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 tran
smitter spillover between mitral cells in olfactory bulb slices. Dendritic
glutamate release from individual mitral cells causes self-excitation via l
ocal activation of N-methyl-D-aspartate (NMDA) receptors. Paired recordings
reveal that glutamate release from one cell generates NMDA receptor-mediat
ed responses in neighboring mitral cells that are enhanced by blockade of g
lutamate 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 syn
chronizing olfactory principal cells.