Urinary 6-sulphatoxymelatonin excretory rhythms in laboratory rats: effects of photoperiod and light.

The excretion rhythm of the melatonin metabolite, 6-sulphatoxymelatonin, was determined in rats maintained on contrasting 14 h L:10 h D and 10 h L:14 h D photoperiods. The novel use of a high protein liquid diet together with an automatic urine collection system facilitated the monitoring of the onset, offset and total production of metabolite before, during and after a 57.5% reduction in the dark period (5.75 h and 8 h delay in lights off). In 14 L:10 D, melatonin metabolite excretion increased 2.7 +/- 0.2 h after lights off, whereas in 10 L:14 D, the onset occurred 5.1 +/- 0.2 h after lights off. Duration of melatonin metabolite excretion was not different between the two photoperiods. Reduction of the dark period by 57.5% delayed the appearance of melatonin metabolite excretion until 2.15 +/- 0.4h and 2.9 +/- 0.4h after darkness in both long and short photoperiods. Upon restoration of the original photoperiod on the third day, the onset of metabolite excretion was significantly delayed by 1.4 +/- 0.4 h (P < 0.05) in long day animals and 2.2 +/- 0.2 h (P < 0.05) in short day-length compared to the first day. This study highlights the utility of urinary 6-sulphatoxymelatonin determinations in rats and provides new information on the acute effects of extended evening light on pineal melatonin secretion.