Correspondence Between Odorant-Evoked Patterns of Receptor Neuron Input and Intrinsic Optical Signals in the Mouse Olfactory Bulb

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Abstract
We compared odorant-evoked patterns of receptor neuron input to the mouse olfactory bulb, imaged with a calcium-sensitive dye, with those of intrinsic optical signals imaged from the same preparations. Both methods yielded patterns of glomerular activity that showed a strong concentration dependence, a loosely organized chemotopy, and involved widely distributed glomeruli. Presynaptic calcium and intrinsic signals showed similar odorant concentration thresholds. Intrinsic signal foci were larger than their corresponding calcium signals, and input to multiple adjacent glomeruli often appeared as a single intrinsic focus. Nonetheless, at near-threshold concentrations, the correspondence between the glomerular calcium and intrinsic signals averaged 75%, with a 71% correspondence between the most strongly activated glomeruli. The correspondence between strongly activated glomeruli decreased as odorant concentration increased, dropping to 51% at 5- to 15-fold higher concentrations. Intrinsic signal foci often saturated at lower concentrations than the calcium signal, implying a smaller dynamic range, and suprathreshold concentrations could recruit strong intrinsic signals in areas showing little or no calcium signal. These differences were such that, at suprathreshold concentrations, the chemotopy of calcium and intrinsic signal response maps often differed. These results suggest that intrinsic optical signals closely reflect receptor neuron input to glomeruli at low odorant concentrations but reflect additional processes at higher concentrations (activation of second-order neurons, centrifugal input, or constraints on the coupling between neuronal activity and hemodynamic changes). Intrinsic signals that are not associated with receptor neuron input have the potential to impact the interpretation of spatial coding strategies in the olfactory bulb.