Localisation of neurokinin 3 (NK3) receptor immunoreactivity in the rat gastrointestinal tract

The localisation of the neurokinin 3 receptor (NK3r) in the rat gastrointestinal tract has been studied by using a polyclonal antiserum against the C-terminal portion (amino acids 388–452) of the rat NK3r. In the oesophagus, immunoreactivity for the NK3r was found on smooth muscle cells of the muscularis mucosae. NK3r immunoreactivity was not present on muscle cells of other regions. Nerve cell bodies immunoreactive for NK3r were seen in the myenteric and submucous plexuses of the small and large intestine, but not in the stomach or oesophagus. Immunoreactivity was largely confined to nerve cell surfaces. The reaction product was on the cell soma and initial parts of axons. Reactivity was not seen on nerve terminals. Immunoreactive nerve cells had Dogiel Type II morphology. Patterns of co-localisation of NK3r and immunoreactivity for other markers were examined in the ileum, to provide a basis from which to deduce the functional identity of NK3r-immunoreactive nerve cells. Most of the NK3r-immunoreactive nerve cells were also immunoreactive for the calcium-binding proteins, calretinin and calbindin, and all were immunoreactive for the NK1 receptor (NK1r). Nerve cells that were immunoreactive for nitric oxide synthase were not immunoreactive for either NK3r or NK1r. The projections of the calbindin and calretinin neurons were determined by nerve lesion studies. Their morphology, projections to the mucosa and other ganglia and immunoreactivity for the calcium-binding proteins suggest that the NK3r-immunoreactive neurons are intrinsic sensory neurons.