INFLUENCE OF THE ESTROUS-CYCLE ON C-FOS AND CRH GENE-TRANSCRIPTION INTHE BRAIN OF ENDOTOXIN-CHALLENGED FEMALE RATS

The purpose of this study was to investigate whether the ovulatory cyc le interferes with the effect of the acute-phase response of a systemi c immune activation on the transcription of the immediate early gene c -fos and the stress-related neuropeptide corticotropin-releasing hormo ne (CRH) in the brains of female rats. Throughout the day of proestrus and diestrus-2 (09.00, 12.00, 15.00 h), adult rats received either a single intraperitoneal injection of the endotoxin lipopolysaccharide ( LPS, 200 mu g/100 g body weight) or the vehicle solution and were kill ed 3 h later (12.00, 15.00, 18.00 h). Frozen brains were mounted on a microtome, cut in 30-mu m slices and then processed for the detection of c-fos mRNA and CRH primary transcript (heteronuclear [hnRNA]) by me ans of in situ hybridization histochemistry using S-35-labeled exonic and intronic probes, respectively. LPS injection induced a profound ex pression of c-fos mRNA in the several nuclei and areas of the brain, s uch as the organum vasculosum of the lamina terminalis/medial preoptic area, supraoptic nucleus, parvo- and magnocellular divisions of the h ypothalamic paraventricular nucleus (PVN), arcuate nucleus/median emin ence, central nucleus of the amygdala, locus coeruleus, nucleus of the solitary tract, area postrema and ventrolateral medulla. Interestingl y, the intensity of expression of c-fos mRNA depended on the phase of the estrous cycle and/or the time of the day. Indeed, in several of th e structures described above, LPS induced a more pronounced c-fos sign al in the morning of proestrus than the afternoon and diestrus-2. CRH primary transcript was significantly increased by LPS treatment select ively in the parvocellular division of the PVN and the highest hybridi zation signal was observed in the morning of proestrus, a period where a large number of c-fos-positive cells were colocalized in CRH-immuno reactive neurons. A significant increase in the levels of AVP hnRNA wa s also observed in the parvocellular PVN of animals sacrificed at noon and early afternoon of both pro- and diestrus days. These results pro vide evidence that the neuroendocrine events regulating the reproducti ve cyclicity influence the endotoxin-induced activation of the early g ene c-fos in selective structures of the brain and the stimulation of neurons directly involved in the regulation of the HPA axis. It is pos sible that the gonadal status of female mammals plays a crucial role i n the integration of the organism in the presence of foreign material in preventing an exaggerated immune response during particular phases of the ovulatory cycle. The capacity of female animals to modulate the intensity through which the neuronal circuitry activated during immun ogenic processes is likely to be an elegant sexual dimorphism particip ating in the adjustment of the responses in line with the physiologica l demand.