Modulation of physiological brain hyperthermia by environmental temperature and impaired blood outflow in rats.

To study the role of ambient temperature and brain blood outflow in modulating physiological brain hyperthermia, temperatures in two brain structures (nucleus accumbens or NAcc and hippocampus or Hippo) and a non-locomotor head muscle (musculus temporalis) were monitored in rats exposed to three arousing stimuli (placement in the cage or environmental change, 3-min social interaction with a female rat, 3-min innocuous tail-pinch) under three conditions (intact animals at 23 degrees C or control, intact animals at 29 degrees C, animals with chronically occluded jugular veins at 23 degrees C). While each stimulus in each condition induced hyperthermia, with more rapid and stronger changes in brain structures than muscle, there were significant differences between conditions. At 29 degrees C, animal placement in the cage resulted in stronger temperature increase and larger brain-muscle differentials, while basal temperatures in Hippo and muscle (but not in NAcc) were higher than control. At 29 degrees C, hyperthermia during social interaction was smaller but more prolonged, while the response to tail-pinch was similar to that seen at normal environmental temperatures. Animals with chronically occluded jugular veins had similar basal temperatures but showed much weaker hyperthermia than intact animals during each stimulus presentation; temperature increases in brain structures, however, were much stronger than in the muscle. Our data suggest that the brain is able to decrease neural activation induced by environmental challenges under conditions of impaired blood outflow and restricted heat dissipation to the external environment.