Olfactory responses of aquatic and terrestrial tiger salamanders to airborne and waterborne stimuli

Learning the native pond odor as one of the mechanisms of olfactory orientation in juvenile smooth newt Lissotriton vulgaris

Olfaction is an important mechanism of orientation in amphibians toward the breeding site. It is known that anurans can memorize the odor of the native pond during larval development and prefer this odor prior to the beginning of dispersion. However, such a mechanism in urodeles has not been studied yet. We conducted experiments on recognition of the odor of a native water body in juveniles of the smooth newt Lissotriton vulgaris. One group of larvae were reared in pure water (control), the other group in water with morpholine (10-7 mol/L). A few days after metamorphosis, the newts were tested under paired-choice conditions in a T-maze. A total of 73 newts from the experimental group and 47 newts from the control group were tested. The results of the experiment show that the newts in the experimental group preferred the morpholine solution, whereas the individuals of the control group made the choice randomly. We conclude that newts can learn the odor of the environment in which they developed and use this memory for orientation in later stages of life.

What do oysters smell? Electrophysiological evidence that the bivalve osphradium is a chemosensory organ in the oyster, Magallana gigas

The sensing of chemical cues is essential for several aspects of bivalve biology, such as the detection of food and pheromones. However, little is known about chemical communication systems in bivalves or the possible role of the osphradium as a chemosensory organ. To address this, we adapted an electrophysiological technique extensively used in vertebrates-the electro-olfactogram-to record from the osphradium in the Pacific oyster, Magallana gigas. This technique was validated using amino acids as stimulants. The osphradium proved to be sensitive to most proteinogenic L-amino acids tested, evoking tonic, negative, concentration-dependent 'electro-osphradiogram' (EOsG) voltage responses, with thresholds of detection in the range of 10^- 6 to 10^- 5 M. Conversely, it was insensitive to L-arginine and L-glutamic acid. The current study supports the hypothesis that the osphradium is, indeed, a chemosensory organ. The 'electro-osphradiogram' may prove to be a powerful tool in the isolation and characterization of pheromones and other important chemical cues in bivalve biology.

Why did the invasive walking catfish cross the road? Terrestrial chemoreception described for the first time in a fish

Clarias batrachus (walking catfish) is an invasive species in Florida, renowned for its air-breathing and terrestrial locomotor capabilities. However, it is unknown how this species orients in terrestrial environments. Furthermore, while anecdotal life history information is widespread for this species in its nonnative range, little of this information exists in the literature. The goals of this study were to identify sensory modalities that C. batrachus use to orient on land, and to describe the natural history of this species in its nonnative range. Fish (n = 150) were collected from around Ruskin, FL, and housed in a greenhouse, where experiments took place. Individual catfish were placed in the center of a terrestrial arena and were exposed to nine treatments: two controls, L-alanine, quinine, allyl isothiocynate, sucrose, volatile hydrogen sulphide, pond water and aluminium foil. These fish exhibited significantly positive chemotaxis toward alanine and pond water, and negative chemotaxis away from volatile hydrogen sulphide, suggesting chemoreception - both through direct contact and through the air - is important to their terrestrial orientation. Additionally, 88 people from Florida wildlife-related Facebook groups who have personal observations of C. batrachus on land were interviewed for information regarding their terrestrial natural history. These data were combined with observations from 38 YouTube videos. C. batrachus appear to emerge most frequently during or just after heavy summer rains, particularly from stormwater drains in urban areas, where they may feed on terrestrial invertebrates. By better understanding the full life history of C. batrachus, we can improve management of this species.

