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

Understanding olfactory and behavioural responses to dietary cues in age-1 lake sturgeon Acipenser fulvescens

Detection of environmental cues is essential for all vertebrates and is typically established by the olfactory epithelium and olfactory sensory neurons (OSNs). In fishes, microvillous and ciliated OSNs are the principal types, typically detecting amino acids and bile salts, respectively. Activation of OSN receptors by specific ligands initiate downstream signal processing often leading to behavioural responses. In this study we used electrophysiological and behavioural techniques to evaluate olfactory detection and behaviour in juvenile lake sturgeon Acipenser fulvescens in response to hatchery- and natural dietary cues. We hypothesized that electro-olfactogram (EOG) and behavioural responses would be dependent on diet type. We predicted that inhibition of the phospholipase C/inositol 1,4,5-triphosphate (PLC/IP3) secondary transduction pathway would reduce EOG responses to dietary cues and, inhibition of the adenylyl cyclase/adenosine 3,5-cyclic monophosphate (cAMP) pathway, would have no effect. Furthermore, we predicted a strong EOG response would be manifested in a change in behaviour. We observed that both the PLC/IP3 and cAMP pathways were significantly involved in the detection of dietary cues. However, EOG responses did not manifest to behavioural responses, although the foraging activity to the hatchery cue was significantly greater compared to the control. Our results support the notion that lake sturgeon raised in a hatchery and fed a commercial pelleted diet may become accustomed to it prior to release into the wild. Further, this study suggests that, in conservation aquaculture settings, lake sturgeon should be exposed to natural dietary cues prior to release as one strategy to promote food recognition.

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.

The Phenomenon of Compensatory Cell Proliferation in Olfactory Epithelium in Fish Caused by Prolonged Exposure to Natural Odorants

It was previously shown that activation of the processes of neurogenesis in the olfactory epithelium (OE) can be caused after intranasal administration of toxic or neurotrophic factors, after axon transection, or as a result of bulbectomy. Our study showed for the first time that a significant increase in olfactory cell renewal can also occur in animals due to periodic chemostimulation with natural odorants (amino acids and peptides) for 15 days. Using electron and laser confocal microscopy in fish (Paracottus knerii (Cottidae), Dybowski, 1874) from Lake Baikal, we showed that periodic stimulation of aquatic organisms with a water-soluble mixture of amino acids and peptides causes stress in OE, which leads to programmed death cells and compensatory intensification of their renewal. We estimated the level of reactive oxygen species, number of functionally active mitochondria, intensity of apoptosis processes, and mitosis activity of cells in the OE of fish in the control group and after periodic natural odorants exposure. This study showed that new stem cells are activated during enhanced odor stimulation and subsequent degenerative changes in the cells of the sensory apparatus. Those new activated stem cells are located in previously proliferatively inactive regions of OE that become involved in compensatory processes for the formation of new cells.

Developmental evolution and developmental plasticity of the olfactory epithelium and olfactory skills in Mexican cavefish

The fish Astyanax mexicanus comes in two forms: the normal surface-dwelling (SF) and the blind depigmented cave-adapted (CF) morphs. Among many phenotypic differences, cavefish show enhanced olfactory sensitivity to detect amino-acid odors and they possess large olfactory sensory organs. Here, we questioned the relationship between the size of the olfactory organ and olfactory capacities. Comparing olfactory detection abilities of CF, SF and F1 hybrids with various olfactory epithelium (OE) sizes in behavioral tests, we concluded that OE size is not the only factor involved. Other possibilities were envisaged. First, olfactory behavior was tested in SF raised in the dark or after embryonic lens ablation, which leads to eye degeneration and mimics the CF condition. Both absence of visual function and absence of visual organs improved the SF olfactory detection capacities, without affecting the size of their OE. This suggested that developmental plasticity occurs between the visual and the olfactory modalities, and can be recruited in SF after visual deprivation. Second, the development of the olfactory epithelium was compared in SF and CF in their first month of life. Proliferation, cell death, neuronal lifespan, and olfactory progenitor cell cycling properties were identical in the two morphs. By contrast, the proportions of the three main olfactory sensory neurons subtypes (ciliated, microvillous and crypt) in their OE differed. OMP-positive ciliated neurons were more represented in SF, TRPC2-positive microvillous neurons were proportionately more abundant in CF, and S100-positive crypt cells were found in equal densities in the two morphs. Thus, general proliferative properties of olfactory progenitors are identical but neurogenic properties differ and lead to variations in the neuronal composition of the OE in SF and CF. Together, these experiments suggest that there are at least two components in the evolution of cavefish olfactory skills: (1) one part of eye-dependent developmental phenotypic plasticity, which does not depend on the size of the olfactory organ, and (2) one part of developmental evolution of the OE, which may stem from embryonic specification of olfactory neurons progenitor pools.

