Response in peripheral plasma melatonin to photoperiod change and the effects of exogenous melatonin on seasonal quiescence in the tammar, Macropus eugenii
in ReproductionSummary. During the light phase of each of 3 photoperiods tested, plasma melatonin concentrations were < 16 to 62 pg/ml and during the dark phase they were 31 to 169 pg/ml. When the photoperiod to which the tammars were exposed was altered from 15 h light: 9 h dark to 12L:12D the onset of the nocturnal rise in melatonin was advanced from the first day, thereby extending its duration, and the females gave birth 32 ± 0·4 (mean ± s.e.m.) days later. To test whether melatonin mediated this effect of photoperiod change, tammars in a second group were injected s.c. with melatonin (400 ng/kg, N = 6) or the arachis oil vehicle (N = 6), 2·5 to 2·25 h before dark during 15L:9D for 15 days before exposure to 12·5L:11·5D. The melatonin injections mimicked the endogenous melatonin profile of 12L:12D and the melatonin-injected tammars gave birth 32 ± 0·8 days after the start of injections, which was the same as the interval from photoperiod change in Exp. 1 but was significantly different (P < 0·005) from the interval in the control group (46·0 ± 1·1 days). These results show that exogenous melatonin given 2·5 to 2·25 h in advance of the endogenous rise fully mimics the response of the tammar to photoperiod change.
An official journal of
