CONCENTRATION-DEPENDENT LICKING OF SUCROSE AND SODIUM-CHLORIDE IN RATS WITH PARABRACHIAL GUSTATORY LESIONS

The medial zone of parabrachial nuclei (PBN) serves as an obligatory synapse in the central gustatory system in rodents. Lesions in the PBN impair taste aversion learning and depletion-induced sodium appetite in rats, and also alter the ingestion of sapid stimuli. Interpretation of these lesion-induced behavioral deficits requires an evaluation of whether taste function is compromised. The present study examined whether rats with PBN lesions could show normal concentration-dependent changes in licking behavior to very small volumes of NaCl and sucrose. Physiological state was also varied; taste responsivity was examined in water-deprived and nondeprived rats. In a specially designed gustometer, nine rats with electrophysiologically guided lesions in the PBN and five surgical controls were trained to lick a drinking spout to receive 10-s access to various concentrations of NaCl (0.03-1.0 M) and sucrose (0.01-1.0 M) during 30-min sessions. Water-deprived control rats progressively decreased their responses compared with water as the concentration of NaCl was raised. In contrast, water-deprived PBNX rats did not decrease their licking responses to NaCl relative to water until the concentration reached 1.0 M. In the nondeprived state, control and PBNX rats decreased their responsiveness as a function of NaCl concentration, and the two groups did not differ. The licking responses of water-deprived PBNX rats did not differ from control rats when sucrose was the stimulus. In the nondeprived condition, both groups monotonically increased their licking to sucrose as a function of concentration, but PBNX rats were significantly less responsive than controls. These findings demonstrate that rats with lesions centered in the parabrachial gustatory area are not ageusic, but under certain physiological conditions these lesions do appear to blunt responsivity to sapid stimuli.