Effects of low degree of high ambient temperature on neural pathways from median preoptic nucleus to arcuate nucleus in related to food intake.

Abstract

High ambient temperature (HTa) is one important environmental factor that decreases food intake (FI). It is well accepted that high degree HTa induced heat stress decreased FI by the activation of corticotropin-releasing factor (CRF) via hypothalamic pituitary adrenal axis. However, short-term low-degree HTa exposure has been demonstrated to decrease FI earlier than activated physiological and stress responses. This effect was related to the neuronal connection between the median preoptic nucleus (MnPO) and arcuate nucleus (Arc). Therefore, the present study aimed to investigate the effect of short-term low-degree HTa exposure on the activity of hypothalamic Arc in regarding to FI, and the involvement of neuronal connection between the MnPO and Arc. It was firstly hypothesized that proopiomelanocortin (POMC) neurons in the Arc can mediate the reduction of FI under the effect of short-term low-degree HTa exposure. Second, the neuronal pathway from MnPO to Arc may related with this effect. In order to test the hypothesis, the present study refined the condition of short-term low-degree HTa. This study demonstrated that the short-term low-degree HTa exposure (90 min) based on the control ambient temperature at 23˚C (CTa = 23˚C) and the HTa at 30˚C. The bout of CTa and HTa conditions is 7°C temperature difference (ΔT = 7 ˚C). The results showed that rats exposed to this condition decreased FI without changing in rectal temperature and hematocrit. The effect of short-term low-degree HTa exposure on the Arc could enhanced the number of c-Fos expression, POMC positive neurons and POMC neurons containing c-Fos positive nuclei in the Arc under feeding condition. Moreover, the injection with POMC receptor antagonist (SHU9119) into the lateral ventricle in the rat brain could reverse the effect of short-term low-degree HTa exposure on FI. Finally, to prove the neuronal connection of MnPO-Arc in related with the effect of short-term low-degree HTa exposure. The fluorogold (FG) retrograde tracer was injected to the Arc. Seven days after FG injection found FG-retrograde labeled neurons in the MnPO. The present of FG-labeled neurons in the MnPO imply that the Arc can receive the afferent signal from the MnPO. In conclusion, this study concludes that the Arc POMC neurons are a crucial hypothalamic nucleus on FI under the effect of low-degree HTa exposure. The effect of low degree HTa exposure in reduction on FI is mediated in part by activation of Arc POMC neurons and may relate with the MnPO-Arc pathway.