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 immediate early genes (IEGs) and neuropeptides in the brain of conscious rats. One, 3, 6, 9, and 12 h after a single intraperitoneal (i.p.) administration of either the immune activator lipopolysaccharide (LPS) or the vehicle solution, adult male rats were sacrificed and their brains cut in 30‐μm coronal sections, mRNA encoding the IEGs c‐fos and nerve growth factor inducible‐B (NGFI‐B), and neuropeptides corticotropin‐releasing factor (CRF), oxytocin (OT), and vasopressin (AVP) were assayed by in situ hybridization histochemistry using a 35S‐labeled riboprobes. The primary transcripts {heteronuclear (hn)RNA} for these neuropeptides were also detected using intronic probe technology, and colocalization of c‐fos mRNA within CRF, AVP, and OT neurons was determined by means of a combination of immunocytochemistry and in situ hybridization techniques on same the brain sections. One h after LPS treatment, both c‐fos and NGFI‐B genes were expressed in the parvocellular division of the paraventricular nucleus (PVN) of the hypothalamus. The medial preoptic area/organum vasculosum of the lamina terminalis, the supraoptic nucleus (SON), the magnocellular division of the PVN, the arcuate nucleus/median eminence, the locus coeruleus, the nucleus of the solitary tract, and the area postrema also exhibited a strong signal for these two transcripts 3 h after endotoxin administration. A smaller but a significant c‐fos expression was observed in various structures, including the dorsomedial hypothalamic area, the central nucleus of the amygdala, the ventral part of the tuberomammillary nucleus, the laterodorsal tegmental nucleus, the external lateral part of the parabrachial nucleus, the dorsal division of the ambiguus nucleus, and the lateral reticular nucleus of LPS‐injected rats. The signal for c‐fos and NGFI‐B mRNA in most of these brain nuclei reached a maximum at 3 h postinjection, 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 numerous OT‐ir and few AVP‐ir neurons. Relative levels of CRF mRNA in the parvocellular PVN were also significantly increased 6 h following LPS, but endotoxin did not alter the genetic expression of this stress‐related neuropeptide in other brain regions. This was confirmed by the CRF primary transcript (hnRNA), which was expressed only in the PVN but 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 PVN or the SON of immune‐challenged rats because of the high basal levels of each transcript in these nuclei, LPS caused significant induction 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 neuropeptide 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.