Characterization of neurokinin-1 receptors in the submucosal plexus of guinea pig ileum

This study combined immunohistochemical double-labeling techniques with functional studies to characterize the neurokinin-1 (NK₁) receptors mediating neuronal and vasodilator responses in submucosal guinea pig ileum. NK₁ receptor distribution in whole mount preparations of the submucosa was examined using a rabbit polyclonal antibody directed against the COOH terminus of the rat NK₁ receptor. Results showed that 97% of neuropeptide Y immunoreactive submucosal neurons colocalized NK₁ receptor immunoreactivity, whereas vasoactive intestinal polypeptide immunoreactive neurons were not NK₁ immunoreactive. Intracellular recordings were made using neurobiotin- filled electrodes to enable reidentification of recorded neurons for immunohistochemical study. The selective NK₁ agonists [Sar⁹,Met(O₂)¹¹]substance P (SP) and septide depolarized S-type submucosal neurons. Of these neurons, 36% were NK₁ immunoreactive and 64% were not. NK₁ immunoreactivity was not observed on submucosal arterioles, but superfusion of [Sar⁹,Met(O₂)¹¹]SP and septide dilated preconstricted submucosal arterioles. Agonist-evoked responses in both neurons and blood vessels were blocked by the selective NK₁ antagonist CP-99994. These findings suggest that NK₁ receptors are found on submucosal neurons and arterioles and that electrophysiological and immunohistochemical techniques may identify conformational variants of the receptor.