Seasonal relationship between pineal hydroxyindole-O-methyltransferase (HIOMT) activity and reproductive status in the pony

Abstract

Pony pineal glands and female reproductive tracts were collected monthly for 1 year from a local slaughterhouse. Pineal gland weights did not change significantly throughout the year. Pineal gland tissue homogenates were assayed for hydroxyindole-O-methyltransferase (HIOMT) activity with N-acetylserotonin as the primary substrate. The greatest HIOMT activity was obtained with N-acetylserotonin as substrate, but three other related 5-OH indole substrates (5-hydroxytryptophol, serotonin, and 5-hydroxy-2-methylindole) were also methylated. HIOMT activity with all substrates was highest during the late fall and winter (November–February) and lowest during the spring, summer, and early fall (April–October). Percentage of mares ovulatory was inversely related to pineal HIOMT activity, except that there was a 1- to 2-month lag phase. These findings are consistent with a hypothesized antigonadal activity of both melatonin and the pineal in mammals.