Inhibition of Backpropagating Action Potentials in Mitral Cell Secondary Dendrites

Abstract

The mammalian olfactory bulb is a geometrically organized signal-processing array that utilizes lateral inhibitory circuits to transform spatially patterned inputs. A major part of the lateral circuitry consists of extensively radiating secondary dendrites of mitral cells. These dendrites are bidirectional cables: they convey granule cell inhibitory input to the mitral soma, and they conduct backpropagating action potentials that trigger glutamate release at dendrodendritic synapses. This study examined how mitral cell firing is affected by inhibitory inputs at different distances along the secondary dendrite and what happens to backpropagating action potentials when they encounter inhibition. These are key questions for understanding the range and spatial dependence of lateral signaling between mitral cells. Backpropagating action potentials were monitored in vitro by simultaneous somatic and dendritic whole cell recording from individual mitral cells in rat olfactory bulb slices, and inhibition was applied focally to dendrites by laser flash photolysis of caged GABA (2.5-μm spot). Photolysis was calibrated to activate conductances similar in magnitude to GABAA-mediated inhibition from granule cell spines. Under somatic voltage-clamp with CsCl dialysis, uncaging GABA onto the soma, axon initial segment, primary and secondary dendrites evoked bicuculline-sensitive currents (up to −1.4 nA at −60 mV; reversal at ∼0 mV). The currents exhibited a patchy distribution along the axon and dendrites. In current-clamp recordings, repetitive firing driven by somatic current injection was blocked by uncaging GABA on the secondary dendrite ∼140 μm from the soma, and the blocking distance decreased with increasing current. In the secondary dendrites, backpropagated action potentials were measured 93–152 μm from the soma, where they were attenuated by a factor of 0.75 ± 0.07 (mean ± SD) and slightly broadened (1.19 ± 0.10), independent of activity (35–107 Hz). Uncaging GABA on the distal dendrite had little effect on somatic spikes but attenuated backpropagating action potentials by a factor of 0.68 ± 0.15 (0.45–0.60 μJ flash with 1-mM caged GABA); attenuation was localized to a zone of width 16.3 ± 4.2 μm around the point of GABA release. These results reveal the contrasting actions of inhibition at different locations along the dendrite: proximal inhibition blocks firing by shunting somatic current, whereas distal inhibition can impose spatial patterns of dendrodendritic transmission by locally attenuating backpropagating action potentials. The secondary dendrites are designed with a high safety factor for backpropagation, to facilitate reliable transmission of the outgoing spike-coded data stream, in parallel with the integration of inhibitory inputs.