Glossopharyngeal nerve transection eliminates quinine-stimulated Fos-like immunoreactivity in the nucleus of the solitary tract: Implications for a functional topography of gustatory nerve input in rats

The relationship between specific gustatory nerve activity and central patt erns of taste-evoked neuronal activation is poorly understood. To address t his issue within the first central synaptic relay in the gustatory system, we examined the distribution of neurons in the nucleus of the solitary trac t (NST) activated by the intraoral infusion of quinine using Fos immunohist ochemistry in rats with bilateral transection of the chorda tympani (CTX), bilateral transection of the glossopharyngeal nerve (GLX), or combined neur otomy (DBLX), Compared with nonstimulated and water-stimulated controls, qu inine evoked significantly more Fos-like-immunoreactive (FLI) neurons acros s the rostrocaudal extent of the gustatory NST (gNST), especially within it s dorsomedial portion (subfield 5). Although the somatosensory aspects of f luid stimulation contributed to the observed increase in FLI neurons, the e levated number and spatial distribution of FLI neurons in response to quini ne were remarkably distinguishable from those in response to water. GLX and DBLX produced a dramatic attenuation of quinine-evoked FLI neurons and a s hift in their spatial distribution such that their number and pattern were indiscernable from those observed in water-stimulated controls. Although CT X had no effect on the number of quinine-evoked FLI neurons within subfield 5 at intermediate levels of the gNST it produced intermediate effects else where; yet, the spatial distribution of the quinine-evoked FLI neurons was not altered by CTX. These findings suggest that the GL provides input to al l FLI neurons responsive to quinine, however, some degree of convergence wi th CT input apparently occurs in this subpopulation of neurons. Although th e role of these FLI neurons in taste-guided behavioral responses to quinine remains speculative, their possible function in oromotor reflex control is considered.