Effects of heat stress on endocrine functions & behaviour in the pre-pubertal rat

BACKGROUND & OBJECTIVES: Heat stress related hyperthermia may cause damage to various organ systems. There are very few studies on the effects of hyperthermia on the endocrine system. We therefore, investigated effects of exogenously induced hyperthermia on adrenal, testicular and thyroid functions and behavioural alterations in pre-pubertal male Sprague-Dawley rats. METHODS: Three groups of 30-day old rats (n=7 per group) were used. Body temperature was increased to 39°C (Group I) and 41°C (Group II) in a hyperthermia induction chamber for 30 min. The rats in the Group III served as control (36 °C). All animals received saline and were decapitated 48 h after the experiments. Serum free triiodothyronin (fT3), free thyroxine (fT4), total testosterone and dehydroepiandrosterone sulphate (DHEA-S) levels were determined by chemiluminescence assay, and corticosterone by enzyme immunoassay. Testes, pituitary and adrenal glands were dissected out and processed for histopathological examination. To assess activity and anxiety of the animals, the open field test and elevated-0-maze test, respectively, were used in all groups 24 h before (day 29) and after (day 31) hyperthermia induction. RESULTS: Serum corticosterone levels (3.22±1.3) were significantly reduced in the 39°C (1.3±0.9) and 41°C (1.09±0.7) hyperthermia groups (P<0.01) compared to controls. Serum levels of thyroid hormones did not significantly differ among the groups. DHEA-S and testosterone values were below the limit of detection in all groups. Histopathological examination revealed that there was mild hydropic degeneration in the pituitary and adrenal glands. Apoptotic germ cells were seen in the seminiferous tubules of pre-pubertal male rats exposed to hyperthermia (41°C). Progression time in the open field test was significantly decreased and anxiety test scores increased in animals exposed to 39°C compared to the control group (P<0.01). These parameters were more pronounced in the 41°C hyperthermia group. INTERPRETATION & CONCLUSIONS: Our results show that heat exposure-induced stress may cause delayed reduction in serum corticosterone levels which may be associated with behavioural deficits in pre-pubertal male rats.