Implication of tryptophan in the stimulatory effect of delta-sleep-inducing peptide on indole secretion from perifused rat pineal glands.

We have recently demonstrated that delta-sleep-inducing peptide (DSIP) stimulates indolamine secretion from rat pineal glands. In the present study, we show that tryptophan (TRP), as well as DSIP, stimulate melatonin (MEL) and 5-methoxy-tryptophol (5-ML) secretion in a dose-dependent manner between 5 x 10(-6) and 10(-4) M. The kinetic characteristics of the MEL and 5-ML secretion and the response induced by the two substances were similar. The increase in MEL secretion in response to 10(-4) M DSIP was completely inhibited by pretreatment of the pineals with 10(-5) M phenanthroline (amino-peptidase inhibitor), suggesting that stimulatory effect of DSIP was due to TRP liberated by peptide degradation. This mechanism occurring in the pineal was confirmed using 10(-4) M para-chlorophenylalamine (TRP hydroxylase inhibitor), which reduced the pineal response to 10(-4) and 10(-5) M DSIP by 50 and 100%, respectively.