Changes in properties of neuronal responses in two cortical taste areas in rats of various ages

Changes in the features of responses of cortical taste neurons were studied in granular insular (GI) and dysgranular insular (DI) areas of urethane-anesthetized SD rats of four age groups (4-10, 14-20, 25-35 and > 90 days). Stimuli included four basic taste stimuli, 0.1 M NH4Cl, and the water extract of a food pellet, applied to the entire oral cavity. At the age of 4-10 days, taste neurons showed no spontaneous discharge, and most responded to NH4Cl in a phasic fashion. In older age groups, however, there was a spontaneous discharge, and tonic taste responses were evident; some responded better to the four basic stimuli and to the food than to NH4Cl. With only a few days' difference in age, inhibitory responses were also noted in both areas. Average taste responses increased or decreased at different ages, depending on the type of taste stimulus and recording area. When taste responses were plotted against the four basic taste stimuli in the order of sucrose, NaCl, HCl and quinine along the abscissa, some neurons showed double-peaked response profiles. This suggests the convergence and mutual inhibition of afferents at single cortical neurons. The fraction of neurons with double-peaked response profiles increased at the age of 25-35 days in both areas. Neurons in rats aged 25-35 days were predominantly sensitive to either NH4Cl or NaCl, or to either food or sucrose. Neurons in rats of other ages, during the preweanling and the adult stages, tended to respond to both these stimuli. Our observations suggest that cortical taste neurons develop in the rat brain in accordance with the maturation of both the taste receptors and the neural networks.