Sandra J. Legan

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Estrogens play a central role in regulating female reproduction throughout the reproductive axis, and the pituitary is one of the major targets of estrogen action. We hypothesized that estrogen receptor alpha (ERalpha) mediates estrogen action in the pituitary gonadotroph. To test this hypothesis, we generated a mouse line with a selective ERalpha deletion(More)
The major environmental parameter controlling seasonal breeding in ewes is photoperiod. Short days stimulate, and long days inhibit breeding activity. One of the more intriguing enigmas of neuroendocrinology is the endocrine mechanism whereby a mere change in daylength initiates or prevents estrous cycles. Recent experiments have begun to solve this problem(More)
In rodents, females but not males, in response to escalating levels of estrogen, express a luteinizing hormone (LH) surge that is prompted by a surge in luteinizing hormone-releasing hormone (LHRH). It cannot take place if estrogen-sensitive afferents located in the anteroventral periventricular nucleus (AVPV) are either absent or disabled. Males appear to(More)
We have demonstrated that a brief step-like increment in circulating estradiol concentrations to approximately 100 pg/ml, achieved by SC insertion of Silastic capsules containing estradiol-17 beta and their withdrawal after 29 1/2 hr, elicited a daily LH surge on 4 consecutive afternoons in rats ovariectomized 2 weeks previously. Since the duration of this(More)
Seasonal variations in the behavioural and feedback actions of oestradiol in the ewe were examined by determining the ability of various physiological oestradiol concentrations to elicit oestrous behaviour, induce an LH surge, and suppress tonic LH secretion at four times of the year. These tests were performed in acutely ovariectomized animals pretreated(More)
We have analyzed the mechanisms by which several known regulators of the LHRH release process may exert their effects. For each, we have attempted to determine how and where the regulatory input is manifest and, according to our working premise, we have attempted to identify factors which specifically regulate the LHRH pulse generator. Of the five(More)