In several seasonally breeding species, gonadectomy of animals in the nonbreeding condition results in an increase in serum LH and FSH levels, which is greatly attenuated when compared to that observed after castration of animals in the breeding condition. Additionally, exposure of seasonal breeders to nonstimulatory daily photoperiods can induce an increase in the sensitivity of the hypothalamic-pituitary axis to the negative feedback effect of steroid hormones, as determined by measurements of serum LH and FSH titers. The present study was designed to determine if the attenuated castration response is due to a hypersensitivity of the hypothalamic- pituitary axis to extragonadal steroids. The attenuated increase in serum LH and FSH levels after castration of male hamsters kept on a nonstimulatory photoperiod was also observed in hamsters which were castrated and adrenalectomized and in hamsters maintained on a low steroid diet. Thus, the inhibitory photoperiod appears to suppress serum LH and FSH levels independently of steroid hormones. Further evidence for the steroid-independent photoperiodic regulation of pituitary gonadotropin release is the increase in serum LH and FSH levels in castrated hamsters which are shifted from inhibitory short days to stimulatory long days. In these photostimulated castrates, serum LH and FSH rose to levels 3-4 times greater than those observed in hamsters kept on short days after castration. On the other hand, when castrated hamsters were transferred from a stimulatory to a nonstimulatory photoperiod, no significant decline in serum LH levels occurred; serum FSH levels declined only moderately. The results suggest that steroid-independent photoperiodic regulation of neuroendocrine activity may be more important in the seasonal transition from the nonbreeding condition to the breeding condition than in the breeding to nonbreeding transition. The present data and results from previous studies indicate that both a steroid-independent and a steroid-dependent mechanism are involved in the photoperiodic control of neuroendocrine- gonadal activity in seasonally breeding animals.
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