts and Fos protein in numerous structures of the brain. The signal for both immediate-early gene transcripts was low to moderate Ih after lipopolysaccharide administration, maximal at 3 h and decline at 6 h post-injection, whereas at that time, Fos-immunoreactive nuclei were s till detected in most of the c-fos messenger RNA-positive structures. Interestingly, the strong and widespread induction of both immediate-e arly gene transcripts was almost totally inhibited by para-chloropheny lalanine treatment; in the hypothalamic paraventricular nucleus for ex ample, c-Sos messenger RNA signal and the number of Fos-immunoreactive positive cells were reduced by 80 and 48%, respectively, in serotonin -depleted rats treated with the bacterial endotoxin. This blunted neur onal response was also associated with an attenuated stimulation of ne uroendocrine corticotropin-releasing factor transcription and plasma c orticosterone release. Indeed, lipopolysaccharide caused a selective e xpression of corticotropin-releasing factor primary transcript in the paraventricular nucleus of the hypothalamus and this effect was si,sig nificantly reduced by treatment with the serotonin inhibitor. However, basal expression of corticotropin-releasing factor messenger RNA acro ss the brain (bed nucleus of the stria terminalis, medial preoptic are a, paraventricular nucleus of the hypothalamus, central nucleus of the amygdala, etc.) was not affected by the para-chlorophenylalanine trea tment. These results suggest that the integrity of serotonin pathways plays a role in the neuronal activity triggered by the systemic endoto xin insult. The fact that serotonin depletion largely prevented activa tion of neurosecretory parvocellular neurons of the paraventricular nu cleus of the hypothalamus and neuroendocrine corticotropin-releasing f actor gene transcription in response to immunogenic challenge provides the evidence that serotonergic system is part of the brain circuitry involved in the corticotroph axis-immune interface. (C) 1998 IBRO. Pub lished by Elsevier Science Ltd.