Plasma prolactin concentrations in barren, pregnant and lactating red deer ( Cervus elaphus) given melatonin to advance the next breeding season

Abstract

The objectives of this trial were to investigate the effects of melatonin treatment initiated in May on plasma prolactin concentrations, growth rate of suckled calves and onset of seasonal oestrous cyclicity in red deer hinds which were either barren or gave birth on average 37 days after the start of treatment. Six barren and nine pregnant/lactating hinds were each given 5 mg melatonin daily in a feed at 15.30 h from 6 May to 22 September. Six barren and nine pregnant/lactating deer served as controls by receiving the feed unsupplemented. Plasma prolactin concentrations in control deer increased during May (from about 10 to 40 ng/ml), remained high in June and July (barren: 30–40 ng/ml, lactating: 50–80 ng/ml), and decreased by the end of August to reach basal values (0–7 ng/ml) thereafter. Melatonin treatment significantly lowered plasma prolactin concentrations to basal levels after 21 days (i.e. end of May) in barren hinds, but not until 14 days post-partum in the pregnant/lactating animals (i.e. end of June) after an average of 51 days of treatment. Calves grew at the same rate (g/day, mean ± SEM) whether suckled by melatonin-treated (321 ± 5.9) or control (326 ± 8.1) hinds. Mean onset of seasonal ovarian activity was advanced by melatonin treatment in lactating hinds (38 days, P < 0.001) and non-lactating hinds (33 days, P < 0.01). In a second concurrent experiment, barren hinds kept indoors in natural daylength were given melatonin as above ( n = 2) or remained as untreated controls ( n = 2). Prolactin showed two or three distinctive phases of increased plasma concentrations in blood samples taken hourly over intermittent 24-h periods, even when baseline values were undetectable for some of the time, and treatment with melatonin apparently induced a phase-shift in this diurnal pattern.