Transition from sea to land: olfactory function and constraints in the terrestrial hermit crab Coenobita clypeatus

The ability to identify chemical cues in the environment is essential to most animals. Apart from marine larval stages, anomuran land hermit crabs (Coenobita) have evolved different degrees of terrestriality, and thus represent an excellent opportunity to investigate adaptations of the olfactory system needed for a successful transition from aquatic to terrestrial life. Although superb processing capacities of the central olfactory system have been indicated in Coenobita and their olfactory system evidently is functional on land, virtually nothing was known about what type of odourants are detected. Here, we used electroantennogram (EAG) recordings in Coenobita clypeatus and established the olfactory response spectrum. Interestingly, different chemical groups elicited EAG responses of opposite polarity, which also appeared for Coenobita compressus and the closely related marine hermit crab Pagurus bernhardus. Furthermore, in a two-choice bioassay with C. clypeatus, we found that water vapour was critical for natural and synthetic odourants to induce attraction or repulsion. Strikingly, also the physiological response was found much greater at higher humidity in C. clypeatus, whereas no such effect appeared in the terrestrial vinegar fly Drosophila melanogaster. In conclusion, our results reveal that the Coenobita olfactory system is restricted to a limited number of water-soluble odourants, and that high humidity is most critical for its function.

Olfactory response termination involves Ca2+-ATPase in vertebrate olfactory receptor neuron cilia

In vertebrate olfactory receptor neurons (ORNs), odorant-induced activation of the transduction cascade culminates in production of cyclic AMP, which opens cyclic nucleotide–gated channels in the ciliary membrane enabling Ca²⁺ influx. The ensuing elevation of the intraciliary Ca²⁺ concentration opens Ca²⁺-activated Cl⁻ channels, which mediate an excitatory Cl⁻ efflux from the cilia. In order for the response to terminate, the Cl⁻ channel must close, which requires that the intraciliary Ca²⁺ concentration return to basal levels. Hitherto, the extrusion of Ca²⁺ from the cilia has been thought to depend principally on a Na⁺–Ca²⁺ exchanger.

In this study, we show using simultaneous suction pipette recording and Ca²⁺-sensitive dye fluorescence measurements that in fire salamander ORNs, withdrawal of external Na⁺ from the solution bathing the cilia, which incapacitates Na⁺–Ca²⁺exchange, has only a modest effect on the recovery of the electrical response and the accompanying decay of intraciliary Ca²⁺ concentration. In contrast, exposure of the cilia to vanadate or carboxyeosin, a manipulation designed to block Ca²⁺-ATPase, has a substantial effect on response recovery kinetics. Therefore, we conclude that Ca²⁺-ATPase contributes to Ca²⁺ extrusion in ORNs, and that Na⁺–Ca²⁺exchange makes only a modest contribution to Ca²⁺ homeostasis in this species.

Chemosensory reception, behavioral expression, and ecological interactions at multiple trophic levels

Chemoreception may function throughout an entire animal lifetime, with independent, stage-specific selection pressures leading to changes in physiological properties, behavioral expression, and hence, trophic interactions. When the California newt (Taricha torosa) metamorphoses from an entirely aquatic larva to a semi-terrestrial juvenile/adult form, its chemosensory organs undergo dramatic reorganization. The relationship between newt life-history stage and chemosensory-mediated behavior was established by comparing responses of adults (as determined here) to those of conspecific larvae (as studied previously). Bioassays were performed in mountain streams,testing responses of free-ranging adults to 13 individual l-amino acids. Relative to stream water (controls), adults turned immediately upcurrent and moved to the source of arginine, glycine or alanine release. These responses were indicative of predatory search. Arginine was the strongest attractant tested, with a response threshold (median effective dose)of 8.3×10–7 mol l–1 (uncorrected for dilution associated with chemical release and delivery). In contrast to adult behavior, arginine suppressed cannibal-avoidance and failed to evoke search reactions in larvae. For a common set of arginine analogs, the magnitudes of adult attraction and larval suppression were not positively correlated. Suppression of cannibal-avoidance behavior in larvae was unaffected by most structural modifications of the arginine molecule. Adult behavior, on the other hand, was strongly influenced by even subtle alterations in the parent compound. Reactions to arginine in both adults and larvae were eliminated by blocking the external openings of the nasal cavity.

