Thermoregulatory responses of guinea pigs with anteroventral third ventricle lesions

Abstract

Guinea pigs with anteroventral third ventricle region (AV3V) lesions fail to develop fever and the associated rise in acute-phase plasma protein levels following systemic injections of lipopolysaccharide (LPS). Since endogenous pyrogen (EP) injected directly into the preoptic area of animals with AV3V lesions causes appropriate elevations in core temperature (Tco) and acute-phase plasma proteins levels, the blocked responses to LPS probably are not due to damage to the adjacent preoptic area. We proposed, therefore, that EP may pass from blood into brain in the AV3V, presumably through the organum vasculosum laminae terminalis. However, the possibility that a more generalized impairment due to damaged pathways within the AV3V could account for the observed effects was not examined. To investigate this possibility, guinea pigs were given AV3V lesions. Pending histological verification of the ablated sites, AV3V lesions were presumed to be placed correctly if the animals did not develop fever following LPS (Salmonella enteritidis, 2 μg/kg i.p., at ambient temperature (Ta) 22 °C); those failing to meet this criterion were designated as sham-operated. Two experiments were conducted. In the first, metabolic rates, Tco, and two skin temperatures (Tsk) were measured at Ta 12°, 22°, and 32 °C over an 8-month postlesion period during which failure to fever persisted; the data were collected during a 30-min period after thermal balance had been achieved at any given Ta. There were no differences in the variables measured between sham-operated and AV3V-lesioned animals at Ta 22 °C. At Ta 12° and 32 °C, the Tsk of both the sham-operated and the AV3V-lesioned guinea pigs were reduced and elevated, and metabolic rate increased and decreased, respectively. However, Tco at Ta 32 °C and metabolic rate at Ta 12° and 32 °C were significantly higher in the AV3V-lesioned than in the sham-operated animals. In the second experiment, only Tco was measured, but under more severe ambient conditions and over a briefer duration. Thus, 11 and 14 days postlesion, the animals were exposed to Ta 35.5 ± 1.8 °C (means ± SD) until their Tco stabilized between 40.5 and 41.5 °C, and to Ta 2.7 ± 1.5 °C for 2.5 h, respectively. In addition, as indices of the acute-phase plasma protein response, their plasma levels of copper (Cu) and N-acteylneuraminic acid (NANA) were assayed 28 h after the onset of these exposures. There were no significant differences in the Tco changes between sham-operated and AV3V-lesioned guinea pigs induced by each of these conditions. Plasma Cu and NANA levels also were not affected by the lesions or the environments. It would appear, therefore, that ablation of the AV3V causes enhanced metabolic responses to thermal stress; but since these changes lead to elevated rather than to depressed Tco and do not alter acute-phase plasma protein levels, they cannot account for the blocked febrile and acute-phase plasma protein responses to LPS.