Nimodipine potentiates the light-induced suppression of melatonin

In mammals the phase shifting response of the circadian clock to light can be enhanced by administration of the calcium channel antagonist nimodipine. In the present study we assessed the potential for nimodipine to affect th e responsiveness of the human circadian clock to light by measuring the lig ht-induced suppression of melatonin levels in plasma. Seven healthy young s ubjects (3M, 4F, 27.3 +/- 1.8 years old) were admitted on four occasions to the Clinical Research Center at Northwestern University Medical School. Bl ood was collected during the night to assess the effect of nimodipine (30 m g, orally, 01:30 h) on plasma melatonin levels in the presence or absence o f light (500 lux, 2-3 am). Melatonin levels in plasma were measured by radi oimmunoassay. Exposure to light for 1 h suppressed melatonin levels in plas ma by nearly 38% relative to samples obtained at the same time in the absen ce of light (P = 0.013). Nimodipine administration did not modify plasma me latonin levels. However, combined treatment with nimodipine and light suppr essed melatonin levels in plasma by 59%. Levels of plasma melatonin were si gnificantly lower following treatment with nimodipine and light than follow ing treatment with placebo/light (P = 0.014). Thus, the calcium channel ant agonist nimodipine potentiated the suppressive effect of light on melatonin levels in plasma. These results suggest that the calcium channel antagonis t nimodipine may also potentiate the response of the human circadian clock to light, and might thus be useful in combination with phototherapy for the treatment of sleep and circadian rhythm disorders. (C) 1999 Elsevier Scien ce Ireland Ltd. All rights reserved.