In order to define the patterns of gonadotropin-releasing hormone (GnRH) secretion during the estrous cycle of the sheep we sampled hypophysial portal blood from conscious animals on day 1 of the cycle (n= 1), during the luteal phase (n = 8), during the follicular phase (n = 6) and during the preovulatory luteinizing hormone (LH) surge (n = 6). At the same time, we sampled jugular blood to measure plasma LH concentrations. During day 1 we noted regular GnRH pulses, whereas GnRH pulse amplitude and frequency were more variable in the luteal phase of the cycle. In the transition from the luteal phase to the follicular phase the GnRH pulse frequency increased and the amplitude decreased. Around the time of the LH surge we noted 3 types of secretory profiles for GnRH. In one sheep (type 1) there was a large GnRH pulse at the onset of the LH surge followed by very little activity during the surge. In two sheep (type 2) the GnRH profile did not change between the late follicular phase and the onset of the LH surge. In the remaining three sheep (type 3) there was a clear increase in the secretion of GnRH at the onset of the LH surge. With the exclusion of the type 1 sheep the GnRH pulse frequency was maximal (2 pulses/h) at the time of the LH surge; average portal GnRH levels were also maximal at this time. These data show that changes in LH secretion during the estrous cycle largely reflect changes in GnRH secretion but the modest and variable increases seen at the time of the LH surge do not wholly account for the rise in LH secretion at this time.

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