In vivo gonadotropin-releasing hormone secretion in female rats during peripubertal development and on proestrus

Pubertal development in female rats is characterized by increased LH levels and the appearance of estrogen-dependent afternoon LH mini-surges. In thes e studies we performed the first analysis of GnRH patterns in peripubertal rats to determine whether there are similar changes in pulsatile GnRH relea se. Microdialysis samples were collected at 8-min intervals throughout a 5- h afternoon period from 22 rats sampled on a single day between 30-47 days of age. Adult female rats were sampled on proestrus for comparison. In 30- to 33-day-old rats, GnRH release was infrequent (2.7 pulses/5 h; n = 3), wh ereas intermediate pulse frequencies were observed in 34- to 37-day-old rat s (6.4 pulses/5 h; n = 9)and 38- to 42-day-old (5.0 pulses/5 h; n = 5) rats . The highest GnRH pulse frequencies were observed in 43- to 47-day old rat s (9.4 pulses/5 h; n = 5). Mean GnRH pulse amplitude did not vary significa ntly with age. Animals sampled before vaginal opening (VO) exhibited signif icantly slower GnRH pulse frequencies than those sampled after vaginal open ing (1.3 pulses/5 h pre-VO us. 7.6 pulses/5 h post-VO; P = 0.01). An aftern oon increase in GnRH secretion, defined operationally as a greater than 25% increase in mean GnRH levels in the last half of the sampling period and t entatively termed a mini-surge, was observed in 0%, 33%, 40%, and 60% of 30 - to 33-, 34- to 37-, 38- to 42-, and 43- to 47-day-old rats, respectively. An overall increase in GnRH pulse frequency was observed in females displa ying a mini-surge (9.0 pulses/5 h with mini-surge compared with 4.7 pulses/ 5 h with no mini-surge). The mini-surge itself, however, was associated wit h a late afternoon increase in GnRH pulse amplitude and not in pulse freque ncy. In adult proestrous rats, peak levels during the GnRH surge were an or der of magnitude greater than those reached in pubertal animals. Our findin gs demonstrate that pubertal maturation in the female rat is associated wit h an acceleration of GnRH pulse generator activity and that later stages of pubertal maturation are characterized by the appearance of afternoon incre ases in GnRH release that may underlie previously reported mini-surges in L H.