Impact of malocclusion on oral health-related quality of life: insights from children with and without hypodontia.

The glomeruli of the olfactory bulb are the input channels of the olfactory system, where OSNs connect to the brain. A given glomerulus receives input from OSNs expressing a given odorant receptor gene, of which there are approximately 1,000 encoded in the mouse genome. The large size of the odorant receptor gene family endows the glomerular array with an identity code of enormous combinatorial capacity. In addition to this combinatorial identity code, individual glomeruli convey information about odor stimuli in their amplitude of activation and the timing of activity relative to the sniff cycle. However, given that odors typically evoke activity from many glomeruli, it has not been possible to determine the contribution of single glomeruli to odor perception. To determine how a single glomerulus contributes to perception, we used optogenetics to express channelrhodopsin-2 in sensory neurons that express a specific odorant receptor and that project to defined glomeruli. We found that stimulation of a single glomerulus can drive perception, even when many glomeruli are activated simultaneously by an odor stimulus. In addition, we found that different amplitudes of glomerular stimulation can be discriminated, as well as different timing of activation during the sniff cycle. Thus, individual glomeruli can drive behavior using identity, amplitude and temporal coding signals. To stimulate a single glomerulus, we used gene targeting in the mouse to express a fusion protein of channelrhodopsin-2 and yellow fluorescent protein (ChR2-YFP) in OSNs that expressed the odorant receptor (also known as ; ). OSNs expressing the M72 receptor have been extensively characterized in terms of development, anatomy and function, and their properties are similar to those of other characterized OSN types. In these mice, OSNs that express the native, intact M72 receptor also express ChR2-YFP, making them light sensitive (). The axons of ChR2-YFP–expressing sensory neurons recapitulate the normal pattern of M72 OSN axonal projections, innervating a medial and a lateral glomerulus on the dorsal surface of each olfactory bulb.