The juvenile-peripubertal transition period in the female rat: establishment of a diurnal pattern of pulsatile luteinizing hormone secretion.

Previous descriptions of pulsatile LH release in immature female rats have been mostly inaccurate because of infrequent blood sampling, discontinuous and unbalanced blood replacement procedures, and possibly because of the use of a heterologous RIA. The present study was undertaken to circumvent these problems and to characterize more precisely the changes in episodic LH secretion previously observed during the juvenile-peripubertal transitional period (27-38 days of age). Changes in LH levels were measured in plasma samples obtained every 5 min from unrestrained conscious animals using a recently developed technique that permitted continuous withdrawal of blood for 4-5 h and simultaneous replacement with an artificial blood mixture. In virtually all of the rats, LH release was pulsatile. In both the juvenile (27-29 days old) and peripubertal (30-38 days old) animals, mean plasma LH levels and LH pulse amplitude were low in the mornings. A similar pattern of LH secretion was found in the afternoon during the juvenile period. In contrast, most of the peripubertal animals exhibited an afternoon increase in LH pulse amplitude. A change in pulse frequency was not detected at this time, but a 1.8-fold increase in mean LH levels suggests that large amplitude pulses may be accompanied by low amplitude, high frequency pulses not revealed by the 5-min sampling paradigm. Some of the peripubertal rats (5 of 12) had plasma LH profiles in which the large amplitude pulses were accompanied by a midafternoon minisurge of secretion which lasted for up to 2 h and reached peak levels of 174 +/- 19 ng/ml; the LH surge at first proestrus was greater than 1000 ng/ml. The results demonstrate the prepubertal development of an afternoon increase in LH pulse amplitude and the appearance of a more prolonged and larger secretory episode of LH secretion (minisurge). Disclosure of a concomitant increase in pulse frequency may necessitate blood sampling at more frequent intervals. It is suggested that the changes in the mode of LH release described here may enhance peripubertal ovarian steroidogenesis and provide an appropriate stimulus for initiating the final stages of ovarian development.