Sound sequence discrimination learning motivated by reward requires dopaminergic D2 receptor activation in the rat auditory cortex

Abstract

We have previously reported that sound sequence discrimination learning requires cholinergic inputs to the auditory cortex (AC) in rats. In that study, reward was used for motivating discrimination behavior in rats. Therefore, dopaminergic inputs mediating reward signals may have an important role in the learning. We tested the possibility in the present study. Rats were trained to discriminate sequences of two sound components, and licking behavior in response to one of the two sequences was rewarded with water. To identify the dopaminergic inputs responsible for the learning, dopaminergic afferents to the AC were lesioned with local injection of 6-hydroxydopamine (6-OHDA). The injection attenuated sound sequence discrimination learning, while it had no effect on discrimination between the sound components of the sequence stimuli. Local injection of 6-OHDA into the nucleus accumbens attenuated sound discrimination learning. However, not only discrimination learning of sound sequence but also that of the sound components were impaired. SCH23390 (0.2 mg/kg, i.p.), a D1 receptor antagonist, had no effect on sound sequence discrimination learning, while it attenuated the licking behavior to unfamiliar stimuli. Haloperidol (0.5 mg/kg, i.p.), a D2 family antagonist, attenuated sound sequence discrimination learning, while it had no clear suppressive effect on discrimination of two different sound components and licking. These results suggest that D2 family receptors activated by dopaminergic inputs to the AC are required for sound sequence discrimination learning.

Figure 1.

Sound sequence discrimination test. (A) Experimental setup for sound sequence discrimination test. Rats were trained to discriminate between sound sequences in a Skinner box. Licking a spout during sound presentation of a particular sequence was rewarded with water containing 0.1% saccharine. (B) An example of rewarded (S⁺) and unrewarded (S⁻) sound sequence. A PT and an AM tone with a sawtooth wave were sequentially presented five times at 2-sec intervals. In each trial, either S⁺ or S⁻ was randomly presented. (C) Schedule of discrimination test. After procedural training for rats to respond to the sound stimuli with licking, 720 trials were repeated at 1-min intervals during a test session of 12 h. The test session was conducted on four consecutive days. (D) Percentage of trials with licking to sound sequence presentation was calculated separately for S⁺ and S⁻ every 2 h during the session (mean ± SEM). (E) Test performance was estimated as the difference in the percentage of trials with licking between S⁺ and S⁻. The results for the first 6 h of the session (mean ± SEM) are shown.

Figure 2.

Effects of electrolytic lesions in the AC on sound sequence discrimination learning. (A) Schematic drawing of the AC. Numbers in the figure represent Krieg’s brain areas (Krieg 1946). (B) The lesions produced bilaterally in the AC 1 wk prior to the test sessions were histologically confirmed after the behavioral test. The lateral view (left) and frontal section (right) of the brain. Hatched and filled areas represent maximal and minimal extent of the lesions, respectively. (C) The AC lesions attenuated the test performance of sound sequence discrimination compared with that in sham-operated rats (*P < 0.05, repeated measures ANOVA). The test performance in the first 6 h of each session is plotted. (D) The AC lesions had no significant effect on discrimination between the PT and AM sound sequence components. (E) The lesions attenuated discrimination learning between AMs with modulation depths of 70% and 30% (*P < 0.05).

Figure 3.

Effects of 6-OHDA lesions in the AC on sound sequence discrimination learning. (A) Immunohistochemical staining of tyrosine hyroxylase in the AC of a control rat (left) and a rat injected with 6-OHDA into the AC (right). (B) Bilateral 6-OHDA lesions in the AC attenuated sound sequence discrimination (*P < 0.05, repeated measures ANOVA with post hoc Scheffe’s test). (C) Total number of trials with licking during the test sessions was not attenuated by the 6-OHDA lesions. (D) 6-OHDA lesions had no significant effect on discrimination between the PT and AM. (E) 6-OHDA lesions had no effect on discrimination learning between AMs with modulation depths of 70% and 30%.

Figure 4.

Effects of 6-OHDA lesions in the NAc on sound discrimination learning. (A) Bilateral 6-OHDA lesions in the NAc attenuated sound sequence discrimination learning (*P < 0.05). (B) Total number of trials with licking during the test session was not attenuated by the 6-OHDA lesions. (C) 6-OHDA lesions attenuated discrimination between the PT and AM sound sequence components (*P < 0.05). (D) 6-OHDA lesions attenuated discrimination learning between AMs with modulation depths of 70% and 30% (*P < 0.05). (E) Immunostaining of tyrosine hydroxylase in the mPFC of a control rat (left) and a rat injected with 6-OHDA to the mPFC (right). (F,G) 6-OHDA lesions in the mPFC had no significant effect on sound sequence discrimination learning (F) or licking behavior (G).

Figure 5.

Effects of concurrent application of 6-OHDA plus 192IgG-saporin to the AC on sound sequence discrimination learning. A mixture of 192IgG-saporin plus 6-OHDA was injected into the AC 1 wk prior to the test sessions. Attenuation of test performance produced by 192IgG-saporin plus 6-OHDA was similar to that induced by 6-OHDA or 192IgG-saporin alone (*P < 0.05). Data for 192IgG-saporin alone are reproduced from our previous paper (Kudoh et al. 2004).

Figure 6.

Effects of D1 receptor antagonist on licking behavior. (A) SCH23390 (0.2 mg/kg, i.p.) reduced the total number of trials with licking behavior (*P < 0.05). (B,C) Rats were overtrained to lick the spout in response to sound presentation for 2 d before the sound sequence discrimination test. In the overtrained rats, SCH23390 had no significant effect on licking behavior (B) or on sound sequence discrimination (C).

Figure 7.

Effects of D2 family antagonist on sound sequence discrimination learning. (A) Haloperidol (0.5 and 2.0 mg/kg, i.p.) attenuated sound sequence discrimination (*P < 0.05). (B) Haloperidol (0.5 mg/kg) had no significant effect on licking behavior, while haloperidol at a higher dose (2.0 mg/kg) suppressed licking (*P < 0.05). (C) Haloperidol had no significant effect on discrimination between the sound sequence components. (D,E) Rats were overtrained to lick the spout in response to sound presentation for 2 d before the sound sequence discrimination test. Haloperidol attenuated sound sequence discrimination (D, *P < 0.01), while it had no effect on total licking (E).

Figure 8.

Effects of D2- and D4-specific antagonists on sound sequence discrimination learning. (A) D2 antagonist raclopride (1 mg/kg, i.p.) had a slight but nonsignificant effect on sound sequence discrimination learning. (B) D4 antagonist clozapine (5 mg/kg, i.p.) had a slight but nonsignificant effect. Simultaneous application of raclopride plus clozapine significantly attenuated sound sequence discrimination (*P < 0.05). (C,D,E) Licking behavior was not significantly suppressed by raclopride alone (C), clozapine alone (D), or simultaneous application of both agents (E).

Figure 9.

Effects of dopaminergic antagonists on the acquired performance of sound sequence discrimination. (A) SCH23390 (0.5 mg/kg, i.p.) had no significant effect on the acquired test performance achieved during four consecutive test sessions conducted 1 wk previously. (B) Haloperidol (0.5 mg/kg, i.p.) had no effect on the acquired test performance.