Delta-sleep-inducing peptide and two of its analogs reduce nocturnal increase of N-acetyltransferase activity in rat pineal gland.

Serotonin N-acetyltransferase, an enzyme of the pineal gland, converts serotonin to N-acetylserotonin. The activity of this enzyme is induced by norepinephrine in the evening to reach high levels during the dark phase. Delta-sleep-inducing peptide, a humoral sleep factor, also seems to affect circadian rhythms. Intravenous injection of this peptide or either of two of its analogs in the evening significantly reduced the increase of N-acetyltransferase 4 h later. The dose-response relationship of the peptides showed an inverted U-shaped pattern with the active dose about 30 nmol/kg. The effect appears to be dependent on the time of day of administration, as injections in the morning did not change the enzymatic activity. These findings indicate that delta-sleep-inducing peptide (and two of its analogs) can affect enzymatic activities and that these influences probably vary throughout a time period of 24 h.