Circannual rhythms in the ewe: patterns of ovarian cycles and prolactin secretion under two different constant photoperiods.

The purpose of this experiment was to determine whether circannual rhythms of reproductive activity and prolactin secretion are expressed differently in ewes housed under two different constant photoperiods and restricted temperatures. Eleven ovary-intact ewes housed previously under constant 12L:12D were used. One group (n = 6) was switched to 8L:16D while the other group (n = 5) remained on 12L:12D. Ovarian cycles and prolactin concentrations were monitored for more than 3 yr. The switch from 12L:12D to 8L:16D caused a change from cycles of varied length to regular 17-day cycles within 25 +/- 3 days in all 6 ewes. Subsequently, one complete circannual cycle (300.8 +/- 10.2 days) of breeding and nonbreeding activity was expressed in all ewes, but it was repeated in only 2 ewes. Ewes maintained on 12L:12D cycled at irregular intervals, and only 2 exhibited a distinct circannual rhythm of anestrus and breeding cycles. Episodic variations in ovarian cycle density were observed in most ewes in both groups. The period of this rhythm was similar between groups (290 +/- 19 and 278 +/- 16 days) and differed from 365 days (p < 0.05). Plasma prolactin exhibited a circannual rhythm under both 8L:16D and 12L:12D with a periodicity different from 365 days. We conclude that photoperiod length does not influence the expression of circannual reproductive and prolactin rhythms in the ewe. The absence of a circannual rhythm of breeding and nonbreeding activity, in the presence of circannual changes in ovarian cycle density, suggests that expression of an underlying endogenous oscillator controlling LH secretion was masked or damped.