Estrous Cycle Variations in Gonadotropin-Releasing Hormone, Substance P and Beta-Endorphin Contents in the Median Eminence, the Arcuate Nucleus and the Medial Preoptic Nucleus in the Rat: A Detailed Analysis of Proestrus Changes

The concentrations of gonadotropin-releasing hormone, substance P and β-endorphin were measured in the median eminence, the arcuate nucleus and the medial preoptic nucleus of 4-day cycling female rats. Very well marked estrous cycle-related fluctuations were registered for these neuropeptides in these areas. The largest variations in concentrations of peptide levels were observed in the median eminence. Substance P concentration was highest throughout the day of proestrus as compared to the three others days of the cycle. At the time of the preovulatory luteinizing hormone surge, on the afternoon of proestrus, there was a marked increase in gonadotropin-releasing hormone concentration and a marked decrease in β-endorphin concentration.