Timing of puberty by photoperiod.

Photoperiod cues play an important role in the timing of puberty in the female lamb. Removal and replacement of photoperiod cues by denervation of the pineal gland and timed melatonin infusions, respectively, indicate that the pathway for transmission of photoperiod information develops well before puberty. This is reinforced by manipulation of artificial photoperiods during various periods of development. Such approaches reveal that even in the first few weeks of life, the pattern of melatonin secretion accords with daylength and modulates prolactin secretion. Several months later, after internal, growth-related cues indicate that sufficient body size has been achieved to initiate reproduction, photoperiod history is used as an important predictor of reproductive success, and thus, whether puberty should occur. In the female spring-born lamb, the decrease in daylength in autumn is the critical cue for the initiation of estrous cycles. Experimentally, this may be achieved by surgically disrupting the pathway for transmission of photic cues after appropriate long-day exposure. In the autumn-born lamb and in the slowly growing lamb, sexual maturation may be masked by the transition into seasonal anestrus the following spring. In these young females, a decreasing photoperiod or "removal of long days" (surgical) is not necessary for puberty the following autumn. Sufficient photoperiod history may be acquired in such lambs that they enter puberty as a consequence of becoming refractory to the long days of summer. We hypothesize that the phenomenon of refractoriness reflects the expression of an innate rhythm of reproductive activity and that changes in daylength experienced early in life serve to synchronize this rhythm with the seasonal environment.