Season-Related Differences in the Biosynthetic Activity of the Neonatal Chicken Pineal Gland

Influence of the season of hatch on the functional characteristics of the pineal gland was examined in neonatal Hi-Line male chickens. The pineal glands from 2-day-old birds hatched in summer and winter, and kept from the day of hatch in artificial lighting conditions (12L:12D), were isolated under dim red light in the middle of the day or night. The pineal glands were analyzed to characterize their melatonin biosynthetic activity: (1) expression of the Arylalkylamine- N-acetyltransferase and Hydroxyindole-O-methyltransferase genes, encoding the final two enzymes of the melatonin biosynthesis pathway; (2) the activity of AA-NAT and HIOMT; and (3) the content of the main substrates of this pathway, tryptophan (TRY) and serotonin (5-HT). Daily changes in pineal AA-NAT activity were observed in chickens hatched in both seasons, with a more pronounced nocturnal increase in summer. In contrast, the level of Aa-nat gene expression, although exhibited the same nocturnal/diurnal pattern in both seasons, was much lower in the summer. The activity of HIOMT was season- and daytime-independent. In "winter" chickens the pineal content of 5-HT was low and stable, while in "summer" birds it was correlated with levels of AA-NAT activity and Aa-nat gene expression. TRY content was very high and exhibited neither daily nor seasonal changes. The pineal gland of newly hatched chickens kept in controlled 12L:12D conditions exhibits daily variations in melatonin biosynthetic activity influenced by the season. This suggests a maternal effect on the perinatal/postnatal development of the circadian clock residing in the chicken pineal gland.