NEURONAL-ACTIVITY AND NEUROPEPTIDE GENE-TRANSCRIPTION IN THE BRAINS OF IMMUNE-CHALLENGED RATS

The present study investigated the effect of the acute-phase response of a systemic immune activation on the transcription of various immedi ate early genes (IEGs) and neuropeptides in the brain of conscious rat s. One, 3, 6, 9, and 12 h after a single intraperitoneal (i.p.) admini stration of either the immune activator lipopolysaccharide (LPS) or th e vehicle solution, adult male rats were sacrificed and their brains c ut in 30-mu m coronal sections. mRNA encoding the IEGs c-fos and nerve growth factor inducible-B (NGFI-B), and neuropeptides corticotropin-r eleasing factor (CRF), oxytocin (OT), and vasopressin (AVP) were assay ed by in situ hybridization histochemistry using a S-35-labeled ribopr obes. The primary transcripts (heteronuclear (hn)RNA) for these neurop eptides were also detected using intronic probe technology, and coloca lization of c-fos mRNA within CRF, AVP, and OT neurons was determined by means of a combination of immunocytochemistry and in situ hybridiza tion techniques on same the brain sections. One h after LPS treatment, both c-fos and NGFI-B genes were expressed in the parvocellular divis ion of the paraventricular nucleus (PVN) of the hypothalamus. The medi al preoptic area/organum vasculosum of the lamina terminalis, the supr aoptic nucleus (SON), the magnocellular division of the PVN, the arcua te nucleus/median eminence, the locus coeruleus, the nucleus of the so litary tract, and the area postrema also exhibited a strong signal for these two transcripts 3 h after endotoxin administration. A smaller b ut a significant c-fos expression was observed in various structures, including the dorsomedial hypothalamic area, the central nucleus of th e amygdala, the ventral part of the tuberomammillary nucleus, the late rodorsal tegmental nucleus, the external lateral part of the parabrach ial nucleus, the dorsal division of the ambiguus nucleus, and the late ral reticular nucleus of LPS-injected rats. The signal for c-fos and N GFI-B mRNA in most of these brain nuclei reached a maximum at 3 h post injection, declined at 6 h, and vanished 9 to 12 h after LPS treatment . In the parvocellular nucleus of the PVN, c-fos was largely expressed in CRF-immunoreactive (ir) neurons, whereas in the magnocellular part of that nucleus and in the SON, this transcript was colocalized in nu merous OT-ir and few AVP-ir neurons. Relative levels of CRF mRNA in th e parvocellular PVN were also significantly increased 6 h following LP S, but endotoxin did not alter the genetic expression of this stress-r elated neuropeptide in other brain regions. This was confirmed by the CRF primary transcript (hnRNA), which was expressed only in the PVN bu t not in any other nucleus in the brains of LPS-treated rats. Although AVP and OT mRNA levels were not significantly altered in either the P VN or the SON of immune-challenged rats because of the high basal leve ls of each transcript in these nuclei, LPS caused significant inductio n of AVP hnRNA in the parvoPVN. These results provide evidence that i. p. endotoxin administration can produce a strong activation of various IEG transcripts in selective structures of the brain, which could in turn play a determinant role in the integration of immune input to the central nervous system. It is possible that the stimulation of neurop eptide biosynthesis in the hypothalamic nuclei involved in the control of the hypothalamic-pituitary adrenal axis during immune challenge is an important step in this integration.