Neurally Released GABA Acts via GABAC Receptors to Modulate Ca2+ Transients Evoked by Trains of Synaptic Inputs, but Not Responses Evoked by Single Stimuli, in Myenteric Neurons of Mouse Ileum

Katerina Koussoulas, Mathusi Swaminathan, Candice Fung, Joel C. Bornstein, Jaime P. P. Foong

γ-Aminobutyric Acid (GABA) and its receptors, GABAA,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 usedWnt1-Cre;R26R-GCaMP3mice, which express a genetically encoded fluorescent calcium indicator in all enteric neurons and glia. Exogenous GABA increased the intracellular calcium concentration, [Ca2+]iof 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 [Ca2+]itransients were reduced by GABAA,B,Cand nicotinic receptor blockade. Electrical stimulation of interganglionic fiber tracts was used to examine possible effects of endogenous GABA release. [Ca2+]itransients evoked by single pulses were unaffected by specific antagonists for each of the 3 GABA receptor subtypes. [Ca2+]itransients evoked by 20 pulse trains were significantly amplified by GABACreceptor blockade. These data suggest that GABAAand GABABreceptors are not involved in synaptic transmission, but suggest a novel role for GABACreceptors in modulating slow synaptic transmission, as indicated by changes in [Ca2+]itransients, within the ENS.