Metabotropic Glutamate Receptors Induce a Form of LTP Controlled by Translation and Arc Signaling in the Hippocampus

Abstract

Activity-dependent bidirectional modifications of excitatory synaptic strength are essential for learning and storage on new memories. Research on bidirectional synaptic plasticity has largely focused on long-term potentiation (LTP) and long-term depression (LTD) mechanisms that rely on the activation of NMDA receptors. In principle, metabotropic glutamate receptors (mGluRs) are also suitable to convert synaptic activity into intracellular signals for synaptic modification. Indeed, dysfunction of a form of LTD that depends on Type I mGluRs (mGluR-LTD), but not NMDARs, has been implicated in learning deficits in aging and mouse models of several neurological conditions, including Fragile X syndrome and Alzheimer's disease. To determine whether mGluR activation can also induce LTP in the absence of NMDAR activation, we examined in hippocampal slices from rats and mice, an NMDAR-independent form of LTP previously characterized as dependent on voltage-gated Ca(2+) channels. We found that this form of LTP requires activation of Type I mGluRs and, like mGluR-LTD but unlike NMDAR-dependent plasticity, depends crucially on protein synthesis controlled by fragile X mental retardation protein and on Arc signaling. Based on these observations, we propose the coexistence of two distinct activity-dependent systems of bidirectional synaptic plasticity: one that is based on the activity of NMDARs and the other one based on the activation of mGluRs.

Significance statement: Bidirectional changes of synaptic strength are crucial for the encoding of new memories. Currently, the only activity-dependent mechanism known to support such bidirectional changes are long-term potentiation (LTP) and long-term depression (LTD) forms that relay on the activation of NMDA receptors. Metabotropic glutamate receptors (mGluRs) are, in principle, also suitable to trigger bidirectional synaptic modifications. However, only the mGluR-dependent form of LTD has been characterized. Here we report that an NMDAR-independent form of LTP, initially characterized as dependent on voltage-gated Ca(2+) channels, also requires the activation of mGluRs. These finding suggest the coexistence of two distinct activity-dependent systems of bidirectional synaptic plasticity: one that is based on the activity of NMDARs and the other one based on the activation of mGluRs.

Keywords: arc; bidirectional; metabotropic glutamate receptors; protein synthesis; synaptic plasticity; translation.

Figure 1.

Induction of non-NMDAR-LTP in hippocampal pathways depends on mGluR and protein synthesis. A, A 200 Hz tetanus delivered in the presence of 100 μm APV induces robust LTP of the CA3→CA1 field EPSPs (open circles), which is blocked by the addition of antagonists to Type I mGluRs (100 μm LY-367385 and 10 μm MPEP; filled circles). B, LTP induced by the 200 Hz tetanus in APV is blocked by the protein synthesis inhibitor anisomycin (10 μm). Open circles represent control DMSO. Filled circles represent anisomycin. A, B, Superimposed traces represent example experiments. Each trace is the average of 10 responses recorded before (thin traces) and 60 min after the tetanus (thick traces). Calibration: 1 mV, 10 ms. C, D, In slices from aged rats (18–20 months), the 200 Hz LTP is also blocked by mGluR antagonists in both CA3→CA1 synapses (C) and in CA3→CA3 synapses (D). The number of experiments is indicated in parentheses.

Figure 2.

TBS induces mGluR-LTP in CA3→CA3 synapses. A, Four TBS epochs (open circles) induce a form of LTP that is completely blocked by 100 μm APV (filled circles). B, LTP induced with 6 TBS epochs (open circles) is not blocked by either 100 μm APV (gray circles) or 10 μm nifedepine (gray triangles), yet it is completely blocked by the coapplication of 100 μm APV and 10 μm nifedepine (black circles). C, In CA3→CA1 synapses, LTP induced with 6 TBS (open circles) is partially blocked by 10 μm nifedepine (black triangles) and completely blocked by 100 μm APV (black circles). The number of experiments is indicated in parentheses.

Figure 3.

Role of FMRP and Arc in the induction of mGluR-LTP in CA1→CA3 synapses. The induction mGluR-LTP with 200 Hz tetanus is blocked by the translation inhibitor anisomycin in control FMRP^(+/+) mice (A) but not in FMRP^(−/−) mice (B). C, D, Role of Arc. Only the transient potentiation normally observed in Arc^(+/+) mice after blockade of protein synthesis (C) is induced in Arc^(−/−) mice. Superimposed traces in each represent example experiments. Each trace represents the average of 10 responses recorded before (thin traces) and 60 min after the tetanus (thick traces). Calibration: 1 mV, 10 ms. The number of mice and experiments is indicated in parentheses.

Figure 4.

Increase in Arc protein after 200 Hz tetanus. Western blot analysis of Arc expression in control (open circle) and tetanized (black circles) slices. Arc levels were relativized to β-actin through densitometry analysis and presented as relative units (RU). Individual data points are laterally displaced for illustrative purposes. Horizontal bars indicate mean ± SEM. *p < 0.01. Top, Example blots from control (C) and tetanized (T) minislices.