Abstract
Reversal of long term potentiation (LTP) may function to increase the flexibility and storage capacity of neuronal circuits; however, the underlying mechanisms remain incompletely understood. We show that depotentiation induced by low frequency stimulation (LFS) (2 Hz, 10 min, 1200 pulses) was input-specific and dependent on N-methyl-d-aspartate (NMDA) receptor activation. The ability of LFS to reverse LTP was mimicked by a brief application of NMDA. This NMDA-induced depotentiation was blocked by adenosine A(1) receptor antagonist. However, the reversal of LTP by LFS was unaffected by metabotropic glutamate receptor antagonism. This LFS-induced depotentiation was specifically prevented by protein phosphatase (PP)1 inhibitors, okadaic acid, and calyculin A but not by the PP2A or PP2B inhibitors. Furthermore, by using phosphorylation site-specific antibodies, we found that LFS-induced depotentiation is associated with a persistent dephosphorylation of the GluR1 subunit of amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor at serine 831, a protein kinase C and calcium/calmodulin-dependent protein kinase II (CaMKII) substrate, but not at serine 845, a substrate of cAMP-dependent protein kinase. This effect was mimicked by bath-applied adenosine or NMDA and was specifically prevented by okadaic acid. Also, the increased phosphorylation of CaMKII at threonine 286 and the decreased PP activity seen with LTP were overcome by LFS, adenosine, or NMDA application. These results suggest that LFS erases LTP through an NMDA receptor-mediated activation of PP1 to dephosphorylate amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and CaMKII in the CA1 region of the hippocampus.
Highlights
We have demonstrated previously that the low frequency stimulation (LFS)-induced depotentiation at Schaffer collateral-CA1 synapses may attribute to an increase of extracellular adenosine acting on the A1 adenosine receptors to interrupt the cAMP-protein kinase A (PKA)-dependent signaling cascades leading to the development of long term potentiation (LTP) [7]
The increased phosphorylation of calmodulin-dependent protein kinase II (CaMKII) at threonine 286 and the decreased PP activity seen with LTP were overcome by LFS, adenosine, or NMDA application
2) The time-dependent reversal of LTP by LFS was mimicked by a brief application of NMDA. 3) NMDA-induced depotentiation was blocked by A1 adenosine receptor antagonist DPCPX. 4) The ability of LFS to reverse LTP was not affected by metabotropic glutamate receptor (mGluR) antagonism
Summary
Slice Preparation—Hippocampal slices were prepared from 4- to 5-week-old male Sprague-Dawley rats as described previously [7, 18]. Immunoblot Analysis—Nitrocellulose membrane was incubated in blocking buffer solution containing 5% nonfat dry milk and 0.1% Tween 20 in PBS (124 mM NaCl, 4 mM KCl, 10 mM Na2HPO4, and 10 mM KH2PO4, pH 7.2) for 1 h and blotted for 2 h at room temperature with antibodies that recognize phosphorylated serine 831 (1:500 dilution) or serine 845 (1:500 dilution) on AMPA receptor GluR1 subunit (Upstate Biotechnology, Inc., Lake Placid, NY) or with a monoclonal antibody to threonine 286-phosphorylated CaMKII ␣-subunit (1:2000 dilution) (Affinity Bioreagents, Golden, CO). Phosphatase activity of 20 ng of protein of CA1 homogenate was measured with the Protein Phosphatase Assay System (Invitrogen) essentially as described by Blitzer et al [20]
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