Differential distribution of two ATP-gated ion channels (P_2X receptors) determined by immunocytochemistry

Several P_2X receptor subunits were recently cloned; of these, one was cloned from the rat vas deferens (P_2X1) and another from pheochromocytoma (PC12) cells differentiated with nerve growth factor (P_2X2). Peptides corresponding to the C-terminal portions of the predicted receptor proteins (P_2X1 391-399 and P_2X2 460-472) were used to generate antisera in rabbits. The specificities of antisera were determined by staining human embryonic kidney cells stably transfected with either P_2X1 or P_2X2 receptors and by absorption controls with the cognate peptides. In the vas deferens and the ileal submucosa, P_2X1 immunoreactivity (ir) was restricted to smooth muscle, whereas P_2X2-ir was restricted to neurons and their processes. Chromaffin cells of the adrenal medulla and PC12 cells contained both P_2X1 and P_2X2-ir. P_2X1-ir was also found in smooth muscle cells of the bladder, cardiac myocytes, and nerve fibers and terminals in the superficial dorsal horn of the spinal cord. In contrast, P_2X2-ir was observed in scattered cells of the anterior pituitary, neurons in the hypothalamic arcuate and paraventricular nuclei, and catecholaminergic neurons in the olfactory bulb, the substantia nigra, ventral tegmental area, and locus coeruleus. A plexus of nerve fibers and terminals in the nucleus of the solitary tract contained P_2X2-ir. This staining disappeared after nodose ganglionectomy, consistent with a presynaptic function. The location of the P_2X1 subunit in smooth muscle is consistent with its role as a postjunctional receptor in autonomic transmission, while in neurons, these receptors appear in both postsynaptic and presynaptic locations.