Effects of melatonin on spontaneous electrical activity of neurons in rat suprachiasmatic nuclei: an in vitro iontophoretic study.

Circadian rhythms, endogenously generated in suprachiasmatic nuclei (SCN), seem to be under the direct influence of melatonin. Therefore, the effect of iontophoretically applied melatonin on electrical activity of SCN neurons was investigated in vitro. Usually, melatonin had an inhibitory effect. In the 3-h periods before (2.00-5.00 p.m.) or after (5.00-8.00 p.m.) the light-dark transition the percentage of SCN neurons sensitive to melatonin was very high (80% and 100%, respectively). However, efficacy of melatonin was low in the periods preceeding (20%) and following (33%) this 6-h time interval.