Role of melatonin and circadian rhythms in seasonal reproduction in rams
Lincoln, G.A.; Almeida, O.F.; Arendt, J.
Journal of Reproduction and Fertility. Supplement 30: 23-31
1981
ISSN/ISBN: 0449-3087 PMID: 6962843 Document Number: 176045
In the ram, changes in daylength influence testicular activity by modifying the release of LHRH, and thus the gonadotropins. To investigate the nature of this response, the hourly fluctuations in the circulating levels of prolactin, LH and melatonin were measured in rams kept under various artificial lighting conditions. In the 1st of 2 experiments, 8 Soay rams (4 from which the superior cervical ganglia had been removed and 4 intact controls) were exposed to alternating 16-wk periods of short days (8-h light:16-h darkness) and long days (16L:8D) for over 2 yr, and blood samples were collected hourly for 25 h on 2 occasions. The light regime resulted in marked testicular and endocrine changes in the controls but not in the ganglionectomised rams, which had low or undetectable levels of melatonin ( less than or equal to 33pg/ml) and an unusual diurnal rhythm in prolactin. In the 2nd experiment, 8 intact Soay rams were exposed to an ahemeral light regime of 8L:28D for 16 wk; at the end of this period, blood samples were collected hourly for 52 h, and were assayed for prolactin. During the pretreatment period of long days (16L:8D), the testes regressed fully. During the 16 wk of 8L:28D, redevelopment occurred, but the growth of the testes was slow compared with that normally occurring under 8L:16D. The prolactin profiles showed evidence of circadian rhythm in hormone secretion, with a correlation of the timing and duration of the rhythm with the degree of testicular development. These combined results support the idea that the photoperiodic response in the ram involves an interplay between the secretory activity of the pineal gland, and a light/dark-entrained circadian mechanism in the brain.