Male DAT Val559 Mice Exhibit Compulsive Behavior under Devalued Reward Conditions Accompanied by Cellular and Pharmacological Changes

Abstract

Identified across multiple psychiatric disorders, the dopamine (DA) transporter (DAT) Ala559Val substitution triggers non-vesicular, anomalous DA efflux (ADE), perturbing DA neurotransmission and behavior. We have shown that DAT Val559 mice display a waiting impulsivity and changes in cognitive performance associated with enhanced reward motivation. Here, utilizing a within-subject, lever-pressing paradigm designed to bias the formation of goal-directed or habitual behavior, we demonstrate that DAT Val559 mice modulate their nose poke behavior appropriately to match context, but demonstrate a perseverative checking behavior. Although DAT Val559 mice display no issues with the cognitive flexibility required to acquire and re-learn a visual pairwise discrimination task, devaluation of reward evoked habitual reward seeking in DAT Val559 mutants in operant tasks regardless of reinforcement schedule. The direct DA agonist apomorphine also elicits locomotor stereotypies in DAT Val559, but not WT mice. Our observation that dendritic spine density is increased in the dorsal medial striatum (DMS) of DAT Val559 mice speaks to an imbalance in striatal circuitry that might underlie the propensity of DAT Val559 mutants to exhibit compulsive behaviors when reward is devalued. Thus, DAT Val559 mice represent a model for dissection of how altered DA signaling perturbs circuits that normally balance habitual and goal-directed behaviors.

Keywords: animal model; compulsivity; dopamine transporter; goal; habit; impulsivity; instrumental learning; psychiatric disorder.

Figure 1

DAT Val559 males display compulsive reward seeking in devalued states for both goal-directed and habit driven behavioral contexts. WT (n = 12) and DAT Val559 (n = 10) males were subjected to a within subject lever pressing paradigm that utilizes contextual cues coupled to random ratio (RR) or random interval (RI) reinforcement schedules to bias animals toward goal-directed or habitual actions, respectively. (A) Overview of the training paradigm. (B) Representation of outcome expectations based on training schedule. (C) Normalized lever presses in the RR valued, RR devalued, RI valued, and RI devalued contexts. Two-way ANOVA revealed a significant impact of schedule/valuation (F(3, 80) = 32.32, p < 0.0001). (D) Normalized head entries into the reward delivery magazine. Two-way ANOVA revealed a significant effect of genotype (F(1, 37) = 14.82, p = 0.0005). (E) Reward (Ensure^®) or chow consumption in the home cage prior to testing. No significant effects of genotype on reward or chow consumption were detected (F(1, 40) = 2.661, p = 0.1107). Data were analyzed by two-way ANOVA with Sidak’s multiple comparison’s test. * p < 0.05, **** p < 0.0001. ns = not significant. Data are presented as mean ± SEM.

Figure 2

DAT Val559 males show elevated reward seeking phenotypes during devalued progressive ratio. The progressive ratio paradigm was performed in WT (n = 22) and DAT Val559 (n = 21) males following reward devaluation. (A) Progressive ratio break point. (B) Head entries into the reward delivery magazine. (C) Experimental session length. (D) Response rate. Data were analyzed by two-tailed Student’s t-test. * p < 0.05. ns = not significant. Data are presented as mean ± SEM.

Figure 3

Reversal learning is intact in male DAT Val559 mice. WT (n = 14) and DAT Val559 (n = 15) males underwent pairwise discrimination training followed by a reversal phase. The two stimulus images are depicted in (A). (B) Stage progression throughout the duration of the training paradigm. (C) % Accuracy during pairwise discrimination (left) and days to reach accuracy criterion (right). Two-way RM ANOVA revealed a significant effect of genotype (F (1, 29) = 8.089, p = 0.0081, ** on graph). (D) Correction trials by day (left) and total correction trials (right) during pairwise discrimination. (E) % Accuracy during reversal learning (left) and days to reach accuracy criterion (right). (F) Correction trials by day (left) and total correction trials (right) during pairwise discrimination. Note that, for this experiment, only a few animals (<5/group) took longer than 6 days to progress. As a result, days 7–10 are not included in the graphs due to the potential for the low N to artificially inflate genotype effects. Data were analyzed by two-tailed Student’s t-test or two-way RM-ANOVA with Sidak’s multiple comparison’s test. * p < 0.05, ** p < 0.01. ns = not significant. Data are presented as mean ± SEM.

Figure 4

DAT Val559 males exhibit enhanced sensitivity to apomorphine-dependent repetitive motor movements. WT (n = 13) and DAT Val559 (n = 15) mice were given a single injection of the DA agonist apomorphine (5 mg/kg, s.c.) and locomotor activity recorded for 60 min post-injection. Datasets are presented in 5 min time bins across the recording period and as summary data adding up all activity post-injection. (A) Horizontal distance traveled. (B) Vertical locomotor activity. (C) Stereotypic motor movements. Data were analyzed by two-way ANOVA with Sidak’s multiple comparison’s test. *** p < 0.001. ns = not significant. Data are presented as mean ± SEM.

Figure 5

Dendritic spine density is increased in the DMS of male DAT Val559 mice. Spine density in sections from WT (n = 4) and DAT Val559 (n = 4) mice was assessed utilizing Golgi staining coupled to brightfield microscopy. (A) Representative whole coronal slice and notation of regions analyzed. Representative images [scale bar = 100 μm], total spine density, and spine densities for each distinct morphology (thin, stubby, and mushroom) are provided for both the (B) DMS and (C) DLS. Data were analyzed by two-tailed Student’s t-test or two-way ANOVA with Sidak’s multiple comparison’s test. * p < 0.05. ns = not significant. Data are presented as mean ± SEM.