Non-reciprocal cross-adaptation of spiking responses of individual olfactory receptor neurons of spiny lobsters: evidence for two excitatory transduction pathways

Understanding responses to chemical mixtures: looking forward from the past

Our goal in this article is to provide a perspective on how to understand the nature of responses to chemical mixtures. In studying responses to mixtures, researchers often identify "mixture interactions"-responses to mixtures that are not accurately predicted from the responses to the mixture's individual components. Critical in these studies is how to predict responses to mixtures and thus to identify a mixture interaction. We explore this issue with a focus on olfaction and on the first level of neural processing-olfactory sensory neurons-although we use examples from taste systems as well and we consider responses beyond sensory neurons, including behavior and psychophysics. We provide a broadly comparative perspective that includes examples from vertebrates and invertebrates, from genetic and nongenetic animal models, and from literature old and new. In the end, we attempt to recommend how to approach these problems, including possible future research directions.

Reduced mating success of female tortricid moths following intense pheromone auto-exposure varies with sophistication of mating system

Mating disruption is a valuable tool for the management of pest lepidopteran species in many agricultural crops. Many studies have addressed the effect of female pheromone on the ability of males to find calling females but, so far, fewer have addressed the effect of pheromone on the mating behavior of females. We hypothesized that mating of female moth species may be adversely affected following sex pheromone auto-exposure, due to abnormal behavioral activity and/or antennal sensitivity. Our results indicate that, for Grapholita molesta and Pandemis pyrusana females, copulation, but not calling, was reduced following pre-exposure to sex pheromone. In contrast, for Cydia pomonella and Choristoneura rosaceana, sex pheromone pre-exposure did not affect either calling or copulation propensity. Adaptation of female moth antennae to their own sex pheromone, following sex pheromone auto-exposure, as measured by electroantennograms, occurred in a species for which identical exposure reduced mating success (G. molesta) and in a species for which such exposure did not affect mating success (C. rosaceana). These results suggest that pre-exposure of female moths of certain species to sex pheromone may further contribute to the success of pheromone-based mating disruption. Therefore, we conclude that, in some species, mating disruption may include a secondary mechanism that affects the mating behavior of female moths, in addition to that of males.

Olfactory coding in Drosophila larvae investigated by cross-adaptation

In order to reveal aspects of olfactory coding, the effects of sensory adaptation on the olfactory responses of first-instar Drosophila melanogaster larvae were tested. Larvae were pre-stimulated with a homologous series of acetic esters (C3-C9), and their responses to each of these odours were then measured. The overall patterns suggested that methyl acetate has no specific pathway but was detected by all the sensory pathways studied here, that butyl and pentyl acetate tended to have similar effects to each other and that hexyl acetate was processed separately from the other odours. In a number of cases, cross-adaptation transformed a control attractive response into a repulsive response; in no case was an increase in attractiveness observed. This was investigated by studying changes in dose-response curves following pre-stimulation. These findings are discussed in light of the possible intra- and intercellular mechanisms of adaptation and the advantage of altered sensitivity for the larva.

Olfactory coding in a simple system: adaptation in Drosophila larvae

Drosophila melanogaster larvae were pre-stimulated with high concentrations of six homologous alcohols (C4-C9) and then tested for adaptation and cross-adaptation using these same alcohols, four aliphatic n-acetates and three acids. Pre-stimulation with hexanol effectively reduced to zero (abolished) test responses to all six alcohols, whereas test responses to hexanol were only affected by pre-stimulation with hexanol. This substance appears to play a fundamental role in the organization of the larval olfactory system. Test responses to butanol and pentanol, and the effect of pre-stimulation with butanol and pentanol, were not significantly different, indicating that they are sensory equivalents. Heptanol, octanol and nonanol induce a complex set of responses among one another. Cross-adaptation between functional groups was observed, in particular following pre-stimulation with hexanol, but there was also evidence that functional groups are coded separately. A model of olfactory processing in the fruitfly maggot is presented that explains the data and provides predictions for future anatomical, genetic and electrophysiological studies.

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.

The neural code for taste in the nucleus of the solitary tract of the rat: effects of adaptation

Adaptation of the tongue to NaCl, HCl, quinine or sucrose was used as a tool to study the stability and organization of response profiles in the nucleus of the solitary tract (NTS). Taste responses in the NTS were recorded in anesthetized rats before and after adaptation of the tongue to NaCl, HCl, sucrose or quinine. Results showed that the magnitude of response to test stimuli following adaptation was a function of the context, i.e., adaptation condition, in which the stimuli were presented. Over half of all taste responses were either attenuated or enhanced following the adaptation procedure: NaCl adaptation produced the most widespread, non-stimulus-selective cross-adaptation and sucrose adaptation produced the least frequent cross-adaptation and the most frequent enhancement of taste responses. Adaptation to quinine cross-adapted to sucrose and adaptation to HCl cross-adapted to quinine in over half of the units tested. The adaptation procedure sometimes unmasked taste responses where none were present beforehand and sometimes altered taste responses to test stimuli even though the adapting stimulus did not itself produce a response. These effects demonstrated a form of context-dependency of taste responsiveness in the NTS and further suggest a broad potentiality in the sensitivity of NTS units across taste stimuli. Across unit patterns of response remained distinct from each other under all adaptation conditions. Discriminability of these patterns may provide a neurophysiological basis for residual psychophysical abilities following adaptation.

