Intrinsic cellular and molecular properties of in vivo hippocampal synaptic plasticity are altered in the absence of key synaptic matrix molecules

Hippocampal synaptic plasticity comprises a key cellular mechanism for information storage. In the hippocampus, both long‐term potentiation (LTP) and long‐term depression (LTD) are triggered by synaptic Ca 2+ ‐elevations that are typically mediated by the opening of voltage‐gated cation channels, such as N ‐methyl‐ d ‐aspartate receptors (NMDAR), in the postsynaptic density. The integrity of the post‐synaptic density is ensured by the extracellular matrix (ECM). Here, we explored whether synaptic plasticity is affected in adult behaving mice that lack the ECM proteins brevican, neurocan, tenascin‐C, and tenascin‐R (KO). We observed that the profiles of synaptic potentiation and depression in the dentate gyrus (DG) were profoundly altered compared to plasticity profiles in wild‐type littermates (WT). Specifically, synaptic depression was amplified in a frequency‐dependent manner and although late‐LTP (>24 hr) was expressed following strong afferent tetanization, the early component of LTP (<75 min post‐tetanization) was absent. LTP (>4 hr) elicited by weaker tetanization was equivalent in WT and KO animals. Furthermore, this latter form of LTP was NMDAR‐dependent in WT but not KO mice. Scrutiny of DG receptor expression revealed significantly lower levels of both the GluN2A and GluN2B subunits of the N ‐methyl‐ d ‐aspartate receptor, of the metabotropic glutamate receptor, mGlu5 and of the L‐type calcium channel, Ca v 1.3 in KO compared to WT animals. Homer 1a and of the P/Q‐type calcium channel, Ca v 1.2 were unchanged in KO mice. Taken together, findings suggest that in mice that lack multiple ECM proteins, synaptic plasticity is intact, but is fundamentally different.