Interactions of odorants with olfactory receptors and receptor neurons match the perceptual dynamics observed for woody and fruity odorant mixtures.

The present study aimed to create a direct bridge between observations on peripheral and central responses to odorant mixtures and their components. Three experiments were performed using mixtures of fruity (isoamyl acetate; ISO) and woody (whiskey lactone; WL) odorants known to contribute to some of the major notes in Burgundy red wine. These experiments consisted of (i) calcium imaging of human embryonic kidney cells (HEK293T) transfected with olfactory receptors (ORs); (ii) single-unit electrophysiological recordings from olfactory receptor neurons (ORNs) and analyses of electro-olfactogram (EOG) responses in the rat nose in vivo; and (iii) psychophysical measurements of the perceived intensity of the mixtures as rated by human subjects. The calcium imaging and electrophysiological results revealed that ISO and WL can act simultaneously on single ORs or ORNs and confirm that receptor responses to mixtures are not the result of a simple sum of the effects of the individual mixture compounds. The addition of WL to ISO principally suppressed the ORN activation induced by ISO alone and was found to enhance this activation in a subset of cases. In the human studies, the addition of high concentrations of WL to ISO decreased the perceived intensity of the ISO. In contrast, the addition of low concentrations of WL enhanced the perceived intensity of the fruity note (ISO) in this mixture, as it enhanced EOG responses in ORNs. Thus, both OR and ORN responses to ISO + WL mixtures faithfully reflected perceptual response changes, so the odour mixture information is set up after the peripheral stage of the olfactory system.

Orexin A effects on the olfactory bulb spontaneous activity and odor responsiveness in freely breathing rats.

The mitral cells (MCs) of the olfactory bulb (OB) are relay neurons between the periphery and the central nervous structures. MCs receive in turn a centrifugal control from several higher brain centers that depends on the nutritional state. In this study, we investigated the effects of orexin A (ORX), a novel molecule known to regulate food intake and whose receptors are present in the OB, on the electrophysiological activity of single MCs. Using icv-injections and direct applications on the OB, we determined the respective central and local effects of this molecule on the MCs' spontaneous firing activity and responsiveness to different odors. Icv-injections and local OB-applications were found to induce a significant decrease in spontaneous firing activity in 14% and 50% of the recorded MCs, respectively. In one case, ORX application on the OB caused a significant firing increase. Effects of OB-applications had shorter delays. The responsiveness of some MCs to food and non-food odors was also changed, but the proportion of changes was not statistically significant. Icv-injection effects likely resulted from a local action of ORX on the OB. Changes of spontaneous firing activity and odor responsiveness are discussed in terms of regulation of the functioning of the olfactory system.