The stimulatory effect of vasopressin on the luteinizing hormone surge in ovariectomized, estradiol-treated rats is time-dependent

The luteinizing hormone surge in the female rat is not only induced by the positive feedback of estradiol, but also by circadian signals originating i n the suprachiasmatic nucleus (SCN). In a previous study we showed that adm inistration of vasopressin. an SCN transmitter present in preoptic projecti ons, induced an LH surge in animals bearing complete lesions of the SCN. Th is strongly suggests vasopressin as a stimulatory circadian signal for the timing of the LH surge. In the present study we investigated during which t ime window vasopressin may act in the medial preoptic area to stimulate LH secretion in SCN-intact female rats. Vasopressin or a specific Via receptor antagonist was administered into the MPO by a reverse microdialysis techni que during different time windows, and plasma LH concentrations were measur ed. Vasopressin stimulated the LH surge in 30% of the animals, when adminis tered during the second half of the light period, but during the first half of the light period no effects were observed. Administration of the Via re ceptor antagonist, however, did not affect the LH surge. These data confirm our previous results that vasopressin is a stimulatory factor for the LH s urge also in SCN-intact animals, and indicate that a certain time window is available for such stimulation. We hypothesize that vasopressin in the SCN -intact animal may act as a circadian signal during a specific time window to induce the LH surge. The time window is the result of other SCN regulato ry systems that are involved in the preparation of the LH surge. (C) 2001 E lsevier Science B.V. All rights reserved.