Neural coding of binary mixtures in a structurally related odorant pair

The encoding of odorant mixtures by olfactory sensory neurons depends on molecular interactions at peripheral receptors. However, the pharmacological basis of these interactions is not well defined. Both competitive and noncompetitive mechanisms of receptor binding and activation, or suppression, could contribute to coding. We studied this by analyzing responses of olfactory bulb glomeruli evoked by a pair of structurally related odorants, eugenol (EG) and methyl isoeugenol (MIEG). Fluorescence imaging in synaptopHluorin (spH) mice revealed that EG and MIEG evoked highly overlapped glomerular inputs, increasing the likelihood of mixture interactions. Glomerular responses to binary mixtures of EG and MIEG mostly showed hypoadditive interactions at intermediate and high odorant concentrations, with a few near threshold responses showing hyperadditivity. Dose-response profiles were well fitted by a model of two odorants competitively binding and activating a shared receptor linked to a non-linear transduction cascade. We saw no evidence of non-competitive mechanisms.