The vertebrate olfactory bulb processes olfactory stimuli within a two-stage network, the first located within the glomerular input layer and the second in the external plexiform layer below. These subnetworks are bridged via the principal mitral and tufted cells; both strongly draw on dendrodendritic interactions, with the axonless inhibitory granule cells being the main players in the second stage. Granule cells are directing their sole output towards the long lateral dendrites of mitral and tufted cells via reciprocal dendrodendritic synapses.
In spite or because of this apparently reduced anatomy, granule cells are capable of various modes of dendritic signalling, including local sodium spikes within their large dendrodendritic spines and global Ca2+ and sodium spikes. To dissect the reciprocal microcircuit, we are using two-photon Ca2+ imaging and uncaging of glutamate in acute slices of juvenile rat olfactory bulbs in conjunction with whole cell recordings. I will briefly discuss mechanisms underlying the local postsynaptic spine signals and the subsequent release of GABA from the spine back onto the exciting mitral cell dendrite. As to the interaction between local and global signals, we are currently investigating coincidence detection within these excitable spines and summation of inputs from simultanously activated spines within the granule cell dendrite. These findings broaden our view on the functionality of olfactory bulb granule cells and their role in synaptic processing.