Elevated Environmental Temperature Alters the Responses of the Reproductive and Thyroid Axes of Female Syrian Hamsters to Afternoon Melatonin Injections

Female Syrian hamsters were kept in a light (14: 10 h light: dark cycle, lights on 0600 h)‐ and temperature (22 or 30°C)‐controlled room; some groups were treated with an afternoon s. c. injection of melatonin (6.25, 12.5, or 25 μg/day) for 11 or 14 weeks. The melatonin‐induced suppression of the reproductive system in hamsters maintained at 22°C (as measured by vaginal cycles, uterine weights, ovarian histology, and plasma and pituitary prolactin levels) was delayed if hamsters were kept at 30°C. The dose‐related depression of thyroxine (T₄) after melatonin injections for 14 weeks in 22°C was not seen at 30°C. Rather, the depression of plasma T₄ and triiodothyronine (T₃) seen at the end of 11 or 14 weeks of exposure to 30°C without melatonin injections (vs. control levels at 22°C) was offset by melatonin injections, raising T₄ and T₃ particularly at the lower doses. In contrast, there was no consistent effect of higher temperature alone on reproductive variables. The interactive effects of temperature and melatonin on the reproductive and thyroidal systems in female hamsters are apparently complex and probably provide a fine‐tuning mechanism for the environmental control of endocrine physiology.