Inhibitory interneurons in the olfactory bulb are thought to play an important role in shaping neuronal representation of odours. Recently, we have shown that altering granule cell-mediated inhibition in behaving animals affects their performance in difficult odour discrimination tasks (Neuron 2010, 65:399). Mechanistically, inhibitory inputs to mitral and tufted cells have been suggested to have diverse functions, including enhancing contrast between similar odours, maintaining invariance of responses to a given odour over different concentrations, establishing temporal coherence and improving precision in spike timing.
Here, using a combination of in vivo whole-cell recordings, pharmacological and optogenetic agents and computational approaches, we investigate the impact of the inhibitory circuitry on odour representation.
We find that odour-evoked responses are dominated by feed-forward inhibition. Measurement of responses to different odours and concentrations and computational modeling suggests that such feed-forward inhibition might boost odour discrimination in selected affinity ranges.
Furthermore, assessing the temporal structure of odour-evoked activity we find tight coupling of mitral and tufted cell activity to the breathing rhythm. Surprisingly, mitral and tufted cells phase lock to distinct phases of the respiratory cycle and thus send information to olfactory cortical areas in distinct bouts of activity. Mechanistically, again GABAergic olfactory bulb interneurons play an important role in shifting the preferred phase of mitral cell activity away from tufted cells.
We conclude that the inhibitory circuitry of the olfactory bulb is key in shaping odour-evoked responses on different time scales. This might serve as the basis for the observed alterations in odour discrimination ability of animals with altered inhibition.