Nitric oxide and fever: immune-to-brain signaling vs. thermogenesis in chicks
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Nitric oxide (NO) plays a role in thermogenesis but does not mediate immune-to-brain febrigenic signaling in rats. There are suggestions of a different situation in birds, but the underlying evidence is not compelling. The present study was designed to clarify this matter in 5-day-old chicks challenged with a low or high dose of bacterial LPS. The lower LPS dose (2 mu g/kg im) induced fever at 3-5 h postinjection, whereas 100 mu g/kg im decreased core body temperature (T-c) (at 1 h) followed by fever (at 4 or 5 h). Plasma nitrate levels increased 4 h after LPS injection, but they were not correlated with the magnitude of fever. The NO synthase inhibitor (NG-nitro-L-arginine methyl ester, L-NAME; 50 mg/kg im) attenuated the fever induced by either dose of LPS and enhanced the magnitude of the Tc reduction induced by the high dose in chicks at 31-32 degrees C. These effects were associated with suppression of metabolic rate, at least in the case of the high LPS dose. Conversely, the effects of L-NAME on Tc disappeared in chicks maintained at 35-36 degrees C, suggesting that febrigenic signaling was essentially unaffected. Accordingly, the LPS-induced rise in the brain level of PGE(2) was not affected by L-NAME. Moreover, L-NAME augmented LPS-induced huddling, which is indicative of compensatory mechanisms to run fever in the face of attenuated thermogenesis. Therefore, as in rats, systemic inhibition of NO synthesis attenuates LPS-induced fever in chicks by affecting thermoeffector activity and not by interfering with immune-to-brain signaling. This may constitute a conserved effect of NO in endotherms.