Role of metabotropic glutamate receptors in amygdala

The aim of the proposed study is to elucidate physiological roles of metabotropic glutamate receptors in the amygdala, which is deeply related to emotional memory formation in the brain. The results obtained in this study may be useful for the understanding and treatment of human anxiety disorders. The metabotropic glutamate (mGlu) receptors have various roles in regulating synaptic transmission in the brain, learning and memory formation. Particularly for the expression and consolidation of fear-related behaviours, mGlus in the amygdala may play a critical role. However, little is known concerning where in the amygdala and how these receptors mediate these emotional behaviours. In the present study, we plan to address these questions focusing on morphological alteration of neuronal synapses and mGlu localization in the amygdala related to the behavioural changes made by emotional memory formation. We have already found by combining neuronal tracer labeling and immunoelectron microscopy for mGlus that mGlu1 is expressed in neurons of the intercalated cell clusters and terminals synapsing on these neurons have abundant mGlu7 and mGlu8 at the presynaptic active zone. Furthermore, these mGlu7-positive terminals are derived from the thalamus and the basolateral amygdaloid nucleus, but not from the mediat prefrontal cortex (unpublished). Input from the medial prefrontal cortex to the intercalated cell clusters has been recently reported to facilitate extinction of conditioned fear responses. In preliminary experiments, we have found that an agonist for mGlu7 injected into the amygdala before the extinction training, facilitates the extinction whereas an agonist for mGlu8 markedly suppresses it (unpublished). Therefore, in the present study, by combining immunoelectron microscopy including the newly developed SDS freeze-fracture replica labeling and behavioural pharmacological studies using mGlu knockout mice, we plan to examine: i) which inputs to the intercalated and basolateral neurons posses group III mGlus; ii) the extent of co-localization and relative content of these mGlus in the same terminals using SDS-FRL; iii) whether the expression and consolidation of both cued and contextual fear extinction are modulated by pharmacological activation or inhibition of group III mGlus and in mice lacking these receptors.

The aim of this study has been to elucidate the physiological roles of metabotropic glutamate receptors in the amygdala, which is deeply related to emotional memory formation in the brain. The results obtained in this study may be useful for the understanding and treatment of human anxiety disorders. The metabotropic glutamate (mGlu) receptors have various roles in regulating synaptic transmission in the brain, learning and memory formation. Particularly for the expression and consolidation of fear-related behaviours, mGlus in the amygdala may play a critical role. However, little was known concerning where in the amygdala and how these receptors mediate these emotional behaviours. In the present study, we have found that an mGlu7 agonist injected into the basolateral amygdala before the extinction training facilitates fear extinction, whereas an agonist for mGlu8 sustained freezing during the extinction acquisition trial. We have also elucidated at the ultrastructural level the mGlu localization in the amygdala. Our study showed that axon terminals containing mGlu7 and mGlu8 receptors at the presynaptic active zone selectively target neurons expressing mGlu1 in the intercalated cell clusters (ITC) and basolateral complex (BLA). Furthermore, we demonstrated that mGlu7-positive terminals originate from the thalamus and BLA, whereas the source of axon containing mGlu8 receptors could not be identified. In addition, in a further attempt to unravel the mechanisms of fear memory formation in the amygdala, we have revealed that GABAergic synapses in the BLA undergo structural and functional plasticity after fear conditioning. Such plastic changes, that consisted in an enlarged synaptic area and altered kinetics of GABAA receptors, were fully reversed by extinction training.