A Detailed Analysis of the Serum Luteinizing Hormone Secretory Profile in Conscious, Free-Moving Female Rats during the Time of Puberty*

To determine if the temporal pattern of LHsecretion changes during the days preceding the first preovulatory surge of gonadotropins, peripubertal female rats (28–37 days of age) were bled through indwelling jugular cannulae every 15 min for 3 h either in the mornings (900–1200 h) or the afternoons (1400–1700 h). In all pubertal stages plasma LH levels fluctuated in a pulsatile manner. In late juvenile rats (anestrous phase of puberty), the magnitude of the pulses in the afternoon was greater than in the morning and a drop in afternoon LH baseline levels became apparent. This pattern became established after day 28 and persisted throughout the late juvenile period and the early proestrous phase of puberty, a stage in which uterine fluid appears for the first time. During the first proestrus (late proestrous phase of puberty), morning LH levels fluctuated in a manner similar to that seen in anestrous and early proestrous animals. In the afternoon a preovulatory surge occurred, with LH levels increasing 40 t...