Stimulating adult predatory search in one case and inhibiting larval cannibal avoidance in the other, arginine is a chemical signal with opposing behavioral effects and varying ecological consequences. Significant differences between responses of adults and larvae to changes in arginine structure suggest alternative, chemosensory receptor targets. Although arginine reception functions throughout an entire newt lifetime, an ontogenetic shift in larval and adult chemoreceptive ability changes behavioral expression, and thus, reflects the unique selection pressures that act at each life-history stage.

Olfactory metamorphosis in the Coastal Giant Salamander (Dicamptodon tenebrosus)

This study examined the gross morphology and ultrastructure of the olfactory organ of larvae, neotenic adults, and terrestrial adults of the Coastal Giant Salamander (Dicamptodon tenebrosus). The olfactory organ of all aquatic animals (larvae and neotenes) is similar in structure, forming a tube extending from the external naris to the choana. A nonsensory vestibule leads into the main olfactory cavity. The epithelium of the main olfactory cavity is thrown into a series of transverse valleys and ridges, with at least six dorsal and nine ventral valleys lined with olfactory epithelium, and separated by ridges of respiratory epithelium. The ridges enlarge with growth, forming large flaps extending into the lumen in neotenes. The vomeronasal organ is a diverticulum off the ventrolateral side of the main olfactory cavity. In terrestrial animals, by contrast, the vestibule has been lost. The main olfactory cavity has become much broader and dorsoventrally compressed. The prominent transverse ridges are lost, although small diagonal ridges of respiratory epithelium are found in the lateral region of the ventral olfactory epithelium. The posterior and posteromedial wall of the main olfactory cavity is composed of respiratory epithelium, in contrast to the olfactory epithelium found here in aquatic forms. The vomeronasal organ remains similar to that in large larvae, but is now connected to the mouth by a groove that extends back through the choana onto the palate. Bowman's glands are present in the main olfactory cavity at all stages, but are most abundant and best developed in terrestrial adults. They are lacking in the lateral olfactory epithelium of the main olfactory cavity. At the ultrastructural level, in aquatic animals receptor cells of the main olfactory cavity can have cilia, short microvilli, a mix of the two, or long microvilli. Supporting cells are of two types: secretory supporting cells with small, electron-dense secretory granules, and ciliated supporting cells. Receptor cells of the vomeronasal organ are exclusively microvillar, but supporting cells are secretory or ciliated, as in the main olfactory cavity. After metamorphosis two distinct types of sensory epithelium occur in the main olfactory cavity. The predominant epithelium, covering most of the roof and the medial part of the floor, is characterized by supporting cells with large, electron-lucent vesicles. The epithelium on the lateral floor of the main olfactory cavity, by contrast, resembles that of aquatic animals. Both types have both microvillar and ciliated receptor cells. No important changes are noted in cell types of the vomeronasal organ after metamorphosis. A literature survey suggests that some features of the metamorphic changes described here are characteristic of all salamanders, while others appear unique to D. tenebrosus.

Ultrastructure of the olfactory organ in the clawed frog, Xenopus laevis, during larval development and metamorphosis

Development of the olfactory epithelia of the African clawed frog, Xenopus laevis, was studied by scanning and transmission electron microscopy. Stages examined ranged from hatching through the end of metamorphosis. The larval olfactory organ consists of two chambers, the principal cavity and the vomeronasal organ (VNO). A third sensory chamber, the middle cavity, arises during metamorphosis. In larvae, the principal cavity is exposed to water-borne odorants, but after metamorphosis it is exposed to airborne odorants. The middle cavity and the VNO are always exposed to waterborne odorants. Electron microscopy reveals that in larvae, principal cavity receptor cells are of two types, ciliated and microvillar. Principal cavity supporting cells are also of two types, ciliated and secretory (with small, electron-lucent granules). After metamorphosis, the principal cavity contains only ciliated receptor cells and secretory supporting cells, and the cilia on the receptor cells are longer than in larvae. Supporting cell secretory granules are now large and electron-dense. In contrast, the middle cavity epithelium contains the same cell types seen in the larval principal cavity. The VNO has microvillar receptor cells and ciliated supporting cells throughout life. The cellular process by which the principal cavity epithelium changes during metamorphosis is not entirely clear. Morphological evidence from this study suggests that both microvillar and ciliated receptor cells die, to be replaced by newly generated cells. In addition, ciliated supporting cells also appear to die, whereas there is evidence that secretory supporting cells transdifferentiate into the adult type. In summary, significant developmental additions and neural plasticity are involved in remodeling the olfactory epithelium in Xenopus at metamorphosis.

