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Kenneth Tovar" Molecular composition of synaptic NMDA receptors "

Abstract :

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is one of the major neurotransmitter receptor subtypes and is present in almost all excitatory pathways in the central nervous system.
The NMDA receptor has an obligate heteromeric quarternary structure, composed of two NR1 and two NR2 subunits. From mutational studies in heterologous expression systems and by analogy with the crystal structure of the AMPA glutamate receptor subtype, NMDA receptors are likely composed of 4 subunits. The NR2A and NR2B NMDA receptor subunits are the predominant NR2 receptors in the cortex and hippocampus and have been associated with different, and sometimes competing and divergent, forms of synaptic plasticity and signaling pathways. Many of these studies assumed that native NMDA receptors are diheteromeric molecules, composed of NR1 and a single type of NR2 subunit. NMDA receptors containing two different NR2 subunits can be created in heterologous expression systems and have been isolated from whole brain tissue. The question remains, however, of the extent to which triheteromeric receptors may contribute to synaptic transmission.
To address this question we used cultured hippocampal neurons from mice that lack NR2A or NR2B. We demonstrate that no other NR2 subunits are expressed in these cells. Thus in the absence of NR2A or NR2B, our mutant neurons present an ideal model for studying the behavior of EPSCs composed of NR1/NR2B or NR1/NR2A diheteromeric receptors, respectively. Whole-cell voltage clamp recordings from these neurons demonstrated that the predominant time constant of EPSC decay from NR2A and NR2B KO neurons differed from each other by almost an order of magnitude while wild-type EPSC kinetics fell in between the extremes of our diheteromeric receptors. Despite the intermediate EPSC kinetics of wild-type NMDA receptor-mediated EPSCs, they could not be well-fitted using the constituents from the fits of the diheteromeric EPSCs, implying that something other than the combination of different amounts of diheteromeric channels was contributing to the kinetics of wild-type EPSCs. We use the microscopic kinetics and sensitivity of wild-type NMDA receptors to different antagonists to argue that most of the NMDA receptors that open in response to synaptically released glutamate are triheteromeric.

Selected publications

Tovar KR, Maher BJ, Westbrook GL. "Direct actions of carbenoxolone on synaptic transmission and neuronal membrane properties." J Neurophysiol. 2009, 102: 974-8.
Harms KJ, Tovar KR, Craig AM. "Synapse-specific regulation of AMPA receptor subunit composition by activity. J Neurosci. 2005, 25: 6379-88.
Lévi S, Logan SM, Tovar KR, Craig AM. "Gephyrin is critical for glycine receptor clustering but not for the formation of functional GABAergic synapses in hippocampal neurons." J Neurosci. 2004, 24:207-17.
Tovar KR, Westbrook GL. "Mobile NMDA receptors at hippocampal synapses." Neuron. 2002, 34: 255-64.

Tovar KR, Sprouffske K, Westbrook GL. "Fast NMDA receptor-mediated synaptic currents in neurons from mice lacking the epsilon2 (NR2B) subunit." J Neurophysiol. 2000, 83: 616-20.
Tovar KR, Westbrook GL. "The incorporation of NMDA receptors with a distinct subunit composition at nascent hippocampal synapses in vitro." J Neurosci. 1999, 19: 4180-8.