BACKGROUND: Photoperiodic changes mediate certain physiological or pathological alterations in organisms. Solstices represent either the longest or shortest photoperiod of a year.
OBJECTIVES: Intact and pinealectomized rats were used to investigate the potential changes of reproductive hormones in the hypothalamus-pituitary-testis (HPT) axis including GnRH, FSH, LH, testosterone and melatonin, and their receptors at summer solstice (SS) and winter solstices (WS).
METHODS: The levels of reproductive hormones in HPT axis and the binding characteristics of their receptors were examined using radioimmunoassay and radioreceptor assay techniques, respectively.
RESULTS: The results indicate that in the intact male rat, GnRH, LH and testosterone are higher at the SS than at the WS. However, FSH exhibited no significant seasonal changes. In the testis, Bmax and Kd of LH receptors are higher at the WS than at the SS while those of FSH receptors are higher at the SS than at the WS. In addition, the melatonin in HPT axis appeared significant differences between WS and SS. Bmax and Kd of melatonin receptors in the hypothalamus and pituitary also showed higher at the WS than at the SS. Moreover, reproductive hormone production lost their seasonal rhythms after pinealectomy.
CONCLUSION: The most important discovery in this study is that we first reported that pinealectomy had profound effects on the binding characteristics of melatonin with its subtype receptors. Especially at the hypothalamus, the dominated melatonin receptors shifted from MT1 to MT2 after pinealectomy at the two solstices.