Suppression of odorant responses by odorants in olfactory receptor cells

Odorants activate an inward current in vertebrate olfactory receptor cells. Here it is shown, in receptor cells from the newt, that odorants can also suppress this current, by a mechanism that is distinct from inhibition and adaptation. Suppression provides a simple explanation for two seemingly unrelated phenomena: the anomalously long latency of olfactory transduction and the existence of an "off response" at the end of a prolonged stimulus. Suppression may influence the perception of odorants by masking odorant responses and by sharpening the odorant specificities of single cells.

Modulation of Cl-, K+, and nonselective cation conductances by taurine in olfactory receptor neurons of the mudpuppy Necturus maculosus

Odors are transduced by processes that modulate the membrane conductance of olfactory receptor neurons. Olfactory neurons from the aquatic salamander, Necturus maculosus, were acutely isolated without enzymes and studied with a resistive whole-cell method to minimize loss of soluble intracellular constituents. 55 of 224 neurons responded to the test compound taurine at concentrations between 10 nM and 100 microM. Four different conductance changes were elicited by taurine: an increased Cl- conductance (33%), an increased nonselective cation conductance (15%), a decreased Cl- conductance (15%), and a decreased K+ conductance (15%); in addition, responses too small to be characterized were elicited in some neurons. In most cases, taurine appeared to modulate only a single conductance in any particular cell. Modulation of each conductance was dose dependent, and each response ran down quickly in the normal whole-cell mode, presumably due to washout of a diffusible component in the transduction pathway. Modulation of taurine-sensitive conductances caused either inhibitory or excitatory responses. A similar diversity of responses in vivo would produce a complex pattern of electrical activity that could encode the identity and characteristics of an odor.

Chemosensory responses in isolated olfactory receptor neurons from Necturus maculosus

Olfactory receptor neurons were isolated without enzymes from the mudpuppy, Necturus maculosus, and tested for chemosensitivity. The cells responded to odorants with changes in firing frequency and alterations in excitability that were detected with tight-seal patch electrodes using on-cell and whole-cell recording conditions. Chemosensitive cells exhibited two primary response characteristics: excitation and inhibition. Both types of primary response were observed in different cells stimulated by mixtures of amino acids as well as by the single compound L-alanine, suggesting that there may be more than one transduction pathway for some odorants. Using the normal whole-cell recording method, the chemosensitivity of competent cells washed out rapidly; a resistive whole-cell method was used to record odorant responses under current-clamp conditions. In response to chemical stimulation, excitability appeared to be modulated in several different ways in different cells: odorants induced hyperpolarizing or depolarizing receptor potentials, elicited or inhibited transient, rhythmic generator potentials, and altered excitability without changing the membrane potential or input resistance. These effects suggest that olfactory transduction is mediated through at least three different pathways with effects on four or more components of the membrane conductance. Polychotomous pathways such as these may be important for odor discrimination and for sharpening the "odor image" generated in the olfactory epithelium.

