Differences in hypothalamic Fos expressions between two heat stress conditions in conscious mice.

Hyperthermia and dehydration were important physiological phenomena in heat stress. But, the degrees of these phenomena were changed by heat stress conditions, and the distinction between both phenomena is necessary for investigation of response for individual phenomenon. Heat stress at 34 degrees C for 60 min increased rectal temperature, and heat stress at 38.5 degrees C for 60 min further increased rectal temperature and increased osmolality in mice. We investigated the activated region in hypothalamus, which played a role in thermoregulation, fluid regulation and so on, using immunostaining for Fos protein under these conditions in conscious mice. At 34 degrees C, Fos-positive neurons increased in the median preoptic nucleus, lateral preoptic area and anterior hypothalamic area, which were known to be the thermoregulatory center, and the dorsomedial hypothalamic nucleus, which was known to control eating. At 38.5 degrees C, Fos-positive neurons further increased in the regions mentioned above and appeared in the lateral septal nucleus, medial preoptic area, lateral hypothalamic area and zona incerta, which were thought to be involved in thermoregulation and/or fluid regulation, and the paraventricular hypothalamic nucleus, supraoptic nucleus and supraoptic nucleus in the retrochiasmatic part, which were known to be involved in neuroendocrine effector systems. These results support that the activated regions in hypothalamus differed with heat stress conditions, which induced only hyperthermia and both hyperthermia and dehydration.