Coding of odor molecules by neurons in mammalian olfactory bulb and cortex : Functional role of olfactory glomeruli

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1) Molecular receptive range properties of mitral/tufted cells in the rabbit main olfactory bulbRecording of extracellular spike responses were made from single mitral/tufted cells in various regions of the main olfactory bulb of urethane-anesthetized rabbits. Examination with an expanded panel of stimulus odor molecules that included both aromatic and aliphatic compounds indicated that single mitral/tufted cells show excitatory spike responses to a range of odor molecules (molecular receptive range) having similar conformations. These results support the hypothesis that conformational parameters of ligand odor molecules play a key role in sensory processing in the olfactory bulb. In addition, the results are in agreement with the hypothesis that sensory neurons expressing the same type of odor receptor converge on one or a few glomeruli.2) Functional role of bulbar local circuit neurons in coding of odor moleculesUsing single unit recording in rabbit main bulb, we showed that inhibitory responses are evoked in mitral/tufted cells by a defined subset of odor molecules with structures closely related to the excitatory odor molecules. Pharmacological blockade of the dendrodendritic synaptic transmission between mitral/tufted and granule cells markedly decreased the odor evoked suppression, suggesting that the inhibitory responses are evoked by lateral inhibition via the dendrodendritic synaptic pathway. The synaptic interaction s within the local circuits in the bulb, thus, greatly enhance the tuning specificity of odor responses and contribute to discrimination of odor molecule information.