Autoradiographic localization of 3H‐digoxin binding by neural cells in the medulla

The purpose of this investigation was to localize binding sites for the cardiac glycoside digoxin in the medulla of the rat in vivo. Adult male Sprague‐Dawley rats were injected (IV) with 3H‐digoxin and killed 30 minutes later. Autoradiographs of medullas showed evidence of 3H‐digoxin binding to small‐ and medium‐sized neural cells in the regions of the nucleus solitarius, dorsal motor nucleus of the vagus, area postrema, and in the zone between the area postrema and the underlying neuropil. However, the parasympathetic preganglionic neurons of the dorsal motor nucleus were not labeled. The 3H‐digoxin‐labeled cells in the medulla were located mainly in the commissural and medial portions of nucleus solitarius at the level of the area postrema. Animals injected with unlabeled digoxin followed by 3H‐digoxin showed reduced binding of radioactivity. The small‐ and medium‐sized neurons of the caudal portions of the nucleus solitarius are internuncial in position with respect to cardiovascular afferents of the glossopharyngeal and vagus nerves and sympathetic and parasympathetic cardiovascular efferent neurons of the medulla. The results of this study suggest that these 3H‐digoxin‐labeled cells, presumably neurons of nucleus solitarius, may possess high affinity binding sites for digoxin. Further, the area postrema, which lacks a blood‐brain barrier, may provide a portal of entry for 3H‐digoxin into regions of the medulla known to contain neurons that play a role in the regulation of cardiac rhythm.