Olfactory Nerve Stimulation-Induced Calcium Signaling in the Mitral Cell Distal Dendritic Tuft

Olfactory receptor neuron axons form the olfactory nerve (ON) and project to the glomerular layer of the olfactory bulb, where they form excitatory synapses with terminal arborizations of the mitral cell (MC) tufted primary dendrite. Clusters of MC dendritic tufts define olfactory glomeruli, where they involve in complex synaptic interactions. The computational function of these cellular interactions is not clear. We used patch-clamp electrophysiology combined with whole field or two-photon Ca²⁺ imaging to study ON stimulation-induced Ca²⁺ signaling at the level of individual terminal branches of the MC primary dendrite in mice. ON-evoked subthreshold excitatory postsnaptic potentials induced Ca²⁺ transients in the MC tuft dendrites that were spatially inhomogeneous, exhibiting discrete "hot spots." In contrast, Ca²⁺ transients induced by backpropagating action potentials occurred throughout the dendritic tuft, being larger in the thin terminal dendrites than in the base of the tuft. Single ON stimulation-induced Ca2 transients were depressed by the NMDA receptor antagonist D-aminophosphonovaleric acid (D-APV), increased with increasing stimulation intensity, and typically showed a prolonged rising phase. The synaptically induced Ca²⁺ signals reflect, at least in part, dendrodendritic interactions that support intraglomerular coupling of MCs and generation of an output that is common to all MCs associated with one glomerulus.