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M. Forsberg and A. Madej

Summary. Melatonin administration in the form of slow-release implants advanced breeding activity in silver fox males when treatment began in June. Plasma testosterone concentrations were significantly higher in treated animals than in controls from September to November, whereas in February and March they were significantly lower. Plasma prolactin concentrations were significantly reduced immediately following melatonin treatment in June but increased to greater levels than control values and 'peaked' after 7 months. This 'peak' was associated with a rapid decrease in testosterone secretion. The normal seasonal spring rise in prolactin secretion was prevented by melatonin administration. Thyroxine values decreased and were significantly lower after 2 months of melatonin treatment.

Keywords: melatonin; reproduction; silver fox; male; prolactin; thyroxine; testosterone

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Y. Xiao, M. Forsberg, J. T. Laitinen and M. Valtonen

Six male raccoon dogs were treated with constant-release implants of melatonin on 29 March, when the animals still had mature spermatozoa and a winter coat. Six untreated animals served as controls. High serum concentrations of melatonin were found in the treated animals throughout the study period (182.2 and 38.5 pg ml−1 in April and August, respectively). The high serum concentrations of prolactin in April and May (18.9 and 15.7 ng ml−1, respectively) in the controls were not seen in the treated animals. Testicular regression, judged by width of the testis and stage of spermatogenesis, was slowed after melatonin administration. Testis width from April to June and stage of spermatogenesis in May and June were greater in the treated animals than in the controls; however, from July onwards the differences between the two groups were no longer significant. Serum concentrations of testosterone remained low (below the detection limit in most cases) in both treated and control animals throughout the study period. Melatonin treatment also affected moulting, which was assessed by changes in the number of growing and mature underfur hairs per bundle and the percentage of bundles with a growing guard hair. The treated animals showed a more rapid shedding of mature underfur hairs and growth of new underfur hairs 6 weeks after melatonin implantation compared with the controls. During the study period, there were significantly more growing underfur hairs per bundle in May to August (4–9 per bundle) and a lower percentage of bundles with a growing guard hair in May and August (38 and 74%, respectively) in the treated animals compared with controls (1–4 per bundle; 79 and 93%), indicating that melatonin treatment stimulated the initiation of growth of underfur hairs and inhibited that of guard hairs. Although melatonin administration in March slowed testicular regression and maintained an unchanged winter coat in the initial stage, earlier melatonin implantation may delay testicular regression and spring moult more effectively in raccoon dogs.

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G. Lagerkvist, E. J. Einarsson, M. Forsberg and H. Gustafsson

Summary. Plasma concentrations of oestradiol-17β and progesterone were studied in yearling mink females. The blood samples were collected from 2 March until 13 April in females not subjected to mating and in females mated on two consecutive days, early or late in the breeding season, or with 8–9 days between matings. Peaks in oestradiol-17β were recorded on the day of first mating, in relation to the second wave of growing follicles, and in early April, around the time when implantation should have occurred. Significant rises in progesterone were recorded from 17 to 21 March and were slightly later in females mated late in the season.

Histological studies of ovaries from unmated females revealed that the number of 'active' follicles exceeded the number of degenerated or luteinized follicles until 7 April, after which the number of degenerated follicles increased rapidly. Degeneration was followed by luteinization. On 15 April, ovaries were collected from two females having 15 luteinized follicles each. These females had increased plasma concentrations of progesterone.

These studies indicate that, in female mink, peaks in oestradiol-17β coincide with the first mating as a result of the copulatory act and that unmated females appear to experience a luteal phase in the absence of ovulation.

Keywords: oestradiol; progesterone; reproduction; mink; ovary; steroids

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M. Forsberg, J. A. Founger, P. O. Hofmo and E. J. Einarsson

Summary. 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.

Keywords: light; melatonin; reproduction; silver fox; male

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M. Forsberg, J. A. Fougner, P. O. Hofmo, Malgorzata Madej and E. J. Einarsson

Summary. Six silver fox males were exposed to short days (6L:18D) from February, when the testes were fully developed, until June 1986 (Group 6L). Eight males maintained in natural daylight served as controls (Group N). Histological sections from the testes of 2 males in Group 6L killed in June indicated full spermatogenic activity. Three blue fox vixens inseminated the following year with semen collected and frozen in June from 3 males in Group 6L failed to produce litters. One possible explanation for the reproductive failures could have been that the high environmental temperatures in June influenced semen quality. There was no significant difference (P > 0·05) in LH release in response to GnRH stimulation in June, but testosterone response to LH release was significantly higher (P < 0·01) in animals subjected to a restricted photoperiod, demonstrating that testicular testosterone production was maintained longer than in control animals. Two males in Group 6L were retained in 6L:18D from June until December 1986 and then exposed to natural daylight until the end of the study in May 1987 (Group 6L:6L:N). These males started to shed their winter coat and showed clinical signs of testicular regression in December, i.e. after ∼11 months exposure to 6L:18D. The 2 remaining males in Group 6L were moved to cages with natural daylight in June 1986, where they were kept until the end of the experiment (Group 6L: N:N). These males displayed testicular regression soon after the change in photoperiod but maintained their capacity for testicular redevelopment during the following breeding season.

