Effect of melatonin on the vestibulocollic reflex in humans

Melatonin has been reported to decrease nerve activity of the medial vestibular nuclei in rats. The purpose of the present study was to determine if melatonin alters the vestibulocollic reflex (VCR) in humans. Based on our previous findings with melatonin's affect on the vestibulosympathetic reflex (VSR), we hypothesized that exogenous melatonin ingestion would attenuate the VCR. To test this hypothesis, vestibular evoked myogenic potentials (VEMP) were recorded in 10 healthy subjects (26 ± 1 yr; 4 male, 6 female) before and 45 min after ingestion of 3 mg melatonin. Additionally, we recorded VEMP in 8 healthy subjects (25 ± 1 yr; 1 male, 7 female) who underwent the same experiments but without melatonin to serve as time controls. Melatonin did not alter the timing of the first VEMP peak (13 ± 0.4 and 14 ± 0.3 ms, pre and post melatonin, respectively) nor the second VEMP peak (21 ± 0.6 and 21 ± 0.7 ms, pre and post melatonin, respectively). More importantly, VEMP interpeak amplitude was not altered by melatonin (before: 22 ± 5 vs. after: 23 ± 5 a.u.). Comparable results were observed in the control group (before: 26 ± 6 vs. after: 26 ± 7 a.u.) These data indicate that melatonin does not alter the VCR in humans. In conjunction with our previous findings demonstrating an attenuated VSR with exogenous melatonin, the present findings suggest that melatonin's affect on the VCR and VSR are different in humans. NIH HL077670 , NASA Space Fellowship