Effects of odorant mixtures on olfactory receptor cells

Odor Enrichment Increases Hippocampal Neuron Numbers in Mouse

The hippocampus and olfactory bulb incorporate new neurons migrating from neurogenic regions in the brain. Hippocampal atrophy is evident in numerous neurodegenerative disorders, and altered hippocampal neurogenesis is an early pathological event in Alzheimer's disease. We hypothesized that hippocampal neurogenesis is affected by olfactory stimuli through the neural pathway of olfaction-related memory. In this study, we exposed mice to novel pleasant odors for three weeks and then assessed the number of neurons, non-neuronal cells (mainly glia) and proliferating cells in the hippocampus and olfactory bulb, using the isotropic fractionator method. We found that the odor enrichment significantly increased the neuronal cell numbers in the hippocampus, and promoted cell proliferation and neurogenesis in the olfactory bulb. In contrast, the glial cell numbers remained unchanged in both of the regions. Our results suggest that exposure to novel odor stimuli promotes hippocampal neurogenesis and support the idea that enriched environments may delay the onset or slow down the progression of neurodegenerative disorders.

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.

Chemosensory event-related potentials in the investigation of interactions between the olfactory and the somatosensory (trigeminal) systems

The aim of the study was to investigate the interaction of the olfactory and somatosensory systems in the perception of chemical stimuli. Stimuli were chosen so as to selectively activate the olfactory (hydrogen sulphide, H2S) and trigeminal (carbon dioxide, CO2) nerves. In addition, carvone was included as a stimulus with mixed properties. Thirty healthy volunteers participated in the experiments. Subjects rated the intensity of each of the stimulants when presented alone and as a component of binary mixtures. Chemosensory event-related potentials (CSERPs) were obtained from 5 recording positions. Analysis of the intensity ratings indicated that there was no difference between the 3 stimulants when used as single components. In binary mixtures intensity estimates of H2S were suppressed by CO2 and carvone. In addition, while estimates of CO2 were suppressed by carvone estimates of the latter were enhanced in the same mixture. CSERP data confirmed earlier findings with regard to the topographic distribution of amplitudes, i.e., if the olfactory system had been activated largest amplitudes were observed at position Pz, whereas activation of the trigeminal nerve produced largest amplitudes at Cz. Moreover, the suppression of CO2 estimates by carvone was reflected in a corresponding suppression of the CSERP amplitudes. In addition, when CO2 was mixed with H2S or carvone there was a decrease in the CSERP latency indicating interactions of both sensory systems in the time domain.

Mixture suppression in behavior: the antennular flick response in the spiny lobster towards binary odorant mixtures

The behavioral responses of Florida spiny lobsters towards various concentrations of binary mixtures and their constituents (AMP, betaine, cysteine, succinate, and taurine) were measured using an antennular flicking assay. The rate of flicking increases with dose and has low thresholds: flick rates towards each of the five chemicals increased with concentration with thresholds between 1 nM and 100 microM. A mixed receptor composition model, which incorporates knowledge of the composition of receptor site types and their distribution across receptor cells (15), was used to predict responses to binary mixtures based on responses to the individual constituents. Nine of the ten binary mixtures elicited response magnitudes which were less than predicted by this model, suggesting mixture suppression. These mixture interactions appear to be independent of the concentration of the mixture tested; rather, they occur with the same magnitude at all concentrations. These behavioral results corroborate findings in olfactory receptor cell studies which indicate the significant prevalence of mixture suppression towards some of the same binary mixtures (19,20).

Electro-olfactogram and multiunit olfactory receptor responses to binary and trinary mixtures of amino acids in the channel catfish, Ictalurus punctatus

In vivo electrophysiological recordings from populations of olfactory receptor neurons in the channel catfish, Ictalurus punctatus, clearly showed that responses to binary and trinary mixtures of amino acids were predictable with knowledge obtained from previous cross-adaptation studies of the relative independence of the respective binding sites of the component stimuli. All component stimuli, from which equal aliquots were drawn to form the mixtures, were adjusted in concentration to provide for approximately equal response magnitudes. The magnitude of the response to a mixture whose component amino acids showed significant cross-reactivity was equivalent to the response to any single component used to form that mixture. A mixture whose component amino acids showed minimal cross-adaptation produced a significantly larger relative response than a mixture whose components exhibited considerable cross-reactivity. This larger response approached the sum of the responses to the individual component amino acids tested at the resulting concentrations in the mixture, even though olfactory receptor dose-response functions for amino acids in this species are characterized by extreme sensory compression (i.e., successive concentration increments produce progressively smaller physiological responses). Thus, the present study indicates that the response to sensory stimulation of olfactory receptor sites is more enhanced by the activation of different receptor site types than by stimulus interaction at a single site type.

Effects of antibodies against odorant binding proteins on electrophysiological responses to odorants

Monoclonal antibodies against two olfactory mucosal proteins, one with affinity for anisole-like and the other for benzaldehyde-like compounds, were applied to mouse olfactory epithelium. Responses to three odorants (anisole, benzaldehyde and amyl acetate) were measured. Of 26 antibodies, three (12%) inhibited responses only to the odorant with affinity for the antigen, nine (35%) inhibited responses to all three odorants, and 14 (54%) were without effect. None reduced responses by as much as 50%. The data support the hypothesis that there is a class of related proteins in olfactory neuronal cell membranes that function as receptor molecules and that other mechanisms also mediate odorant stimulation.