Synaptic plasticity in the adult visual cortex is regulated by the metabotropic glutamate receptor, mGLUR5

In the intact adult animal, synaptic plasticity in the visual cortex (VC) is a dynamic and naturalistic phenomenon, the mechanisms of which are not yet fully understood. Given its intrinsic role in hippocampal plasticity, we investigated the effects of pharmacological antagonism of mGluR5 on synaptic plasticity and receptor expression in the VC of freely moving adult pigmented rats, and compared this with hippocampal effects. Persistent long-term potentiation (LTP, > 24 h) in layer II/III of the primary VC, and LTP in the dentate gyrus, were impaired by application of the mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP). Long-term depression in VC was unaffected. Twenty-four hours after MPEP treatment, mGluR1a monomer expression was reduced in the VC but not in the hippocampus, whereas dimer expression was unaffected; mGluR2/3 and mGluR5 monomers were unaffected, but dimers were reduced in the VC. Our data suggest that mGluR5 is engaged in the regulation of synaptic plasticity in the adult VC and hippocampus: the mechanisms for this may be quite distinct, however. While only LTP is affected by mGluR5-antagonism in the VC, both LTP and LTD are affected in the hippocampus. Furthermore, the higher sensitivity of mGluR expression to antagonism of mGluR5 in VC compared to the hippocampus suggests that mGluR5 regulates plasticity phenomena in these structures by means of distinct mGluR-dependent processes.