Light suppression of melatonin in the squirrel monkey (Saimiri sciureus)

This study examined plasma melatonin levels and the suppressant effect of light on melatonin production in the squirrel monkey. Monkeys were maintained on a 12:12 light-dark cycle (LD 12:12) with lights on from 07:00 to 19:00. Plasma levels of melatonin were determined by gas chromatography negative chemical ionization mass spectrometry. Melatonin levels at 00:00 (99.5 +/- 18.9 pg/ml) were significantly higher than at 02:00 (57.21 +/- 7.7 pg/ml; Student's t = 2.859; P less than or equal to 0.021). Baseline values at 02:00 were compared with levels at the same time of day after exposure to 2 hours of 200 lux of light (30.6 +/- 13.1 pg/ml), which caused an average suppression of 54.8% in melatonin levels. One animal did not show light suppression. Results indicated that the squirrel monkey suppressant response to light, as well as baseline values of melatonin, varied between animals.