  Trafficking of glutamate receptors into and out of synapses is critically
involved in the plasticity of excitatory synaptic transmission. Endocytosis and
exocytosis of receptors have initially been thought to account alone for this
trafficking. However, membrane proteins also traffic through surface lateral
diffusion in the plasma membrane. We describe developments in
electrophysiological and optical approaches that have allowed for the real time
measurement of glutamate receptor surface trafficking in live neurons. These
include (i) specific imaging of surface receptors using a pH sensitive
fluorescent protein, (ii) design of a photoactivable drug to inactivate locally
surface receptors and monitor electrophysiologically their recovery, and
(iii)application of single molecule fluorescence microscopy to directly track
the movement of individual surface receptors with nanometer resolution inside
and outside synapses. Altogether, these approaches have demonstrated that
glutamate receptors diffuse at high rates in the neuronal membrane and suggest
a key role for surface diffusion in the regulation of receptor numbers at
synapses.
