SEASONAL RELATIONSHIP BETWEEN PINEAL HYDROXYINDOLE-O-METHYLTRANSFERASE (HIOMT) ACTIVITY AND REPRODUCTIVE STATUS IN THE PONY

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. © 1979.