Transient overexpression of striatal D2 receptors impairs operant motivation and interval timing

Abstract

The striatum receives prominent dopaminergic innervation that is integral to appetitive learning, performance, and motivation. Signaling through the dopamine D2 receptor is critical for all of these processes. For instance, drugs with high affinity for the D2 receptor potently alter timing of operant responses and modulate motivation. Recently, in an attempt to model a genetic abnormality encountered in schizophrenia, mice were generated that reversibly overexpress D2 receptors specifically in the striatum (Kellendonk et al., 2006). These mice have impairments in working memory and behavioral flexibility, components of the cognitive symptoms of schizophrenia, that are not rescued when D2 overexpression is reversed in the adult. Here we report that overexpression of striatal D2 receptors also profoundly affects operant performance, a potential index of negative symptoms. Mice overexpressing D2 exhibited impairments in the ability to time food rewards in an operant interval timing task and reduced motivation to lever press for food reward in both the operant timing task and a progressive ratio schedule of reinforcement. The motivational deficit, but not the timing deficit, was rescued in adult mice by reversing D2 overexpression with doxycycline. These results suggest that early D2 overexpression alters the organization of interval timing circuits and confirms that striatal D2 signaling in the adult regulates motivational process. Moreover, overexpression of D2 under pathological conditions such as schizophrenia and Parkinson's disease could give rise to motivational and timing deficits.

Figure 1.

A, In situ hybridization against the transgenic D₂ mRNA. Expression was detected in the striatum and olfactory tubercle of D₂OE mice and was completely absent in D₂OE-Dox and control mice. B, In situ hybridization of two adjacent 16 μm sections hybridized with an antisense oligonucleotide against the mouse D₂ receptor coding sequence (top) or an antisense oligonucleotide directed against the transgenic D₂ mRNA (bottom). Whereas endogenous D₂ receptors are expressed in dopaminergic midbrain neurons, transgenic D₂ receptors are absent. C, Double in situ hybridization using cRNA probes against acetylcholinesterase (AChE; left) and the transgenic D₂ receptor (trD2R; middle). The right panel shows the overlay (with DAPI nuclear stain).

Figure 2.

Mean response rate on FI trials as a function of session (A) and averaged across all sessions (B). Response rate was reduced in D₂OE mice, and this reduction was partially ameliorated in D₂OE-Dox mice. *p < 0.05. ns, Not significant.

Figure 3.

The development of operant response timing as a function of training session. A, To assess timing of operant responding, we compared the response rate during seconds 22–26 (Window 1) with that of seconds 54–58 (Window 2). B, The window 1/window 2 ratio increased as a function of sessions, but the increase was greater in control mice than in D₂OE and D₂OE-Dox mice. *p < 0.05.

Figure 4.

Timing of operant responses during the final four sessions of the peak interval procedure. A, Mean response rate during peak trials as a function of time in the trial. In B, response rates are expressed as a proportion of the per-trial maximum response rate. Both D₂OE and D₂OE-Dox mice have broader distributions of responding, consistent with reduced timing precision. Response rate was reduced in D₂OE mice, and this deficit was partially rescued in D₂OE-Dox mice.

Figure 5.

Mathematical modeling of peak interval performance. Peak trial data were fit to a Gaussian probability density function, illustrated in A. B–E show the mean best-fit parameter values from the Gaussian fits. B, Peak height was reduced in D₂OE mice but rescued in D₂OE-Dox mice. C, Peak width was significantly increased in D₂OE-Dox mice; the D₂OE group did not differ significantly from either of the other groups. D, Peak location was significantly increased (right-shifted) in D₂OE mice, and this increase was rescued in D₂OE-Dox mice. E, The Gaussian function provided a better fit for the control data than for data from either D₂OE or D₂OE-Dox mice. *p < 0.05.

Figure 6.

On FI trials, mean number of rewards earned per session (A) and mean latency to earn the reward after the FI had elapsed (B). D₂OE mice earned slightly but significantly fewer rewards and took longer to earn rewards than the other groups. Both of these deficits were rescued in the D₂OE-Dox mice. *p < 0.05.

Figure 7.

Performance in the progressive ratio task. A, Kapan–Meier survival function plotting the percentage of subjects continuing to respond on the progressive ratio schedule as a function of session time. D₂OE mice ceased responding significantly sooner than the other groups. B, Mean number of rewards earned per session and break point (last ratio completed). D₂OE mice earned fewer rewards and had a lower break point than the other groups. C, Mean number of lever presses per session. D₂OE and D₂OE-Dox mice made fewer responses than controls. The difference between D₂OE and D₂OE-Dox approached significance (p = 0.08). D, Mean number of head entries to the food compartment while food rewards were available (i.e., the dipper was up). The groups did not differ on this measure, indicating that all groups approached rewards when they were available. *p < 0.05.

Figure 8.

Preference for sucrose solution in the two-bottle choice test. Amount of sucrose solution consumed is expressed as a proportion of total liquid consumed (sucrose plus water). Both control and D₂OE mice exhibited a strong preference for the sucrose solution that was not affected by food deprivation.