Activity differentially regulates the surface expression of synaptic AMPA and NMDA glutamate receptors

Abstract

Distinct subtypes of glutamate receptors often are colocalized at individual excitatory synapses in the mammalian brain yet appear to subserve distinct functions. To address whether neuronal activity may differentially regulate the surface expression at synapses of two specific subtypes of ionotropic glutamate receptors we epitope-tagged an AMPA (␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor subunit (GluR1) and an NMDA (N-methyl-Daspartate) receptor subunit (NR1) on their extracellular termini and expressed these proteins in cultured hippocampal neurons using recombinant adenoviruses. Both receptor subtypes were appropriately targeted to the synaptic plasma membrane as defined by colocalization with the synaptic vesicle protein synaptophysin. Increasing activity in the network of cultured cells by prolonged blockade of inhibitory synapses with the ␥-aminobutyric acid type A receptor antagonist picrotoxin caused an activity-dependent and NMDA receptor-dependent decrease in surface expression of GluR1, but not NR1, at synapses. Consistent with this observation identical treatment of noninfected cultures decreased the contribution of endogenous AMPA receptors to synaptic currents relative to endogenous NMDA receptors. These results indicate that neuronal activity can differentially regulate the surface expression of AMPA and NMDA receptors at individual synapses.Information about the mechanisms of synaptic transmission and synaptic plasticity in the mammalian brain derives primarily from electrophysiological studies of excitatory synapses in the hippocampus. These synapses use the neurotransmitter glutamate, which can act on distinct subtypes of ionotropic and metabotropic receptors that frequently colocalize at individual synapses but appear to subserve distinct functions (1-3). Two major subtypes of ionotropic receptors, AMPA (␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) and NMDA (Nmethyl-D-aspartate) receptors, have been found at virtually all excitatory synaptic connections in the mammalian brain. AMPA receptors (AMPARs) are heteromers of the homologous subunits GluR1-4 and mediate the bulk of synaptic transmission during basal neural activity (1-3). NMDA receptors (NMDARs) also exist as heteromers formed from the NR1 subunit and one or more NR2A-D subunits (1-3). Because of their voltage dependence and high calcium permeability, NMDARs are particularly important for triggering several different forms of synaptic plasticity, including longterm potentiation and long-term depression. When inappropriately activated during a variety of pathological conditions, NMDARs also contribute to neuronal injury and death.It has commonly been assumed that AMPARs and NMDARs are colocalized at individual synapses (1-4), although it is now clear that these receptor subtypes interact with different proteins at the synapse (5). The distinct molecular interactions and functions of these receptor subtypes raise the possibility that their surface expression at synapses may be independently regulated....

Distinct molecular mechanisms and divergent endocytotic pathways of AMPA receptor internalization

“…To date, studies of AMPA receptor trafficking have focused on the presence or absence of AMPA receptors at synaptic sites, or on internal versus surface expression [3][4][5][6][7][8][9][10][15][16][17][18] . However, the multiple subunits of AMPA receptors interact differentially with diverse cytoplasmic proteins including GRIP/ABP, PICK1, NSF and SAP97, all of which have been implicated in synaptic targeting of AMPA receptors 3,5,17,[19][20][21][22][23][24][25][26][27] .…”

“…Moreover, changes in postsynaptic membrane trafficking or in synaptic targeting of AMPA receptors have been correlated with alterations in synaptic efficacy [2][3][4][5][6][7][8][9] .…”

“…Mature neurons in culture also exhibit slow changes in synaptic AMPA receptor expression in response to chronic changes in endogenous activity 9,13,14 . On a faster time scale, induction of long-term depression (LTD) or acute application of AMPA or insulin can induce a loss of AMPA receptors from the cell surface of cultured hippocampal neurons within minutes 3,6,9,15 . Conversely, induction of long-term potentiation (LTP) and activation of CaMKII are correlated with a rapid recruitment of AMPA receptors to the postsynaptic membrane 5,8 .…”

Dendritic spine geometry is critical for AMPA receptor expression in hippocampal CA1 pyramidal neurons

“…Substantial biochemical evidence indicates that glutamate receptors in postsynaptic densities (PSDs) are regulated by various protein machineries that link the receptors to the cytoskeleton 3,4,5 and that control the insertion [6][7][8][9][10] and phosphorylation of the receptors 11,12 . Individual dendritic spines have been thought to act as functional compartments of glutamate receptor expression, given that they are physically and thus metabolically separated from the body of the dendrite by the narrow spine neck [13][14][15][16] .…”

Regulation of AMPA receptors and synaptic plasticity

“…We will review here how changes in postsynaptic AMPAR accumulation contribute to synaptic scaling of excitatory synapses. Chronic manipulations of neuronal activity in dissociated cultured neurons produce effects on the number of synaptic AMPARs, and on AMPAR-mediated excitatory postsynaptic currents (Lissin et al, 1998;O'Brien et al, 1998;Turrigiano et al, 1998). Increasing activity by chronically blocking inhibitory synaptic transmissision using picrotoxin leads to a decrease in the number of surface AMPARs in hippocampal (Lissin et al, 1998), spinal (O'Brien et al, 1998 or cortical neurons in culture.…”