EFFECTS OF MELATONIN IMPLANTS ON THE CIRCADIAN RHYTHM OF PLASMA MELATONIN AND PROLACTIN IN SHEEP.

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Abstract
The effects of implanting Silastic capsules containing melatonin on plasma melatonin and prolactin levels were investigated in pinealectomized (Ptimes;) and sham-operated sheep (SPtimes;). Prior to implanatation, melatonin was found in plasma samples obtained during the night period from SPtimes; sheep (mean value 150 pg/ml), but could not be (<25 pg/ml) detected in plasma samples obtained during the day in SPtimes; sheep or in any sample obtained during the night or day period in Ptimes; 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 SPtimes; 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 SPtimes; and P× 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.