Effect of methionine-enkephalin on xanthopore aggregation

The intracranial injection of an opioid antagonist (naloxone) caused aggregation of xanthophores in goldfish scales. This aggregating effect produced by naloxone was inhibited by an injection of methionine-enkephalin (M-ENK). M-ENK, when injected together with melatonin which normally produces aggregation of xanthophores, interfered with the effect of melatonin by inhibiting the aggregation. An injection of naloxone together with melatonin showed no difference in the aggregation from that of either naloxone or melatonin alone. The possibility of interaction between melatonin and M-ENK was discussed.