Dendrodendritic recurrent excitation in mitral cells of the rat olfactory bulb.

Most neuronal interactions within the olfactory bulb network are mediated by dendrodendritic synapses. Dendritic transmitter release potentially could affect the parent dendrite as well as local neuronal elements that have receptors for the released transmitter. Here we report that under conditions that facilitate N-methyl-D-aspartate (NMDA) receptor activity (reduced GABAA inhibition and extracellular Mg2+), a single action potential evoked by brief intracellular current pulses in mitral cells is followed by a prolonged depolarization, which is blocked by an NMDA receptor antagonist. This depolarization also is evoked by a presumed calcium spike in the presence of tetrodotoxin. A similar NMDA-receptor-dependent prolonged depolarization is elicited by stimulation of the lateral olfactory tract at current intensities subthreshold for antidromic activation of the recorded neuron. These observations suggest that glutamate released from the dendrites of mitral cells excites the same and neighboring mitral cell dendrites. Further evidence suggests that both the apical and lateral dendrites of mitral cells participate in this recurrent excitation. These dendrodendritic interactions may play a role in the prolonged, NMDA-receptor-dependent depolarization of mitral/tufted cells evoked by olfactory nerve stimulation.