Effects of varying intensities of heat stress on neuropeptide Y and proopiomelanocortin mRNA expression in rats.

The aim of the present study was to investigate the effects of varying intensities of heat stress on the mRNA expression levels of neuropeptide Y (NPY), proopiomelanocortin (POMC) and stress hormones in rats. To establish a rat model of heat stress, the temperature and time were adjusted in a specialized heating chamber. Sprague-Dawley (SD) rats were randomly divided into four groups; control (CN; temperature, 24±1˚C); moderate strength 6 h (MS6; temperature, 32±1˚C time, 6 h), moderate strength 24 h (MS24; temperature, 32±1˚C; time, 24 h) and high strength 6 h (HS6; temperature, 38±1˚C; time, 6 h) groups. SD rats were exposed to heat for 14 consecutive days. The levels of heat stress-related factors, including corticotropin-releasing hormone (CRH), cortisol (COR), epinephrine (EPI) and heat shock protein 70 (HSP70), were measured in the rat blood using ELISA. In addition, the weight of the spleen, thymus, hypophysis and hypothalamus were determined. The mRNA expressions levels of NPY and POMC were detected using quantitative PCR. The results showed that the CRH, COR and HSP70 levels were increased in the three heat stress groups compared with the CN group. Notably, the levels of CRH, EPI and HSP70 were increased in the HS6 group compared with the CN and MS6 groups (P<0.05). Furthermore, the weights of the hypophysis and hypothalamus in the HS6 group were significantly lower compared with the CN group (P<0.05). In addition, NPY and POMC expression levels were downregulated in the MS24 group compared with the CN group. The mRNA expression levels of NPY and POMC were altered in response to different intensities of heat stress. Therefore, their levels were downregulated and upregulated following long-time and moderate-time heat exposure, respectively. The results of the present study suggested that the reduced mRNA expression levels of NPY may be partially responsible for the heat-induced injuries in rats following long-time heat exposure.