Vasopressin induces a luteinizing hormone surge in ovariectomized, estradiol-treated rats with lesions of the suprachiasmatic nucleus

The luteinizing hormone surge in the female rat is the result of the integr ation of multiple signals within the medial preoptic area. The medial preop tic area contains gonadotropin-releasing hormone neurons that are responsib le for the release of luteinizing hormone, neurons containing estrogen rece ptors and terminals originating from the suprachiasmatic nucleus with, for example, vasopressin as neurotransmitter. Both the medial preoptic area and suprachiasmatic nucleus are crucial for the occurrence of luteinizing horm one surges, since lesioning of either nucleus prevents pre-ovulatory and st eroid-induced luteinizing hormone surges. In this study, we investigated wh ether vasopressin in the medial preoptic area could he the daily neuronal s ignal from the suprachiasmatic nucleus responsible for the timing of the lu teinizing hormone surge. Vasopressin (50 ng/mu l) or Ringer solution was ad ministered by reverse microdialysis from Zeitgeber times 7.5 to 12.5 into t he medial preoptic area of ovariectomized, estradiol-treated rats. The supr achiasmatic nucleus was lesioned to remove all cyclic luteinizing hormone s ecretion. This was evaluated by monitoring behavioral activity; animals tha t were arrhythmic were included in the experiments. Hourly blood samples we re taken to measure plasma luteinizing hormone levels. Preoptic vasopressin administration induced a surge-like luteinizing hormone pattern in suprach iasmatic nucleus-lesioned animals, whereas constant, basal luteinizing horm one levels were found in the control animals. These data show that vasopressin, by itself, is able to trigger the luteini zing hormone surge in suprachiasmatic nucleus-lesioned rats. We propose tha t vasopressin is a timing signal from the suprachiasmatic nucleus responsib le for the activation of the hypothalamo-pituitary-gonadal axis in the fema le rat. (C) 1999 IBRO. Published by Elsevier Science Ltd.