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Melatonin was administered intravaginally in Silastic tubing to adult and prepubertal ewes. In Exp. 1, ewe lambs (born early March) were given intravaginal melatonin implants at a mean age (+/- s.e.m.) of 7.5 +/- 0.1 weeks (Group E, N = 10) or 19.4 +/- 0.2 weeks (Group L, N = 10). The third group (Group C, N = 10) received empty implants. In Exp. 2 mature ewes were given implants on 13 May (Group E, N = 10) or 18 July (Group L, N = 10) or received empty implants (Group C, N = 10) on one of these two dates. Blood samples were taken twice weekly for progesterone assay. In Exp. 1 the mean age (+/- s.e.m.) at puberty (progesterone greater than 2 nmol/l for two consecutive samples) was 35.4 +/- 0.8 weeks. Puberty was advanced by 5.2 weeks in Group L lambs, occurring at a mean age of 30.2 +/- 0.7 weeks (P less than 0.001). In Group E lambs the timing of puberty was unaltered, occurring at a mean age of 34.8 +/- 0.6 weeks. Mature ewes in Group L (Exp. 2) showed increased incidence of ovarian activity (9/10 ewes cycling by 26 September) compared with the control ewes (1/10) (P less than 0.001), but there was no effect in Group E ewes (3/10). The results demonstrate that continuous melatonin administration to adult and prepubertal ewes can mimic the effect of short days in terms of the reproductive response, and that the present and previous exposure to melatonin is critical in determining the response.

Effect of intravaginal implants of melatonin on the onset of ovarian activity in adult and prepubertal ewes