MAPPING OF CAROTID BARORECEPTOR SUBTYPE PROJECTIONS TO THE NUCLEUS-TRACTUS-SOLITARIUS USING C-FOS IMMUNOHISTOCHEMISTRY

This study has combined physiological pressure stimulation of carotid baroreceptors via a vascularly isolated carotid sinus and anodal block of baroreceptor afferent fibers in the carotid sinus nerve to examine the medullary projections of type I vs. type II (large A- vs. small A - and C-fiber afferent) baroreceptors. The control distribution of cel ls in the nucleus tractus solitarius expressing c-fos in response to p hysiological activation of carotid baroreceptors in the isolated sinus was compared to that during anodal block of large A-fibers in the car otid sinus nerve. Carotid baroreceptor stimulation primarily activated neurons in the ipsilateral commissural and medial subnuclei of the ca udal nucleus tractus solitarius and the dorsolateral, dorsomedial and medial subnuclei in the intermediate and rostral levels of the nucleus tractus solitarius. Elimination of large A-fiber carotid baroreceptor afferents, during similar carotid baroreceptor stimulation resulted i n a decrease in the number of cells expressing c-fos in the dorsomedia l subnucleus of the rostral nucleus tractus solitarius. These data ind icate that projections of larger A-fiber (type I) carotid baroreceptor s are localized primarily to the rostral dorsomedial subnucleus, while those of smaller A- and C-fiber baroreceptors are more widely distrib uted to the commissural, medial and dorsal subnuclei of the nucleus tr actus solitarius.