Daily rhythm of cortisol, and evidence for a photo-inducible phase for prolactin secretion in nonpregnant mares housed under non-interrupted and skeleton photoperiods.

The research article discusses the patterns of cortisol and prolactin secretion in horses based on different light-dark photoperiods. The aim is to understand if there’s an influence of light exposure on these hormones and to better comprehend the possible role of these hormonal rhythms in the regulation of reproductive activities in horses.

Overview of Study

• The research involved studying mares (female horses) in a state of anestrus, meaning they were not in their period of sexual receptivity or estrus.
• Various light-dark cycles were examined, both non-interrupted, with ambient light and a cycle of 16 hours of light and 8 hours of darkness, and skeleton photoperiods, with a short period of darkness interspersed between two segments of light exposure.
• One of the main objectives of the study was to find out if there was a specific photosensitive phase for the secretion of prolactin, a hormone that plays a key role in a variety of essential bodily functions, including reproduction.

Results of the Study

• Through the study, the researchers found that the peak and lowest concentrations of cortisol, and the time duration for these concentrations to occur, did not vary amongst the different photoperiod treatments.
• However, significant differences were noticed in the daily concentrations of serum prolactin based on the photoperiods. Increased secretion of prolactin was observed under 16 hours of light + 8 hours of darkness and 10 hours of light + 8 hours of darkness + 2 hours of light + 4 hours of darkness photoperiods, as compared to other photoperiod treatments. The lowest prolactin levels were found under the ambient light treatment.
• The researchers also found that the timing of the first seasonal ovulation in these mares in February was inversely related to mean daily concentration of prolactin among different photoperiod treatments. This means that the higher the prolactin levels, the fewer days were required to reach the first ovulation of the season.

Interpretation of Results

• The results suggested that the horses in the 10 hours of light + 8 hours of darkness + 2 hours of light + 4 hours of darkness photoperiod did not readjust their internal rhythm or timing in response to the additional 2-hour light pulse. They did not interpret it as the beginning of their ‘subjective day’.
• Further, the study provided evidence supporting the hypothesis that the increased secretion of prolactin and the triggering of seasonal reproductive activity occurred approximately 8 to 10 hours following the beginning of the dark period. This suggests that the onset of darkness plays an essential role in activating certain hormonal and reproductive processes in mares.