The pineal gland is known to synthesize numerous indolamines. Since delta-sleep-inducing peptide (DSIP, a tryptophan nonapeptide) is found in the pineal gland, its effect on the secretion of indolamines was investigated. DSIP stimulated melatonin (MEL), 5-methoxytryptophol (5-ML) and serotonin (5-HT) synthesis and release, whereas it did not affect pineal cyclic AMP levels. The stimulatory effect of DSIP on MEL secretion was dose dependent between 5 X 10–6 and 10–4M, whereas the minimal effective concentration of DSIP on 5-ML secretion was higher than 10–5M. The effect of DSIP (10–4M) was compared to the effect of isoproterenol (ISO, 10–6M) on MEL and 5-HT release. ISO stimulated MEL secretion and concomitantly decreased 5-HT release. With regard to kinetic characteristics, the effect of DSIP (10–4M) on MEL release was faster and of shorter duration than the effect of ISO (10–6M; 2and 4 h, respectively). At 10–4 M, DSIP potentiated the ISO-induced increase of MEL secretion. The DSIP-stimulated release of MEL was not significantly altered when the pineal glands were treated with 10-5M propranolol (a β-adrenergic antagonist), 10–5M prazosin (an α1-adrenergic antagonist) or 10–5M naloxone (an opioidergic antagonist). This study demonstrates that the DSIP-induced secretion of indolamines from rat pineal glands may not be elicited through the well-known noradrenergic or opioid systems.

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