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REDUCED NON-NMDA CURRENTS AND SYNAPTIC TRANSMISSION IN
HIPPOCAMPAL CA1 NEURONS OF NMDA RECEPTOR MUTANT MICE.
R.-S. Chen, S. Hong, and Y. Li. Neuroscience Program, Beckman Institute, Dept. of Molecular
& Integrative Physiology, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.
Studies using cortical and hippocampal brain slices suggest that many young central synapses
initially contain only NMDA receptors, and are thus functionally silent. The expression of
AMPA receptors in the formerly silent synapses requires NMDA receptor-mediated calcium
influx. However, whether NMDA receptor plays the same role in vivo is less clear. We have
made transgenic mice lacking functional NMDA receptors in cerebral cortex and hippocampus.
These region-specific knockout mice provide a unique opportunity to address this question. On
the other hand, knocking out of NMDA receptors would also prevent us from directly identifying
silent synapses in brain slices. Instead, we tested the hypothesis by recording whole-cell
glutamate current from CA1 pyramidal neurons, which were acutely isolated by combined
enzymatic digestion and mechanical dissociation. Glutamate and NMDA responses were elicited
by rapidly moving the cells into different drug-containing solutions.The inactivation of NMDA
receptors was confirmed by the lacking of NMDA response in the knockout neurons isolated
from 11-day to 18-day old animals. More interestingly, recording of glutamate current showed
that its non-NMDA component was also greatly reduced (0.32±0.04 nA, n=12) in the mutant
neurons, as compared to the control group (4.2±0.5 nA, n=12). Examination of synaptic
transmission in mutant Schaffer collateral pathway showed a similar deficict with AMPAmediated synatpic transmission at (64±10pA, n=5 animals) about half of the control level
(125±22pA, n=8 animals). Western blot analysis showed same levels of GluR1, GluR2, GluR3,
or GluR4 proteins both in control and mutant hippocampi (n=3 each). These results suggest that
maturation of the AMPA receptors to the surface of the CA1 neurons, the conductance of the
AMPA receptors, or both, were affected in the absence of NMDA receptors. The present study
provides a supporting evidence for silent hypothesis in vivo. Furthermore, the dramatic reduction
of the somatic AMPA currents implies a novel mechanism that regulation of the surface nonNMDA receptors by NMDA receptors is not confined locally near the synaptic sites, rather is
executed globally throughout the neuronal processes including the soma surface as a major site
of regulation.