Distribution of GABA-like immunoreactivity in myenteric plexus of carp, frog and chicken.

The distribution of GABA-like immunoreactivity was studied by means of indirect immunocytochemical methods in some lower vertebrate species (carp, frog, chicken). An immunoreactive network was revealed in the myenteric plexus of the alimentary canal of carp. GABA-positive nerve cells were attached closely to the fibres in the stomach. In other gut regions immunostained neurons were less frequent. Immunoreactive fibres often formed baskets on the surfaces of immunonegative neurons along the whole length of the alimentary canal. The number of immunopositive nerve fibres and pericellular baskets seemed to be lower in the mid- and hingut than in the foregut region. A similar distribution of GABA-immunoreactivity was revealed in the frog myenteric plexus. The ganglionated foregut region possessed a relatively dense GABAergic innervation. This part of the gut contained immunostained nerve cells and fibres, while the mid- and hindgut possessed only a scanty fibre system. Chicken exhibited an extensive immunoreactive plexus for GABA, although the GABA-stained perikarya were restricted mainly to the duodenum. Further regions of the small intestine were poor in immunoreactive cell bodies, which suggests a segmental origin and arrangement of GABAergic innervation within the plexus. In all three species studied, GABA-positive fibres run into the circular muscle layer. The varicosity suggests their influence on the movement of the smooth muscles through modifying the transmitter release of neighbouring terminals.