Localization of cortical gustatory area in rats and its role in taste discrimination

1. The distribution of the spots on the cerebral cortex at which a surface positive potential could be evoked by electrical stimulation of the chorda tympani (CT), the glossopharyngeal nerve (GN), and the lingual nerve (LN) was mapped in rats. Horseradish peroxidase (HRP) was injected into the cortical projection areas of these tongue afferents, and the labeled cells were localized in the thalamus to determine the thalamocortical connections of these nerves. Further, the functional significance of the cortical gustatory area was studied by monitoring the effects of ablations of the cortical areas for tongue afferents on conditioned taste aversion. 2. Both the CT and the LN project to two separate areas in the cortex; one is anterior to the middle cerebral artery (MCA) and the other is posterior to the MCA. The anterior CT area is just dorsal to the rhinal sulcus and more ventral than the interior LN area, while the posterior CT and LN areas overlap each other. For both the CT and LN, the latency of the initial positive phase of the evoked potential recorded in the anterior area was 2-5 ms faster than that in the posterior area. The projection area of the GN is single and is located posterior to the MCA, extending dorsoventrally. The dorsal third of the GN area overlaps the posterior CT and LN areas, the middle third is just posterior to the anterior LN area, and the ventral third is just posterior to the anterior CT area. Judging from the extent of the cortical response area and the amplitude and latency of the responses, the major projection of the CT was bilaterally symmetrical, whereas that of the LN and the GN was contralateral. 3. Cortical unitary responses were recorded in response in taste, thermal, and tactile stimulations of the anterior part of the tongue. Cortical units sensitive to taste only or both to taste and cooling stimuli were found in the anterior CT area, while the units in the posterior CT area were sensitive to different modalities of stimuli. 4. A small amount (0.01-0.05 μl) of HRP solution was injected into varying parts of the projection areas for tongue afferents. Following injections into the anterior CT area and ventral GN area, labeled cells were found in the most medial part of the ventral nucleus of the thalamus. Labeled cells were located just lateral to this most medial part and at the more rostral levels when injections were centered in the anterior LN area. Within the above-mentioned thalamic locations, labeled cells were also found following injections of HRP into the posterior CT and LN areas (which correspond to the dorsal GN area). 5. Bilaterally symmetrical ablations of the combined projection areas of three tongue afferents completely abolished the conditioned taste aversion (to NaCl solution) that had been acquired before ablations. Ablations including both the anterior CT area and the ventral GN area also abolished the conditioning effects, whereas ablations of the posterior CT and LN areas and the dorsal GN area had no effects on acquired conditioned taste aversion. 6. These results demonstrate that taste inputs project to two different locations in the rat's cortex. One, which consists of the anterior CT area and the ventral third of the GN area, may receive direct thalamocortical inputs and play an important role in taste discrimination, as shown by the present behavioral assessment. The other, which is an overlapped area of the posterior CT area and the dorsal third of the GN area, may in part receive direct thalamocortical inputs, but its functional importance is still vague. Further functional significance of these locations is discussed.