Olfactory sensitivity to steroid glucuronates in Mozambique tilapia suggests two distinct and specific receptors for pheromone detection

Cichlids offer an exciting opportunity to understand vertebrate speciation; chemical communication could be one of the drivers of African cichlid radiation. Chemical signals mediate key aspects in the lives of vertebrates and often are species specific. Dominant male Mozambique tilapia [Oreochromis mossambicus (Peters 1852)] release a sex pheromone, 5β-pregnan-3α,17α,20β-triol 3-glucuronate and its 20α-epimer, via their urine. The objective of this study was to assess the sensitivity, specificity and versatility of the olfactory system of O. mossambicus to other steroids and their conjugates using the electro-olfactogram. Oreochromis mossambicus was sensitive to several 3-glucuronidated steroids, but did not respond to prostaglandins, unconjugated steroids or 17- or 20-conjugated steroids. Stimulation of the olfactory epithelium with increasing concentrations (1 pmol l(-1) to 10 μmol l(-1)) of 5β-pregnan-3α,17α,20β-triol 3-glucuronate, 5β-pregnan-3α,17α,20α-triol 3-glucuronate, 3α,17α-dihydroxy-5β-pregnan-20-one 3-glucuronate, etiocholanolone 3α-glucuronate and 17β-estradiol 3-glucuronate produced characteristic sigmoidal concentration-response curves. However, tilapia were most sensitive to 17β-estradiol-3-glucuronate, which also had the lowest apparent EC50 and maximal response amplitude. Cross-adaptation and binary mixture experiments suggested that 5β,3α-reduced pregnan- and androstan-3-glucuronates share (a) common olfactory receptor(s), whereas 17β-estradiol 3-glucuronate is detected via (a) distinct olfactory receptor(s). In conclusion, the Mozambique tilapia has evolved high olfactory sensitivity and specificity to 3-glucuronidated steroids through two distinct olfactory receptor types; one detecting a male sex pheromone and a second detecting 17β-estradiol 3-glucuronate, a putative female-derived signal. However, O. mossambicus differs markedly in its olfactory perception from the more recently derived East African cichlid Astatotilapia burtoni, suggesting that chemical communication could, indeed, be involved in speciation.

The perception of odor objects in everyday life: a review on the processing of odor mixtures

Smelling monomolecular odors hardly ever occurs in everyday life, and the daily functioning of the sense of smell relies primarily on the processing of complex mixtures of volatiles that are present in the environment (e.g., emanating from food or conspecifics). Such processing allows for the instantaneous recognition and categorization of smells and also for the discrimination of odors among others to extract relevant information and to adapt efficiently in different contexts. The neurophysiological mechanisms underpinning this highly efficient analysis of complex mixtures of odorants is beginning to be unraveled and support the idea that olfaction, as vision and audition, relies on odor-objects encoding. This configural processing of odor mixtures, which is empirically subject to important applications in our societies (e.g., the art of perfumers, flavorists, and wine makers), has been scientifically studied only during the last decades. This processing depends on many individual factors, among which are the developmental stage, lifestyle, physiological and mood state, and cognitive skills; this processing also presents striking similarities between species. The present review gathers the recent findings, as observed in animals, healthy subjects, and/or individuals with affective disorders, supporting the perception of complex odor stimuli as odor objects. It also discusses peripheral to central processing, and cognitive and behavioral significance. Finally, this review highlights that the study of odor mixtures is an original window allowing for the investigation of daily olfaction and emphasizes the need for knowledge about the underlying biological processes, which appear to be crucial for our representation and adaptation to the chemical environment.

