Chorda tympani nerve stimulation evokes Fos expression in regionally limited neuron populations within the gustatory nucleus of the solitary tract.

The distribution of neurons in the rostral nucleus of the solitary tract (rNST) that respond to gustatory input from the anterior tongue was visualized by Fos protein immunohistochemistry following electrical stimulation of the chorda tympani (CT) nerve in rats. Maps of Fos-immunoreactive (Fos-ir) neurons were compared with the distribution of CT afferent terminal fields labeled by transganglionic transport of rhodamine-dextran in a separate group of animals. The primary concentration of Fos-ir neurons localized in register with the major terminal fields of CT afferent fibers, in the central third of the rostral 1.0 mm of the NST ipsilateral to the stimulated nerve. A similar correspondence in location and degree of labeling of Fos-ir neurons and afferent terminals was observed in the ipsilateral dorsal spinal trigeminal complex (Sp5) pars caudalis, near the obex, and the Sp5 pars oralis near the rostral pole of the rNST. Thus, the magnitude of Fos upregulation in brainstem targets of the CT nerve having chemosensory or nociceptive function, was proportional to the relative density of the CT afferent input. This correspondence, and the absence of labeling in neurons known to be one additional synapse away from the afferent input within gustatory or oral reflex pathways, suggests that the cell map obtained represents mainly neurons that are directly activated via primary afferent synapses from CT fibers. The availability of a method to histochemically identify a population of putative second-order taste neurons will facilitate analysis of the cellular/molecular properties of these neurons and of synaptic circuitry in the rNST.