Dendritic integration in olfactory bulb granule cells: Thresholds for lateral inhibition and role of active conductances upon simultaneous activation.

The inhibitory axonless olfactory bulb granule cells (GCs) form reciprocal dendrodendritic synapses with mitral and tufted cells via large spines, mediating recurrent and lateral inhibition. Rat GC dendrites are excitable by local Na+ spine spikes and global Ca2+- and Na+-spikes. To investigate the transition from local to global signaling without Na+ channel inactivation we performed simultaneous holographic two-photon uncaging in acute brain slices, along with whole-cell recording and dendritic Ca2+ imaging. Less than 10 coactive reciprocal spines were sufficient to generate diverse regional and global signals that also included local dendritic Ca2+- and Na+-spikes (D-spikes). Individual spines could sense the respective signal transitions as increments in Ca2+ entry. Dendritic integration was mostly linear until a few spines below global Na+-spike threshold, where often D-spikes set in. NMDARs strongly contributed to active integration, whereas morphological parameters barely mattered. In summary, thresholds for GC-mediated bulbar lateral inhibition are low.