Branch-specific Ca2+ influx from Na+-dependent dendritic spikes in olfactory granule cells

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 microM DNQX. They were affected only slightly by 100 microM 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 approximately 50-80 microm 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.