Melatonin implants prevent the onset of seasonal quiescence and suppress the release of prolactin in response to a dopamine antagonist in the Bennett's wallaby (Macropus rufogriseus rufogriseus)

Abstract

Three groups of adult female wallabies were maintained out of doors under conditions of natural photoperiod and temperature from late December to mid-August. One group (M1; N = 6) received Silastic elastomer melatonin implants on 14 December, a second group (M2; N = 5) were given implants on 16 February and a third group (C; N = 7) were unimplanted controls. Group C animals had all ceased cycling by 15 March and the subsequent breeding season commenced on 5 July ± 6.9 days. Group M1 wallabies continued to cycle throughout the experimental period and did not exhibit ovarian quiescence. In Group M2, 2/5 animals continued to undergo repeated oestrous cycles and 3/5 ceased cycling between 14 December and 27 January and began again after the insertion of melatonin implants on 16 February. The prolactin response 30 min after s.c. administration of the dopamine antagonist domperidone was determined approximately every 4 weeks. In Group C, peak responses were high during the period of seasonal quiescence (January-June; mean range 14.2-19.6 ng/ml) and fell significantly (P < 0.02) at the beginning of the breeding season in early July to 7.4 ± 3.1 ng/ml. In Group M1, prolactin levels remained low (2.8-8.2 ng/ml) throughout the course of the experiment while in Group M2, response to domperidone fell following the insertion of the implants and subsequently remained at levels similar to those in Group M1. Our data support the hypothesis that photoperiod-induced changes in the secretion of melatonin after the winter solstice drive this species into seasonal quiescence by influencing the dopaminergic control of prolactin secretion.