Neurally Released GABA Acts via GABA(C) Receptors to Modulate Ca(2+) Transients Evoked by Trains of Synaptic Inputs, but Not Responses Evoked by Single Stimuli, in Myenteric Neurons of Mouse Ileum

γ-Aminobutyric Acid (GABA) and its receptors, GABA(A,B,C), are expressed in several locations along the gastrointestinal tract. Nevertheless, a role for GABA in enteric synaptic transmission remains elusive. In this study, we characterized the expression and function of GABA in the myenteric plexus of the mouse ileum. About 8% of all myenteric neurons were found to be GABA-immunoreactive (GABA+) including some Calretinin+ and some neuronal nitric oxide synthase (nNOS+) neurons. We used Wnt1-Cre;R26R-GCaMP3 mice, which express a genetically encoded fluorescent calcium indicator in all enteric neurons and glia. Exogenous GABA increased the intracellular calcium concentration, [Ca(2+)](i) of some myenteric neurons including many that did not express GABA or nNOS (the majority), some GABA+, Calretinin+ or Neurofilament-M (NFM)+ but rarely nNOS+ neurons. GABA+ terminals contacted a significantly larger proportion of the cell body surface area of Calretinin+ neurons than of nNOS+ neurons. Numbers of neurons with GABA-induced [Ca(2+)](i) transients were reduced by GABA(A,B,C) and nicotinic receptor blockade. Electrical stimulation of interganglionic fiber tracts was used to examine possible effects of endogenous GABA release. [Ca(2+)](i) transients evoked by single pulses were unaffected by specific antagonists for each of the 3 GABA receptor subtypes. [Ca(2+)](i) transients evoked by 20 pulse trains were significantly amplified by GABA(C) receptor blockade. These data suggest that GABA(A) and GABA(B) receptors are not involved in synaptic transmission, but suggest a novel role for GABA(C) receptors in modulating slow synaptic transmission, as indicated by changes in [Ca(2+)](i) transients, within the ENS.