Impact of light pulses on 6-sulphatoxymelatonin rhythms in rats

The acute and residual entraining effects of a 15 min pulse of light on rat pineal function in individual animals were investigated using the excretion rate of the melatonin metabolite, 6-sulphatoxymelatonin, as an index of melatonin production. In animals maintained in a 12-hr L:12-hr D photoperiod (lights off 1800), 15 min light exposure at 2000 had no significant effect on the metabolite rhythm. Light pulses at 2200 (i.e., after the onset of excretion) decreased the melatonin metabolite excretion rate for 3 hr, after which the excretion rate increased to normal. In contrast, pulses at 2400 and 0200 suppressed 6-sulphatoxymelatonin for the rest of the night such that total excretion was 58 +/- 8% and 66 +/- 6% of the amount excreted on the night before the pulse. A light pulse at 0400 had no significant effect on the 6-sulphatoxymelatonin excretion when compared with the first night collection. When the 6-sulphatoxymelatonin rhythm was assessed on the night following light pulses at 2000 and 0400, the onset of metabolite excretion was unaffected (0.2 +/- 0.75-hr advance and 0.75 +/- 0.15-hr delay, respectively). In contrast, pulses at 2200, 2400, and 0200 resulted in significant delays in the onset of 2.4 +/- 0.2-hr, 1.5 +/- 0.1-hr, and 2.1 +/- 0.3-hr, respectively (P < 0.05). The offset of metabolite excretion was not significantly affected by a prior light pulse, except in animals receiving light treatment at 0400 when the offset was advanced by 1.5 +/- 0.3 hr.(ABSTRACT TRUNCATED AT 250 WORDS)