Chronic metabotropic glutamate receptor 5 inhibition corrects local alterations of brain activity and improves cognitive performance in fragile X mice.

BACKGROUND: Fragile X syndrome (FXS) is the most common genetic cause for intellectual disability. Fmr1 knockout (KO) mice are an established model of FXS. Chronic pharmacological inhibition of metabotropic glutamate receptor 5 (mGlu5) in these mice corrects multiple molecular, physiological, and behavioral phenotypes related to patients' symptoms. To better understand the pathophysiology of FXS and the effect of treatment, brain activity was analyzed using functional magnetic resonance imaging in relation to learning and memory performance.
METHODS: Wild-type (WT) and Fmr1 KO animals receiving chronic treatment with the mGlu5 inhibitor CTEP or vehicle were evaluated consecutively for 1) learning and memory performance in the inhibitory avoidance and extinction test, and 2) for the levels of brain activity using continuous arterial spin labeling based functional magnetic resonance imaging. Neural activity patterns were correlated with cognitive performance using a multivariate regression analysis. Furthermore, mGlu5 receptor expression in brains of untreated mice was analyzed by autoradiography and saturation analysis using [(3)H]-ABP688.
RESULTS: Chronic CTEP treatment corrected the learning deficit observed in Fmr1 KO mice in the inhibitory avoidance and extinction test and prevented memory extinction in WT and Fmr1 KO animals. Chronic CTEP treatment normalized perfusion in the amygdala and the lateral hypothalamus in Fmr1 KO mice and furthermore decreased perfusion in the hippocampus and increased perfusion in primary sensorimotor cortical areas. No significant differences in mGlu5 receptor expression levels between Fmr1 WT and KO mice were detected.
CONCLUSIONS: Chronic mGlu5 inhibition corrected the learning deficits and partially normalized the altered brain activity pattern in Fmr1 KO mice.