212 J. Santiago-Moreno et al. Animal Reproduction Science 64 2000 211–219
1. Introduction
Prolactin PRL shows an endogenous rhythm of secretion, which is modulated by photo- toperiod among other influences, and clearly affected by time of year in mammals. Seasonal
changes in plasma concentrations of PRL have been shown to be associated with changes in daylength Thimonier et al., 1978, with highest and lowest concentrations of PRL occurring
during summer long days and winter short days, respectively Pelletier, 1973; Ravault and Ortavant, 1977. The regulation of this seasonal rhythm of PRL is under pineal control
by the secretion of melatonin Reiter, 1991; Misztal et al., 1994; Lincoln and Tortonese, 1995 whose natural suppressive effect on PRL secretion is predominantly caused by a di-
rect action on the pituitary gland Lincoln and Clarke, 1994. There is also direct evidence of a genetic influence on the seasonal changes in plasma concentrations of PRL during
the light-phase, between rams of wild Mouflon, feral Soay and various domesticated breeds of sheep, which shows that selection for a more continuous pattern of reproduction
is accompanied by variations on the annual PRL profile Lincoln, 1990.
In addition to a seasonal pattern of PRL secretion, earlier studies have shown a marked variation in plasma PRL levels between the light and dark-phases of the day Walton et al.,
1980. Seasonal changes in these circadian rhythms in plasma PRL of the ram have been confirmed by Ravault et al. 1987; however, whether melatonin participates in the circadian
regulation of PRL secretion is still unclear. In the domestic sheep, the earliest studies showed that the major daily change in plasma PRL concentrations is a rise around dusk Davis and
Borger, 1974; Forbes et al., 1975, although this increase may vary depending on seasonal influences Misztal et al., 1997; Walton et al., 1980.
The Mouflon is the European wild sheep Ovis gmelini musimon originated in Sardinia and Corsica, and represents the wild ancestor of the current domesticated breeds of sheep
Ovis aries, Hiendleder et al., 1998. Although, marked seasonal changes in blood plasma concentrations of PRL have been established Lincoln, 1990, the nyctohemeral variations
in prolactinemia have not been estimated in this species.
The aims of the current study were to assess the existence of a nocturnal rhythm of secretion of PRL in the Mouflon, and whether possible seasonal differences in the nocturnal
rhythms of PRL could be affected by breed in sheep. Manchega breed was selected for this study because, despite it is originated in the Mediterranean area and at similar latitude than
Mouflon 40
◦
N, the domestication has modified its reproductive activity in comparison with the wild ancestor Santiago-Moreno et al., 2000. In addition, this breed is a valuable
model to study if the selection for a more continuous pattern of reproduction is accompanied by nyctohemeral variations of the annual PRL profile when compared with the wild ancestor.
The temporal relationship between the secretion of PRL and melatonin was also studied.
2. Material and methods
