To ascertain whether changes in the pattern of GnRH release from the hypothalmus occur during the 4-day rat estrous cycle, the pattern of LH release was characterized on each day of the estrous cycle, and the results were interpreted in light of the changes in pituitary responsiveness to GnRH previously described by this laboratory to occur during this time. Blood samples were taken from intact, freely moving rats via venous catheters at 6– to 10-min intervals for 3–4 h. LH pulse height and LH interpulse interval were quantified on each day of the cycle, and the transition on the afternoon of pfoestrus from tonic LH release to the preovulatory LH surge was detailed. The effects on the pattern of LH release during estrus of small doses of GnRH (0.4 ng) and the continuous infusion of the opioid antagonist naloxone were also examined. Plasma LH concentrations (NIAMDD rat LH-RP–1) were determined with a highly sensitive LH RIA. LH pulses were identified using the PULSAR algorithim. The LH interpulse intervals of 46 ± 2 min on diestrous-1 day, 49 ± 4 min on diestrous day 2, and 60 ± 8 min on proestrus immediately before the LH surge were not significantly different. There were no changes immediately preceding the preovulatory LH surge on the afternoon of proestrus in either the LH interpulse interval or the LH pulse height. Instead, the transition from tonic LH secretion to the preovulatory LH surge was found to occur abruptly. These data suggest that an abrupt increase in GnRH secretion during the afternoon of proestrus initiates the dramatic rise in LH concentrations. The pattern of LH secretion during the day of estrus differed significantly from that on the other days of the cycle in that no LH pulses were observed. However, the administration of small pulses of GnRH elicited physiological elevations in LH release. Furthermore, the continuous infusion of naloxone to estrous rats immediately stimulated a pulsatile pattern of LH secretion, with a LH interpulse of 56 ± 4 min. These data indicate that the absence of LH pulses during estrus may result from a deficit in GnRH release. Similar modifications in GnRH release during the other days of the cycle were inferred from the observed changes in LH pulse heights. The LH pulse height of 21 ± 3 ng/ml on diestrous day 2 was significantly less than the LH pulse height of 41 ± 4 ng/ml on diestrous day 1 or 35 ± 4 ng/ml on proestrus before the surge. The changes in LH pulse height from diestrous day 1 through proestrus before the surge do not parallel the increases in pituitary responsiveness to GnRH and pituitary GnRH receptor content that occur during this time. Taken together, these studies suggest that changes in tonic LH secretion during the rat estrous cycle do not result from changes in LH pulse frequency. Rather, the fluctuations in LH concentrations seem to result largely from changes in LH pulse height, possibly effected by changes in the quantity of GnRH released per pulse of GnRH.
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