Glutamate spillover mediates excitatory transmission in the rat olfactory bulb.

In the CNS, glutamate typically mediates excitatory transmission via local actions at synaptic contacts. In the olfactory bulb, mitral cell dendrites release glutamate at synapses formed only onto the dendrites of inhibitory granule cells. Here, I show excitatory transmission mediated solely by transmitter spillover between mitral cells in olfactory bulb slices. Dendritic glutamate release from individual mitral cells causes self-excitation via local activation of N-methyl-D-aspartate (NMDA) receptors. Paired recordings reveal that glutamate release from one cell generates NMDA receptor-mediated responses in neighboring mitral cells that are enhanced by blockade of glutamate uptake. Furthermore, spillover generates spontaneous NMDA receptor-mediated population responses. This simultaneous activation of neighboring mitral cells by a diffuse action of glutamate provides a mechanism for synchronizing olfactory principal cells.