Immunohistochemical localisation of cholinergic markers in putative intrinsic primary afferent neurons of the guinea-pig small intestine

Antibodies against choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (VAChT) were used to determine whether neurons that have previously been identified as intrinsic primary afferent neurons in the guinea-pig small intestine have a cholinergic phenotype. Cell bodies of primary afferent neurons in the myenteric plexus were identified by their calbindin immunoreactivity and those in the submucous plexus by immunoreactivity for substance P. High proportions of both were immunoreactive for ChAT, viz. 98% of myenteric calbindin neurons and 99% of submucosal substance P neurons. ChAT immunoreactivity also occurred in all nerve cell bodies immunoreactive for calretinin and substance P in the myenteric plexus, but in only 16% of nerve cells immunoreactive for nitric oxide synthase. VAChT immunoreactivity was in the majority of calbindin-immunoreactive varicosities in the myenteric ganglia, submucous ganglia and mucosa and also in the majority of the varicosities of neurons that were immunoreactive for calretinin and somatostatin and that had been previously established as being cholinergic. We conclude that the intrinsic primary afferent neurons are cholinergic and that they may release transmitter from their sensory endings in the mucosa.