Abstract

Immunologic challenge with lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) produces a functional response within the paraventricular nucleus of the hypothalamus (PVN) and leads to changes in gene expression within PVN neurons. Regulated expression of neuropeptide genes within neurons of the PVN is a potential mechanism by which an organism can adapt to stressful challenges. Here, the authors used a transgenic mouse model in which expression of a readily measurable beta-galactosidase reporter was driven in PVN neurons by human proenkephalin regulatory sequences. This proenkephalin-beta-galactosidase transgene has been demonstrated previously to respond appropriately to a variety of stressors. It is demonstrated that expression of the proenkephalin transgene product was up-regulated significantly in a subset of PVN neurons 6 hours following intraperitoneal LPS (16-400 microg/kg) administration, remained elevated at 12 hours, and fell below basal levels by 24 hours. A more rapid and transient pattern of transgene up-regulation in the PVN followed administration of intraperitoneal IL-1beta (10 microg/kg) with significant induction by 2 hours, peak levels reached by 4 hours, and a return toward basal levels by 6 hours. IL-1beta (10-50 ng/mouse) administered intracerebroventricularly also led to up-regulation of the transgene 6 hours following infusion. Transgene expression was not up-regulated in hypothalamic slice cultures treated directly with IL-1beta (5-10 ng/ml media). Up-regulation of transgene expression does not appear to result from local action of IL-1beta at the level of the PVN but, rather, through as yet unidentified intermediates. The authors demonstrate phosphorylation of the cyclic amino-3-hydroxy-5-methyl-4-isoxazolepropionate response element binding protein, a transcription factor known to interact with proenkephalin regulatory sequences within the transgene, in the PVN following LPS administration. LPS induced up-regulation of the transgene was blocked by pretreatment with naltrexone, indicating an additional role for endogenous opioid systems in regulation of the PVN response to immune challenge.