Neurons bearing NK3 tachykinin receptors in the guinea-pig ileum revealed by specific binding of fluorescently labelled agonists

The localisation of NK₃ tachykinin receptors in guinea-pig ileum was studied using the fluorescently labelled agonists, Cy3.5-neurokinin A and Cy3.5-kassinin. Binding to nerve cell bodies in the myenteric and submucosal plexuses was visualised using confocal microscopy. Binding to NK₁ receptors was blocked by the NK₁ receptor antagonist, CP-99994. NK₃ receptors, demonstrated by binding in the presence of CP-99994, occurred in 72% of myenteric and 38% of submucosal neurons. Colocalisation with other markers was examined to deduce the classes of neurons with NK₃ receptors. In myenteric ganglia, NK₃ receptors were present on the following: 73% of calbindin-immunoreactive (IR) intrinsic primary afferent neurons, 63% of calretinin-IR excitatory motor neurons and ascending interneurons, 63% of nitric oxide synthase-IR inhibitory motor neurons and descending interneurons, 79% of strongly neuropeptide Y (NPY)-IR secretomotor neurons, 67% of weakly NPY-IR descending interneurons and motor neurons, and 46% of NK₁ receptor-IR neurons. In submucosal ganglia, NK₃ receptors were on 65% of calretinin-IR secretomotor/vasodilator neurons, 81% of NPY-IR cholinergic secretomotor neurons, 2% of vasoactive intestinal peptide-IR non-cholinergic secretomotor neurons and were completely absent from substance P-IR intrinsic primary afferent neurons. The results support physiological studies suggesting that NK₃ receptors mediate tachykinin transmission between myenteric sensory neurons and to interneurons and/or motor neurons in descending inhibitory and ascending excitatory pathways. Accepted: 22 June 1999