Effects of melatonin implants on insulin-like growth factor 1 in male red deer ( Cervus elaphus)

Abstract

Red deer stags have a seasonal pattern of growth, alternating between periods of summer weight gain and winter weight loss that are influenced by photoperiod and by exogenous melatonin. A seasonal pattern of plasma insulin-like growth factor 1, also influenced by photoperiod, underlies the seasonal growth pattern. The present studies aimed to determine the influence of exogenous melatonin, administered at various times of the year, on plasma IGF1 in adult red deer stags in New Zealand at 45°S. In one study, 7-year-old stags ( N = 9) were allocated to one of three treatment groups, either control or subcutaneous melatonin (3 × 18-mg coated implants (Regulin) per month) from November to February or from December to February. Blood was sampled, the stags were weighed, and antler status was recorded over 17 months. Melatonin treatment advanced the seasonal patterns of rise and fall of plasma IGF1 and of weight gain and loss. The cessation of melatonin treatment in February produced early antler casting and a second (out-of-season) antler and increased IGF1. In a second study, 4-year-old stags ( N = 30) were allocated to one of six treatment groups as follows: three melatonin implants per month for 6 consecutive months beginning on 22 June, i.e., winter solstice, 4 August, 16 September, and 23 October; three melatonin implants per month for 12 months beginning on 22 June; and an untreated group. All animals were sampled as before for 12 months. Melatonin treatment beginning in July and August did not prevent the seasonal peak in IGF1, but the amplitude was lowered and antler casting delayed. In contrast, melatonin treatment beginning in September and October advanced the seasonal decrease in plasma IGF1, thus the duration of elevated IGF1 was shortened. In the stags treated with melatonin for 1 year, the amplitude of the IGF1 peak was reduced but the seasonal pattern was unaffected. Treatment with melatonin tended to reduce antler size and liveweight gain in all groups. The results indicate that the effect of melatonin treatment on IGF1 differs in June and December. Initially melatonin does not influence the seasonal increase in IGF1 although its amplitude is lowered. Later melatonin treatment advances the seasonal decrease in IGF1 to shorten the duration of elevated IGF1. Melatonin treatment uncoupled antler growth from the IGF1 seasonal pattern either by delaying casting or advancing cleaning to give a decreased antler size.