Highly specific olfactory receptor neurons for types of amino acids in the channel catfish

Odorant specificity to l-alpha-amino acids was determined electrophysiologically for 93 single catfish olfactory receptor neurons (ORNs) selected for their narrow excitatory molecular response range (EMRR) to only one type of amino acid (i.e., Group I units). These units were excited by either a basic amino acid, a neutral amino acid with a long side chain, or a neutral amino acid with a short side chain when tested at 10(-7) to 10(-5) M. Stimulus-induced inhibition, likely for contrast enhancement, was primarily observed in response to the types of amino acid stimuli different from that which activated a specific ORN. The high specificity of single Group I ORNs to type of amino acid was also previously observed for single Group I neurons in both the olfactory bulb and forebrain of the same species. These results indicate that for Group I neurons olfactory information concerning specific types of amino acids is processed from receptor neurons through mitral cells of the olfactory bulb to higher forebrain neurons without significant alteration in unit odorant specificity.

Neural representation of olfactory mixtures in the honeybee antennal lobe

Natural olfactory stimuli occur as mixtures of many single odors. We studied whether the representation of a mixture in the brain retains single-odor information and how much mixture-specific information it includes. To understand mixture representation in the honeybee brain, we used in vivo calcium imaging at the level of the antennal lobe, and systematically measured odor-evoked activity in 24 identified glomeruli in response to four single odorants and all their possible binary, ternary and quaternary mixtures. Qualitatively, mixture-induced activity patterns always contained glomeruli belonging to the pattern of at least one of the components, suggesting a high conservation of component information in olfactory mixtures. Quantitatively, glomerular activity saturated quickly and increasing the number of components resulted in an increase of cases in which the response of a glomerulus to the mixture was lower than that to the strongest component ('suppression'). This shows global inhibition in the antennal lobe, probably acting as overall gain control. Single components were not equally salient (in terms of number of active glomeruli) and mixture activity patterns were always more similar to the more salient components, in a way that could be predicted linearly. Thus, although a gain control system in the honeybee antennal lobe prevents saturation of the olfactory system, mixture representation follows essentially elemental rules.

Responses of olfactory forebrain units to amino acids in the channel catfish

A paucity of information exists concerning the processing of odorant information by single neurons in any vertebrate above the level of the olfactory bulb (OB). In this report, odorant specificity to four types of L-alpha-amino acids (neutral with long side-chains, neutral with short side-chains, basic and acidic), known biologically relevant odorants for teleosts, was determined for 217 spontaneously active forebrain (FB) neurons in the channel catfish. Group I FB units were identified that were excited by only one of four types of amino acids; no Group I unit was encountered that was excited by an acidic amino acid. The Group I FB units exhibited similar preferences as described previously for OB neurons, suggesting that no major modifications of olfactory information for at least some of these units occurred between the OB and FB. Evidence, however, for the convergence of odor information between the OB and FB was suggested by Group II FB units that exhibited a broader excitatory molecular receptive range (EMRR) than those of previously recorded types of OB units or the Group I FB units. Group II FB units were excited by both neutral and basic amino acids and a few also by acidic amino acids, EMRRs not observed previously in OB units. Stimulus-induced inhibition, likely for contrast enhancement, was also often observed for the many of the FB units encountered. The observed EMRRs of the FB units presently identified and those of the OB units previously studied are consistent with the ability of catfish to behaviorally discriminate these compounds.

