EFFECT OF MELATONIN AND METHIONINE-ENKEPHALIN ON SURFACING RESPONSES IN GOLDFISH

Intracranial injection of 5 to 20 μg of methionine-enkephalin (M-ENK) increased surfacing responses in a dose-related manner in goldfish. This behavioral effect of M-ENK was completely inhibited by an injection of 8 μg naloxone, indicating that the surfacing response produced by M-ENK was mediated by naloxone-sensitive opiate receptors. The possibility of melatonin affecting opiate receptors directly, or indirectly through M-ENK was examined. The injection of melatonin caused surfacing responses and this effect was inhibited by naloxone. Although the behavioral effect of M-ENK was reduced by inhibition of melatonin synthesis by scotophobin A, the increased inhibition of melatonin synthesis did not cause increased inhibition of the M-ENK effect. It was also found that M-ENK did not affect the activity of the enzyme (hydroxyindole-O-methyltransferase) in vitro, which is responsible for the conversion of N-acetylserotonin to melatonin. The results from these experiments suggested that melatonin may directly (or indirectly through M-ENK) affect opiate receptors. © 1979.