Biochemical and morphological characterization of an intracellular membrane compartment containing AMPA receptors

AMPA receptors cycle rapidly in and out of the postsynaptic membrane, while NMDA receptors are relatively immobile. Changing the distribution of AMPA receptors between intracellular and surface synaptic pools is an important means of controlling synaptic strength. However, little is known about the intracellular membrane compartments of neurons that contain AMPA receptors. Here we describe biochemical and morphological characteristics of an intra cellular pool of AMPA receptors in rat brain. By velocity gradient centrifu gation of microsomal light membranes from rat brain, we identified a membra ne fraction enriched for AMPA receptor subunits GluR2/3 but lacking NMDA re ceptors. This membrane compartment sedimented. more slowly than synaptosome s but faster than synaptic vesicles and cofractionated with GRIP, PICK-1 an d syntaxin-13. Morphological examination of this fraction revealed round an d tubular vesicles ranging from similar to 50 to 300 nm in diameter. Immuno cytochemistry of cultured hippocampal neurons showed that a significant por tion of AMPA receptors colocalized with syntaxin-13 (a SNARE protein associ ated with tubulovesicular recycling endosomes) and with transferrin recepto rs. Taken together, these results suggest that a pool of intracellular GluR 2/3 resides in a syntaxin 13-positive tubulovesicular membrane compartment, which might serve as a reservoir for the dendritic recycling of AMPA recep tors. (C) 2001 Elsevier Science Ltd. All rights reserved.