Inhibitory neurotransmission in the circular muscle layer of mouse colon

Intracellular electrophysiological techniques were used to record the spontaneous myoelectric activity in the circular muscle layer of an in vitro preparation of whole mouse colon. In 34 out of 58 preparations, spontaneous depolarisations (myoelectric complexes, MCs) were recorded cyclically, about every 4 min. In these preparations, apamin (250 nM) and N(G)-nitro-L-arginine (NOLA, 100 μM) depolarised the membrane potential between MCs by 8 mV or 13 mV, respectively. Tetrodotoxin (1.6 μM) abolished MCs and also induced depolarisation (17 mV). In the remaining 24 preparations. MCs were not recorded and the membrane potential was significantly depolarised compared to the membrane potential between MCs. NOLA (100 μM), apamin (250 nM) and tetrodotoxin (1.6 μM) were without significant effect on membrane potential. It is suggested that in preparations that exhibit MC cycling, membrane potential between MCs is maintained in a state of tonic inhibition, predominantly mediated by nitrergic mechanisms generated via spontaneously active inhibitory neurons. Apamin-sensitive channels may also be involved in the inhibition.