Seasonal variation and circadian rhythmicity of the prolactin profile during the summer months in repeat-breeding Murrah buffalo heifers.

The present study was undertaken to determine a detailed endocrine profile for prolactin and progesterone during the oestrous cycle in repeat-breeding Murrah buffalo heifers during summer and winter. Hormone concentrations were quantified in blood plasma samples collected over the oestrous cycle in both winter and summer, as well as in samples collected during the summer months to observe circadian rhythmicity. The mean plasma prolactin concentration during the winter months ranged from 3.10 +/- 0.48 to 9.17 +/- 1.39 ng mL(-1); during the summer months, plasma prolactin concentrations ranged from 248.50 +/- 16.03 to 369.63 +/- 25.13 ng mL(-1). During the winter months, the mean plasma progesterone concentration ranged from 0.20 +/- 0.00 to 3.04 +/- 0.34 ng mL(-1), which was significantly higher (P < 0.01) than the prolactin concentrations recorded in the summer months (range 0.20 +/- 0.00 to 1.48 +/- 0.13 ng mL(-1)). Plasma prolactin and progesterone concentrations were negatively correlated (r = -0.24) during the summer oestrous cycle, which indicates prolactin-induced suppression of progesterone secretion through poor luteal development. During the summer, a circadian rhythmicity was observed in buffaloes and the results indicate that high prolactin secretion contributes to poor fertility by lowering gonadal hormone (progesterone) secretion. It was concluded from the present study that prolactin and progesterone profiles during the summer and winter months are directly correlated with the reproductive performance of buffaloes. The finding also validates the hypothesis that hyperprolactinaemia may cause acyclicity/infertility in buffaloes during the summer months due to severe heat stress.