Behavioral and electrophysiological experiments suggest that the antennular outer flagellum is the site of pheromone reception in the male helmet crab Telmessus cheiragonus

Sexually competent females of Telmessus cheiragonus (helmet crab) release two pheromones that elicit grasping and copulation behaviors in males (Kamio et al., 2000, 2002, 2003). Our study aimed to use behavioral and electrophysiological techniques to identify the site of reception of these sex pheromones. In behavioral experiments, either the inner or the outer flagella of the antennules were ablated bilaterally from male crabs, and responses of male crabs to female odor were examined. When the inner flagella were surgically ablated, the sexual response (i.e., grasping and copulation behavior) of male crabs was not significantly changed relative to control animals that had their second antennae ablated. In contrast, the sexual response was significantly reduced when the outer flagella of the antennules were ablated, suggesting that the outer flagellum is the receptor organ that detects the sex pheromones. In electrophysiological experiments, urine, which in females contains the pheromone that elicits grasping behavior by males but does not contain the pheromone eliciting copulation, whose release site is not known, was tested. Female and male urine as well as shrimp extract evoked phasic responses of chemosensory afferents innervating aesthetasc sensilla on the outer flagellum of male crabs. The response of the afferents had significantly higher magnitude and lower threshold when female urine was applied. Thus, behavioral and electrophysiological observations suggest that in male helmet crabs, the outer flagellum of the antennule is the chemosensory organ that detects female sex pheromone.

Classes and narrowing selectivity of olfactory receptor neurons of Xenopus laevis tadpoles

In olfactory receptor neurons (ORNs) of aquatic animals amino acids have been shown to be potent stimuli. Here we report on calcium imaging experiments in slices of the olfactory mucosa of Xenopus laevis tadpoles. We were able to determine the response profiles of 283 ORNs to 19 amino acids, where one profile comprises the responses of one ORN to 19 amino acids. 204 out of the 283 response profiles differed from each other. 36 response spectra occurred more than once, i.e., there were 36 classes of ORNs identically responding to the 19 amino acids. The number of ORNs that formed a class ranged from 2 to 13. Shape and duration of amino acid-elicited [Ca2+]i transients showed a high degree of similarity upon repeated stimulation with the same amino acid. Different amino acids, however, in some cases led to clearly distinguishable calcium responses in individual ORNs. Furthermore, ORNs clearly appeared to gain selectivity over time, i.e., ORNs of later developmental stages responded to less amino acids than ORNs of earlier stages. We discuss the narrowing of ORN selectivity over stages in the context of expression of olfactory receptors.

Electrophysiological demonstration of independent olfactory receptor types and associated neuronal responses in the trout olfactory bulb

The present study attempts to highlight the principles by which peripheral olfactory information of across- and within-class odorant signals is transformed into bulbar neuron responses. For this purpose, we performed electro-olfactogram cross-adaptation and mixture experiments as well as single unit recording of olfactory bulb neurons using amino acid, bile acid and F-prostaglandin stimulants in brown and rainbow trout. The results show that amino acids, a bile acid and a F-prostaglandin activate independent receptor types. However, within the class of amino acids, different receptor types are only partially independent. Neurons responsive to bile acid and amino acids were segregated to the mid-dorsal and latero-posterior olfactory bulb, respectively. Of the 43 responsive olfactory bulb neurons studied in brown trout, 41 showed specificity for one odorant class. Olfactory bulb neurons gained responsiveness to new amino acids with increasing stimulant concentration. We conclude that different odorant classes activate specific neurons located in different regions of the trout olfactory bulb, and that information distinguishing related amino acids can be represented in a limited number of bulbar neurons with distinct response profiles under the conditions investigated.

