Hourly Fluctuations in the Blood Levels of Melatonin, Prolactin, Luteinizing Hormone, Follicle-Stimulating Hormone, Testosterone, Tri-Iodothyronine, Thyroxine and Cortisol in Rams Under Artificial Photoperiods, and the Effects of Cranial Sympathectomy

Abstract

Eight adult Soay rams (four control and four cranially sympathectomized by removal of the superior cervical ganglia) were exposed to alternating 16-week periods of short days of 8 h light: 16 h darkness (8L : 16D) and long days (16L : 8D) for more than 3 years, and the changes in the diameter of the testes were recorded. Once during short days and once during long days blood samples were collected hourly for 28 h, and the concentrations of melatonin, prolactin, FSH, LH, testosterone, tri-iodothyronine (T3), thyroxine (T4) and cortisol were measured by radioimmunoassay. In the control rams the testes were reduced in size during long days compared to short days, and the blood concentrations (24-h mean values) of FSH, LH and testosterone were decreased. The levels of prolactin were much increased, while there was no significant change in the mean levels of melatonin, T3, T4 and cortisol. During both photoperiods there was considerable hour-to-hour variation in all eight hormones indicative of episodic secretion, as well as a consistent variation related to the time of day which was most pronounced for melatonin and T3. There was a clear difference in the daily profile of plasma melatonin levels between short and long days. In the superior cervical ganglionectomized rams there were no significant changes in the size of the testes or in the hormone titres between short and long days. Compared to the controls the plasma levels of LH, FSH, testosterone and prolactin were in the intermediate range. Some consistent diurnal variation was evident in the levels of all the hormones measured, with a pattern similar to the controls for a few of the hormones (e.g. T3) but quite different for others (e.g. melatonin).

Inhibition of reproduction in rams by long daylengths and the acute effect of superior cervical ganglionectomy

“…Melatonin is one of the principal products of the pineal which is thought to influence the secretion of the anterior pituitary gland and affect reproduction (Wurtman & Moskowitz, 1977;Turek & Campbell, 1979;Reiter, 1980). In the sheep, there is a marked change in the circulating levels of melatonin from day to night and this daily pattern of secretion under long days is different from that under short days (Rollag et al, 1978;Lincoln & Short, 1980;Arendt et al, 1980;Lincoln, Almeida, Klandorf & Cunningham, 1982). The regulation of the pattern of melatonin secretion may thus be involved in the photoperiodic responses.…”

Seasonal variations of LH, prolactin, androstenedione, testosterone and testicular FSH binding in the male blue fox (Alopex lagopus)

“…Changes in the secretion of gonadotrophins in response to changes in photoperiod are seen in many seasonal breeders (Ortavant, Mauleon & Thibault, 1964;Lincoln & Short, 1980 (Lincoln, Almeida & Arendt, 1981;Lincoln et al, 1982). Melatonin implants in sexually inactive male mink have been shown to advance testicular redevelopment (Allain et al, 1981).…”

Testicular function and semen characteristics of Awassi rams treated with melatonin out of the breeding season

“…The exogenous photoperiodic signal is translated through a neuroendocrine pathway to the daily alteration of melatonin level. Exogenous administration of melatonin by slow-release implants may be a reliable method for controlling reproductive rhythm (Lincoln et al, 1982;Arendt, 1995). Melatonin can be used alone, or combined with other hormonal treatments, after an artificial light (photoperiodic) treatment.…”