Combination of inflammatory cytokines increases nitrite and nitrate levelsin the paraventricular nucleus of conscious rats

Inflammatory cytokines stimulate glial cells in vitro to produce nitric oxi de (NO) from inducible NO synthase (iNOS). Whether the stimulation with cyt okines produces NO derived from iNOS has not hitherto been demonstrated in the vivo brain. Nitrite and nitrate (NOx(-)) levels in the rat paraventricu lar nucleus (PVN) were measured before and after intraparenchymal microinje ction of cytokines with a microdialysis technique. The cytokines, tumor nec rosis factor (TNF)-alpha (10 ng), interleukin (IL)-1 beta (2 ng), and inter feron (IFN)-gamma (2 ng) were microinjected. None of the cytokines alone ha d any effect on the NOx(-) levels for 8 h. But a combination of TNF-alpha a nd IFN-gamma gradually increased NOx(-) levels beginning at 140 min after t he microinjection, and NOx(-) levels reached 1.8 times the basal level at 3 80 min. A combination of TNF-alpha and IL-1 beta increased NOx(-) beginning at 340 min. reaching 1.7 times the basal level at 440 min, whereas a combi nation of IL-1 beta and IFN-gamma had no effect. Microinjection of a mixtur e of all three cytokines increased NOx(-) levels beginning at 120 min, reac hing 3.3 times the basal level at 400 min. Aminoguanidine. which is a selec tive inhibitor of iNOS, reduced NOx levels induced by the mixed cytokine tr eatment. Semi-quantitative RT-PCR for iNOS mRNA was done. The intensity of the iNOS mRNA band for the cytokine-treated PVN was stronger than that for the vehicle-treated PVN. These results suggest that the increased NOx after the treatment with mixed cytokines were dependent on iNOS activity. This i s the first report to indicate that only cytokines induce NOS in vivo in th e brain. (C) 2001 Elsevier Science B.V. All rights reserved.