The role of nutrition and photoperiod in the timing of puberty.

In animals that are kept in temperate latitudes and that are seasonal in their breeding activity, nutritional and photoperiodic signals operate in harmony to time the activation of the gonadotrophin-releasing hormone (GnRH) pulse generator in the brain and trigger puberty. These signals ensure that within the constraints imposed by the circannual changes in the environment, body size and time of puberty are appropriate for the overall success of the reproductive process. Although photoperiodic influences on the timing of puberty in farmed species occur in sheep (Foster et a f . 1986), goats (Deveson et al. 1992), red deer (Cervus elaphzu; Webster & Barrell, 1985), cattle (Schillo et al. 1992) and pigs (Paterson et al. 1992), it is the short-day breeders (sheep, goats and red deer) that are most photodependent. If the natural timings of their birth during either the increasing daylength of the spring (sheep, goats) or the long days of early summer (red deer) are followed by natural photoperiodic changes and normal growth rates then puberty usually occurs during the declining daylengths of either the first (sheep, goats) or second (red deer) autumn. Recent studies for all three species, and in particular for sheep and deer, have added to our understanding of the interdependence of nutrition and photoperiod in the timing of puberty. These have varied from empirical descriptions of the effects of restricted growth rates and artificial photoperiods on the activation of the GnRH pulse generator in both the female and male, to mechanistic approaches which attempt to identify the biochemical and molecular signals involved in recording and transmitting metabolic information on nutritional status and photoperiod history to the neuroendocrine axis. Recent research also involves attempts to identify blood-borne metabolites that mediate the nutritional activation of the GnRH pulse generator. In the case of photoperiod, for which there is unequivocal proof that the mediator of this environmental cue in timing puberty is the pineal indoleamine, melatonin (Yellon & Foster, 1986), research effort is now directed to identifying its receptor sites and mode of action. In this regard the recent observations that sheep and deer possess melatonin receptors during fetal life (Helliwell & Williams, 1991; Helliwell et al. 1991) are particularly interesting in view of the demonstration that the photoperiodic signal received by the fetus in utero can alter age at puberty in both species (Adam et al. 1992~; Helliwell et al. 1992), albeit in ways that are specific for each sex.