GENETIC-VARIABILITY IN MELATONIN CONCENTRATIONS IN EWES ORIGINATES INITS SYNTHESIS, NOT IN ITS CATABOLISM

We investigated whether the genetic difference in plasma melatonin con centration in ewes was due to differences in the synthesis pathway fro m the pineal gland or in the catabolism of the hormone. Two groups of ewes [9 low (L) and 10 high (H)] were selected according to the breedi ng value of their mean nighttime plasma melatonin concentrations estim ated at winter and summer solstices. In response to an identical dose of melatonin administered intravenously at 9:00 AM, no differences bet ween groups were observed for any of the kinetic parameters: clearance rate, steady-state volume of distribution, terminal half-lives, and m ean residence times. In the second experiment, two series of frequent blood samples were performed, one in the middle of the dark. phase wit h samples taken every 5 min, and the other over 24 h with hourly sampl es. Highly significant differences between groups in nocturnal melaton in production rate were observed (L: 25.7 +/- 2.8 vs. H: 63.1 +/- 8.9 mu g kg(-1).h(-1), P < 0.01). Thus the genetic differences in plasma m elatonin concentrations in ewes originate in the synthesis pathway of the melatonin from the pineal gland rather than from differences in th e catabolism of the hormone.