Fractionation of spatial memory in GRM2/3 (mGlu2/mGlu3) double knockout mice reveals a role for group II metabotropic glutamate receptors at the interface between arousal and cognition

Abstract

Group II metabotropic glutamate receptors (mGluR2 and mGluR3, encoded by GRM2 and GRM3) are implicated in hippocampal function and cognition, and in the pathophysiology and treatment of schizophrenia and other psychiatric disorders. However, pharmacological and behavioral studies with group II mGluR agonists and antagonists have produced complex results. Here, we studied hippocampus-dependent memory in GRM2/3 double knockout (GRM2/3(-/-)) mice in an iterative sequence of experiments. We found that they were impaired on appetitively motivated spatial reference and working memory tasks, and on a spatial novelty preference task that relies on animals' exploratory drive, but were unimpaired on aversively motivated spatial memory paradigms. GRM2/3(-/-) mice also performed normally on an appetitively motivated, non-spatial, visual discrimination task. These results likely reflect an interaction between GRM2/3 genotype and the arousal-inducing properties of the experimental paradigm. The deficit seen on appetitive and exploratory spatial memory tasks may be absent in aversive tasks because the latter induce higher levels of arousal, which rescue spatial learning. Consistent with an altered arousal-cognition relationship in GRM2/3(-/-) mice, injection stress worsened appetitively motivated, spatial working memory in wild-types, but enhanced performance in GRM2/3(-/-) mice. GRM2/3(-/-) mice were also hypoactive in response to amphetamine. This fractionation of hippocampus-dependent memory depending on the appetitive-aversive context is to our knowledge unique, and suggests a role for group II mGluRs at the interface of arousal and cognition. These arousal-dependent effects may explain apparently conflicting data from previous studies, and have translational relevance for the involvement of these receptors in schizophrenia and other disorders.

Figure 1

GRM2/3^−/− mice were impaired in appetitive SRM (left panels, a and c) and SWM tasks (right panels, b and d). (a) GRM2/3^−/− mice (n=14) displayed slower acquisition of a Y-maze SRM task than wild-type (wt; n=18) mice and (b) GRM2/3^−/− mice (n=14) were persistently impaired in an appetitive T-maze rewarded alternation SWM task compared with wild-types (wt; n=18). Data shown are percent correct responses (mean±SEM) for each block of 10 trials. Chance performance is indicated by the dashed line. (c) GRM2/3^−/− mice (n=13) displayed slower acquisition of the SRM component of an appetitive six-arm radial maze task than wt (n=13) and (d) GRM2/3^−/− mice subsequently made significantly more SWM errors than wt. Data shown are mean errors (±SEM) per trial in each block of four trials. Asterisks indicate statistical significance at p<0.05.

Figure 2

GRM2/3^−/− mice (n=13) did not differ from wild-type mice (wt; n=18) in the water maze SRM task. (a) Path length for each day of testing during acquisition. (b) Percentage time spent in the adjacent left (AdjL), training (TRA), adjacent right (AdjR), and opposite (Opp) quadrants in probe test 1 (day 7). (c) Percentage time spent in each quadrant in probe test 2 (day 11). Chance performance is indicated by the dashed line. Values are mean±SEM.

Figure 3

GRM2/3^−/− mice show a dissociation between performance on appetitive and aversive spatial memory tasks. (a) GRM2/3^−/− mice (n=25) learned the swimming SRM Y-maze task at the same rate as wild-type mice (wt; n=25). Data shown are mean percent correct responses (±SEM) for each block of 5 trials. Inset: in a probe test on day 7, GRM2/3^−/− mice spent an equal or greater (p=0.051) proportion of their time searching in the arm that had previously held the platform, relative to wild-type mice. (b) GRM2/3^−/− mice (n=25) displayed slower acquisition of the appetitive SRM Y-maze task than wild-type controls (n=25). Data shown are mean percent correct responses (±SEM) for each block of 10 trials. Order of testing and the rooms in which the tests were performed were fully counterbalanced. Chance performance is indicated by the dashed line. ^*p<0.05.

Figure 4

Cognitive deficits in GRM2/3^−/− mice cannot be explained simply by reduced appetitive motivation for the milk reward. (a) GRM2/3^−/− mice (n=14) did not differ from wild-type mice (wt; n=18) in acquisition of an appetitively motivated, non-spatial gray vs black–white stripes visual discrimination task (experiment 6). Data shown are mean percent correct responses (±SEM) for each block of 10 trials. Chance performance is indicated by the dashed line. (b) GRM2/3^−/− mice (n=13) show a reduced spatial novelty preference compared with wild-type animals (n=18; experiment 7). Data shown are mean discrimination ratios ((novel arm/(novel+other arm)) (±SEM) calculated for the number of arm entries (left) and the time spent in the arms (right) during the test phase. The discrimination ratio was significantly lower for GRM2/3^−/− mice on both measures. A discrimination ratio of 0.5 reflects chance performance. (^*p<0.05 and ^**p=0.005).

Figure 5

GRM2/3^−/− mice are hypoactive. (a) GRM2/3^−/− mice (n=14) displayed less spontaneous locomotor activity (LMA) than wild-type mice (wt; n=18). (b) In all, 2.5 mg/kg amphetamine increased LMA in both wild-type (n=10) and GRM2/3^−/− mice (n=10) mice, such that GRM2/3^−/− mice remained hypoactive relative to wild-types. (i) Summarizes the total number of beam breaks over a 2-h period while (ii) shows the time course of changes in LMA. (c) In total, 10 mg/kg amphetamine induced stereotypy in both wt (n=6) and GRM2/3^−/− (n=6) mice, resulting in a decrease in LMA followed later by an increase in activity levels relative to saline-injected controls (n=5 wt and n=6 GRM2/3^−/− mice). At all-time points GRM2/3^−/− mice were hypoactive relative to wild-types.

Figure 6

Effect of injection stress on the appetitive T-maze SWM task. Injection stress impaired performance in the wild-types (wt; n=12) but facilitated it in GRM2/3^−/− mice (n=11).