A low degree of high ambient temperature decreased food intake and activated median preoptic and arcuate nuclei

High ambient temperature (HTa) activates both behavioral and autonomic responses. Decreased food intake (FI) is a well-known behavioral response under HTa. In general, this behavioral response seems to be mediated under the activation of the stress axis. The present study, however, revealed that decreased FI after a low degree of HTa appeared to be activated earlier than stress axis activation. By comparing low and high levels of HTa exposure in rats, the present experiment demonstrated that the HTa-induced decrease in FI was clear in both conditions. However, only the high degree HTa rats showed prominent physiological responses to HTa. The results indicated that low degree HTa exposure decreased FI earlier than activated physiological responses. Using the low degree HTa condition, the next experiment revealed that only the median preoptic nucleus (MnPO) and arcuate nucleus (Arc) were activated. These results suggested that the MnPO and Arc (MnPO-Arc) are apparently involved in the low degree HTa effect on FI and that the paraventricular hypothalamic nucleus, which is the upper part of the stress axis, was not associated with this effect. A separate experiment also showed no differences in the corticotropin-releasing factor (CRF) concentrations from many selected nuclei. This result suggested that the effect of HTa to decrease FI via the MnPO-Arc was not associated with CRF. The current findings introduce a new behavioral aspect of the HTa effect on FI, representing important information to explain the mechanism of obesity relative to environmental conditions.