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The Journal of Neuroscience, January 4, 2006, 26(1):30-40; doi:10.1523/JNEUROSCI.1419-05.2006

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Cellular/Molecular
Branch-Specific Ca²⁺ Influx from Na⁺-Dependent Dendritic Spikes in Olfactory Granule Cells

Tibor Zelles,² Jamie D. Boyd,¹ Alexandre B. Hardy,¹ and Kerry R. Delaney¹

¹Department of Biology, University of Victoria, Victoria, British Columbia V8W 3N5, Canada, and ²Institute of Experimental Medicine, Hungarian Academy of Sciences, Budapest H-1083, Hungary

Two-photon laser scanning microscopy was used to correlate electrical events detected with whole-cell somatic recordings to Ca²⁺ transients in dendrites of olfactory bulb granule cells. A subset of spontaneous subthreshold depolarizing events recorded at the soma were shown to correspond to suprathreshold dendritic, Na-dependent action potentials [APs; dendritic spikes (D-spikes)]. These potentials were blocked by intracellular QX-314 (lidocaine N-ethyl bromide), hyperpolarizing current injection at the soma, and by partial inhibition of AMPA/kainate receptors with 0.75 µM DNQX. They were affected only slightly by 100 µM NiCl₂. The majority of D-spikes recorded at the soma had a time to peak of <4 ms, comparable with somatic APs, a nonexponential decay, and amplitudes between 3 and 21 mV. Somatically recorded APs produced Ca²⁺ transients that were observed in spines and dendrites in all parts of the cell. Ca²⁺ transients from D-spikes were restricted to subsets of distal dendrites and their associated spines but were absent from the soma and dendrite within 50–80 µm of the soma. Ca²⁺ transients in different branches could be correlated with different-sized D-spikes. D-spike and backpropagating AP-induced Ca²⁺ transients summed in dendrites, provided the interval between them was >5–6 ms. Generation of a D-spike in a particular dendrite <5–6 ms before a somatic AP blocked backpropagation of the somatic AP into that dendrite. The temporally specific interplay between D-spikes and backpropagating APs may play a role in regulating feedback and feedforward inhibition of groups of mitral cells synapsing on different granule cell dendrites.

Key words: dendrite; dendritic action potential; backpropagating action potential; calcium imaging; olfactory bulb; granule cell

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Received April 11, 2005; revised October 25, 2005; accepted November 3, 2005.

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