Odorant Specificity of Single Olfactory Bulb Neurons to Amino Acids in the Channel Catfish

Odorant specificity to l-α-amino acids was determined for 245 olfactory bulb (OB) neurons recorded from 121 channel catfish. The initial tests included 4 amino acids representing acidic [monosodium glutamate (Glu)], basic [arginine (Arg)], and neutral [possessing short: alanine (Ala) and long: methionine (Met) side chains] amino acids that were previously indicated to bind to independent olfactory receptor sites. Ninety-one (37%) units (Group I) tested at 1, 10, and 100 μM showed high selectivity and were excited by only one of the 4 amino acids. Odorant specificity for the vast majority of Group I units did not change over the 3 s of response time analyzed. A total of 154 OB units (63%) (Group II) were excited by a second amino acid, but only at ≥10× odorant concentration. An additional 69 Group I units were tested with related amino acids and derivatives from 10−9 to 10−5 M to determine their excitatory odorant thresholds and selectivities. Two groups of units originally selective for Met were evident: those most sensitive to neutral amino acids having branched and linear side chains, respectively. OB units originally selective for Ala responded at low concentration to other similar amino acids. Units originally selective for Arg were excited at low concentration by amino acids possessing in their side chains at least 3 methylene groups and a terminal amide or guanidinium group. The specificities of the OB units determined electrophysiologically are sufficient to account for many of the previous results of behavioral discrimination of amino acids in this and related species.

Mitral cell temporal response patterns evoked by odor mixtures in the rat olfactory bulb

Mammals generally have the ability to extract odor information contained in complex mixtures of molecular components. However, odor mixture processing has been studied electrophysiologically only in insects, crustaceans, and fish. As a first step toward a better understanding of this processing in high vertebrates, we studied the representation of odor mixtures in the rat olfactory bulb, i.e., the second-order level of the olfactory pathways. We compared the single-unit responses of mitral cells, the main cells of the olfactory bulb, to pure odors and to their binary mixtures. Eighty-six mitral cells were recorded in anesthetized freely breathing rats stimulated with five odorants and their 10 binary mixtures. The spontaneous activity and the odor-evoked responses were characterized by their temporal distribution of activity along the respiratory cycle, i.e., by cycle-triggered histograms. Ninety percent of the mixtures were found to evoke a response when at least one of their two components evoked a response. Mixture-evoked patterns were analyzed to describe the modalities of the combination of patterns evoked by the two components. In most of the cases, the mixture pattern was closely similar to one of the component patterns. This dominance of a component over the other one was related to the responsiveness of the cell to the individual components of the mixture, to the molecular nature of the stimulus, and to the coarse shape of individual response patterns. This suggests that the components of binary mixtures may be encoded simultaneously by different odor-specific temporal distributions of activity.

Chemical orientation of brown bullheads, Ameiurus nebulosus, under different flow conditions

The spatiotemporal information in chemical signals provides critical information for organisms during chemical orientation. Information in chemical signals is influenced by the hydrodynamic conditions of the environment. Hydrodynamically distinct environments will contain different types of information, which will influence how organisms orient. This study was designed to examine how the orientation behavior of the brown bullhead (Ameiurus nebulosus) is influenced by flow regime. The experiment was conducted in a flume under two different flow conditions. Treatments consisted of control (no odor) and plain gelatin (odor). Percent success, swimming speed, turning angle, heading angle, heading angle upstream, and net-to-gross ratio were analyzed. Brown bullheads were 100% successful in finding the odor source under no flow and 57% successful in flow. Bullheads swam differently in the no-flow condition when compared to the flow condition. Since, these fish did not orient the same under different flow conditions, it appears that hydrodynamics plays a role in shaping their behavior.

Characteristic features and ligand specificity of the two olfactory receptor classes from Xenopus laevis

Amphibia have two classes of olfactory receptors (ORs), class I (fish-like receptors) and class II (mammalian-like receptors). These two receptor classes correspond to the two classes identified in other vertebrates, and amphibians thus provide a unique opportunity to compare olfactory receptors of both classes in one animal species, without the constraints of evolutionary distance between different vertebrate orders, such as fish and mammals. We therefore identified the complete open reading frames of class I and class II ORs in Xenopus laevis. In addition to allowing a representative comparison of the deduced amino acid sequences between both receptor classes, we were also able to perform differential functional analysis. These studies revealed distinct class-specific motifs, particularly in the extracellular loops 2 and 3, which might be of importance for the interaction with odorants, as well as in the intracellular loops 2 and 3, which might be responsible for interactions with specific G-proteins. The results of functional expression studies in Xenopus oocytes, comparing distinct receptor types, support the idea that class I receptors are activated by water-soluble odorants, whereas class II receptors are activated by volatile compounds.

