The cellular traffic of neurotransmitter receptors has captured a lot of attention over the last decade, mostly because synaptic receptor number is adjusted during synaptic development and plasticity. Although each neurotransmitter receptor family has its own trafficking characteristics, two main modes of receptor delivery to the synapse have emerged: endoexocytotic cycling and surface diffusion [eg, for glutamatergic receptors, see Bredt and Nicoll (2003) and Groc and Choquet (2006)]. Receptor cycling through endo-exocytotic processes can be measured by several experimental means, from biochemical to imaging assays. The use of fluorescent protein (XFP)-tag imaging provides a powerful approach to investigate the trafficking of receptor clusters between neuronal compartments (eg, soma, dendrite, spine)(Kennedy and Ehlers, 2006). A disadvantage of the

XFP-tag approach in live experiment is extreme difficulty in detecting XFP fluorescence signals from small nonclustered receptor pool (Cognet et al., 2002; Lippincott-Schwartz and Patterson, 2003). XFP-tagged neurotransmitter receptors are often present in several cellular compartments from the endoplasmic reticulum to the plasma membrane with various relative contents. For instance, surface XFP-tagged neurotransmitter receptors represent only a minor fraction of the total receptor population, precluding their specific detection. Alternative livecell imaging approaches were thus required to specifically isolate surface receptors. Interestingly, a variant of the green fluorescent protein (GFP), ecliptic pHluorin, shows a reversible excitation ratio change between pH 7.5 and 5.5, and its absorbance decreases as the pH is lowered. Most neurotransmitter receptors, including the ionotropic glutamate ones, display an extracellular N-terminal region, implying that the N terminus will always be in an acidic environment inside the cell, whereas it will be exposed to a neutral pH after insertion into the plasma membrane. By this means, surface receptors can be specifically detected and tracked with live-imaging approaches (Ashby et al., 2004). Alternatively, surface receptors can be labeled and detected by immunocytochemical approaches using