The vagus nerve in the thermoregulatory response to systemic inflammation

Experimentally, systemic inflammation induced by a bolus intravenous injection of lipopolysaccharide (LPS) may be accompanied by three different thermoregulatory responses: monophasic fever (the typical response to low doses of LPS), biphasic fever (medium doses), and hypothermia (high doses). In our recent study [Romanovsky, A. A., V. A. Kulchitsky, C. T. Simons, N. Sugimoto, and M. Székely. Am. J. Physiol. (Regulatory Integrative Comp. Physiol.). In press], monophasic fever did not occur in subdiaphragmatically vagotomized rats. In the present work, we asked whether vagotomy affects the two other types of thermoregulatory response. Adult Wistar rats were vagotomized (or sham operated) and had an intravenous catheter implanted. On day 28 postvagotomy, the thermal responses to the intravenous injection of Escherichia coli LPS (0, 1, 10, 100, or 1,000 micrograms/kg) were tested in either a neutral (30 degrees C) or slightly cool (25 degrees C) environment. Three major results were obtained. 1) In the sham-operated rats, the 1 microgram/kg dose of LPS caused at 30 degrees C a monophasic fever with a maximal colonic temperature (Tc) rise of approximately 0.6 degree C; this response was abated (no Tc changes) in the vagotomized rats. 2) At 30 degrees C, all responses to higher doses of LPS (10-1,000 micrograms/kg) were represented by biphasic fevers (the higher the dose, the less pronounced the first and the more pronounced the second phase was); none of these biphasic fevers was altered in the vagotomized animals. 3) In response to the 1,000 micrograms/kg dose at 25 degrees C, hypothermia occurred: Tc changed by -0.5 +/- 0.1 degree C (nadir); this hypothermia was exaggerated (-1.1 +/- 0.1 degrees C) in the vagotomized rats. It is concluded that vagal afferentation may be important in the mediation of the response to minor amounts of circulating LPS, whereas the response to larger amounts is brought about mostly (if not exclusively) by nonvagal mechanisms. This difference may be explained by the dose-dependent mechanisms of the processing of exogenous pyrogens. Vagotomized animals also appear to be more sensitive to the hypothermizing action of LPS in a cool environment; the mechanisms of this phenomenon remain speculative.