Abstract
We have reported that reward-driven discrimination learning of sound order is dependent on D2-like dopamine receptors in the rat. In the present study, we investigated the responsible targets of dopaminergic inputs in the brain and their roles in the discrimination learning of sound order. Water-deprived rats were trained to discriminate order of two sounds, and licking a spout during presentation of a particular sequence (S+) was rewarded with water while the opposite sequence (S-) was not. Either S+ or S- was presented randomly in a trial, which was repeated every one minute for 12 hours in 4 days. Percentage of trials in which rats licked the spout was calculated separately for S+ and S-, and test performance was estimated as the difference. 6-hydroxydopamine (6-OHDA) was locally injected into the brain one week before the test sessions. 6-OHDA injections into the bilateral nucleus accumbens suppressed sound discrimination in a nonspecific manner, so that discrimination of two sounds was impaired as that of sound order was. However, 6-OHDA injections into the bilateral auditory cortex specifically suppressed the test performance of sound order discrimination, while it has no clear effect on discrimination of two sounds. Licking itself was not impaired in both groups of rats. These findings suggest that dopaminergic inputs to the auditory cortex are critical for acquisition of sound order discrimination, while those to the nucleus accumbens are required for association between sensory stimuli and reward. [Jpn J Physiol 54 Suppl:S188 (2004)]
