Daily variation in the content of indoleamines, catecholamines and related compounds in the pineal gland of Syrian hamsters kept under long and short photoperiods.

This study examined the diurnal changes in the content of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), N-acetylserotonin (NAS), melatonin, 5-methoxytryptophol (5-ML), noradrenaline (NA), dopamine (DA), and 3, 4-dihydroxyphenylacetic acid (DOPAC) in the pineal gland of Syrian hamsters kept under long (14L:10D) and short (10L:14D) photoperiods. The nocturnal increase in NAS and melatonin levels was dependent upon the prevailing photoperiod, with a prolonged duration when the night lengthened. In both photoperiods, NAS and melatonin contents increased several hours after the onset of darkness, and, in animals kept in short photoperiod, the levels of both compounds began to decrease before light onset. On the contrary, decreases were noted in 5-HT, 5-HIAA, and 5-ML contents during the night, which was directly proportional to the dark phase. 5-HTP levels did not show a rhythmic variation. Correlations between the mean values of 5-HT-related compounds showing daily rhythms were very high when group means were compared, but they decreased when values from individual animals were considered. In addition, when correlations were calculated on per-animal basis during the night phase, a weak negative correlation was found for 5-HT vs NAS and 5-HT vs melatonin, although the correlation of 5-HT with positively 5-HT-correlated compounds (5-HIAA and 5-ML) continued to be high. These results indicate that the nocturnal increase in the N-acetyl transferase activity is the major factor generating the rhythm of pineal 5-HT content, but that other photoperiod-dependent mechanisms (i.e., 5-HT synthesis or release) seem to be also implicated. On the other hand, this study shows that NA content in the Syrian hamster pineal gland does not exhibit daily variations, although marked nocturnal increases in the levels of DA and DOPAC were evident. These results suggest the existence of parallel daily alterations in pineal catecholamine synthesis and release, and suggest a role for DA in the pineal activation at night.