Electrophysiological evidence for two transduction pathways within a bitter-sensitive taste receptor

Among the sapid stimuli, those that elicit bitter taste are the most abundant and structurally diverse. To accommodate this diversity, animals are thought to use multiple bitter transduction pathways. We examined the role of individual taste receptor cells (TRCs) in this transduction process by focusing on one of the taste organs, or chemosensilla, of a caterpillar (Manduca sexta). This chemosensillum (the lateral styloconicum) contains four functionally distinct TRCs: the salt, sugar, inositol, and deterrent TRCs, which are known to respond strongly to, in respective order, salts, sugars, inositol, and compounds humans describe as bitter. Using an extracellular recording technique, we tested three hypotheses for how a structurally diverse array of bitter compounds (salicin, caffeine, and aristolochic acid) could excite the same chemosensillum: several TRCs within the lateral styloconica respond to the bitter compounds; only the deterrent TRC responds to the bitter compounds, through a single transduction pathway; and only the deterrent TRC responds to the bitter compounds, but through multiple transduction pathways. To discriminate among these hypotheses, we tested five predictions. The first addressed how many TRCs within the lateral styloconica responded to the bitter compounds. Subsequent predictions were based on the results of the test of the first prediction and assumed that only the deterrent TRC responded to these compounds. These latter predictions addressed whether the bitter compounds acted through one or multiple transduction pathways. We obtained evidence consistent with the third hypothesis: only the deterrent TRC responded to the bitter compounds; the temporal patterns of firing and concentration-response curves elicited by caffeine and salicin were similar to each other, but different from those elicited by aristolochic acid; the patterns of sensory adaptation and disadaptation elicited by caffeine and salicin were similar to each another, but different from those elicited by aristolochic acid; reciprocal cross-adaptation occurred between caffeine and salicin, but not between aristolochic acid and caffeine or aristolochic acid and salicin; and the responsiveness of individual deterrent TRCs to caffeine and salicin correlated significantly, whereas that to aristolochic acid and caffeine or aristolochic acid and salicin did not. Taken together, these results indicate that the deterrent TRC contains at least two excitatory transduction pathways: one responds to caffeine and salicin and the other to aristolochic acid. To our knowledge, this is the first direct support for the existence of two bitter transduction pathways within a single TRC.

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

Coding of binary mixtures by a population of olfactory receptor neurons in the spiny lobster (Panulirus argus) was examined. Extracellular single-unit responses of 50 neurons to seven compounds and their binary mixtures were recorded. The ability of a noncompetitive model with correction for binding inhibition to predict responses to mixtures based on responses to their components was compared with the predictive abilities of other models. This model assumes that different compounds activate different transduction processes in the same neuron leading to excitation or inhibition, and it includes a term quantifying the degree to which binding of an odorant to its receptor sites is inhibited by other compounds. The model accurately predicted the absolute response magnitude of the population of neurons for 13 of 15 mixtures assessed, which is superior to the predictive power of any of the other models. The model also accurately predicted the across neuron patterns generated by the binary mixtures, as evaluated by multidimensional scaling analysis. The results suggest that there is no emergence of unique qualities for binary mixtures relative to components of these mixtures.

Mixture suppression without inhibition for binary mixtures from whole cell patch clamp studies of in situ olfactory receptor neurons of the spiny lobster

Whole cell patch clamping was used to investigate mechanisms of mixture suppression for in situ olfactory receptor neurons (ORNs) of the spiny lobster Panulirus argus. We used a set of single compounds and binary mixtures that have been used in previous biochemical studies of receptor-odorant binding, electrophysiological studies of spiking output from ORNs, and behavioral studies. These odorants were adenosine 5'-monophosphate (AMP), betaine (Bet), L-cysteine (Cys), L-glutamate (Glu), taurine (Tau), ammonium chloride, D,L-succinate, binary mixtures of these compounds, as well as a 33-component artificial oyster mixture (AOM). For the 40 ORNs studied, these stimuli more frequently elicited inward than outward currents. AMP, Glu, Tau and Bet evoked the largest and most numerous inward currents; Cys most commonly evoked outward currents. Na+ was an important charge-carrying ion for the Glu-evoked response in one ORN and the Bet-evoked response in another ORN. Mixture suppression, defined conservatively in this study as cases where the response to a binary mixture was less than the response to the more excitatory component of that mixture, was observed in 6 ORNs. In all 6 cases, neither component of the mixture evoked an outward conductance (i.e. neither was inhibitory). Five of these cases of mixture suppression involved a mixture containing two excitatory compounds (i.e. producing inward conductances): four ORNs were excited by both Glu and AMP, and one ORN was excited by both Tau and Glu. One case of mixture suppression occurred for a compound (Tau) tha did not produce a current when presented alone but which when added to Bet suppressed the inward current generated by Bet. Mechanisms for these suppressions are discussed, including inhibition of receptor binding by the components of a binary mixture and effects on second messengers or ion channels.