Melatonin formation in different parts of the guinea-pig pineal complex as assessed over 24 hours

There is morphological evidence that the pineal gland is not a uniformly built organ but rather a complex. In the guinea-pig the gland is 6--7 mm in length and dumbbell-shaped, the proximal part coming into intimate contact with central commissural fibres. The aim of the present 24-hour study was to examine in male guinea-pigs whether the proximal, intermediate and distal areas of the gland are involved in melatonin formation and to compare their rhythmicities. Levels of melatonin in serum and the whole pineal gland as assessed by RIA show day/night ratios of 1 : 1.25 and 1 : 3.3, respectively. Serotonin N-acetyltransferase activity was found to exhibit ratios of 1 : 1.75 (Experiment I) and 1 : 4.4 (Experiment II). All three pineal regions are involved in melatonin formation, and to the same extent. Whether the rhythms in the different regions are identical could not be clarified as the curves obtained exhibited striking oscillations and the day/night differences were rather small. As the extrapineal portion of the habenular commissure was found to contain melatonin there is a possibility that melatonin may be taken up when the commissural fibres pass through the pineal parenchyma.