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Abstract
Regulation of LH and FSH secretion by estradiol was evaluated during sexual development in the bovine. Involvement of opioid neuropeptides in modulating estradiol inhibition of gonadotropin secretion during puberty in heifers was investigated in the first two experiments. Low levels of estradiol inhibited gonadotropin secretion in prepubertal ovariectomized heifers. Administration of the opioid antagonist, naloxone, blocked the inhibitory effects of estradiol and caused an increase in LH secretion. Naloxone had no effect on gonadotropin secretion in prepubertal heifers that were ovariectomized and not administered estradiol. As heifers matured, estradiol inhibition of gonadotropin secretion waned as did the ability of naloxone to disinhibit LH secretion in ovariectomized heifers treated with estradiol. By the time control heifers had attained puberty, estradiol no longer inhibited gonadotropin secretion and opioids had only minor inhibitory effects on LH secretion in ovariectomized heifers administered estradiol.
Estradiol feedback on gonadotropin secretion in bovine males during the time of the peripubertal change in estradiol feedback in bovine females was examined in the second experiment. Administration of low levels of estradiol inhibited LH and FSH secretion in gonadectomized bovine males and females. Inhibition occurred during the time intact heifers were prepuberty. During the time estradiol inhibition of gonadotropin secretion was abating in females, a simultaneous decline was observed in males. Moreover, at the end of the study, estradiol increased tonic LH secretion in gonadectomized males and females above concentrations detected in gonadectomized animals not receiving estradiol. This was at a time when intact heifers had initiated estrous cycles.
The final experiment investigated estradiol feedback on gonadotropin secretion in adult gonadectomized males and females administered various physiological concentrations of estradiol. Administration of low concentrations of estradiol increased mean concentrations of LH and amplitude of LH pulses in both sexes and increased mean concentrations of FSH in males, but did not alter FSH in females compared to administration of sham implants. High physiological concentrations of estradiol suppressed LH and FSH secretion in bovine males and females.
