Circadian variations in plasma concentrations of melatonin and prolactin during breeding and non-breeding seasons in yak (Poephagus grunniens L.)

Circadian variations of plasma melatonin and prolactin concentrations were determined during breeding as well as non-breeding seasons in yak. Blood samples (5 ml) were collected during different phases of estrous cycle, viz. early (0-6 days), mid (7-12 days) and late luteal (13-19 days) at 2 h interval for 24 h from eight yaks during one breeding month (November); the same yaks were bled at 2 h interval during one non-breeding month (February) for 24 h. Plasma melatonin concentrations rose sharply (P < 0.01) after sunset to record peak concentrations between midnight and 2 a.m. declining sharply thereafter in both breeding as well as non-breeding seasons. Basal melatonin concentrations were recorded between 0600 and 1600 h. Stage of luteal phase did not influence the diurnal hormone change (P < 0.01). In the breeding season, mean plasma prolactin concentrations displayed circadian variations with maximum value at 0400 h (41.22+ /- 1.5 ng/ml) and minimum at 1400 h (12.0 +/- 4.02 ng/ml). In the non-breeding season plasma prolactin concentrations showed circadian variation with maximum value at 0000 h (59.9 +/- 10.5 ng/ml) and minimum at 1200 h (32.13 +/- 3.2 ng/ml). A positive correlation in breeding (r = 0.75) and in non-breeding season (r = 0.65) between circadian changes in mean plasma prolactin and melatonin concentrations were seen. Circadian changes of mean plasma melatonin concentrations during breeding and non-breeding seasons were not different (P > 0.05). However, mean plasma prolactin concentrations were found to be higher (P < 0.01) in the non-breeding season. Three conclusions were drawn from the study: (i) melatonin and prolactin concentrations followed a circadian pattern of secretion (ii) melatonin and prolactin secretion may be closely interrelated and (iii) higher prolactin concentrations during the non-breeding season could be due to nutritional and environmental stress and hence might be contributing to lack of cyclicity.