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Effects of exogenous melatonin on prolactin secretion, lactogenesis and reproductive seasonality of adult female red deer (Cervus elaphus)

The effects of administration of exogenous melatonin to pregnant red deer hinds on prolactin secretion, lactogenesis and reproductive seasonality were studied. Mature hinds (n = 23) were allocated to one of four treatments. Hinds in treatment 1 (n = 6) each received two subcutaneous melatonin implants (Regulin) at monthly intervals starting on 2 October, about 80 days before expected parturition. Hinds in treatment 2 (n = 6) received similar treatment starting on 2 November, about 40 days before calving, whereas hinds in treatment 3 (n = 5) received treatment starting on the actual day of calving (about 10 December). Final implants were delivered on 1 February, with overall treatment durations of 150, 120 and 90 days for treatments 1–3, respectively. Hinds in treatment 4 (n = 6) served as controls and received no melatonin treatment. Blood samples were taken twice a week from September to May, and plasma was analysed for progesterone and prolactin. Mammary development was assessed by palpation score (0–5) twice a week from October to April inclusive, and liveweights were recorded at least every two weeks throughout the trial. Calving occurred between 28 November and 24 December, with no significant differences among treatments (P > 0.10). Hinds in treatment 1 exhibited significant retardation of mammary gland development and liveweight gain leading up to parturition (P < 0.01). Furthermore, sex-adjusted calf birth weights were on average 3 kg lighter for treatment 1 (P < 0.05), with all calves either removed for bottle-rearing or having died within a few hours of birth. Failure of lactogenesis in treatment 1 was characterized by the presence of underdeveloped, hard mammary tissue devoid of expressible milk. Hinds in treatments 2–4 all exhibited full lactation and successfully reared their calves, and there were no significant differences in calf weaning weight and growth rates. Likewise, there were no significant differences in mean liveweight or lactation score profiles. Mean plasma prolactin concentrations varied significantly between treatments (P < 0.05), and control hinds exhibited a marked seasonal pattern of secretion which reached a peak at calving. However, hinds in treatments 1 and 2 failed to show any discernible seasonal increase in mean plasma prolactin concentrations, whereas there was a marked increase in mean prolactin concentrations in hinds in treatment 3 up to parturition, but concentrations decreased rapidly thereafter relative to those of control hinds. Melatonin treatment significantly advanced the date of first oestrus and decreased the postpartum–oestrus interval (P < 0.05). It was concluded that initiation of melatonin implant treatment about 80 days before parturition compromises mammary and fetal development in red deer hinds. However, the role of prolactin was not demonstrated conclusively.