Diazepam and melatonin effects upon circadian variation of cultured murine myocardiocytes

We determined that melatonin and diazepam affected the persisting rhythm in contraction rates in cultured murine myocardiocytes. The effect of different concentrations of melatonin (10^-4M, 10^-6M and 10^-8M) and diazepam (10^-4M, 10^-6M and 10^-8M) was tested at six different times of day 4h apart in continuous light allowing a condition free-running from the alternation of light and darkness. Circadian variations with graded concentrations of melatonin or diazepam were observed by the cosinor fit of a 24h cosine function and the rejection of the zero-amplitude (no-rhythm) assumption. Melatonin and diazepam induced decreases in time structure or chronome-adjusted averages (MESORs) and amplitudes (measures of extent of predictable change), and generally delayed acrophases (peak time) by several hours. At the highest melatonin concentration (10^-4M), the circadian rhythm could not be detected. In combination, melatonin and diazepam also reduced the MESOR, with phase delays similar to treatment with diazepam alone. The free-running circadian rhythm in peripheral tissue, in the beating of murine myocardiocytes in vitro, is under endogenous coordination, affected by melatonin and diazepam. In doses that are physiological, in the sense that effects occur within the usual range of variability, melatonin and diazepam can affect characteristics of a persisting circadian rhythm, with altered MESOR, amplitude and/or phase.