Ca2(+)-dependent adaptive properties in the solitary olfactory receptor cell of the newt

The time-dependent decay of the olfactory receptor potential was analyzed with a solitary cell preparation by using the whole-cell patch clamp technique. During prolonged stimulation by 10 mM N-amylacetate under standard conditions, 17 out of 63 isolated olfactory cells responded with slow depolarization. Of these 17 cells, response amplitudes in 14 cells ('phasic/tonic' response) gradually decayed within 9 s, with a half-decay time of 1.71 +/- 1.10 s (mean +/- S.D.). The relative amplitude (ratio of tonic component to peak amplitude, Vtonic/Vmax) was 0.29 +/- 0.10. The response decay was attributed to the inactivation of the odorant-activated conductance. The recovery after inactivation, which was determined with double pulse experiments, was dependent on the resting interval. The inactivation of the odorant-activated conductance was found to be observed only when the external medium contained Ca2+. In addition, it was found that the odorant-activated conductance was capable of permeating Ca2+ (PCa/PNa = 6.5), and a rise in the internal EGTA concentration (to 50 mM) inhibited the inactivation. These observations suggest that the decay of the olfactory response to prolonged stimulation is mediated by Ca2+ influx.

Activation by odorants of cation-selective conductance in the olfactory receptor cell isolated from the newt

1. Ionic selectivity of the conductance activated by n-amyl acetate (odorant-activated conductance) was analysed in isolated olfactory receptor cells under the whole-cell voltage clamp condition. 2. Solitary receptor cells had a resting membrane potential of -44.7 +/- 7.0 mV (mean +/- S.D.; n = 70). Application of 10 mM-n-amyl acetate caused a depolarizing response in about 30% of the cells. Sensitivity to the odorant was maximum at around the apical dendrite. 3. Odorant induced an inward current to cells voltage clamped at their resting potential and bathed in the standard medium. The response amplitude was voltage dependent, and the polarity reversed at +2.5 +/- 2.2 mV (n = 6). The I-V relation was almost linear at membrane potentials more positive than -20 mV, with an average slope of 3.14 +/- 1.59 nS (measured at 0 mV), but showed a marked outward rectification at voltages more negative than -30 mV. 4. Removal of external Ca2+ increased the amplitude of the odorant-induced current and prolonged response duration, but did not cause a significant change on the reversal potential. Thus, Ca2+ affected the kinetics of the conductance, but did not seem to be a dominant charge carrier in the physiological condition. 5. Reduction of external Na+ concentration [( Na+]o) (replaced with choline) shifted the reversal potential by about 57 mV per 10-fold change of [Na+]o. Removal of external Cl- (replaced with glutamate ions) did not affect the reversal potential. 6. The odorant-activated conducting channels were permeable to all alkali metal ions. The permeability ratios were: PLi:PNa:PK:PRb:PCs = 1.25:1:0.98:0.84:0.80. 7. The present study strongly suggests that the olfactory receptor potential is generated by an increase in the membrane conductance to alkali metal ions.

A comparison of the discriminatory ability and sensitivity of the trigeminal and olfactory systems to chemical stimuli in the tiger salamander

Trigeminal receptors can respond to a wide variety of chemical stimuli, but it is unknown whether these receptors mediate discrimination between chemical stimuli matched for equal perceptual intensity. The present electrophysiological and behavioral experiments address this issue using tiger salamanders, Ambystoma tigrinum, and four compounds (amyl acetate, cyclohexanone, butanol, and d-limonene). In addition, the relative sensitivities of the trigeminal and olfactory systems to these compounds are compared. In electrophysiological cross-adaptation experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), there was complete cross adaptation such that only concentrations above the background (cross-adapting) stimulus concentration elicited responses, suggesting that chemical stimuli may stimulate trigeminal receptors nonspecifically. In behavioral experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), only animals with intact olfactory nerves could discriminate between perceptually equivalent concentrations, that is concentrations that elicited the same level of responding. Both electrophysiologically and behaviorally, the trigeminal system exhibited higher thresholds than the olfactory system. We conclude that trigeminal chemoreceptors, at least in salamanders, are unable to discriminate between these two pairs of compounds when matched for equal perceptual intensity, and that trigeminal chemoreceptors are less sensitive than olfactory receptors.