The overall objective of the research is to determine neuroendocrine mechanisms governing onset of puberty. The domestic sheep, a seasonal breeder, will be used as the animal model to study internal and external determinants for initiation of ovulation. The working hypothesis will be tested that photoperiod and growth-associated cues, by impinging on systems that control pulsatile LH secretion time first ovulation. Experiments will be conducted to determine if melatonin is the pineal product that transduces photoperiodic information in the lamb; prolonged infusions of melatonin by portable pumps will be carried out in maturing females that have denervated pineal glands. This technique, coupled with artificial light treatments, will be used to determine the role of each component of the novel tripartite photoperiod sequence that initiates puberty. This photoperiod sequence, as well as photoperiods that have been developed to delay puberty, will be used as noninvasive probes to examine steroid-dependent and steroid-independent control of LH pulse frequency. The effect of growth rate on photoperiodic requirements for first ovulation will be evaluated under conditions in which body weight is regulated by level of nutrition. Patterns of circulating melatonin will be monitored to determine if photoperiod cues are perceived when growth ceases under limited nutrition. The growth-retarded model, in which puberty is delayed, will be used to study the effects of limited nutrition on control of pulsatile LH secretion by steroid-dependent and steroid-independent mechanisms. During the catch-up growth phase when puberty occurs, it will be determined whether level of nutrition can be used to set the frequency of LH pulses. The relationship between level of tonic LH secretion and the ability to respond to the positive feedback action of estradiol will be examined in growth-restricted lambs treated with a pulsatile infusion of GnRH. Patterns and levels of growth-associated hormones will be monitored during normal sexual development and in delayed puberty induced by inappropriate photoperiod or by inadequate growth. The research will fill and important gap in our knowledge of how the timing of puberty is influenced by external environment in a seasonally breeding species. The results of investigations of nutrition and weight regulation of pulsatile LH secretion will have broad application to the understanding of normal and delayed puberty in children.
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