Refinement of odor molecule tuning by dendrodendritic synaptic inhibition in the olfactory bulb.

TLDR

Mitral/tufted cells and granule cells form reciprocal dendrodendritic synapses in the olfactory bulb, where glutamate‑activated granule cells inhibit neighboring mitral/tufted cells via GABA, a lateral inhibition mechanism thought to refine olfactory signals but whose precise role in odor discrimination remains unclear. This study investigates how dendrodendritic synaptic inhibition contributes to olfactory discrimination by recording single‑unit responses of rabbit mitral/tufted cells to a series of normal aliphatic aldehydes. By blocking glutamate or GABA receptors, the authors showed that odor‑evoked inhibition in mitral/tufted cells is mediated by reciprocal synaptic transmission, confirming that lateral inhibition underlies the observed inhibitory responses. The results demonstrate that inhibitory responses are triggered by odor molecules structurally related to excitatory ones, and that this synaptic regulation sharpens odor tuning and enhances discrimination of olfactory information.

Abstract

Mitral/tufted cells (M/T cells) and granule cells form reciprocal dendrodendritic synapses in the main olfactory bulb; the granule cell is excited by glutamate from the M/T cell and in turn inhibits M/T cells by gamma-aminobutyrate. The trans-synaptically excited granule cell is thought to induce lateral inhibition in neighboring M/T cells and to refine olfactory information. It remains, however, elusive how significantly and specifically this synaptic regulation contributes to the discrimination of different olfactory stimuli. This investigation concerns the mechanism of olfactory discrimination by single unit recordings of responses to a series of normal aliphatic aldehydes from individual rabbit M/T cells. This analysis revealed that inhibitory responses are evoked in a M/T cell by a defined subset of odor molecules with structures closely related to the excitatory odor molecules. Furthermore, blockade of the reciprocal synaptic transmission by the glutamate receptor antagonist or the gamma-aminobutyrate receptor antagonist markedly suppressed the odor-evoked inhibition, indicating that the inhibitory responses are evoked by lateral inhibition via the reciprocal synaptic transmission. The synaptic regulation in the olfactory bulb thus greatly enhances the tuning specificity of odor responses and would contribute to discrimination of olfactory information.