The role of the pineal gland in seasonality

Abstract

The life time reproductive performance of 2 flocks of Merino crossbred ewes pinealectomized at 7-60 days of age, and maintained in South Australia did not differ from that of sham-operated control animals kept in the same flocks. The pineal gland is therefore not a major determinant of reproductive success, but a role for the pineal in adjusting breeding activity to season is not excluded. It was confirmed that pineal denervation of adult ewes by cranial sympathectomy could have disruptive effects and evidence of a seasonal change in pineal function was obtained from studies of plasma levels of melatonin, a putative pineal hormone, and a pineal peptide with gonadotrophin releasing activity. That melatonin stems mainly, but not exclusively, from the pineal gland was confirmed by studies of urinary excretion values. Adult Merino ewes produced 39-90 micrograms melatonin/day, mostly (80%) during the dark period, whereas pinealectomized animals produced less than one-fifth this amount. A link was provided between the pineal gland and diet. Melatonin and other 5-methoxyindoles influenced the activity of alpha-chymotrypsin and may regulate the availability of precursor substances required for the synthesis of neurotransmitters important in reproduction.