Olfactory Nerve Stimulation-Evoked mGluR1 Slow Potentials, Oscillations, and Calcium Signaling in Mouse Olfactory Bulb Mitral Cells

Fast synaptic transmission between olfactory receptor neurons and mitral cells (MCs) is mediated through AMPA and NMDA ionotropic glutamate receptors. MCs also express high levels of metabotropic glutamate receptor 1 (mGluR1) whose functional significance is less understood. Here we characterized a slow mGluR1-mediated potential that was evoked by high-frequency (100-Hz) olfactory nerve (ON) stimulation in the presence of NBQX and D-APV, blockers of ionotropic glutamate receptors, and that was associated with a local Ca²⁺ transient in the MC dendritic tuft. High-frequency ON stimulation in the presence of NBQX and D-APV also evoked a slow, nearly 2-Hz oscillation of MC membrane potential that was abolished by the mGluR1 antagonist LY367385 (50 μM). Both mGluR slow potential and slow oscillation persisted in the presence of gabazine (10 μM), a GABA_A receptor antagonist, and intracellular QX-314 (10 mM), a Na⁺ channel blocker. In contrast to a slow mGluR1 potential in cerebellar Purkinje neurons, the MC mGluR1 potential was not depressed by SKF96365 (≤250 μM) and thus is likely not mediated by TRPC1 cation channels, nor was it potentiated by an elevation of intracellular Ca²⁺ level. Imaging with the Na⁺ indicator SBFI revealed a Na⁺ transient in the MC dendrite accompanying the mGluR1 slow potential. We conclude that the MC mGluR1 potential triggered by glutamate released from the ON supports oscillations and synchronizations of MCs associated within one glomerulus.