Abstract
Dendrodendritic synaptic interactions are a hallmark of neuronal processing in the vertebrate olfactory bulb. Many classes of olfactory bulb neurons including the principal mitral cells (MCs) and the axonless granule cells (GCs) dispose of highly efficient propagation of action potentials (AP) within their dendrites, from where they can release transmitter onto each other. So far, backpropagation in GC dendrites has been investigated indirectly via Ca2+ imaging. Here, we used two-photon Na+ imaging to directly report opening of voltage-gated sodium channels due to AP propagation in both cell types. To this end, neurons in acute slices from juvenile rat bulbs were filled with 1 mM SBFI via whole-cell patch-clamp. Calibration of SBFI signals revealed that a change in fluorescence ΔF/F by 10% corresponded to a Δ[Na+]i of ∼22 mM. We then imaged proximal axon segments of MCs during somatically evoked APs (sAP). While single sAPs were detectable in ∼50% of axons, trains of 20 sAPs at 50 Hz always resulted in substantial ΔF/F of ∼15% (∼33 mM Δ[Na+]i). ΔF/F was significantly larger for 80 Hz vs. 50 Hz trains, and decayed with half-durations τ1/2 ∼0.6 s for both frequencies. In MC lateral dendrites, AP trains yielded small ΔF/F of ∼3% (∼7 mM Δ[Na+]i). In GC apical dendrites and adjacent spines, single sAPs were not detectable. Trains resulted in an average dendritic ΔF/F of 7% (16 mM Δ[Na+]i) with τ1/2 ∼1 s, similar for 50 and 80 Hz. Na+ transients were indistinguishable between large GC spines and their adjacent dendrites. Cell-wise analysis revealed two classes of GCs with the first showing a decrease in ΔF/F along the dendrite with distance from the soma and the second an increase. These classes clustered with morphological parameters. Simulations of Δ[Na+]i replicated these behaviors via negative and positive gradients in Na+ current density, assuming faithful AP backpropagation. Such specializations of dendritic excitability might confer specific temporal processing capabilities to bulbar principal cell-GC subnetworks. In conclusion, we show that Na+ imaging provides a valuable tool for characterizing AP invasion of MC axons and GC dendrites and spines.
Highlights
Many neuronal interactions in the vertebrate olfactory bulb are mediated by dendrodendritic synapses at both stages of the olfactory network, the glomerular input layer and the external plexiform layer (EPL)
The principal mitral cell (MC) and the axonless granule cell (GC) are especially interesting because they interact via dendrodendritic reciprocal synapses and both rely on dendritic transmitter release to exchange information
Geometric dimensions were based on previous data and on two-photon fluorescence scans of the neurons in this data set
Summary
Many neuronal interactions in the vertebrate olfactory bulb are mediated by dendrodendritic synapses at both stages of the olfactory network, the glomerular input layer and the EPL. The dendrites of many classes of olfactory bulb neurons dispose of several mechanisms that support dendrodendritic processing, including release of transmitter (Sheperd et al, 1990; Wachowiak and Shipley, 2006). Bulbar dendrites often are endowed with high densities of active conductances since they feature an efficient propagation of action potentials. MC dendrites are accessible to whole-cell patch-clamp; in combination with Ca2+ and voltage-sensitive dye imaging it has been shown that (1) action potentials can be initiated in their apical dendritic tuft and propagate to the soma and that (2) under certain conditions action potentials will travel far out into their lateral dendrites (Chen et al, 1997; Margrie et al, 2001; Xiong and Chen, 2002; Christie and Westbrook, 2003; Debarbieux et al, 2003; Djurisic et al, 2004)
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