SYNAPTIC BEHAVIOR IN THE MYENTERIC PLEXUS OF THE GUINEA-PIG GASTRIC ANTRUM

1. Intracellular recording methods were used to study the synaptic behaviour of neurones in the myenteric plexus of the guinea-pig gastric antrum. Synaptic potentials occurred spontaneously or were evoked by focal electrical stimulation of interganglionic fibre tracts. Synaptic events consisted of fast and slow excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). 2. Fast EPSPs with durations < 20 ms were evoked in every antral neurone in a population sample of 370 cells. Most of the ganglion cells received multiple inputs from axons entering the individual ganglia in several different interganglionic fibre tracts. Many of the neurones also received input from multiple axons projecting in individual fibre tracts. The fast EPSPs behaved like nicotinic cholinergic EPSPs. They were evoked at stimulus frequencies up to 60 Hz without evidence of the run-down characteristic of fast EPSPs in the intestine. 3. Slow EPSPs were evoked by repetitive stimulation of the interganglionic connectives. They consisted of a slowly activating depolarization which persisted for several seconds after termination of the stimulus. The depolarizing responses were associated with an increase in the input resistance, with enhanced excitability and with suppression of hyperpolarizing after-potentials in AH/type 2 neurones. They were observed in 14.4% of the neurones of which 89% were AH/type 2 neurones. AH/type 2 neurones, unlike other myenteric neurones, were identified by action potentials with long lasting after-hyperpolarization. 4. IPSPs were hyperpolarizing potentials evoked by repetitive stimulation of interganglionic fibre tracts. The hyperpolarizing responses were associated with decreased input resistance. They occurred in 1.4% of the antral neurones. 5. Application of acetylcholine (ACh) by micro-ejection mimicked the fast EPSPs in all neurones. This fast nicotinic response to ACh was followed by a slowly activating, long-lasting muscarinic depolarization in 32% of the neurones. The slow muscarinic response was associated with increased input resistance, suppression of hyperpolarizing after-potentials and enhanced excitability. 6. Fast EPSPs were not suppressed by accumulation of ACh at presynaptic transmitter release sites. Unlike the intestine, presynaptic muscarinic autoreceptors appeared to be absent from the microcircuits in the antrum. 7. Synaptic behaviour in the local circuits of the gastric antrum differed from the gastric corpus. This may be a reflection of specialization of the circuits for organization of the distinctive patterns of digestive behaviour found in this region of the stomach.