Fluctuations in Hypothalamic LH-RF (Luteinizing Hormone-Releasing Factor) During the Rat Estrous Cycle

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Abstract
A study has been made of changes in LH-RF content of the rat hypothalamus during the various phases of the estrous cycle. Normally cyclic female Sprague- Dawley rats under a regimen of 14 hr of light and 10 hr of darkness were killed at different times of day during each of the stages of the cycle. The stalk-median eminence region (SME) of the hypothalamus was quickly dissected out, extracted in O.1N HC1 and boiled for 10 min. The supernate was tested for LHreleasing activity in immature Holtzman rats by a variant of the Parlow LH method: the ovarian ascorbic acid depletion induced by the extract was considered the index of LH-RF. The amount of LH-RF present in the SME during late proestrus and early estrus was significantly lower than at other times during the cycle. The drop in SME LH-RF occurred later than the critical period of the day of proestrus, during which there is evidence for the release of pituitary LH. It is postulated that released pituitary LH in the plasma may exert a negative internal feedback action on the SME-secretion of LH-RF, inhibiting the synthesis of the neurohumor after it (LH) reaches a threshold level in the plasma. (Endocrinology76: 282, 1965)

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