A behavioral genetics approach to understanding D1 receptor involvement in phasic dopamine signaling

Abstract

Dopamine-producing neurons fire with both basal level tonic patterns and phasic bursts. Varying affinities of the five dopamine receptors have led to a hypothesis that higher affinity receptors are primarily activated by basal level tonic dopamine, while lower affinity receptors may be tuned to be sensitive to higher levels caused by phasic bursts. Genetically modified mice provide a method to begin to probe this hypothesis. Here we discuss three mouse models. Dopamine-deficient mice were used to determine which behaviors require dopamine. These behaviors were then analyzed in mice lacking D1 receptors and in mice with reduced phasic dopamine release. Comparison of the latter two mouse models revealed a similar failure to learn about and respond normally to cues that indicate either a positive or negative outcome, giving support to the hypothesis that phasic dopamine release and the D1 receptor act in the same pathway. However, the D1 receptor likely has additional roles beyond those of phasic dopamine detection, because D1 receptor knockout mice have deficits in addition to what has been observed in mice with reduced phasic dopamine release.

Figure 1. D1R KO mice have altered motor behavior

(A) Locomotion for 13 D1R KO mice and 14 WT mice was monitored for 72 hr. A two-tailed t-test demonstrates that D1R KO mice were hyperactive each night (p=0.019), while displaying normal locomotion during each day (two-way repeated measures ANOVA F_1,50 = 2.237; p=0.147). (B) Abnormal locomotion resulted from a subset of “high responders”. Night activity for 13 D1R KO mice and 14 WT mice was averaged over 3 nights. Four of the KO mice traveled ~4 times further than WT mice at night, while the rest were the same as controls (two-tailed t-test; p=0.502). (C) D1R KO mice (n = 11) and WT mice (n = 11) were tested in a 4-day accelerating rotarod paradigm (4–40 rpm over 120 sec; 3 trials/day). D1R KO mice had a generalized motor coordination deficit and attenuated motor learning. A Students t-test gave a significant difference between groups on the first day of training (p=0.009). Over days, two-way repeated measures ANOVA revealed significant effects of time (F_3,80 = 3.189; p=0.0281) and genotype (F_1,80 = 45.10; p < 0.0001). Further, one-way repeated-measures ANOVA shows that while WT performance improved during the experiment (F_3,43 = 6.242; p=0.0020), D1R KO mice showed no improvement (F_3,43 = 1.807; p=0.167).

Figure 2. Appetitive learning is attenuated, while preference for sweet rewards remains intact

(A) Learning in a T-maze for a food reward was attenuated in D1R KO mice. D1R KO mice (n = 9) and WT mice (n = 10) were food restricted to 85% body weight, then trained (10 trials/day) for 10 days to turn right to obtain a food reward. The correct arm was denoted by horizontal stripes (Zweifel et al. 2009). Two-way repeated measures ANOVA shows a significant effect of genotype (F_1,153 = 6.32; p= 0.0223), time (F_9,153 = 4.50; p <0.001) and time × genotype interaction (F_9,153 = 2.62; p= 0.0076). One-way repeated-measures ANOVA with Bonferroni’s multiple comparison test shows that while WT mice learned the task by day 6 (p < 0.0001), D1R KO mice failed to acquire the task over 10 days (p=0.3341). (B) Saccharin preference was normal in D1R KO mice. D1R KO mice (n = 6) and WT mice (n = 7) were monitored for saccharin preference over 4 consecutive days. The first 2 days examined preference for 0.033% saccharin vs. H₂O with no difference between genotypes (Students t-test; p=0.152). The final 2 days examined preference for 0.066% saccharin with no difference between genotypes (Students t-test; p=0.059). The locations of the saccharin and H₂O bottles were alternated each day. (C) Instrumental conditioning is attenuated in D1R KO mice. D1R KO mice (n = 7) and WT mice (n = 8) were trained on a simple fixed-ratio schedule in which each lever press delivered a single food pellet up to a maximum of 50 (Zweifel et al., 2009). Two-way repeated- measures ANOVA shows an effect of both day (F_6,91 = 6.284; p < 0.0001) and genotype (F_1,91 = 498.1; p < 0.0001).