Effects of D1 receptor knockout on fear and reward learning

Abstract

Dopamine signaling is involved in a variety of neurobiological processes that contribute to learning and memory. D1-like dopamine receptors (including D1 and D5 receptors) are thought to be involved in memory and reward processes, but pharmacological approaches have been limited in their ability to distinguish between D1 and D5 receptors. Here, we examine the effects of a specific knockout of D1 receptors in associative learning tasks involving aversive (shock) or appetitive (cocaine) unconditioned stimuli. We find that D1 knockout mice show similar levels of cued and contextual fear conditioning to WT controls following conditioning protocols involving one, two, or four shocks. D1 knockout mice show increased generalization of fear conditioning and extinction across contexts, revealed as increased freezing to a novel context following conditioning and decreased freezing to an extinguished cue during a contextual renewal test. Further, D1 knockout mice show mild enhancements in extinction following an injection of SKF81297, a D1/D5 receptor agonist, suggesting a role for D5 receptors in extinction enhancements induced by nonspecific pharmacological agonists. Finally, although D1 knockout mice show decreased locomotion induced by cocaine, they are able to form a cocaine-induced conditioned place preference. We discuss these findings in terms of the role of dopamine D1 receptors in general learning and memory processes.

Keywords: Cocaine; D1 receptor; D5 receptor; Dopamine; Extinction; Fear; Learning.

Figure 1. One shock cued fear conditioning is retained in D1 knockout mice

Mice did not show differences in freezing between genotypes in Acquisition (Acq.), Contextual Extinction (Ctx. Ext.), Test 1 or Test 2 during CS-on or CS-off periods. Male and female wildtype (WT), heterozygote (D1 HET), knockout (D1 KO) mice were pooled within genotypes for statistical analysis.

Figure 2. Cued fear conditioning is retained in D1 knockout mice

(A) Freezing during fear conditioning before (pre-CS), during (CS1 and CS2) and after (post-CS) the two conditioning trials in wildtype (WT), heterozygote (D1 HET), and knockout (D1 KO) mice. (B) Average freezing to the context during acquisition (Acq CTX A) and during an extinction test 24 hr later (Ext CTX A), and freezing to the CS during daily cued extinction sessions in Context B (Tests 1–6 CTX B), and during a test in the acquisition context (Renewal CTX A). (C) Freezing during CS-off periods during the cued extinction tests. D1 KO mice showed increased freezing in response to the CS on Test 1 (compared to WT) and on Test 2 (compared to WT). D1 KO mice showed a trend towards increased freezing during CS-off periods on Test 1. D1 KO mice showed a non-significant decrease in freezing during CS presentations compared to WT during Renewal. Error bars indicate SEM. (*) p < 0.05 significant difference compared to WT. (°) p = non-significant trend for KO compared to WT.

Figure 3. D1 agonist (SKF 81297) effects on contextual fear extinction in D1 knockout mice

Mice were treated with SKF 81297 or saline following extinction. WT and D1 HET mice treated with SKF 81297 generally showed non-significant decreases in freezing from Test 1–3. D1 KO mice showed a decrease in freezing during the first block of Test 1, but no differences were apparent on Test 2 or Test 3.

Figure 4. Cocaine conditioned place preference in D1 knockout mice

Panel A shows locomotor activity of WT, D1 HET and D1 KO during pre-test, training days, and preference test day. Panel B shows percent preference for the cocaine-paired floor during the test day in male and female wildtype (WT), heterozygotes (D1 HET) and knockouts (D1 KO). Panel C shows grid+/grid− analysis of WT, D1 HET and D1 KO during the preference test day. During pre-test, cocaine conditioning sessions, and Test Day, knockout mice showed decreased locomotor activity compared to wildtype mice. Knockout mice showed significantly less preference for cocaine-paired floor compared to heterozygote mice in Panel B. Grid+/grid− analysis did not indicate a main effect of genotype, but showed that all genotypes acquired a conditioned place preference when comparing animals that received cocaine on grid floor (grid+) against animals that received saline on grid floor (grid−) during training. Error bars indicate SEM. (*) p < 0.05 significant difference compared to D1 HET or WT.