As during the adult estrous cycle, the number of pituitary luteinizing hormone-releasing hormone (LHRH) receptors was found to increase prior to the first preovulatory surge of LH. Receptor content, as measured by the binding of the analog 125I-D-Ala6 -Pro9 -LHRH to pituitary membranes, declined markedly at the time of the first LH surge, remaining low during the first estrus and first diestrus. When pituitary membranes from animals undergoing an LH surge were incubated with MgCl2 to dissociate endogenously bound hormone, available binding sites were restored to pre-LH surge values. The increase in LHRH binding capacity was not related to an increase in receptor affinity, and it was not demonstrable in other pubertal phases. MgCl2- may have unmasked receptors made cryptic at the time of the LHRH-induced LH surge. Nevertheless, the results are compatible with the view that the proestrous decline in LHRH receptor content is, at least in part, due to occupancy of binding sites by endogenous hormone.
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