Odor specificity of habituation in the rat anterior piriform cortex

Exposure to odorants results in a rapid (<10 s) reduction in odor-evoked activity in the rat piriform cortex despite relatively maintained afferent input from olfactory bulb mitral cells. To further understand this form of cortical plasticity, a detailed analysis of its odor specificity was performed. Habituation of odor responses in anterior piriform cortex single units was examined in anesthetized, freely breathing rats. The magnitude of single-unit responses of layer II/III neurons to 2-s odor pulses were examined before and after a 50-s habituating stimulus of either the same or different odor. The results demonstrated that odor habituation was odor specific, with no significant cross-habituation between either markedly different single odors or between odors within a series of straight chain alkanes. Furthermore, habituation to binary 1:1 mixtures produced minimal cross-habituation to the components of that mixture. These latter results may suggest synthetic odor processing in the olfactory system, with novel odor mixtures processed as unique stimuli. Potential mechanisms of odor habituation in the piriform cortex must be able to account for the high degree of specificity of this effect.

Physiological evidence for the discrimination of L-arginine from structural analogues by the zebrafish olfactory system

Although it is generally assumed that fish are capable of discriminating amino acid odorants on the basis of differences in side-chain structure, less is known about their ability to discriminate amino acids with modifications to alpha-carboxyl and alpha-amino groups. In this study, the ability of the zebrafish olfactory system to detect and presumably discriminate analogues of the basic amino acid Arg was assessed, by using cross-adaptation and activity-dependent labeling techniques. Electrophysiological recordings established that esterification (L-arginine methyl ester; AME) or deletion (agmatine or amino-4-guanidobutane; AGB) of the alpha-carboxyl group yielded odorants more potent than Arg, whereas deletion of the alpha-amino group (L-argininic acid; AA) yielded a less potent analogue. In cross-adaptation experiments, no test-competitor odorant combination yielded complete cross-adaptation, suggesting the detection of these Arg analogues by multiple odorant receptors (ORs) with partially nonoverlapping specificities. Activity-dependent immunocytochemical labeling of olfactory receptor neurons supported this conclusion. AGB, an ion-channel-permeant probe (and odorant), labeled 4.9 +/- 0.4% (n = 24) of sensory epithelium, whereas the addition of Arg, 1-ethylguanidine sulfate, L-alpha-amino-beta-guanidinopropionate, or AME to AGB resulted in a significant elevation of labeling (8-14%). This study provides evidence that the olfactory system has the potential to discriminate among amino acid odorants with modified alpha-carboxyl and alpha-amino groups.

Facial taste responses of the channel catfish to binary mixtures of amino acids

We investigated the neural processing of binary gustatory mixtures of amino acids by the facial taste system of the channel catfish, Ictalurus punctatus. In vivo electrophysiological recordings indicated that the magnitude of both integrated and single-unit facial taste responses to binary mixtures of amino acids was greatest if the components bound to independent receptor sites. Facial taste responses were obtained from 32 multiunit and 55 single taste fiber preparations to binary mixtures of amino acids whose components bind to independent taste receptor sites (group I) or to the same or highly cross-reactive taste receptor sites (group II). All component stimuli were adjusted in concentration to provide approximately equal response magnitude as determined by either the height of the integrated multiunit taste response or by the number of action potentials generated/3 s of response time/single taste fiber. The mixture discrimination index (MDI), defined as the response to the mixture divided by the average of the responses to the component stimuli, was calculated for each test of a binary mixture. MDIs of group I binary mixtures for both the integrated multiunit and single fiber data were significantly greater than those for either the control or group II binary mixtures. In a subset of multiunit recordings, the MDIs of a group I binary mixture across three log units of stimulus concentration were similar and significantly greater than those of a group II binary mixture. Analysis of the single fiber data also indicated that the MDIs of group I binary mixtures were significantly larger than those of group II binary mixtures for both alanine-best and arginine-best taste fibers; however, the MDIs of group I binary mixtures calculated from recordings from arginine-best taste fibers were significantly greater than those recorded from alanine-best taste fibers.

