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Peripubertal male mink (Mustela vison) were treated with prolactin, melatonin or antibodies against melatonin to determine the effects of altered circulating concentrations of prolactin and melatonin throughout one season of testicular development. Treatment began on 1 October and continued until 4 March. Administration of 0.5 mg ovine prolactin day−1 by minipump increased the circulating concentration of prolactin for the duration of the study and increased serum concentrations of LH. This treatment had no effect on the testosterone concentration or on testis size. Neither chronic treatment with melatonin throughout the period of crudescence nor passive immunization against melatonin for 79 days affected the circulating concentrations of prolactin, LH, testosterone or testis size. These results show clearly that, unlike in other seasonally breeding species, prolactin does not play a significant role in testis growth in the mink. Administration of melatonin to male mink in October did not affect testis growth, presumably because the melatonin signal that cues photoperiodic events had already been received. Administration of antibodies against melatonin did not affect any of the features measured, suggesting that melatonin may have neural but not peripheral effects. Further support for this view can be found in the absence of an influence of melatonin on testis growth or on the plasma concentration of testosterone.
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Summary. Immature female mink, 8 weeks of age in July, were treated with implants releasing melatonin. Mating, which induced ovulation, took place during the normal breeding season in the following March. Circulating prolactin and progesterone concentrations did not undergo the expected gestational increases, and no embryos implanted. A similar absence of gestational changes in prolactin and progesterone values ensued in primiparous mink treated with the melatonin implant 2–3 days after the second of 2 matings. Administration of exogenous sheep prolactin (0·5 mg/day) by minipump induced precocious elevation of progesterone concentrations in mated mink. Prolactin administration overcame the effects of melatonin, in that the corpora lutea were activated and embryos implanted, but exogenous prolactin resulted in degeneration of implanted embryos both in the presence and absence of chronic melatonin. The results suggest that melatonin has a single effect in alteration of gestation in mink; i.e. the prevention of prolactin secretion. Hyperprolactinaemia may inhibit embryo development in this species.
Keywords: melatonin; prolactin; mink; delayed implantation
