Effect of melatonin implants on reproduction in the male silver fox (Vulpes vulpes)

Abstract

In June 1987, when the testes were fully regressed, 5 males were given s.c. implants of 40 mg melatonin. The same treatment was repeated in August and October 1987. Five males served as controls. Plasma concentrations of melatonin increased significantly in treated males and were still elevated at the end of the study in April 1988. The changes in testicular volume and blood plasma concentrations of testosterone in response to GnRH indicated that melatonin administration promoted testicular development. However, testicular regression was observed earlier in treated than control animals, perhaps because of refractoriness to melatonin or to a down-regulation of melatonin receptors. Semen was collected and frozen in November 1987, 2 months ahead of the natural breeding season, from the melatonin-treated males, and 10 blue fox vixens were inseminated the following breeding season: 9 vixens conceived, and the average litter size was 7.6 +/- 0.5. The results demonstrate that melatonin treatment initiated during exposure to naturally long days (a) promotes testicular development in a way similar to an artificial short photoperiod and (b) may induce a refractory condition after an extended period of treatment.

Effects of melatonin implants on plasma concentrations of testosterone, thyroxine and prolactin in the male silver fox (Vulpes vulpes)

“…The effects of constant exposure to high melatonin concentrations on the reproductive cycle result in an initial period of rapid testicular growth followed by involution (Forsberg et al, 1990), in the present study registered as a time shift in peak testosterone secretion.…”

“…Earlier experiments have shown that the reproductive cycle as well as the moulting cycle of the silver fox (Vulpes vulpes) male can be influenced by changes in photoperiod (Forsberg et al, 1989) and melatonin administration (Connor, 1988;Forsberg et al, 1990). Under natural daylight con¬ ditions plasma concentrations of testosterone and thyroxine show distinct seasonal variations (Joffre & Joffre, 1975;Joffre, 1976;Maurel & Boissin, 1981), and prolactin secretion increases during and after the mating season (Maurel et al, 1984).…”

Effects of Superior Cervical Ganglionectomy and Melatonin Replacement on Intra-hypothalamic LHRH Content and Pulsatile Luteinizing Hormone Release in the Mink

“…However, a long latency for the responses of the Downloaded by [University of Saskatchewan Library] at 14:04 27 December 2014 gonadal system has already been reported in mink, either following ganglionectomy (Maurel et al, 1992a), or in intact animals during the seasonal cycle of reproduction (Jallageas et al, 1994a). The occurrence of a refractory state following long-term effects of melatonin administration has also been described in other species: a decrease in testicular activity was observed in the ram and in the fox (Lincoln and Ebling, 1985;Forsberg et al, 1990), while oestrus cycles were abolished in the ewe (Nowak and Rodway, 1985).…”

Integrating evolution in the management of captive zoo populations

“…), and silver fox ( Vulpes vulpes ) (Forsberg et al. ). However, further research is required to develop a better understanding of the impact of changes in latitude on melatonin secretion and the potential benefits of therapeutic melatonin administration to captive breeding programs that are not sustainable.…”

“…Monitoring and supplementing melatonin levels may be a possible method for animal managers to regulate circadian and seasonal rhythms of translocated species to enhance reproduction and welfare. Melatonin implants have been used to regulate reproductive performance in sheep (Ovis aries) (Hanif and Williams 1991;O'Callaghan et al 1991;Haresign 1992), deer (Dama dama) (Asher et al 1988), and silver fox (Vulpes vulpes) (Forsberg et al 1990). However, further research is required to develop a better understanding of the impact of changes in latitude on melatonin secretion and the potential benefits of therapeutic melatonin administration to captive breeding programs that are not sustainable.…”