Absence of metabotropic glutamate receptor-mediated plasticity in the neocortex of fragile X mice

Fragile X syndrome is a common heritable form of mental retardation in humans. Recent neuroanatomical studies indicate an apparent immature appearance of neurons in fragile X syndrome patients and fragile X mental retardation protein (FMRP)-knockout mice, an animal model of this condition. In this work, we investigated possible alterations in synaptic plasticity in the neocortex of FMRP-knockout mice. Extracellular field potentials were recorded from the deep-layer visual neocortex. Long-term potentiation (LTP) was severely attenuated in brain slices from knockout mice relative to that observed in slices from wild-type mice. Considering that neocortical LTP can involve both NMDA receptor-dependent and -independent mechanisms, we attempted to distinguish the nature of LTP attenuated in the knockout condition. In slices from wild-type mice, LTP was partially attenuated by the NMDA receptor antagonist 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphate (CPP); however, the general metabotropic glutamate receptor (mGluR) antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) strongly attenuated LTP, resulting in a response indistinguishable from that observed in slices from knockout mice. The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) attenuated LTP to a similar degree as did MCPG in wild-type slices, but MPEP did not alter the reduced potentiation in knockout slices. Our results suggest that LTP in layer V visual neocortex depends primarily on mGluR5 activation. Our data also indicate that mGluR5-mediated synaptic plasticity is absent in the neocortex of FMRP-knockout mice. Such an alteration may contribute to the cognitive and learning deficits exhibited in these mice as well as in fragile X syndrome.