Five males from Group N were moved from natural daylight to 6L:18D in June 1986, when the testes were fully regressed. The animals were kept in 6L:18D until December 1986 and then exposed to natural daylight until the end of the study (Group N:6L:N). The 3 remaining males in Group N continued to serve as controls (Group N: N:N). Six blue fox vixens inseminated in the natural breeding season with semen collected and frozen in December 1986 from 4 males in Group N:6L:N conceived with an average litter size of 8·8 ± 1·7. Plasma concentrations of LH in response to GnRH stimulation in October and November 1986 indicated no variation of the pituitary to respond by LH release. In contrast, total testosterone release was twice as high in animals subjected to a restricted photoperiod compared with control animals, indicating that short days had a stimulatory effect on testicular redevelopment.

The results demonstrate that artificial illumination can be used to increase the reproductive capacity of silver fox males.

Keywords: light; reproduction; silver fox; male; semen; testes

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E. Ropstad, M. Forsberg, J. E. Sire, H. Kindahl, T. Nilsen, Ø. Pedersen and L-E. Edqvist

Blood samples were collected from eleven 1.5 year old female reindeer three times a week from September 1992 until February 1993 and daily in October and November 1992. Blood samples were collected every third hour for a period of three weeks from two females that showed regular oestrous cycles. Plasma progesterone, oestradiol, LH and 15-ketodihydro-PGF were analysed to characterize variations in ovarian function. Reindeer are seasonally polyoestrous. Average duration of oestrous cycles was 19.4 days (range: 13–33 days). Short periods of high progesterone concentrations (4–8 days duration) occurred in five animals before the onset of regular oestrous cycles. The first regular cycle had a significantly longer luteal phase than did the following cycles. During luteolysis 15-ketodihydro-PGF was released in a pulsatile pattern. Maximal oestradiol concentrations preceded preovulatory LH peaks by about 3 h in the two frequently sampled animals. The duration of LH surges was 12 and 15 h, respectively.

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C Viñoles, M Forsberg, G B Martin, C Cajarville, J Repetto and A Meikle

This study tested whether the effects of a short period of nutritional supplementation given to ewes during the luteal phase on follicle development and ovulation rate is associated with an increase in circulating concentrations of FSH, glucose or metabolic hormones. Oestrus was synchronised with two prostaglandin injections given 9 days apart and the supplement consisted of corn grain and soybean meal. Corriedale ewes with low body condition were randomly assigned to 2 groups: the control group (C; n = 10) received a maintenance diet while the short-term supplemented group (STS; n = 10) received double the maintenance diet over days 9 to 14 of the oestrous cycle (day 0 = ovulation). Ovaries were examined daily by ultrasound and blood was sampled three times a day during the inter-ovulatory interval for measuring reproductive and metabolic hormones. On days 9, 11 and 14 of the oestrous cycle, half of the ewes from each group (n = 5) were bled intensively to determine the concentrations of glucose, insulin, IGF-I and leptin. Plasma FSH, progesterone, oestradiol and androstenedione concentrations were similar among groups. Dietary supplementation increased plasma insulin concentrations from the first to the sixth day of supplementation and increased glucose concentrations on the third day, compared with control ewes. Plasma leptin concentrations were higher in STS ewes from the second to the fifth day of supplementation. The pattern of IGF-I concentrations was similar among groups. In STS ewes, the nutritional treatment prolonged the lifespan of the last non-ovulatory follicle, so fewer follicular waves developed during the cycle. In STS ewes, increased concentrations of glucose, insulin and leptin one day before ovulatory wave emergence were associated with increased numbers of follicles growing from 2 to 3 mm and with stimulation of the dominant follicle to grow for a longer period. We suggest that the mechanism by which short-term nutritional supplementation affects follicle development does not involve an increase in FSH concentrations, but may involve responses to increased concentrations of glucose, insulin and leptin, acting directly at the ovarian level. This effect is acute, since concentrations of all three substances decrease after reaching peak values on the third day of supplementation. The status of follicle development at the time of maximum concentrations of glucose and metabolic hormones may be one of the factors that determines whether ovulation rate increases or not.

