Median eminence (ME) implants of estradiol performed in 26-day-old female rats induce: a) a precocious opening of the vaginal membrane; b) ovulation, as indicated by the presence of corpora lutea; c) a reduction of pituitary LH stores; d) a concomitant increase of plasma LH levels; and e) an increase in uterine weight. These data suggest that estradiol, when implanted in the ME region of immature female rats, may stimulate LH release through a “positive” feedback effect exerted on specific neural receptors. Implants of estradiol performed in the habenular region (HR) of 26-day-old female rats are followed by: a) retardation of puberty; b) reduction of the weights of the ovaries and of the uteri; c) increase of LH stores in the pituitary. LH remains undetectable in the general circulation, however. It is suggested that the HR contains receptors which are sensitive to the “negative” feedback effect of estrogen and which intervene in the control of LH sectetion. (Endocrinology83: 1101, 1968)

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