Horseradish peroxidase study of brain stem projections of carotid sinus and aortic depressor nerves in the cat

Abstract

The connections of carotid sinus (CSN) and aortic depressor (ADN) afferent fibers in the brain stem of the cat were studied using horseradish peroxidase (HRP). Crystalline HRP was applied to the proximal cut end of either the CSN or ADN for 4–10.5 h and after a survival period of 24–120 h the animals were perfused and frozen sections of brain stem, nodose and petrosal ganglia were processed according to the tetramethyl-benzidine method. CSN labeled axons were found to project ipsilaterally to several nuclei of the solitary complex: the dorsal aspect of the medial (Sm), the lateral (Slt), the ventrolateral (Svl), the commissural (Com), the intermediate (Int) and the dorsomedial aspect of the parvocellular (Spc) solitary nuclei. These nuclei, except for the Svl, also received a less intense contralateral projection. Additional terminal labeling was observed on the ipsilateral side along the dorsal border of the dorsal motor nucleus of the vagus (DMV), in the reticular formation ventral to the solitary complex, and in the ventrolateral external cuneate nucleus, and labeled fibers were found in the dorsolateral spinal trigeminal tract. On the other hand, ADN labeled fibers were found to project only to the solitary complex, bilaterally. Terminal labeling was found primarily in the Sm, Slt, Com and dorsal Spc. After HRP application to the CSN few cells were found labeled in the petrosal ganglion; in addition clusters of labelled neurons were found bilaterally in the region of the rostral nucleus ambiguus, retrofacial and facial nuclei, and in the ipsilateral rostral dorsomedial reticular formation. ADN labeled ganglion cells were identified in clusters in the medial aspect of the nodose ganglion primarily near the entry of the superior laryngeal nerve; additional clusters of labeled smaller neurons were found in the ventromedial portion of the ganglion and intermingled with vagal fibers just caudal to the ganglion. No HRP-positive cells were identified in the brain stem after ADN labeling. These data demonstrate that different regions of the solitary complex receive direct inputs from either one or both buffer nerves, suggesting a degree of separation of central pathways carrying CSN and ADN afferent information. Furthermore, the finding of labeled cell bodies in the medulla after CSN labeling suggests a possible route by which the central nervous system may alter activity of receptors in the carotid sinus and body.