Sex pheromones and amino acids evoke distinctly different spatial patterns of electrical activity in the goldfish olfactory bulb

Until now, electrophysiological studies of the vertebrate olfactory bulb have tested only 'generalist' cues. These studies suggest that odorants are discriminated by a broadly distributed spatial map. In this study, we tested for the first time in a vertebrate the hypothesis that 'specialist' cues (pheromones) are discriminated by a more restricted component of the olfactory bulb. Our model is the male goldfish, Carassius auratus, for which five sex pheromones with both behavioral and physiological activity have now been identified. Electrical activity (electroencephalography: EEG) was recorded over a 12-point grid from the surface of the olfactory bulb, while fish were exposed to one of ten stimuli including: five sex pheromones, two amino acids, two bile steroids and a control. Evoked activity was evaluated by time series analysis. Power ratios were calculated by dividing the power of the dominant frequency in the power spectrum before stimulation by the power of the dominant frequency during stimulation. Next, the average magnitudes of odorant responses were compared using analysis of variance (ANOVA). The spatial patterning of these responses was also described using cluster analysis, which grouped odorants based on the similarity of their spatial patterns of activity. Although all odorants elicited EEG responses with similar dominant frequencies, odorant-specific differences were evident in the size and distribution of these responses. Sex pheromones and bile steroids elicited relatively small responses that were spatially restricted in distinctive manners, although some overlap was evident. In contrast, amino acids consistently produced large responses at all positions. These results are consistent with the hypothesis that vertebrate pheromones are discriminated by a distinctive subcomponent of the vertebrate olfactory system comprised of a relatively small number of olfactory neurons.

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.

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).

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.

Inhibition of taurine and 5'AMP olfactory receptor sites of the spiny lobster Panulirus argus by odorant compounds and mixtures

1. The effects of the odorant compounds adenosine-5'-monophosphate (5'AMP), ammonium, betaine, L-cysteine, L-glutamate, DL-succinate, and taurine and of mixtures of these compounds on binding of taurine and 5'AMP to dendritic membrane from the olfactory organ of spiny lobsters (Panulirus argus) were quantified to evaluate the contribution of inhibition of odorant-receptor binding to the generation of physiological responses to mixtures. 2. Taurine binding sites belong to two affinity classes, while 5'AMP binding sites belong to a single affinity class. Binding of either taurine or 5'AMP was partially inhibited in an apparently noncompetitive, concentration dependent fashion by most odorant compounds, with 25-40% inhibition by 1 mM of odorant. Mixtures of two or more odorant compounds also inhibited binding of taurine or 5'AMP to its sites. However, the inhibition by mixtures was often significantly less than expected from the inhibition produced by a mixture's components assuming either a noncompetitive or competitive mechanism. 3. By including this binding inhibition between compounds into models for predicting physiological responses to mixtures from the responses to the components, the predictive power of the models is significantly improved. This result strongly suggests that binding inhibition can influence the physiological responsiveness of chemoreceptor cells to mixtures.

Partitioning non-linearities in the response of honey bee olfactory receptor neurons to binary odors

