Distinct distribution and time-course changes in neuronal nitric oxide synthase and inducible NOS in the paraventricular nucleus following lipopolysaccharide injection

Nitric oxide (NO) is known to be involved in the modulation of neuroendocrine function. To clarify the role of different isoforms of NO synthase (NOS) in the neuroendocrine response to immune challenge, the expressions of neuronal NOS (nNOS) and inducible NOS (iNOS) genes in the hypothalamus following lipopolysaccharide (LPS) injection were examined using in situ hybridization. NOS activity was also determined by NADPH-diaphorase (NADPH-d) histochemistry. LPS (25 mg/kg) or sterile saline was injected intraperitoneally to male Wistar rats and the rats sacrificed 30 min, or 1, 2, 3, 5, 12 or 24 h after injection. nNOS mRNA expression in the paraventricular nucleus (PVN) was significantly increased 2 h after LPS injection. iNOS mRNA, which was not detected until 2 h after LPS injection, was significantly increased in the PVN 3 h after LPS injection. Both RNA expressions had returned to basal levels by 12 h after LPS injection. The number of NADPH-d positive cells was significantly increased 5 h after LPS injection. iNOS expression was more robust in parvocellular PVN, while nNOS was distributed mainly in the magnocellular PVN. Double in situ hybridization histochemistry revealed that some of the iNOS- (48.4%) or nNOS-positive cells (34. 3%) in the parvocellular PVN expressed CRF mRNA. The results demonstrate that LPS-induced sepsis causes significant increases in nNOS and iNOS gene expression with different time-courses and distributions, and that iNOS mRNA was more frequently co-localized with CRF-producing parvocellular neurons in the PVN. Thus, NO produced by iNOS and nNOS may play an important role in the neuroendocrine response to an immune challenge. Distinct differences in the distribution and time-course changes of iNOS and nNOS suggest different roles for the hypothalamic-pituitary-adrenal axis and/or neurohypophyseal system.