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A. J. Smith, M. Mondain-Monval, K. Andersen Berg, P. Simon, M. Forsberg, O. P. F. Clausen, T. Hansen, O. M. Møller and R. Scholler

Summary. Melatonin administration to male blue foxes from August for 1 year resulted in profound changes in the testicular and furring cycles. The control animals underwent 5-fold seasonal changes in testicular volume, with maximal values in March and lowest volumes in August. In contrast, melatonin treatment allowed normal redevelopment of the testes and growth of the winter coat during the autumn but prevented testicular regression and the moult to a summer coat the following spring. At castration in August, 88% of the tubular sections in the testes of the controls contained spermatogonia as the only germinal cell type, whereas in the treated animals 56–79% of sections contained spermatids or even spermatozoa. Semen collection from a treated male in early August produced spermatozoa with normal density and motility.

Measurement of plasma prolactin concentrations revealed that the spring rise in plasma prolactin values (from basal levels of 1·6–5·4 ng/ml to peak values of 4·1–18·3 ng/ml) was prevented; values in the treated animals ranged during the year from 1·8 to 6·3 ng/ml. Individual variations in plasma LH concentrations masked any seasonal variations in LH release in response to LHRH stimulation, but the testosterone response to LH release after LHRH stimulation was significantly higher after the mating season in the treated animals, indicating that testicular testosterone production was maintained longer than in the controls.

The treated animals retained a winter coat, of varied quality and maturity, until the end of the study in August.

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M Muñoz-Gutiérrez, P A Findlay, C L Adam, G Wax, B K Campbell, N R Kendall, M Khalid, M Forsberg and R J Scaramuzzi

An experiment was carried out to determine the pattern of follicular expression of mRNAs for aromatase, IGF-I receptor (IGF-IR), IGF-binding protein (IGFBP)-2, -4 and -5, leptin and the long form of the leptin receptor (Ob-Rb) in ten ewes infused with human recombinant leptin (n = 5; 1 μg/h) or saline (n = 5) for 72 h in the luteal phase of the oestrous cycle. At the end of infusion a follicular phase was induced with a luteolytic dose of a prostaglandin F2α analogue and the ovaries were collected 32 h later. One ovary from each ewe was serially sectioned at 10 μm using a cryostat at −20 °C. All follicles >1 mm in diameter were counted and probed with specific oligoprobes for aromatase, IGF-IR and IGFBP-2, -4 and -5 and specific riboprobes for leptin and Ob-Rb. Leptin mRNA was detected in theca and granulosa cells and Ob-Rb mRNA was detected only in granulosa cells, of some, but not all antral follicles. Leptin doubled the number of follicles with a diameter ≥3.5 mm (1.0 ± 0.36 (s.e.m.) vs 2.4 ± 0.24; control vs leptin; P < 0.02) but had no effect on the number of ≥1 < 3.5 mm follicles. Leptin had no effect on the number of follicles expressing aromatase mRNA but it decreased significantly the number of follicles expressing mRNA for IGF-IR (10.7 ± 0.79 vs 7.4 ± 0.81; control vs leptin; P < 0.05), IGFBP-2 (10.0 ± 0.82 vs 5.2 ± 0.87; control vs leptin; P < 0.05) and IGFBP-5 (5.2 ± 1.60 vs 1.2 ± 0.30; control vs leptin; P < 0.05). Leptin increased the diameter of IGFBP-2 mRNA-positive follicles (1.5 ± 0.15 vs 2.2 ± 0.31 mm; control vs leptin; P < 0.05) and increased follicular mRNA expression for IGFBP-2 (0.30 ± 0.021 vs 0.39 ± 0.027 arbitrary units; control vs leptin; P < 0.05) and IGFBP-5 (0.46 ± 0.019 vs 0.25 ± 0.053 arbitary units; control vs leptin; P < 0.05). The mRNA for IGFBP-4 was detected in the theca of only two follicles from the control group. Leptin increased the number of follicles expressing Ob-Rb mRNA (0.25 ± 0.25 vs 1.40 ± 1.17; control vs leptin; P < 0.05) but had no effect on the number expressing leptin mRNA. Leptin decreased plasma concentrations of oestradiol (P < 0.05) and increased concentrations of FSH (P < 0.001) and insulin (P < 0.001), with no effect on glucose concentrations. These data show that: (i) ovine granulosa cells express mRNA for Ob-Rb and leptin and (ii) leptin increased the number of follicles ≥3.5 mm. Furthermore, the data suggest that suppression of oestradiol production by leptin is not mediated by inhibition of aromatase gene expression. Finally, the data indicate that the action of leptin in ovarian follicles is mediated by the IGF system, because leptin increased mRNA expression of IGFBP-2 and -5. Leptin also decreased the number of follicles expressing IGF-IR and IGFBP-2 and -5. We suggest that these actions of leptin on the IGF system decrease the bioavailability of IGF-I, resulting in decreased oestradiol production.