Activity of gonadotropin-releasing hormone neurons during the preovulatory luteinizing hormone surge

Abstract

We assessed the frequency of luteinizing hormone (LH) pulsatility (reflecting the activity of gonadotropin-releasing hormone [GnRH] neurons in the hypothalamus) in six women during the periovulatory LH surge, in five women during the early follicular phase, and in seven women in the midfollicular phase (MFP) (calculated as being 3 to 8 days before the LH surge). Collection of blood at 5-minute, versus 15-minute, intervals allowed detection of a larger number of LH pulses in both the MFP (16, versus 27) and periovulatory phase (POP) (11, versus 22) groups of women, but it made no difference in the early follicular phase (EFP) (10 pulses with both methods). During the EFP, the mean number of LH pulses per 4 hours (detected by 5-minute sampling) was 2.0 ± 0.7 (± standard deviation [SD]), and the mean LH amplitude (± SD) was 1.3 ± 0.4 IU/l. There was a significant increase in the number of pulses in the MFP group (3.9 ± 1.3 pulses/4 hours; P < 0.05) but no significant change in pulse amplitude (1.1 ± 0.1 IU/l). During the POP, the mean pulse amplitude was increased (8.5 ± 1.4 IU/l; P < 0.001), compared with the MFP and EFP groups, but the mean pulse frequency (3.7 ± 1.2 pulses/4 hours) was not significantly different from the MFP frequency. We conclude that an acceleration of LH pulsatility occurs several days before the LH surge and does not change thereafter. However, there is an increase in LH pulse amplitude during the LH surge; we attribute this to the increase in pituitary sensitivity at this time. These results suggest that the preovulatory LH surge is primarily a pituitary event, although increased activity of GnRH neurons does occur earlier during the follicular phase.