Analysis of purinergic and cholinergic fast synaptic transmission to identified myenteric neurons

Abstract

Types and projections of neurons that received cholinergic, purinergic and other fast excitatory synaptic inputs in myenteric ganglia of the guinea-pig distal colon were identified using combined electrophysiological recording, application of selective antagonists, marker dye filling via the recording microelectrode, and immunohistochemical characterisation. Fast synaptic inputs were recorded from all major subtypes of uniaxonal neurons including Dogiel type I neurons, filamentous interneurons, circular muscle motor neurons and longitudinal muscle motor neurons. Fast excitatory postsynaptic potentials were completely blocked by the nicotinic receptor antagonists hexamethonium or mecamylamine in 62% of neurons tested and were partially inhibited in the remaining neurons. The P2 purine receptor antagonist, pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid, reduced the amplitudes of fast excitatory postsynaptic potentials in 20% of myenteric neurons. The 5-hydroxytryptamine 3 receptor antagonist granisetron reduced the amplitude of fast excitatory postsynaptic potentials in only one of 15 neurons tested. In five of five neurons tested, the combination of a nicotinic antagonist, pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid, granisetron and 6-cyano-7-nitroquinoxaline-2,3-dione did not completely block the fast excitatory postsynaptic potentials. Immunohistochemical studies of the neurons that had been identified electrophysiologically and morphologically imply that P2X 2 receptors may mediate fast transmission in some neurons, and that other P2X receptor subtypes may also be involved in fast synaptic transmission to myenteric neurons of the guinea-pig distal colon. Neurons with nicotinic and pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid-sensitive fast excitatory postsynaptic potentials were present in both ascending and descending pathways in the distal colon. Thus, neither cholinergic nor mixed cholinergic/purinergic synaptic responses are confined to a particular class of neuron. The results indicate that acetylcholine and ATP are the major fast excitatory neurotransmitters in guinea-pig distal colon myenteric ganglia.