| Summary: | Electrophysiological recordings were made from myenteric neurones of the rat colon and were combined with immunohistochemical and intracellular staining techniques with a view to gaining an insight into the nature of regional and species differences in neuronal properties and the determination of enteric neuronal function. Immunohistochemical studies indicated that immunoreactivity (IR) to the biologically active mediators VIP and NPY were contained within three distinct neuronal populations, namely those containing either VIP- or NPY-IR and those containing both VIP- and NPY-IR. Regional differences in numbers of immunoreactive neurones and the extent of co-localization of the two neuropeptides indicated the possibility of regional differences in function. Intracellular recordings of >500 myenteric neurones revealed three electrophysiological classes of neurones (S/Type 1, AH and Type 3 neurones; impalement ratio 5:4:1) based on their active and passive properties and their synaptic inputs. Presynaptic tetanic stimulation evoked either slow membrane depolarizations or hyperpolarizations, but these were not a prominent feature of myenteric neurones being detected in only 8% of neurones. Muscarinic presynaptic autoinhibition of acetylcholine output, detectable as changes in fast ESP amplitude, was not commonly observed. Two presynaptic 5-HT receptors were, however, identified, a 5-HT<SUB>1A</SUB> receptor and 5-HT<SUB>4</SUB> receptor that respectively inhibited and facilitated acetycholine output.
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