"In medio stat virtus": Concentration-dependent effects of GABA on colonic motor patterns in mice

BACKGROUND AND AIMS: In the enteric nervous system, GABA (gamma amino butyric acid) has been shown to modulate neuronal activity via activation of different GABAergic receptors. The consequences of this modulation remain poorly understood and depend on the region of the gastrointestinal (GI) tract or on the animal species examined. Although several studies have been addressed the role of GABA as neuromodulator in the upper part of GI tract, accumulating data suggest that it may play a key role also in GI distal tract function in health and disease. Therefore, the aim of this study was to investigate the role of GABA in both spontaneous and electrically-evoked contractions and in the peristaltic reflex, using as model the mouse distal colon. METHODS: The effects of GABA on circular muscle of mouse distal colon were investigated utilizing a vertical organ bath, in order to record spontaneous and neurally-evoked mechanical activity, and a modified Trendelenburg set-up, through which gradual distension of distal colonic segments evoked peristaltic, rhythmic and aborally migrating contractions. RESULTS: GABA up to 1 mM failed to affect the spontaneous mechanical activity of colonic circular muscle strips. Indeed, depending upon the dose, it differently modulated neurally-evoked cholinergic contractions and peristaltic activity. Low GABA concentrations (10–100 µM) induced a significant increase in the amplitude of the neurally-evoked cholinergic contractions and in the amplitude and frequency of the peristaltic contractions. These excitatory effects were prevented by pretreatment with the selective GABAA-receptor antagonist, bicuculline, which per se reduced the amplitude of EFS-induced cholinergic contractions and the amplitude and frequency of peristaltic waves. These positive GABA effects were mimicked by muscimol, a selective GABAA-receptor agonist. In contrast, higher GABA concentrations (0.3–1 mM) caused a significant decrease of the electrically-evoked cholinergic contractions and an inhibition of peristaltic activity of the distal colonic segments, reducing the amplitude and frequency of peristaltic waves. These inhibitory effects were antagonized by phaclofen (GABAB-receptor antagonist), which per se had no effect. Baclofen, a selective GABAB-receptor agonist, mimicked the effect of higher GABA concentrations. Lastly, GABAC-receptor agonist (CACA) and antagonist (TPMPA) showed no significant effect both on the response to EFS and on the peristaltic activity. CONCLUSIONS: Our study indicates that GABA is a modulator of acetylcholine release from enteric neurons in mouse distal colon, via activation of GABAA and GABAB receptors. GABAA are recruited by low GABA concentrations and appear to be tonically activated, enhancing Ach release from excitatory cholinergic neurons. Indeed, GABAB are recruited at higher GABA concentrations inducing opposite effects on Ach release. Overall GABA effects would in turn influence colonic peristaltic activity.