Distributional pattern and targets of GABA-containing neurons in the porcine small and large intestine.

In the rat and guinea-pig enteric nervous system, gamma-aminobutyric acid (GABA) has been shown to act as a neurotransmitter in interneurons innervating both excitatory and inhibitory motor neurons, thus modulating peristalsis and acid secretion. The present study reports on the distribution of GABA-containing neurons in the porcine intestine by the use of immunocytochemistry. Duodenal, jejunal, ileal and distal colonic segments were exposed in vitro to exogenous GABA (10(-8) M) prior to fixation and immunocytochemical staining to supplement endogenous stores of GABA. In contrast to the guinea-pig intestine, where GABA-immunoreactive (IR) nerve cell bodies were common in myenteric ganglia but very rare in the submucosa, in the pig intestine the nerve cell bodies expressing GABA-immunoreactivity were found predominantly in the ganglia and nerve strands of the inner submucous plexus, while a small number were also found in the myenteric plexus. Most of the neurons were small-sized and had a multidendritic uniaxonal appearance. Their varicose axonal processes protruded within the same ganglion to other nerve cells, including GABA-IR ones, or ran into connecting nerve strands towards neighbouring ganglia. Some of the bigger GABA-IR neurons in the myenteric plexus, however, displayed either a lamellar multidendritic uniaxonal or a bipolar appearance. The density of GABA-IR neurons in the inner submucous plexus varied from duodenum to colon, being highest in ileum and lowest in duodenum. Double-immunolabelling of GABA with substance P revealed that approximately 40% of the GABA-IR neurons in the inner submucous plexus of the ileum also stained for substance P. Our results suggest that in the porcine enteric nervous system, GABA-containing neurons are primarily involved in the modulation of secretory processes rather than in the modulation of peristalsis.