Transient and selective overexpression of D2 receptors in the striatum causes persistent deficits in conditional associative learning

Abstract

Cognitive deficits in schizophrenia are thought to derive from a hypofunction of the prefrontal cortex (PFC), but the origin of the hypofunction is unclear. To explore the nature of this deficit, we genetically modified mice to model the increase in striatal dopamine D(2) receptors (D(2)Rs) observed in patients with schizophrenia. Previously, we reported deficits in spatial working memory tasks in these mice, congruent with the working memory deficits observed in schizophrenia. However, patients with schizophrenia suffer from deficits in many executive functions, including associative learning, planning, problem solving, and nonspatial working memory. We therefore developed operant tasks to assay two executive functions, conditional associative learning (CAL) and nonspatial working memory. Striatal D(2)R-overexpressing mice show a deficit in CAL because of perseverative behavior, caused by interference from the previous trial. D(2)R up-regulation during development was sufficient to cause this deficit, because switching off the transgene in adulthood did not rescue the phenotype. We validated prefrontal dependency of CAL by using neurotoxic lesions. Lesions of the medial PFC including the anterior cingulate, infralimbic, and prelimbic cortices impair CAL because of increased interference from previously rewarded trials, exactly as observed in D(2)R transgenic mice. In contrast, lesions restricted to the infralimbic and prelimbic cortices have no effect on CAL but impair performance in the nonspatial working memory task. These assays not only give us insight into how excess striatal D(2)Rs affect cognition but also provide tools for studying cognitive endophenotypes in mice.

Fig. 1.

Mice with selective overexpression of D₂Rs in the striatum are impaired in conditional associative learning. Developmental overexpression is sufficient to induce the deficit. (A) Percentage correct trials of transgenic and control mice. Each session block consisted of five sessions. Controls included the following: wild-type littermates (n = 5); tetO, mice carrying the tetO-D₂R transgene (n = 5); tTA, mice carrying the CamKII-tTA transgene (n = 7); and D₂R-OE, D₂R-overexpressing mice that carry both transgenes (n = 11). *, P < 0.0001. The solid line represents chance (50%); the dotted line depicts acquisition criterion (78%). (B) Distribution of reinforced lever presses and withholds in the four groups of mice. The number of reinforced responses for each session block is depicted. LP, lever press; WH, withhold. (C) Probability of emitting an error by repeating the previously reinforced response for the four groups (analyzed for last session). *, P < 0.001. (D) Probability of emitting a correct response when repeating a previously reinforced response for the four groups (analyzed for last session). P > 0.05. (E) Percentage correct trials of transgenic and control mice treated with doxycycline: wild-type, tetO, and tTA single transgenic littermates (n = 7); D₂R transgenic, mice carrying both transgenes (n = 5). *, P = 0.05. The solid line represents chance (50%). Error bars represent SEM.

Fig. 2.

Mice with a medial prefrontal lesion including the anterior cingulate are impaired in conditional associative learning. (A) Percentage correct trials of LTA, LNA, and control mice. *, P < 0.006. Each session block consists of five sessions. The solid line represents chance (50%), and the dotted line depicts acquisition criterion (78%). (B) Distribution of reinforced lever presses and withholds for the three groups. The number of reinforced responses for each session block is depicted. LP, lever press; WH, withhold. (C) Probability of emitting an error by repeating the previously reinforced response for the three groups (analyzed for last session). *, P < 0.001. (D) Probability of emitting a correct response when repeating a previously reinforced response for the three groups (last session analyzed). P > 0.05. Error bars represent SEM.

Fig. 3.

Schematic representation of the maximal lesions in the medial PFC for the mice that acquired the task (LTA) and that did not acquire the task (LNA).

Fig. 4.

Working memory is significantly impaired in mice with a lesion restricted to the PL and IL cortices. Percentage correct trials for control and LTA mice when tested without delay (0 s) or variable delays (1–10 s) are shown. *, P < 0.05.