Fear conditioning-induced time- and subregion-specific increase in expression of mGlu5 receptor protein in rat hippocampus

Memory formation involves encoding, consolidation and retention. These proc esses have been the subjects of considerable research, but physiological me chanisms underlying consolidation have proved difficult to dissociate exper imentally. Previous reports have indicated a role for metabotropic glutamat e receptors (mGluRs) in memory formation, and we here examined the specific role of mGluRs in the consolidation phase of memory formation. Particular weight was given to the hippocampus due to a high expression level for grou p I mGluRs and its outstanding role in spatial learning. Rats were first tr ained in a combined context and cue conditioning paradigm. Then, ex vivo an alysis of neuronal tissue taken from hippocampal CA1, CA3 or dentate gyrus of behaviourally trained animals showed a 3-fold hyper-expression of mGluR5 protein in CA3 one day after acquisition training. This increase was trans ient and greatly diminished within ten days. The decline was paralleled by an increase in mGluR5 protein expression in CA1 and, to a lesser extent, in dentate gyrus, ten days posttraining. Overexpression in CA1 was also obtai ned after 9 days of extinction training. These data provide new insight int o the role of the hippocampus and its subregions in memory consolidation. T hey support the notion that mGluRs in CA3 may play a part in short-term, an d those in CA1 may play a part in long-term consolidation of memory. (C) 20 00 Elsevier Science Ltd. All rights reserved.