Immune stress activates putative nitric oxide-producing neurons in rat brain: cumulative effects with restraint

Immune and restraint stresses induce changes in the hypothalamo-pituitary-adrenal axis activity and autonomic function. In the hypothalamus, the paraventricular nucleus (PVN) plays an integral role, and nitric oxide (NO) is hypothesized to participate in this process. We used 1) intravenous injections of lipopolysaccharide (LPS, 125 microg/kg) to identify activated (Fos-positive) putative NO-producing neurons, 2) retrograde tracing to determine if autonomic medullary regions signal the PVN to mediate this activation, and 3) intravenous LPS injections plus restraint stress to determine if responses to restraint are altered by the presence of immune stress. At 2 hours after LPS injections, approximately 15% of putative NO-producing neurons were activated in the nucleus of the tractus solitarius (NTS) and ventrolateral medulla (VLM); about half of the putative NO neurons in the PVN were activated. In LPS + restraint rats, the percentage of activated putative NO neurons in the PVN was not significantly different from LPS-treated rats, but the numbers of putative NO neurons and activated NO neurons per section increased significantly. Retrogradely labeled neurons were found mostly in the middle NTS and VLM, and about 75% were activated. No neurons in the NTS or VLM were triple labeled. The results show that putative NO-producing neurons in the PVN, NTS, and VLM are activated by circulating LPS. However, the LPS-induced signaling to the PVN likely occurs through pathways other than the NO network of neurons in NTS or VLM. Finally, superimposition of restraint stress onto animals already exposed to immune stress stimulates the NO system in the PVN to a greater extent than either stress alone.