Differential trafficking of AMPA receptors following activation of NMDA receptors and mGluRs

Sanderson, Collingridge, Fitzjohn

Description

The removal of AMPA receptors from synapses is a major component of long-term depression (LTD). How this occurs, however, is still only partially understood. To investigate the trafficking of AMPA receptors in real-time we previously tagged the GluA2 subunit of AMPA receptors with ecliptic pHluorin and studied the effects of NMDA receptor activation. In the present study we have compared the effect of NMDA receptor and group I mGluR activation, using GluA2 tagged with super ecliptic pHluorin (SEP-GluA2) expressed in cultured hippocampal neurons. Surprisingly, agonists of the two receptors, which are both able to induce chemical forms of LTD, had clearly distinct effects on AMPA receptor trafficking. In agreement with our previous work we found that transient NMDA receptor activation results in an initial decrease in surface GluA2 from extrasynaptic sites followed by a delayed reduction in GluA2 from puncta (putative synapses). In contrast, transient activation of group I mGluRs, using DHPG, led to a pronounced but more delayed decrease in GluA2 from the dendritic shafts. Surprisingly, there was no average change in the fluorescence of the puncta. Examination of fluorescence at individual puncta, however, indicated that alterations did take place, with some puncta showing an increase and others a decrease in fluorescence. The effects of DHPG were, like DHPG-induced LTD, prevented by treatment with a protein tyrosine phosphatase (PTP) inhibitor. The electrophysiological correlate of the effects of DHPG in the SEP-GluA2 infected cultures was a reduction in mEPSC frequency with no change in amplitude. The implications of these findings for the initial mechanisms of expression of both NMDA receptor- and mGluR-induced LTD are discussed.

Sujets

• Synaptic plasticity
• Long-term depression
• Dihydroxyphenylglycine
• NMDA
• Metabotropic glutamate receptor

Informations

• Publié par: biomed
• Publié le: 01 janvier 2011
• Nombre de lectures: 8
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Sandersonet al.Molecular Brain2011,4:30 http://www.molecularbrain.com/content/4/1/30

R E S E A R C HOpen Access Differential trafficking of AMPA receptors following activation of NMDA receptors and mGluRs 1,2 1,21,3* Thomas M Sanderson, Graham L Collingridgeand Stephen M Fitzjohn

Abstract The removal of AMPA receptors from synapses is a major component of longterm depression (LTD). How this occurs, however, is still only partially understood. To investigate the trafficking of AMPA receptors in realtime we previously tagged the GluA2 subunit of AMPA receptors with ecliptic pHluorin and studied the effects of NMDA receptor activation. In the present study we have compared the effect of NMDA receptor and group I mGluR activation, using GluA2 tagged with super ecliptic pHluorin (SEPGluA2) expressed in cultured hippocampal neurons. Surprisingly, agonists of the two receptors, which are both able to induce chemical forms of LTD, had clearly distinct effects on AMPA receptor trafficking. In agreement with our previous work we found that transient NMDA receptor activation results in an initial decrease in surface GluA2 from extrasynaptic sites followed by a delayed reduction in GluA2 from puncta (putative synapses). In contrast, transient activation of group I mGluRs, using DHPG, led to a pronounced but more delayed decrease in GluA2 from the dendritic shafts. Surprisingly, there was no average change in the fluorescence of the puncta. Examination of fluorescence at individual puncta, however, indicated that alterations did take place, with some puncta showing an increase and others a decrease in fluorescence. The effects of DHPG were, like DHPGinduced LTD, prevented by treatment with a protein tyrosine phosphatase (PTP) inhibitor. The electrophysiological correlate of the effects of DHPG in the SEPGluA2 infected cultures was a reduction in mEPSC frequency with no change in amplitude. The implications of these findings for the initial mechanisms of expression of both NMDA receptor and mGluRinduced LTD are discussed. Keywords:Synaptic plasticity, longterm depression, DHPG, GluA2, NMDA, mGluR, super ecliptic phluorin

Background AMPA receptor trafficking is under exquisite control in excitatory neurons (reviewed in [1,2]). One way to change the efficacy of a synapse is to redistribute AMPA receptors at the postsynaptic membrane so as to either increase or decrease their number and thus alter the responsiveness of the synapse to glutamate. Such changes in synaptic efficacy, termed synaptic plasticity, are crucial for normal brain function, particularly during the development of synaptic connections and memory formation. One form of plasticity, long term depression (LTD), involves a decrease in synaptic strength and can occur via trafficking of AMPA receptors away from

* Correspondence: [e-mail] 1 MRC Centre for Synaptic Plasticity, School of Physiology and Pharmacology, University of Bristol, Bristol, BS8 1TD, UK Full list of author information is available at the end of the article

synapses. Two major forms of LTD have been described in the CNS that are triggered by the activation of NMDA and mGluRs. These are induced physiologically by trains of electrical stimulation [35] but can also be mimicked by the application of specific agonists, in par ticular Nmethyl Daspartate (NMDA) [6,7] and dihy droxyphenylglycine DHPG [810], respectively. For NMDAinduced LTD there is agreement between electrophysiological and imaging studies on the impor tance of AMPA receptor endocytosis in LTD expression [1,3,11,12]. In the case of mGluRinduced LTD (mGluR LTD), however, conflicting evidence has been reported [13]. Immunofluorescence and biochemical studies indi cate that surface AMPA receptor numbers decrease on exposure to DHPG [1416]. However, a range of electro physiological measurements, such as changes in paired pulse facilitation [14,1720], failure rate [17], coefficient

© 2011 Sanderson et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.