Effect of cyclic changes in environmental lighting and ambient temperature on the daily rhythm in melatonin excretion by rats.
Melatonin excretion was measured by radioimmunoassay in 6 h or 12 h urine specimens from individual control rats and from animals previously blinded by bilateral orbital enucleation. Among sighted rats, the rate of melatonin excretion was greatest during the daily 12 h of darkness (1.44 +/- 0.06 ng/12 h dark period vs. 0.53 +/- 0.07 ng/12 h light period; P less than 0.001). Moreover, greater quantities of melatonin were excreted in the latter half of the dark period than in the first half (e.g. 0.55 +/- 0.08 ng/first 6 h vs 0.97 +/- 0.05 ng/second 6 h; P less than 0.001). When the onset of the daily light period was shifted forward by 12 h, 5--7 days were needed for the daily rhythm in melatonin excretion to become re-entrained to the new light--dark cycle. Among blinded rats, the rate of melatonin excretion also varied rhythmically; however, the rhythm was neither synchronized with the light-dark cycle nor influenced by alterations in the lighting schedule. Similarly, artificial cycles in environmental temperature were not effective in entraining the daily rhythm in melatonin excretion among blinded rats.
D000818:Animals; D002940:Circadian Rhythm; D008029:Lighting; D008297:Male; D008550:Melatonin / Q000097:blood / Q000652:urine*; D051381:Rats; D012683:Sensory Deprivation; D013696:Temperature
D008550:Melatonin