Appearance of a nocturnal peak of leptin secretion in the pubertal rat

Whether leptin is involved in the timing of puberty remains highly controve rsial in the rat. Daytime leptin secretion shows little change during the t ransition into adulthood. Because leptin exhibits a diurnal variation in th e adult, it is possible that the ontogeny of such a rhythm provides importa nt information for the timing of puberty. To begin to evaluate this hypothe sis, we determined the development of the diurnal leptin secretion in the r at. The young females were raised in a light-controlled environment (12L, 0 700 h light on). A cannula was placed in the right atrium on the previous d ay, and blood samples were collected every 4 h on Days 21, 24, 28, 32, 36 ( 1 day after vaginal opening), and 48 (adult, diestrus of estrous cycle). In addition to vaginal opening, plasma prolactin levels were determined as an endocrine index of puberty. Changes in food intake were monitored because nocturnal food intake has been considered to be a synchronizer for the lept in rhythm. This pattern of food intake was clearly evident throughout the a ges studied. By contrast, there was no leptin rhythm at 21 and 24 days of a ge. Beginning at 28 days, leptin secretion exhibited a significant nocturna l peak (2300 h); this nocturnal peak increased in amplitude at 32 and 36 da ys and was still apparent in the cycling adult at Day 48. Plasma prolactin did not exhibit a diurnal rhythm but it increased from Days 32 to 48. The p resent findings indicate that in the rat, both the appearance of the noctur nal leptin rhythm and the nocturnal increase in circulating leptin levels d uring development carry information for timing the onset of puberty. (C) 20 00 Academic Press.