Neuronal and glial localization of GABA transporter immunoreactivity in the myenteric plexus

The neurotransmitter gamma-aminobutyric acid (GABA) is removed from the extracellular space by sodium and chloride dependent high affinity plasma membrane transporters. In the rat central nervous system, three GABA transporters, GAT1, GAT2 and GAT3, have been cloned and localized by immunohistochemistry. The purpose of this study was to examine the distribution of these transporters within the myenteric plexus of the rat gastrointestinal tract. We investigated their cellular locations using GAT1-3 specific antisera in lightly fixed segments of rat duodenum, ileum and colon. Immunohistochemistry revealed a large number of GAT2-immunoreactive structures that surrounded neurons within each ganglion of the myenteric plexus. GAT2 was colocalized in these structures with the glial cell marker p75(NTR), suggesting that the predominant high affinity GABA transporter within enteric glia is GAT2. GAT3 immunoreactivity was localized within many nerve cell bodies, and no labeling for GAT1 was detected, although it was present in retina, which was used as a control. Double labeling for calretinin and nitric oxide synthase (NOS) revealed colocalization of GAT3 with approximately 75% of calretinin-immunoreactive neurons and 15% of NOS-immunoreactive neurons. This suggests that a small proportion of inhibitory motor neurons and at least some putative intrinsic primary afferent neurons within the rat gastrointestinal tract express GAT3. Thus NOS neurons, which appear to utilize GABA as a transmitter, and calretinin-immunoreactive neurons, which do not appear to be GABAergic, both express immunoreactivity for GABA transporters.