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 processes have been the subjects of considerable research, but physiological mechanisms underlying consolidation have proved difficult to dissociate experimentally. Previous reports have indicated a role for metabotropic glutamate 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 group I mGluRs and its outstanding role in spatial learning. Rats were first trained in a combined context and cue conditioning paradigm. Then, ex vivo analysis 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 transient 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 obtained after 9 days of extinction training. These data provide new insight into the role of the hippocampus and its subregions in memory consolidation. They support the notion that mGluRs in CA3 may play a part in short-term, and those in CA1 may play a part in long-term consolidation of memory.