Effects of high temperature and high humidity stress on the negative feedback regulation of hippocampus on HPA axis in rats.

Objective: To investigate the negative feedback regulation from rat hippocampus on hypothalamic-pituitary-adrenal (HPA) axis under high temperature and high humidity stress.
Methods: Thirty (30) SD male rats were randomly divided into three groups: control group, high temperature and high humidity group, drug intervention group. The rats in control group were kept in the environment with temperature of 24 ± 1°C and humidity of 50 ± 5%, without any stimulation. The rats in the other groups were exposed to high temperature and high humidity environment for 4 h each day, with temperature of 35±1 °C and humidity of 85±5%. The rats in drug intervention group were intragastrically administered with the glucocorticoid receptor antagonist mifepristone. The administration was continued for 3 weeks. After 3 weeks, the serum levels of corticotropin releasing hormone (CRH), adrenocorticotropic hormone (ACTH) and corticosterone (CORT) were detected by ELISA.The protein and mRNA levels of corticosteroid receptors (MR), glucocorticoid receptors (GR) and inducible nitric oxide synthase (iNOS), transient receptor potential vanilloid 1 (TRPV1) and 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) in hippocampus were determined by immunohistochemistry and in situ hybridization, respectively. The apoptosis of hippocampal cells was examined with TUNEL apoptosis staining.
Results: After stimulation with high temperature and high humidity stress for 3 weeks, the serum levels of CRH, ACTH and CORT in the high temperature and high humidity group were significantly increased compared to that of control group; the levels of these indicators in drug intervention group were decreased compared to that of high temperature and high humidity group (P<0.05). In high temperature and high humidity group, the protein and mRNA levels of MR, GR, iNOS in hippocampus of rats were significantly increased compared with that of control group (p<0.05); and the levels of these indicators in drug intervention group were lower than that of high temperature and high humidity group (p<0.05). In addition, compared with the control group, the TRPV1 protein level in hippocampus of rats in high temperature and high humidity group was not significantly changed (p>0.05), while the TRPV1 mRNA level was significantly increased (p<0.05). Neither the protein nor mRNA levels of 11β-HSD1 showed significant difference compared to control group (p>0.05). The apoptosis of hippocampus cells in the high temperature and high humidity group was significantly increased compared with that of control group (p<0.05); and it was lower in the drug intervention group than that of in high temperature and high humidity group while the result was not significant (p>0.05).
Conclusion: High temperature and high humidity stress may up-regulate the local expression of iNOS in hippocampus and decrease the activity of glucocorticoids (GC) receptor, then the effective binding of GR-GC would be decreased and the negative feedback regulation of hippocampus on HPA axis would be inhibited. The glucocorticoid receptor antagonist can improve the negative feedback regulation of hippocampus on HPA axis in rat.