Diurnal variation and binding characteristics of melatonin in the mouse brain and gastrointestinal tissues.

1. Compared to night time values, levels of melatonin (M), determined by specific RIA, were lower in the brain and the duodenum-jejunum segment of mice sacrificed in mid-photophase. In other parts of the gut no significant diurnal difference was observed. 2. Highest daytime levels were detected in the stomach (569 pg/ml), lowest in the brain (62 pg/ml). 3. Preliminary studies indicate the presence of specific binding sites for [125I]iodo-melatonin in the colon, ileum, jejunum, stomach and brain. These sites, which exhibit an affinity of about 1 nM, may be involved in mediating the gastrointestinal and central effects of melatonin in this species.