Dendritic glutamate autoreceptors modulate signal processing in rat mitral cells.

It has been shown recently that in mitral cells of the rat olfactory bulb, N-methyl-D-aspartate (NMDA) autoreceptors are activated during mitral cell firing. Here we consider in more details the mechanisms of mitral cell self-excitation and its physiological relevance. We show that both ionotropic NMDA and non-NMDA autoreceptors are activated by glutamate released from primary and secondary dendrites. In contrast to non-NMDA autoreceptors, NMDA autoreceptors are almost exclusively located on secondary dendrites and their activation generates a large and sustained self-excitation. Both intracellularly evoked and miniature NMDA-R mediated synaptic potentials are blocked by intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) and result from a calcium-dependent release of glutamate. Self-excitation can be produced by a single spike, and trains of spikes result in frequency facilitation. Thus activation of excitatory autoreceptors is a major function of action potentials backpropagating in mitral cell dendrites, which results in an immediate positive feedback counteracting recurrent inhibition and increasing the signal-to-noise ratio of olfactory inputs.