Effects of melatonin implants on the circadian rhythm of plasma melatonin and prolactin in sheep.

The effects of implanting Silastic capsules containing melatonin on plasma melatonin and prolactin levels were investigated in pinealectomized (Px) and sham-operated sheep (SPx). Prior to implantation, melatonin was found in plasma samples obtained during the night period from SPx sheep (mean value 150 pg/ml), but could not be (less than 25 pg/ml) detected in plasma samples obtained during the day in SPx sheep or in any sample obtained during the night or day period in Px sheep. Following implantation, a constant basal plasma melatonin level of about 165 pg/ml was established in all sheep with a superimposed nighttime rise in SPx animals suggesting no diminution of endogenous melatonin production during the dark period. Following melatonin treatment, there was a marked depression in plasma prolactin levels in both SPx and Px sheep. These results are interpreted to indicate that 1) there is no negative feedback of melatonin upon its own synthesis and release, 2) that there is no circadian change in the rate of metabolism of melatonin and 3) that constant melatonin availability in sheep caused a depression in plasma prolactin levels similar to that found following exposure of animals to a short day.