Evidence for 5-HT-containing intrinsic neurons in the teleost intestine.

The stomach and small intestine of seven species of teleost fish were examined with both fluorescence and ultrastructural histochemical techniques for demonstrating monoamines. Fluorescence histochemistry demonstrated large numbers of yellow fluorescent neurons in the myenteric plexus of the intestine, but not the stomach, of four species. Varicose fibres arising from these neurons ran in the myenteric plexus, circular muscle layer and submucosa. Ultrastructural histochemistry of tissue fixed with a modified chromaffin reaction showed that some neurons in the myenteric plexus contained biogenic amines in vesicles of 40--100 nm diameter. It is concluded that 5-HT-containing neurons are present in the intestine of some teleost species.