Brain stem projections of the aortic nerve in the cat: a study using tetramethyl benzidine as the substrate for horseradish peroxidase.

The intra-axonal transport of horseradish peroxidase (HRP) has been used to trace the nodose ganglion and brain stem projections of a physiologically distinct nerve - the aortic depressor nerve - following electrophysiological identification. Tetramethyl benzidine (TMB) has been used as the substrate for demonstrating the centrally transported HRP15, 16. This sensitive method for horseradish peroxidase histochemistry has permitted the visualization of the central projections of aortic nerve afferents and has also provided information regarding the anatomical localization of cell bodies of these sensory nerve fibers within the nodose ganglion. This study demonstrates the usefulness of using TMB as a substrate for HRP histochemistry in anatomical studies where the detection of anterogradely transported HRP is an essential prerequisite. The uptake of HRP from the cut central ends of sensory nerve fibers and the transport of this enzyme to the sensory ganglion and subsequently into the central processes of these sensory neurons have made possible this study of the central projections of a functionally distinct peripheral nerve. Information has been provided by this study that cell bodies of aortic nerve afferent fibers are localized in the rostrolateral pole of the nodose ganglion. Dense central projections of sensory terminals of aortic afferents have been found in the dorsolateral and medial subdivisions of the nucleus of the tractus solitarius. These central projections of aortic afferents extend for 6 mm rostrocaudally in the medulla with the densest projection being found at the level of the obex. These projections are bilateral at all rostrocaudal levels. This anatomical demonstration of the dorsolateral and medial subdivisions of the nucleus of the tractus solitarius confirms earlier reports based on electrophysiological studies. Of particular interest in this study is the new observation that there exists a dense projection of aortic nerve afferents to the area postrema. The possible physiological implications of a direct input of peripheral chemoreceptor afferents to a region of central chemosensitivity are discussed. The complete absence of any retrogradely labeled cell body in the brain stem from exposure of the aortic nerve to horseradish peroxidase is noteworthy. This indicates that the aortic nerve is purely afferent in function and that reflex control of afferent activity in the aortic nerve is not mediated by brain stem neurons projecting down the same nerve.