Evidence for the presence of two 24-h rhythms 180 degrees out of phase in the pineal gland of male Pirbright-White guinea pigs as monitored by counting "synaptic" ribbons and spherules.

Previous studies have shown that the "synaptic" bodies of mammalian pinealocytes are a heterogeneous group of organelles. Whereas "synaptic" ribbons (SR) exhibit a day/night rhythm with small numbers during daytime and high numbers at night, the so-called "synaptic" spherules (SS) show species-specific differences in their rhythmicity. In the present study the numbers of SR and SS were monitored in male guinea pigs over a full 24-h period at 4-hourly intervals (LD 12:12, lights on at 7:00). The results obtained show that the rhythms of SR numbers and SS numbers are out of phase by 180 degrees. SR numbers are small during daytime and high at night, whereas SS numbers are high during daytime and low at night. As there are no indications that SR are transformed into SS and vice versa and as SR and SS lie in different parenchymal areas it is suggested that they characterize different types of pinealocytes with differing rhythmicities. - Serum melatonin levels were low during daytime (30 pg/ml) and increased at night to reach a peak (84 pg/ml) at 24:00 h.