Extracellular signal-regulated protein kinase activation is required for metabotropic glutamate receptor-dependent long-term depression in hippocampal area CA1

Abstract

Activation of group 1 metabotropic glutamate receptors (mGluRs) induces long-term depression (LTD) of synaptic transmission that relies on dendritic protein synthesis. We investigated the signal transduction pathways required for mGluR-LTD to identify candidate mechanisms for mGluR regulation of synaptic protein synthesis. Our results demonstrate a role for extracellular signal-regulated protein kinase (ERK), a subclass of the mitogen-activated protein kinases (MAPKs), in mGluR-LTD in area CA1 of the rat hippocampus. Inhibitors of the upstream kinase of ERK, MAP/ERK kinase significantly reduce mGluR-LTD induced by the group 1 agonist dihydroxyphenylglycine (DHPG) and synaptic stimulation but do not affect NMDA receptor-dependent LTD. In contrast, inhibitors of p38 MAPK were ineffective against DHPG-induced LTD. Consistent with the role of ERK in mGluR-LTD, we observed that DHPG treatment of hippocampal slices (isolated CA1), at concentrations that induce LTD, results in a robust phosphorylation of ERK but not of p38 MAPK. These results point to ERK as an important regulator of mGluR-LTD and a potential mechanism for mGluR regulation of synaptic protein synthesis.

Figure 2.

p38 MAPK is not required for DHPG-induced LTD in area CA1. A, B, Application of DHPG (100 μm, 5 min, in 0.1% DMSO) to hippocampal slices induced LTD in FPs recorded in the dentate gyrus (A) and area CA1 (C). B, D, Preapplication of the p38 MAPK inhibitor SB203580 (5 μm, 1 hr) prevents LTD in the dentate gyrus (B) but does not affect LTD in area CA1 (D). Dashed lines indicate 100% of baseline.

Figure 1.

MEK inhibitors selectively inhibit mGluR-LTD. Preapplication of the MEK inhibitors U0126 (A, 20 μm) or PD98059 (B, 50 μm) reduced DHPG-LTD. In this and Figure 2, representative EPSP waveforms (2 min average) taken from an experiment at the times indicated by the numbers on the graph are shown. Calibration: 0.5 mV, 5 msec. C, U0126 also inhibited LTD induced with PP-LFS [900 paired pulses (50 msec interpulse interval) delivered at 1 Hz] in the presence of 100 μm dl-AP-5. D, U0126 (20 μm) did not affect NMDA receptor-dependent LTD induced by LFS (900 pulses at 1 Hz). Dashed lines indicate 100% of baseline.

Figure 3.

DHPG at concentrations that induce LTD and activate the ERK cascade, but not p38 MAPK, in CA1. Representative Western blots are shown of untreated [ACSF or basal (B)] or treated [100 μm DHPG (D), 5 min] hippocampal slices. Quantitative group data of each condition are described below. The number of slices (n) is indicated on each bar. Conditions are indicated below each bar (D 50 and D 100 indicate 50 and 100 μm DHPG, respectively, for 5 min). *p < 0.01, statistically different from condition control. A, DHPG-induced ERK phosphorylation does not rely on NMDAR activation or action potentials and is blocked by U0126. B, DHPG-induced ERK phosphorylation is blocked by the group 1 mGluR antagonists LY367385 (LY) and MPEP. C, DHPG increases the phosphorylation of the ERK substrate RSK1 and is blocked by U0126. Representative Western blots show control (C) extract (phorbol ester-treated HeLa cells) and hippocampal slices (basal) or DHPG treated as indicated. D, In contrast, DHPG treatment does not result in an increase in phosphorylated p38 MAPK/total p38 MAPK. Dashed lines indicate 100% of basal.