Evidence for Ca2+-permeable AMPA receptors in the olfactory bulb

Abstract

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs), a subtype of glutamate receptor, contribute to olfactory processing in the olfactory bulb (OB). These ion channels consist of various combinations of the subunits GluR1–GluR4, which bestow certain properties. For example, AMPARs that lack GluR2 are highly permeable to Ca2+ and generate inwardly rectifying currents. Because increased intracellular Ca2+ could trigger a host of Ca2+-dependent odor-encoding processes, we used whole cell recording as well as histological and immunocytochemical (ICC) techniques to investigate whether AMPARs on rat OB neurons flux Ca2+. Application of 1-naphthylacetyl spermine (NAS), a selective antagonist of Ca2+-permeable AMPARs (CP-AMPARs), inhibited AMPAR-mediated currents in subsets of interneurons and principal cells in cultures and slices. The addition of spermine to the electrode yielded inwardly rectifying current-voltage plots in some cells. In OB slices, olfactory nerve stimulation elicited excitatory responses in juxtaglomerular and mitral cells. Bath application of NAS with d,l-2-amino-5-phosphonovaleric acid (AP5) to isolate AMPARs suppressed the amplitudes of these synaptic responses compared with responses obtained using AP5 alone. Co2+ staining, which involves the kainate-stimulated influx of Co2+ through CP-AMPARs, produced diverse patterns of labeling in cultures and slices as did ICC techniques used with a GluR2-selective antibody. These results suggest that subsets of OB neurons express CP-AMPARs, including functional CP-AMPARs at synapses. Ca2+ entry into cells via these receptors could influence odor encoding by modulating K+ channels, N-methyl-d-aspartate receptors, and Ca2+-binding proteins, or it could facilitate synaptic vesicle fusion.