Location of cardiac vagal preganglionic cell bodies in the bullfrog

The bullfrog, Rana catesbeiana, provides a relatively simple model for study of the cardiac innervation. The present study located the origin of cardiac vagal preganglionic cell bodies in the frog with the retrograde tracer, horseradish peroxidase (HRP). HRP was injected subepicardially over the surface of atrial regions and along the major vessels exiting the heart. labeled cell bodies in the brainstem were confined to the nucleus of the glossopharyngeal-vagal complex, from approximately 1.5 mm caudal to 1.5 mm rostral to the calamus scriptorius (obex). Brainstem neurons were labeled ipsilateral to the intact vagus nerve when unilateral vagotomy was performed at the time of HRP injection. No labeled somata were identified in the spinal cord. Comparison of the cardiac pool of neurons with the population of neurons labeled after injection of HRP directly into the cervical vagus nerve demonstrated that cardiac preganglionic neurons are localized to the caudal region of the vagal neuron pool and that there are no dimensional differences between the two populations in terms of major and minor diameters.