Responses of olfactory receptor neurons in the spiny lobster to binary mixtures are predictable using a noncompetitive model that incorporates excitatory and inhibitory transduction pathways

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.

Mixture interactions in moth olfactory physiology: examining the effects of odorant mixture, concentration, distal stimulation, and antennal nerve transection on sensillar responses

The insect olfactory system is challenged to decipher valid signals from among an assortment of chemical cues present in the airborne environment. In the moth, Heliothis virescens, males rely upon detection and discrimination of a unique blend of components in the female sex pheromone to locate mates. The effect of variable odor mixtures was used to examine physiological responses from neurons within sensilla on the moth antenna sensitive to female sex pheromone components. Increasing concentrations of heliothine sex pheromone components applied in concert with the cognate stimulus for each neuronal type resulted in mixture suppression of activity, except for one odorant combination where mixture enhancement was apparent. Olfactory receptor neuron (ORN) responses were compared between moths with intact and transected antennal nerves to determine whether specific instances of suppression might be influenced by central mechanisms. Type A sensilla showed little variation in response between transected and intact preparations; however, recordings from type B sensilla with transected antennal nerves exhibited reduced mixture suppression. Testing by parallel stimulation of distal antennal segments while recording and stimulating proximal segments dismissed the possibility of interneuronal or ephaptic effects upon sensillar responses. The results indicate that increasing concentrations of "noncognate" odorants in an odor mixture or antennal nerve transection can produce variation in the intensity and temporal dynamics of physiological recordings from H. virescens ORNs.

Single olfactory sensory neurons simultaneously integrate the components of an odour mixture

Most odours are complex mixtures. However, the capacities of olfactory sensory neurons (OSNs) to process complex odour stimuli have never been explored in air-breathing vertebrates. To face this issue, the present study compares the electrical responses of single OSNs to two odour molecules, delivered singly and mixed together, in rats in vivo. This work is the first aimed at demonstrating that single OSNs simultaneously integrate several chemical signals and which, furthermore, attempts to describe such processes for the whole concentration range over which single OSNs can work. The results stress that complex interactions occur between components in odour mixtures and that OSN responses to such mixtures are not simply predictable from the responses to their components. Three types of interactions are described. They are termed suppression, hypoadditivity and synergy, in accord with psychophysical terminology. This allows us to draw links between peripheral odour reception and central odour coding. Indeed, events occurring in single OSNs may account for the dominating or even the masking effects of odour molecules in complex mixtures, i.e. for the prevailing action of a minor component in the final qualitative perception of a mixture. We conclude that our observations with binary mixtures anticipate the complexity of processes which may rise at the level of a single OSN in physiological conditions. Following this hypothesis, a natural odour would induce a multi-chemical integration at the level of single OSNs which may result in refining their individual odour-coding properties, leading them to play a crucial role in the final performance of the olfactory system.

Responses in highly selective sensory neurons to blends of pheromone components in the moth Agrotis segetum

Pheromone detecting sensory neurons in moths are known to be highly sensitive and selective. Female-emitted sexual pheromones are normally mixtures of a few to several components. However, not much is known about how receptor neurons respond to blends of compounds. In the present study we investigated how four physiological types of pheromone component-selective neurons responded to binary mixtures or to the complete blend in the turnip moth Agrotis segetum. We found that responses to mixtures only rarely differed from that to the excitatory component alone. The mixture interactions were exclusively suppressive and occurred only at high concentrations. Therefore we conclude that the, in A. segetum, commonly observed mixture interactions observed in higher brain centra are mainly the result of central nervous processing and that information about the pheromone components reaches the antennal lobes virtually unaltered. In addition, we found a physiological type of receptor neuron, responding selectively to one of the female-emitted pheromone components, that has previously not been observed in the Swedish population.

Why do animals have so many receptors? The role of multiple chemosensors in animal perception

Many animals have an abundance and diverse assortment of peripheral sensors, both across and within sensory modalities. Multiple sensors offer many functional advantages to an animal's ability to perceive and respond to environmental signals. Advantages include extending the ability to detect and determine the spatial distribution of stimuli, improving the range and accuracy of discrimination among stimuli of different types and intensities, increasing behavioral sensitivity to stimuli, ensuring continued sensory capabilities when the probability of damage or other loss of function to some sensors is high, maintaining sensory function over the entire sensory surface during development and growth, and increasing the richness of behavioral output to sensory stimulation. In this paper, we use the crustacean chemosensory system as the primary example to discuss these functions of multiple sensors. These principles may be applicable to the function of autonomous robots and should be considered in their design.

Peripheral odor coding in the rat and frog: quality and intensity specification

In mammals, two recent studies have shown recently that one odor molecule can be recognized by several molecular olfactory receptors (ORs), and a single OR can recognize multiple odor molecules. In addition, one olfactory receptor neuron (ORN) may respond to different stimuli chosen as representative of distinct odor qualities. The aim of the present study was to analyze quality and intensity coding abilities of rat single ORNs, comparing them with previous extensive data gathered in the frog to get insight into the generality of olfactory coding mechanisms over vertebrates. Response properties of 90 rat ORNs to different odors or to one odor at different concentrations were analyzed. In the rat and the frog, odor quality appears to be specified through the identity of activated ORNs. However, rat ORNs have higher response thresholds. This lower sensitivity may be interpreted as an increase in selectivity of rat ORNs for low or medium odor intensities. In these conditions, the lower proportion of activated ORNs could be counterbalanced by their number, as well as by their higher glomerular convergence ratio in the olfactory bulb. From amphibians to mammals, the olfactory system appears to use universal mechanisms based on a combinatorial-coding mode that may allow quasi-infinite possibilities of adaptation to various olfactory environments.

