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 (NK 1 ) 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 NK 1 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 [Sar 9 ,Met(O 2 ) 11 ]substance P (SP) and septide depolarized S-type submucosal neurons. Of these neurons, 36% were NK 1 immunoreactive and 64% were not. NK 1 immunoreactivity was not observed on submucosal arterioles, but superfusion of [Sar 9 ,Met(O 2 ) 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.