GUSTATORY AND TACTILE STIMULATION OF THE POSTERIOR TONGUE ACTIVATE OVERLAPPING BUT DISTINCTIVE REGIONS WITHIN THE NUCLEUS OF THE SOLITARY TRACT

Both the gustatory and somatosensory systems provide necessary sensory input for the initiation and control of oromotor behaviors. Behavioral studies indicate that somatosensory input from the posterior tongue (PT) is important in initiating swallowing, whereas PT taste input is particularly important in gustatory rejection reflexes. However, there have been few studies of the central representation of PT gustatory or tactile responses. In the present study, electrophysiological multi-unit recording techniques were used to map the location of PT-mediated taste and tactile responses in the nucleus of the solitary tract (NST) of the rat. A stimulation technique that allows taste stimuli to be introduced directly and specifically into the papillae trenches was used to optimally activate PT taste receptors located within the circumvallate (CV) and foliate (FOL) papillae. The results demonstrated that non-PT responsive sites dominated the rostral half of the rostral division of NST (rNST), while PT-responsive sites dominated the caudal half. Some PT-responsive sites extended into the caudal NST. Both gustatory and tactile stimuli were effective at 28% of PT-responsive locations (taste-tactile sites), whereas at the remaining locations, only tactile stimulation was effective (tactile-only sites). Although these two types of PT-responsive sites exhibited some anatomical overlap, their distributions were distinctive, with taste-tactile sites restricted medially and the laterally located tactile-only sites offset caudally. On the other hand, responses arising from stimulation of the CV and FOL exhibited no anatomical organization, i.e., responses to stimulation of both papillae were coextensive. On average, of the four tastants used (0.01 M Na saccharin, 0.3 M NaCl, 0.01 M quinine hydrochloride, 0.03 M HCl), HCl was the most effective stimulus for both the CV and FOL. The present results delimit the regions of the NST that provide a substrate for the gustatory and somatosensory limbs of PT-mediated oromotor reflexes.