Thresholding of sensory inputs by extrasynaptic glutamate receptors in olfactory bulb glomeruli

The mammalian olfactory bulb has presented a challenging system for understanding information processing, in part because the bulb largely lacks the topographical ordering of neurons that promotes processes such as lateral inhibition. Here we have used dual and triple-cell recordings in rodent bulb slices combined with ultrastructural methods to provide the first experimental evidence for a processing mechanism circumventing this problem that operates at the level of single glomeruli, the bulbs odorant receptor-specific modules. A key feature is non-traditional, extrasynaptic glutamatergic signaling derived from excitatory interneurons and what it means for the local balance between excitation (E) and inhibition (I). We found that the distinct dynamic properties of extrasynaptic excitation versus synaptic inhibition create a thresholding effect whereby only strong stimuli produce a favorable E/I balance enabling an output. This single-glomerulus threshold could have a number of important functions during natural odor responses, for example enhancing stimulus tuning.