Learning from spiny lobsters about chemosensory coding of mixtures

Studies of the peripheral olfactory system of the Caribbean spiny lobster Panulirus argus and related decapod crustaceans have helped us understand mechanisms of coding of mixtures, some of which are discussed in this review. Although the number of cells in the lobster's olfactory system is much lower than in vertebrate olfactory systems, it is a highly complex system. The receptor neurons (RNs) of this olfactory system are complex processors that cannot be categorized into discrete cell types, but rather have a diversity of response profiles. Each RN can have different types of receptor proteins, second messengers, and/or ion channels, which undoubtedly contributes to the functional diversity of these neurons and makes them complex peripheral integrators. The RNs probably encode information about the quality of mixtures as a distributed or population code, providing a basis for behavioral discrimination of natural food stimuli. Analysis of distributed codes for a series of blend ratios of binary mixtures reveals that the qualities of individual compounds are probably not lost when mixed. Such peripheral processing allows spiny lobsters to perceive complex odors as a set of elemental cues if the salience of the components is sufficiently high.

Elemental and configural learning and the perception of odorant mixtures by the spiny lobster Panulirus argus

The present study used a conditioning assay to investigate if the type of learning task that spiny lobsters (Panulirus argus) were required to perform influenced the way that they perceived odorant mixtures. Mixtures were composed of 2 food-related compounds (adenosine-5'-monophosphate, betaine, or L-glutamate) at concentrations that produced the same duration of searching behavior in unconditioned animals. Aversive conditioning of search behavior coupled with generalization testing was used to evaluate perceptual similarity between related mixtures. When animals were conditioned to stop searching to a binary mixture AX, they did not generalize significantly from this mixture to either of its components (A or X), or to a binary mixture containing one novel component (AY). However, when lobsters were conditioned to avoid AX but to continue responding to AY, they generalized between AX and X and between AY and Y. The results support the hypothesis that altering the salience of a mixture's components by giving them different reinforcement contingencies changed the way that the mixtures were perceived. As a result of such conditioning, animals perceived the mixture's components as separate elements, rather than as a configuration, and, as a consequence, animals generalized between binary mixtures and their most salient or predictive components.

In Vivo Responses of Single Olfactory Receptor Neurons of Channel Catfish to Binary Mixtures of Amino Acids

Kang, Jiesheng and John Caprio. In vivo response of single olfactory receptor neurons of channel catfish to binary mixtures of amino acids. J. Neurophysiol. 77: 1–8, 1997. For the first time in any vertebrate, in vivo responses of single olfactory receptor neurons to odorant mixtures were studied quantitatively. Extracellular electrophysiological response of 54 single olfactory receptor neurons from 23 channel catfish, Ictalurus punctatus, to binary mixtures of amino acids and to their components were recorded simultaneously with the electroolfactogram (EOG). For 57% (73 of 128) of the tests, no significant change (N) from spontaneous activity occurred. Responses to the remaining 55 tests of binary mixtures were excitatory (E; 13%) or suppressive (S; 30%). No response type was associated with any specific mixture across the neurons sampled. Eighty-six percent of the responses of catfish olfactory receptor neurons to binary mixtures were classifed similar to at least one of the component responses, a percentage comparable (i.e., 89%) with that observed for single olfactory bulb neurons in the same species to equivalent binary mixtures. The responses of single olfactory receptor neurons to component-similar binary mixtures (i.e., component responses were both E, both S, and both N, respectively) were generally (80% of 59 tests) classified similar to the responses to the components. For E+N and S+N binary mixtures, the N component often (66% of 58 tests) reduced or concealed (i.e., “masked”) the excitatory and suppressive responses, respectively. For the majority (6 of 11 tests) of E+S binary mixtures, null activity resulted. Responses to the remaining five tests were either excitatory ( n = 3) or suppressive ( n = 2).

Generalization among related complex odorant mixtures and their components: analysis of olfactory perception in the spiny lobster

We investigated the processing of odorant mixtures containing two to seven components by the spiny lobster Panulirus argus. The chemicals tested were food-related compounds that are attractive to spiny lobsters, and include adenosine-5'-monophosphate, betaine, L-cysteine, L-glutamate, DL-succinate, taurine, and ammonium. Components were tested at concentrations that produced search behavioral responses of equal magnitude in unconditioned animals. Responses of unconditioned animals to mixtures and their components reveal hypoadditivity, in which the response to a mixture is less than the sum of the responses to that mixture's components. Aversive conditioning coupled with generalization testing was used to evaluate generalization and hence perceptual similarity between related mixtures. Animals were conditioned to either an individual odorant, a four-compound mixture, or a seven-compound mixture, followed by generalization testing with submixtures or larger mixtures containing the conditioned stimulus. Animals tended not to generalize, but significant generalization between a more simple conditioned stimulus and more complex mixtures containing that conditioned stimulus occurred in 2 of 11 cases, and significant generalization between a conditioned mixture and its submixtures was observed in 4 of 9 cases. Both the number and chemical identity of components of mixtures may contribute to the degree of generalization between mixtures. Overshadowing, in which the ability to learn about a chemical is affected by simultaneous presentation of other chemicals, occurred in two of three cases. We discuss implications of these findings with respect to elemental and configural processing of odorant mixtures in the spiny lobster, possible neural mechanisms responsible for these results, and the potential utility of generalization and overshadowing to the spiny lobster's natural behavior.