Effects of sympathicolytic and sympathicomimetic drugs on plasma immunoreactive melatonin in the domestic pig.

Plasma immunoreactive melatonin was monitored for 110 hr in sexually immature female domestic pigs kept under 12L:12D light conditions and treated with two sympathicolytic (alpha-methyl-p-tyrosine, propranolol) and three sympathicomimetic (clorgyline, isoproterenol, dobutamine) drugs. Mean levels of plasma melatonin in the control pigs showed significant diurnal changes with the highest values during the scotophase. In the individual gilts the diurnal changes of plasma melatonin were markedly irregular; melatonin increases were not consistently noted at night, and melatonin levels during light and darkness varied considerably between days. The lack of consistency in the diurnal patterns of plasma melatonin in individual pigs during consecutive days explains the large inter-individual differences observed in previous short-term studies. Administration of alpha-methyl-p-tyrosine resulted in the decrease in both daytime and nighttime melatonin levels, compared with the control group, but it did not eliminate the increases in melatonin during the night. Treatment with other drugs did not significantly change the level of plasma melatonin compared with the control group, however they had modulatory effects on melatonin rhythm. The results suggest that the plasma melatonin in the domestic pig is controlled by the adrenergic nervous system, but the pharmacology of the secretion and/or metabolism of this pineal hormone is different than described in other species.