Abstract
Light acts in two ways to control the duration of the nocturnal melatonin rhythm. It inhibits the production of melatonin from the pineal gland and it entrains the underlying circadian rhythm generators located in the suprachiasmatic nuclei. To investigate the role of these two mechanisms under long and short days, four experiments were carried out using groups of adult Soay rams (n = 6-8). The animals were housed in individual pens in light-controlled rooms and entrained to long (LD 16:8) or short (LD 8:16) days for at least 8 wk. The treatments were as follows: (i) dark period extended by 4 h under long days (L dark-delay), (ii) dark period advanced by 4 h under long days (L dark- advance), (iii) dark period extended by 4 h under short days (S dark-delay), and (iv) dark period advanced by 4 h under short days (S dark-advance). Each treatment was given on a single day and the animals were subsequently maintained in, or transferred to, constant dim red light (DD) for 24 h. A control group (C) was run in parallel with each treatment group. Blood samples were collected every 30 min for 6-9 h during the dark-shift to monitor the light-induced changes in the secretion of melatonin, and during DD to monitor any phase shift in the endogenous rhythm (phase markers provided by onset or offset of melatonin secretion). L dark-delay resulted in a significantly (p < 0.01, ANOVA) later offset of the melatonin peak (3.4-h delay) with no phase shift of the onset of the rhythm under DD. L dark-advance resulted in a significantly (p < 0.01) earlier onset of the melatonin peak compared to C (2.1-h advance), and a mean phase-advance of 1.0 h in the offset (p < 0.05) of the rhythm under DD. S dark-delay did not affect the time of the offset of the melatonin peak, but caused a significant ( p < 0.05) phase delay in the onset of the melatonin rhythm under DD (2.2 h). S dark-advance caused no significant change in the time of the onset of the melatonin peak but resulted in a significant (p < 0.05) phase-advance in the offset of the melatonin rhythm under DD (1.6 h). Overall, the results show that in sheep, entrainment by the LD cycle determines the phase of the melatonin rhythm, while the inhibitory effect of light primarily dictates the change in duration of melatonin secretion between long and short days.