Dendritic integration in olfactory bulb granule cells upon simultaneous multi-spine activation: Low thresholds for non-local spiking activity

The inhibitory axonless olfactory bulb granule cells form reciprocal dendrodendritic synapses with mitral and tufted cells via large spines, mediating recurrent and lateral inhibition. As a case in point for dendritic transmitter release, rat granule cell dendrites are highly excitable, featuring local Na+ spine spikes and global Ca2+- and Na+-spikes. To investigate the transition from local to global signaling we performed holographic, simultaneous two-photon uncaging of glutamate at up to twelve granule cell spines, along with whole-cell recording and dendritic two-photon Ca2+ imaging in acute juvenile rat brain slices. Coactivation of less than ten reciprocal spines was sufficient to generate diverse regenerative signals that included regional dendritic Ca2+-spikes and dendritic Na+-spikes (D-spikes). Global Na+-spikes could be triggered in one third of granule cells. Individual spines and dendritic segments sensed the respective signal transitions as increments in Ca2+ entry. Dendri...