EFFECT OF IMMUNE AND METABOLIC CHALLENGES ON THE LUTEINIZING-HORMONE-RELEASING HORMONE NEURONAL SYSTEM IN CYCLING FEMALE RATS - AN EVALUATION AT THE TRANSCRIPTIONAL LEVEL

The purpose of this study was to investigate the influence of immune ( systemic endotoxin administration) and metabolic (fasting) challenges on LHRH neuronal activity and transcription in the organum vasculosum of the lamina terminalis/medial preoptic area as well as on the expres sion of the LHRH receptor (LHRH-R) in the anterior pituitary of cyclin g female rats. The reproductive stages of adult female rats (200-250 g ; 14 h of light; lights on at 0600 h) were verified by daily vaginal s mears taken every morning for a minimum of three or four cycles before the experiment. The acute-phase response was induced via an ip inject ion of lipopolysaccharide (LPS; 200 mu g/100 g BW), whereas the metabo lic challenge consisted of food deprivation for at least 48 h. Control and challenged rats were killed at specific times in the ovulatory cy cle (1200, 1500, and 1800 h on proestrus and diestrous day 2). Frozen brains and pituitaries were mounted on a microtome, cut into 30-mu m s lices, and then processed for the detection of transcripts encoding ei ther LHRH or LHRH-R by means of in situ hybridization histochemistry u sing intronic (heteronuclear RNA) and exonic [messenger RNA (mRNA)] ri boprobes. Dual immunocytochemistry to detect Fos-immunoreactive (ir) n uclei in LHRH-ir perikarya and colocalization of LHRH mRNA with Fos pr otein during the day of proestrus were performed by using both in situ hybridization and immunocytochemistry techniques on the same brain se ctions. The percentage of LHRH-ir and LHRH-expressing neurons displayi ng positive Fos-ir nuclei during the afternoon of proestrus was signif icantly inhibited 3 h after endotoxin administration. Rats exhibited a n increase in the levels of LHRH primary transcript in the organum vas culosum of the lamina terminalis/medial preoptic area structure at 150 0 h on proestrus, a phenomenon significantly attenuated by LPS injecti on only at this phase of the estrous cycle. On the other hand, fasting did not affect LHRH neuronal activity or gene expression in intact cy cling rats, but affected these cells in animals exhibiting a disruptio n of the ovulatory cycle, Interestingly, LPS caused a profound down-re gulation of LHRH-R gene expression in the anterior pituitary throughou t the entire estrous cycle. Although food deprivation provoked a more variable pattern of LHRH-R mRNA in cycling rats, the signal for this t ranscript in the adenohypophysis was deeply altered in those showing a perturbed cycle. These results provide evidence that immune challenge interferes with the LHRH system at both hypothalamic and pituitary le vels, whereas alteration of that neuroendocrine system in food-deprive d rats seems highly associated with the impairment of reproductive cyc licity.