Estradiol-Induced LH Release in Juvenile Female Rats: Nembutal Blocks both the Decline and the Exposure of Nonavailable Pituitary LHRH Receptor Sites

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Abstract
We have previously shown that, as seen in the adult rat, pituitary LHRH receptor content declines during the hours encompassing the first preovulatory LH surge. This decrease in receptor number was prevented, however, when pituitary membranes were treated with MgCb to dissociate endogenously bound ligand(s). The present study investigates: (a) whether the first proestrous reduction in available LHRH receptors can be reversed by subjecting the pituitary membranes to a less drastic, dilution-washing procedure previously reported to be highly effective in dissociating bound LHRH, and (b) whether Nembutal blockade of the proestrous decline in LHRH receptors also prevents the dissociation-induced exposure of additional LHRH binding sites. A premature LH surge was induced by exposing juvenile female rats to proestrous-type levels of plasma estradiol (E2) via Silastic capsules. A decline in available LHRH receptors was found around the time of this steroid-induced LH surge. A marked increase in receptor number was, however, observed upon dilution-washing of the membranes. The increase in receptor number was not accompanied by any changes in receptor affinity (Ka). Nembutal administration blocked both the LH surge and the decline in available LHRH receptors. Moreover, dilution-washing of pituitary membranes from Nembutal-treated rats failed to uncover additional LHRH binding sites. The results suggest that a significant portion of the proestrous decline in pituitary LHRH receptors is due to a reduced availability of the receptor to binding. Whether such a phenomenon is due to true occupancy by endogenous ligand(s) or to binding-dependent localization of the receptor within the cell membrane is unclear. Nevertheless, the capacity of Nembutal to prevent both the proestrous decline in receptor number and the exposure of unavailable binding sites indicates that increased release of LHRH and/or LHRH-like molecules is necessary for the sequestration of LHRH binding sites into a nonavailable, membrane-located pool during the afternoon of proestrus.

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