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The hippocampus (HC) has been suggested to play a role in transitive inference (TI) on an ordered sequence of stimuli. However, it has remained unclear whether HC is involved in the expression of TI, or rather contributes to TI through its role in the acquisition of the sequence of elements (Frank, Rudy, & O'Reilly, 2003). Presently, the authors compared the effects of excitotoxic dorsal HC lesions in C57BL mice that received surgery before or after they were trained to discriminate between pairs of visual stimuli. Performance on a subsequent TI task was worse in mice with pretraining lesions than in those with posttraining lesions, which showed similar performance to shams without lesions. This indicates that HC is not involved in the expression of TI, but may merely help to acquire the underlying representations required for TI.

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Background:Group III metabotropic glutamate receptors (mGlu4, mGlu7, mGlu8) display differential brain distribution, which suggests different behavioral functions. However, comparison across the available animal studies remains methodologically hazardous and controversial. The present report directly compares knockouts for each group III receptor subtype using a single behavioral test battery and multivariate analysis.Methods:The behavioral phenotypes of C57BL/6J mice lacking mGlu4, mGlu7, or mGlu8 and their respective littermates were examined using a multimetric test battery, which included elements of neuromotor performance, exploratory behavior, and learning and memory. Multivariate statistical methods were used to identify subtype-specific behavioral profiles and variables that distinguished between these mouse lines.Results:It generally appears that mGlu7 plays a significant role in hippocampus-dependent spatial learning and in some fear-related behaviors, whereas mGlu4 is most clearly involved in startle and motivational processes. Excepting its influence on body weight, the effect of mGlu8 deletion on behavior appears more subtle than that of the other group III receptors. These receptors have been proposed as potential drug targets for a variety of psychopathological conditions.Conclusion:On the basis of these controlled comparisons, we presently conclude that the different group III receptors indeed have quite distinct behavioral functions.