Photoperiod

In rams, exposure to an artificial lighting regimen of alternating 16-weekly periods of long and short days induces cyclical changes in gonadotrophin and PRL secretion (Figure 1). An abrupt switch from long to short days promotes an increase in gonadotrophin secretion and a decrease in PRL secretion, while the switch back to long days has the reverse effects. Since FSH acts along with testosterone on the Sertoli cells in the seminiferous tubules of the testis to regulate the efficiency of spermatogenesis (13), the testes become fully enlarged during short days as normally occurs in autumn in animals living outdoors. Circulating FSH concentrations decline at the peak of the tesicular cycle due to the negative feedback effect of testosterone and inhibin acting at the level of the hypothalamus and pituitary gland (45,64). The high PRL concentrations induced by long days have potentially many effects since PRL receptors are expressed in liver, kidney, pancreas, muscle, skin, adipose tissue, gonads and selected sites in brain (7). One conspicuous effect is on the pelage cycle with PRL acting in the

Figure 1. Cyclical changes in: a) FSH secretion and testicular size, and b) PRL secretion and wool growth in Soay rams exposed to alternating 16 weekly periods of long (16L: 8D) and short days (8L : 16D).The timing of the phases of aggressive behaviour (rut) and the pelage moult (horizontal bar) are also shown (32).

Figure 1. Cyclical changes in: a) FSH secretion and testicular size, and b) PRL secretion and wool growth in Soay rams exposed to alternating 16 weekly periods of long (16L: 8D) and short days (8L : 16D).The timing of the phases of aggressive behaviour (rut) and the pelage moult (horizontal bar) are also shown (32).

dermal papilla of the hair follicle to trigger reactivation of hair growth and the associated spring moult (11,14).

Regular changes between long and short days has the effect of accelerating the seasonal cyclicity and two complete reproductive cycles can be induced within 12 months (27). If the light changes are more frequent, however, rams fail to express a decline in gonadotrophin secretion during brief exposure to long days and may remain permanently sexually active state (59). While photoperiod has a clear inductive effect on seasonality, sheep do not need to experience changes in photoperiod to express long-term cyclicity. Under constant long or short days, both rams and ewes continue to show long-term variations in both gonadotrophin and PRL secretion, although the cycles amonst individuals become asynchronous and damp out with time (15,24,25,27). The long-term cycles are thus generated endogenously as circannual rhythms, but under natural conditions the annual cycle in photoperiod acts to induce and entrain the cycles such that the period of the endogenous rhythm is adjusted to match the sidereal year, and phase of each component system is timed to the appropriate season.

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