IS A CRITICAL INTERVAL OF THE CIRCADIAN PACEMAKER AT DUSK RESPONSIVE TO LIGHT AND MELATONIN RESPONSIBLE FOR THE TIMING OF ESTRUS IN THE ROMNEY-MARSH EWE

Two experiments, using Romney Marsh ewes, tested for the existence and role of a critical interval of the circadian pacemaker located near d usk that may be integrally involved in the precise timing of the breed ing season. Groups of Romney Marsh ewes (n = 6) were provided with exo genous melatonin by injection at dusk (Experiment 1) or by infusion at dawn or subjected to extended darkness at dawn (Experiment 2) from th e winter to the summer solstice before being exposed to natural photop eriod at latitude 35 degrees S. Other than the experimental protocols, all animals were held in natural photoperiod. The onset of the breedi ng season (defined as cyclic ovarian activity as indicated by plasma p rogesterone monitoring) was normal in those animals treated with morni ng melatonin but was delayed in those animals treated with melatonin a t dusk or extended darkness at dawn compared to controls in natural ph otoperiod (p < .01). Exogenous melatonin at dusk was associated with a phase advance of the onset of the circadian pacemaker (as measured by endogenous melatonin in acutely extended darkness); additional darkne ss at dawn was associated with a phase delay of both the onset and the offset of the circadian pacemaker. Exogenous morning melatonin did no t change the phase of the circadian pacemaker relative to the controls . The results are consistent with an external coincidence model of sea sonal breeding in which a critical interval of the circadian pacemaker requires exposure to light during spring/summer to time estrus correc tly. The proposed critical interval appears to be located near dusk in this model and is phase locked to the circadian pacemaker. The effect of the exogenous melatonin on the timing of the breeding season is si milar to darkness when administered at dusk but is not equivalent to d arkness at dawn. The timing of anestrus was not affected by any of the experimental treatments and may reflect a common response to an envir onmental influence.