In many organisms, of which honey bees are one example, a general (i.e., non-pheromonal) olfactory receptor neuron may respond to some odorants by increasing its firing rate and to others by decreasing its firing rate. In the latter case, this decrease will be with respect to a background firing rate determined by intrinsic (internal noise) and extrinsic (background odors) factors. To analyse receptor neurons of this complexity, we extend Beidler's model of receptor protein activation dynamics to account for the competition between depolarizing and hyperpolarizing pathways and couple the model to a phenomenological description of the non-linear relationship between the proportion of activate membrane receptors and the receptor cell spike generation rates. We then examine the implications of this theory for predicting the response of receptor neurons to odor mixtures based on their response to pure odorants at concentrations matched to the mixture. We derive inequalities that must be satisfied under our normative model, and propose that deviations from the model be designated as synergisms and inhibitions, depending on the direction in which various equalities and inequalities are violated. We then apply our inequalities to identifying synergisms and inhibitions in data analysed in a different way elsewhere (Akers, R.P. and Getz, W.M. Response of olfactory receptor neurons in honey bees to odorants and their binary mixtures. J. Comp. Physiol. (in press)). In these data regarding the response of honey bee placode sensilla to a number of odorants and their binary combinations, we demonstrate the presence of synergisms and inhibitions--that is, elevated or repressed responses that are not due to competitive interactions of mixture component odorants for receptor sites or Beidler (Beidler, L.M., 1962. Taste receptor stimulation. Prog. Biophys. Biophys. Chem. 12, 107-151) saturation mechanisms.

Learned olfactory discrimination versus innate taste responses to amino acids in channel catfish (Ictalurus punctatus)

Intact channel catfish conditioned to the L-amino acids, proline, arginine, alanine, and lysine, discriminated these stimuli from all other amino acids tested. Behavioral structure-activity tests indicated that L-pipecolate was the only effective agonist of the L-proline conditioned response. For channel catfish in which one of the paired olfactory organs was surgically removed, the number of turns to the conditioned stimulus was 40% fewer than those of intact catfish; however, these semiosmic channel catfish discriminated the conditioned from nonconditioned stimuli, as evidenced by their responding to the conditioned amino acid, with a two- to threefold greater number of turns than to the nonconditioned amino acids. Irrespective of the number of conditioning trials attempted, catfish with both olfactory organs removed were unable to discriminate the conditioned from the nonconditioned stimuli.

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

Single-unit spiking responses of 72 olfactory receptor neurons (ORNs) in the olfactory organ of the spiny lobster Panulirus argus were recorded extracellularly during presentation of a set of seven odorant stimuli (adenosine-5'-monophosphate, ammonium chloride, betaine, L-cysteine, L-glutamate, D,L-succinate and taurine) and analyzed in order to evaluate the response specificities of single ORNs and the independence of receptor sites. Individual ORNs often had narrow excitatory response spectra, but the most excitatory compound was different from neuron to neuron. These results suggest that these compounds can exert most of their excitatory effects through relatively independent receptor site types. To determine the relative independence of excitatory transduction processes in single ORNs for these stimuli, single-unit spiking responses of these neurons under conditions of self- and cross-adaptation were analyzed. The results demonstrate extensive cross-adaptation between pairs of the seven stimuli. When averaged across all neurons and all cross-adaptation conditions, cross-adaptation resulted in a mean reduction of 81% of the unadapted response. However, there were differences in the degree and pattern of adaptation for different pairs of compounds and for different neuron types (defined by most excitatory or 'best' chemical). For a given neuron type, there were significant levels of non-reciprocal cross-adaptation: neurons cross-adapted more when adapted to their best chemical than when adapted to their non-best chemicals. These results suggest the existence of two excitatory transduction pathways within an olfactory receptor neuron: one pathway activated exclusively by the best chemical and a second pathway activated by a broader spectrum of chemicals.

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).

Chemosensory responses to mixtures: a model based on composition of receptor cell types

Previous mixture models have assumed that members of a population of chemoreceptor cells are homogeneous in type, i.e., with either single shared or multiple independent receptor sites. In reality, many chemosensory systems actually consist of a heterogeneous population of receptor cells, consisting of both highly specific cells as well as more broadly and variably tuned cells. A mixed receptor composition model for binary mixtures is described which can be applied to chemosensory systems with heterogeneous receptor cell compositions. The model incorporates information on a) the number of receptor sites/transduction processes per cell, b) the specificity of receptor cells, and c) the contribution of the magnitude of response of each receptor cell to the overall response magnitude of the population of all receptor cells. The predictions of this model can be compared to behavioral responses of animals towards binary mixtures, or at any level of sensory processing which involves the input of the receptor cell population, in order to detect possible mixture interactions.