Functional coding variation in the presynaptic dopamine transporter associated with neuropsychiatric disorders drives enhanced motivation and context-dependent impulsivity in mice

Abstract

Recent genetic analyses have provided evidence that clinical commonalities associated with different psychiatric diagnoses often have shared mechanistic underpinnings. The development of animal models expressing functional genetic variation attributed to multiple disorders offers a salient opportunity to capture molecular, circuit and behavioral alterations underlying this hypothesis. In keeping with studies suggesting dopaminergic contributions to attention-deficit hyperactivity disorder (ADHD), bipolar disorder (BPD) and autism spectrum disorder (ASD), subjects with these diagnoses have been found to express a rare, functional coding substitution in the dopamine (DA) transporter (DAT), Ala559Val. We developed DAT Val559 knock-in mice as a construct valid model of dopaminergic alterations that drive multiple clinical phenotypes, and here evaluate the impact of lifelong expression of the variant on impulsivity and motivation utilizing the 5- choice serial reaction time task (5-CSRTT) and Go/NoGo as well as tests of time estimation (peak interval analysis), reward salience (sucrose preference), and motivation (progressive ratio test). Our findings indicate that the DAT Val559 variant induces impulsivity behaviors that are dependent upon the reward context, with increased impulsive action observed when mice are required to delay responding for a reward, whereas mice are able to withhold responding if there is a probability of reward for a correct rejection. Utilizing peak interval and progressive ratio tests, we provide evidence that impulsivity is likely driven by an enhanced motivational phenotype that also may drive faster task acquisition in operant tasks. These data provide critical validation that DAT, and more generally, DA signaling perturbations can drive impulsivity that can manifest in specific contexts and not others, and may rely on motivational alterations, which may also drive increased maladaptive reward seeking.

Keywords: Attention-deficit hyperactivity disorder; Dopamine transporter; Impulsivity; Instrumental learning; Motivation; Transgenic model.

Figure 1. DAT VAL559 mice show alterations in 5-CSRTT acquisition and impulsivity

(A)DAT Val559 mice learn 5-choice serial reaction time task (5-CSRTT) faster than their WT littermate counterparts. (WT, n=18; VaL559, n=19; Two-Way RM-ANOVA, time P ˂ 0.0001, interaction P ˂ 0.0001, genotype P < 0.01; post-hoc tests reveal P ˂ 0.05 at day 6 and 27, P ˂ 0.01 at day 9 and 26, and P ˂ 0.0001 at Day 7 and 8). (B) DAT Val559, similar to their faster acquisition of the task, also display better accuracy on the 5-choice task on their first day experience of the full paradigm at a 16 sec stimulus duration (Two-Way RM-ANOVA, Time P ˂ 0.05, interaction P ˂ 0.0001, genotype P < 0.05; post-hoc tests reveal P ˂ 0.01 at day 1 16 sec) (C) No genotype differences were observed for premature responses working down to and at baseline (WT, n=18; VaL559, n=19; Two-Way RM-ANOVA). (D) DAT Val559 mice display a higher impulsivity than WT mice when the delay between the start of a trial and stimulus presentation is increased from 5 sec to 15 sec (two-tailed Student’s t-test, premature responses P < 0.05 (WT. n=15; VaL559, n=19). (E) DAT Val559 mice display reduced impulsivity compared to WT mice when the delay between the start of a trial and stimulus presentation is variable/unpredictable (two-tailed Student’s t-test, premature responses P <0.05 (WT, n=15; Val559 = 19). (F) Additionally, when premature responses were separated out based on the delay duration, DAT Val559 mice specifically show a reduction at the 15 sec duration (delay P ˂ 0.0001, genotype P <0.05, interaction P < 0.05; post-hoc reveals a genotype difference at the 15 sec delay, P ˂ 0.01).

Figure 2. DAT Val559 mice have normal interval timing, but heightened response rates with longer wait times

(A) Training schematic for Peak Interval task. Mice were trained on a fixed interval of 5 sec or 15 sec (WT, n=10 and 9 per training group, VAL559, n=10 per training group). Mice were then tested on a probe trial for accuracy of response timing. (B) Both WT and VAL559 mice display the same response curves at the 5 sec timing (Two-Way RM-ANOVA; interaction P > 0.05, time P > 0.05, genotype P > 0.05). (C) However when the interval is increased to 15 sec, DAT Val559 display an increase in response rate amplitude (Two-Way RM ANOVA; interaction P ˂ 0.0001, time P ˂ 0.0001, genotype P =0.0557).

Figure 3. DAT Val559 mice show alterations in the Go/NoGo task

(A) DAT Val559 mice acquire the Go/NoGo task differently than WT (WT, n=8, Val559, n=12; Two-Way RM-ANOVA, genotype P < 0.05, day P < 0.0001, interaction P < 0.05). (B) DAT Val559 mice progressively make more omissions (Two-Way RM-ANOVA, genotype P < 0.05, day P < 0.0001, interaction P < 0.05; post-hoc tests reveal P < 0.05 at day 8). (C) DAT Val559 mice trend towards making more correct rejections than WT (Two-Way RM-ANOVA, genotype P = 0.07, day P < 0.0001, interaction P > 0.05). (D) No difference was seen in premature responses between genotypes (Two-Way RM-ANOVA, Two-Way RM-ANOVA; interaction P > 0.05, time P > 0.05, genotype P > 0.05). (E) No genotype difference was present in percent omissions during long delay (two-tailed Student’s t-test, P > 0.05). (F) DAT Val559 mice made more correct rejections during long delay (two-tailed Student’s t-test, P < 0.05). (G) No genotype difference was seen in premature responses during long delay (two-tailed Student’s t-test, P > 0.05).

Figure 4. DAT Val559 mice have enhanced motivation as seen in progressive ratio without alterations in sucrose preference

(A) DAT Val559 mice display a higher break point in the Progressive Ratio (PR) task compared to WT (two-tailed Student’s t-test, PR Break P < 0.04 (WT, n = 22; VaL559; n = 21)) and (B) PR met (P < 0.05). (C) DAT Val559 made more correct responses (P < 0.05) and (D) received more rewards (P < 0.05). (E) Higher response rates were also seen (P < 0.05). (F) Sucrose preference was calculated showing no statistical difference between genotype (Two-Way RM ANOVA; P > 0.05; n = 8 per genotype).