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-microns 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 arcurate 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.(ABSTRACT TRUNCATED AT 400 WORDS)