Decreased heat tolerance is associated with hypothalamo-pituitary-adrenocortical axis impairment.

Abstract : When rats are exposed to heat, they adapt themselves to the stressor with a wide inter-individual variability. Such differences in heat tolerance may be related to particularities in the hypothalamo-pituitary-adrenocortical (HPA) axis activation. To further this hypothesis, 80 rats instrumented with a telemetric device for abdominal temperature (Tabd) measurement were separated into two groups. Sixty-eight rats were exposed during 90 min at an ambient temperature of 40 degrees C, and 12 rats to an ambient temperature of 22 degrees C. Heat-exposed rats were then divided into three groups using the a posteriori k-means clustering method according to their Tabd level at the end of heat exposure. Heat tolerant rats (Tol, n=30) exhibiting the lowest Tabd showed a slight dehydration, a moderate triglyceride mobilization, but the highest plasma adrenocorticotropic-hormone (ACTH) and corticosterone levels. Conversely, heat exhausted rats (HE, n=14) presented the highest Tabd, a higher degree of dehydration, a greater metabolic imbalance with the lowest plasma triglyceride level and the highest lactate concentration, as well as a lowest plasma corticosterone and ACTH levels. The fact that the proopiomelanocortin (POMC) mRNA content within the pituitary was low despite of a high c-fos mRNA level is also relevant. Current inflammatory processes in HE rats were underlined by lower inhibitory factor kappaBalpha (IkappaBalpha) mRNA and higher tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) mRNA. In conclusion, data show that intolerance to heat exposure is associated to an HPA axis impairment, possibly related to changes occurring in the IkappaBalpha and TNF-alpha mRNA levels.