Topographic organization of Fos-like immunoreactivity in the rostral nucleus of the solitary tract evoked by gustatory stimulation with sucrose and quinine

Fos immunohistochemistry was used to elucidate the pattern of activation elicited by two qualitatively and hedonically distinct taste stimuli, sucrose and quinine, within the first-order gustatory relay, the rostral division of the nucleus of the solitary tract. Compared to unstimulated controls, both sucrose and quinine elicited significant increases in Fos-like immunoreactivity in the rostral central subnucleus, the region of the rostral solitary nucleus that receives the densest primary afferent input. Within the rostral central subnucleus, neurons that exhibited Fos-like immunoreactivity following quinine stimulation were concentrated medially, but neurons that exhibited Fos-like immunoreactivity following sucrose stimulation were distributed more evenly along the mediolateral axis. Despite their differential distribution, sucrose- and quinine-activated neurons also demonstrated notable intermingling. Further, the chemotopic arrangement was only partially consistent with what would be predicted if chemotopy was merely an outcome of orotopy. Our results suggest that a rough chemotopy characterizes the organization of taste responses in the nucleus of the solitary tract, and that the topographic pattern of taste afferent terminations in this nucleus is related to their chemosensitivity as well as to their peripheral spatial distribution.