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New Mouse Tests Tie Striatal Dopamine to Learning and Memory

10 October 2008. While many people covet the latest power tools at Home Depot, some schizophrenia researchers yearn for powerful new tools to measure cognition in animal models of the disease. They might consider two new tests of executive function in mice, developed by Eric Kandel, Christoph Kellendonk, Eleanor Simpson, and others at Columbia University in New York City. As reported in the October 2 PNAS Early Edition, their tests of conditional associative learning and non-spatial working memory detected impairment in mice with lesions of the prefrontal cortex, a brain region implicated in the executive functioning deficits that occur in schizophrenia. When administered to transgenic mice that overexpressed dopamine D2 receptors in the striatum, the CAL test revealed deficits that arose from a failure to switch responses when circumstances changed.

Patients with schizophrenia or frontal lobe lesions often show deficits in conditional associative learning (CAL) and working memory, but translating these deficits into animal models can prove challenging (see SRF live discussion). Despite the availability of executive function tests for rats, researchers wanting to study mice have had fewer options. In fact, Mary-Elizabeth Bach, first author of the PNAS study, and associates note that only maze tests have been validated in mice by prefrontal lesions, but they just measure spatial working memory. Consequently, she and her colleagues adapted a test previously used in rats to measure CAL and non-spatial working memory in mice.

The CAL task required connecting stimuli to responses through repeated trials in which only correct responses produced rewards. Specifically, mice received reinforcement only if they pressed a lever within five seconds of hearing a particular sound or refrained from pressing it on hearing a different sound. To assess non-spatial working memory, Bach and colleagues simply added to the CAL task a delay between the stimulus and the opportunity to respond.

Various lines of research link abnormal dopamine activity in the prefrontal cortex to the cognitive impairments often seen in schizophrenia (Gray and Roth, 2007). In the striatum, whose major input source is the prefrontal cortex, imaging studies find increased density of dopamine D2 receptors and greater dopamine docking at them in patients with schizophrenia. Previously, Kandel’s group observed that even a transient increase in the number of D2 receptors in the mouse striatum produced lasting deficits in working memory (see SRF related news story). Their new study explored whether striatal D2 receptors affect other types of executive function as well.

When perseverance doesn’t pay
Employing the transgenic method developed for their previous study, Bach and colleagues overexpressed D2 receptors—only in the striatum—and the resulting transgenic mice made more mistakes than their wild-type siblings on the CAL task. Their excess errors resulted from persevering with a previously rewarded response even after a stimulus change signaled the need for a response shift.

In the model used in the study, doxycycline turns off transgene expression. When Bach and colleagues fed doxycycline-spiked food to eight- to 10-week-old mice, expression of striatal D2 receptors returned to normal, but the CAL deficits remained. Having found similar evidence for developmental changes in their study of spatial working memory, the researchers write, “This suggests that both deficits may share common underlying mechanisms.” They further reason that antipsychotic drugs, which act on dopamine D2 receptors (see SRF current hypothesis), may do little to improve cognition in schizophrenia “because they are given too late”—after the developmental changes have occurred.

To determine whether performance on the CAL task depended on the prefrontal cortex, the investigators used N-methyl-D-aspartate to induce lesions in the medial prefrontal cortex (mPFC). Out of 13 lesioned mice, five simply could not learn the task. These animals behaved like the transgenic mice with upregulated D2 receptors: they favored the response that had earned them rewards in the past even when it no longer did.

Next, the researchers asked whether the extent of mPFC damage distinguished lesioned animals that learned the task from those that did not. They report that non-learners had bigger lesions in the anterior cingulate cortex (see SRF related news story), a hot spot of dopamine innervation (for a review of relationships between brain structure and cognition in schizophrenia, see Antonova et al., 2004). In contrast, the two groups showed similar amounts of damage to the infralimbic and prelimbic cortices. “This might suggest that the behavior is mediated by the anterior cingulate in isolation, or alternatively, it is the combined destruction of these regions that is necessary to produce the deficit,” Bach and colleagues write. On the other hand, lesioned mice that learned the task performed worse than controls on the working memory test, even without anterior cingulate damage.

The Columbia team hopes that these new cognitive assays will prove useful for the study of schizophrenia endophenotypes (see SRF live discussion), but whether they will become a mainstay of the schizophrenia researcher’s toolbox remains to be seen. Meanwhile, this study adds weight to the idea that dopamine abnormalities during development contribute to executive function deficits in schizophrenia.

Bach and colleagues write, “Our results also draw new attention to the striatum by demonstrating that the striatum plays a central role in the cognitive processes affected in schizophrenia.” As such, these results fit into the argument the researchers are making for the overall validity of this model for schizophrenia research. In a recent article, they also proposed the striatal D2 overexpression mice as a model for negative symptoms because the animals are impaired in simple assays of motivation (Drew et al., 2007). Unlike the cognitive deficits reported in the new study by Bach and colleagues, the motivational deficit could be rescued in adult mice by reversing D2 overexpression with doxycycline.—Victoria L. Wilcox.

Reference:
Bach M-E, Simpson, EH, Kahn L, Marshall J, Kandel ER, Kellendonk C. Transient and selective overexpression of D2 receptors in the striatum causes persistent deficits in conditional associative learning. PNAS Early Edition. 2008 Oct 2. Abstract

 
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