Daily rhythmicity of circulating melatonin is not endogenously generated in the horse

Melatonin secretion exhibits daily rhythmicity in many vertebrate species. In mammals, the rhythmicity is believed to result from two related processes: inhibition of pineal activity by environmental light during the day and stimulation of pineal activity by the master circadian clock at night. While circadian modulation of melatonin secretion under constant environmental conditions has been documented in numerous mammalian species, no extensive survey has been conducted. In this study, we investigated daily/circadian rhythmicity of plasma melatonin concentration, as well as of body temperature and of plasma sodium concentration, in horses maintained under a light-dark cycle and in extended darkness. All three variables exhibited robust 24-h rhythmicity under the light- dark cycle. Whereas rhythmicity of body temperature and sodium concentration persisted in extended darkness, rhythmicity of melatonin concentration did not. These findings strongly suggest that melatonin secretion in the horse is not under circadian control and is modulated only by environmental light.