Effects of estrogen on luteinizing hormone releasing hormone in a hypothalamic synaptosomal fraction from the ovariectomized rat.

Previous investigations have shown that hypothalamic luteinizing hormone releasing hormone (LHRH) is localized within a subcellular fraction eqivalent or similar to the synaptosome or nerve ending particle. In the present study, the ovariectomized adult rat was employed to investigate the effects of estrogen on the distribution of LHRH among various subcellular fractions of the hypothalamus. In addition, the liberation of LHRH from a synaptosomal fraction by repeated freeze-thaw cycles was examined before and after in vivo estrogen treatment. Following a single injection of 5 micrograms of estradiol benzoate (EB), the LHRH concentration of the synaptosomal fraction rose as much as 100% in 27 h. Increasing the dose of EB to 50 micrograms shortened the latency of the rise and prolonged its duration, but did not alter its magnitude compared to the lower dose. Furthermore, the LHRH in that synaptosomal fraction was significantly more resistant to solubilization by repetitive freezing and thawing than the LHRH in a synaptosomal fraction from rats which had not received EB. These observations led us to conclude that estrogens can alter the physical properties of the hypothalamic LHRH-containing particles or the storage form of the peptide within those particles so that it is less easily released. This phenomenon may represent one aspect of the regulatory effects of estrogens on hypothalamic LHRH secretion.