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

Abstract

The excretion rhythm of the melatonin metabolite, 6-sulphatoxymelatonin, was determined in rats maintained on contrasting14h L: 10h D and10h L: 14h 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). In14L: 10D, melatonin metabolite excretion increased2.7±0.2h after lights off, whereas in10L: 14D, the onset occurred5.1±0.2h 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 until2.15±0.4h and2.9±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 by1.4±0.4h(P < 0.05) in long day animals and2.2±0.2h(P < 0.05) in short day-length compared to the forst 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.