EFFECTS OF SYMPATHICOLYTIC AND SYMPATHOMIMETIC DRUGS ON PLASMA-IMMUNOREACTIVE MELATONIN IN THE DOMESTIC PIG

Plasma immunoreactive melatonin was monitored for 110 hr in sexually i mmature female domestic pigs kept under 12L:12D light conditions and t reated with two sympathicolytic (alpha-methyl-p-tyrosine, propranolol) and three sympathicomimetic (clorgyline, isoproterenol, dobutamine) d rugs. Mean levels of plasma melatonin in the control pigs showed signi ficant diurnal changes with the highest values during the scotophase. In the individual gilts the diurnal changes of plasma melatonin were m arkedly irregular; melatonin increases were not consistently noted at night, and melatonin levels during light and darkness varied considera bly between days. The lack of consistency in the diurnal patterns of p lasma melatonin in individual pigs during consecutive days explains th e large inter-individual differences observed in previous short-term s tudies. Administration of alpha-methyl-p-tyrosine resulted in the decr ease in both daytime and nighttime melatonin levels, compared with the control group, but it did not eliminate the increases in melatonin du ring the night. Treatment with other drugs did not significantly chang e the level of plasma melatonin compared with the control group, howev er they had modulatory effects on melatonin rhythm. The results sugges t that the plasma melatonin in the domestic pig is controlled by the a drenergic nervous system, but the pharmacology of the secretion and/or metabolism of this pineal hormone is different than described in othe r species.