Daily melatonin injections entrain the circadian change of synaptic ribbon number in the pineal organ of the Japanese newt.

The effect of melatonin on the number of synaptic ribbons (SR) in the pineal photoreceptor cells was examined in the Japanese newt using electron microscopy. Under a light-dark cycle of LD 12:12 (lights on at 07:00), a subcutaneous injection of 10 microg of melatonin at two times during the day (at 11:00 and 11:30) did not increase the number of SR. However, daily injections of 10 microg of melatonin in constant darkness at 11:00 for 7 days did reset the phase of the circadian rhythm in SR number. The time of melatonin injection, corresponding to the onset of subjective night, preceded the daily increase of SR number. These data suggest that although melatonin does not directly increase the SR number, daily injections of melatonin can entrain the pineal circadian clock responsible for the circadian change in SR number.