An electrophysiological comparison of substance P-immunoreactive neurons with other neurons in the guinea-pig submucous plexus

Intracellular microelectrodes containing the dye Lucifer yellow CH were used to record from nerve cells of ganglia in isolated preparations of submucosa from the guinea-pig small intestine. The electrophysiological properties of the neurons were investigated by passing intracellular current pulses and by stimulating nerve strands that connect with the ganglia. Following each electrophysiological investigation, the fluorescent dye was injected into the neuron by passing hyperpolarizing currents through the electrode. The preparations were fixed and processed for substance P (SP) immunohistochemistry. When impaled neurons were not immunoreactive for SP the preparations were processed first for vasoactive intestinal peptide immunoreactivity and then for neuropeptide Y immunoreactivity. Data from neurons reactive for either of these peptides were pooled for comparison with the data from SP-immunoreactive neurons. Nerve cells that were immunoreactive for SP were usually multipolar, had broad action potentials (half width 2.6 ms compared with 1.5 ms for other submucous neurons) and lower input resistances than other submucous neurons. Only a minority (6 of 18) had demonstrable fast excitatory synaptic inputs. When such excitatory synaptic inputs were observed, these did not initiate action potentials. Thus, the SP-immunoreactive neurons were readily distinguishable from other submucous neurons. It is concluded that these neurons are likely to be sensory.