INFLUENCE OF NITRIC-OXIDE SYNTHASE INHIBITORS ON THE ACTH AND CYTOKINE RESPONSES TO PERIPHERAL IMMUNE SIGNALS

Nitric oxide (NO) is an unstable gas that participates in the response of the hypothalamic-pituitary-adrenal (HPA) axis to a variety of immu ne signals, including turpentine-induced tissue damage and the systemi c injection of the pro-inflammatory cytokine interleukin 1-beta (IL-1 beta). Studies that have investigated the role of this gas in the inta ct rat have relied on blockade of NO formation with the NO synthase (N OS) inhibitor N(omega)nitro-L-arginine-methylester (L-NAME). They have suggested that endogenous NO blunts the ACTH response to intravenous (iv) IL-1 beta in part by exerting an inhibitory influence on the rele ase of hypothalamic peptides such as corticotropin-releasing factor (C RF) from nerve terminals in the median eminence. It must nevertheless be noted that, at present, evidence for this mode of action remains ci rcumstantial. Significant controversy remains regarding the specificit y of the compounds used to block NO formation, the characteristics of their effect in terms of dose and timing of administration, the possib ility that their effect is restricted to IL-1 beta or can be expanded to other pro-inflammatory cytokines, and the question of whether the p ossibility that they might also influence ACTH release by altering cir culating levels of tumor necrosis factor-alpha (TNF-alpha) and IL-6. T he purpose of the present study was to elucidate these points. In the first series of experiments, we determined the iv IL-1 beta-induced AC TH response to various doses of systematically injected L-NAME (1-100 mg/kg), At 10-100, but not 1 mg/kg, L-NAME significantly (P < 0.01) au gmented the ACTH response to IL-1 beta, with a maximum effect observed at 30 and 100 mg/kg, At the 30 mg/kg dose, L-NAME was equally effecti ve in augmenting the ACTH response when administered between 5 and 240 min prior to the cytokine. The effect of L-NAME was fully mimicked by equivalent doses of other arginine derivatives such as N-omega-monome thyl-L-arginine (L-NMMA) or N-omega-nitro-L-arginine (L-NNA), indicati ng that controversy in the published literature concerning the influen ce of NO on CRF secretion does not appear to be due to the use of diff erent arginine derivatives. The ability of other cytokines such as TNF -alpha and IL-6 to release ACTH and corticosterone was significantly ( P < 0.01) augmented by blockade of NO formation in a manner similar to that found with IL-1 beta. To test the hypothesis that L-NAME might a lter ACTH secretion at least in part by modifying the secretion of pro -inflammatory cytokines, vie measured plasma concentrations of TNF-alp ha and IL-6 following endotoxin injection in the presence or absence o f L-NAME, Blockade of NO formation reduced TNF-alpha but increased IL- 6 levels in rats administered the lipopolysaccharide (25 mu g/kg iv). As L-NAME augments the ACTH response to TNF-a as well as IL-6, it is i mprobable that changes in TNF-alpha and IL-6 secretion during immune s timulation represents an important mechanism mediating the inhibitory influence of endogenous WO on the HPA axis activity. Collectively, the se results indicate that the systemic injection of L-NAME very quickly augments the stimulatory effect of pro-inflammatory cytokines on ACTH 1 secretion, and does so for at least 4 h. Other arginine derivatives known to block the activity of constitutive NO syntheses, such as L-N MMA and L-NNA, exert an effect that is virtually identical to that of L-NAME, The ability of L-NAME to increase the ACTH response to iv IL-1 beta is also observed in rats injected with TNF-alpha and IL-6. Becau se of the opposite effects of L-NAME on the levels of these two cytoki nes, the influence of arginine derivatives on ACTH release is probably not due to changes in cytokines produced during immune stimulation su ch as endotoxemia.