The elusive crypt olfactory receptor neuron: evidence for its stimulation by amino acids and cAMP pathway agonists

Response of Olfactory Sensory Neurons to Mercury Ions in Zebrafish: An Immunohistochemical Study

Olfactory sensory neurons (OSNs) of fish belong to three main types: ciliated olfactory sensory neurons (cOSNs), microvillous olfactory sensory neurons (mOSNs), and crypt cells. Mercury is a toxic metal harmful for olfaction. We exposed the olfactory epithelium of zebrafish to three sublethal Hg2+ concentrations. Molecular markers specific for the different types of OSNs were immunohistochemically detected. Image analysis of treated sections enabled counting of marked cells and measurement of staining optical density indicative of the response of OSNs to Hg2+ exposure. The three types of OSNs reacted to mercury in a different way. Image analysis revealed that mOSNs are more susceptible to Hg2+ exposure than cOSNs and crypt cell density decreases. Moreover, while the ratio between sensory/nonsensory epithelium areas is unchanged, epithelium thickness drops, and dividing cells increase in the basal layer of the olfactory epithelium. Cell death but also reduction of apical processes and marker expression could account for changes in OSN immunostaining. Also, the differential results between dorsal and ventral halves of the olfactory rosette could derive from different water flows inside the olfactory chamber or different subpopulations in OSNs.

Physiology, endocrinology and chemical communication in aggressive behaviour of fishes

Fishes show remarkably diverse aggressive behaviour. Aggression is expressed to secure resources; adjusting aggression levels according to context is key to avoid negative consequences for fitness and survival. Nonetheless, despite its importance, the physiological basis of aggression in fishes is still poorly understood. Several reports suggest hormonal modulation of aggression, particularly by androgens, but contradictory studies have been published. Studies exploring the role of chemical communication in aggressive behaviour are also scant, and the pheromones involved remain to be unequivocally characterized. This is surprising as chemical communication is the most ancient form of information exchange and plays a variety of other roles in fishes. Furthermore, the study of chemical communication and aggression is relevant at the evolutionary, ecological and economic levels. A few pioneering studies support the hypothesis that aggressive behaviour, at least in some teleosts, is modulated by "dominance pheromones" that reflect the social status of the sender, but there is little information on the identity of the compounds involved. This review aims to provide a global view of aggressive behaviour in fishes and its underlying physiological mechanisms including the involvement of chemical communication, and discusses the potential use of dominance pheromones to improve fish welfare. Methodological considerations and future research directions are also outlined.

Multimodal Imaging and Analysis of the Neuroanatomical Organization of the Primary Olfactory Inputs in the Brownbanded Bamboo Shark, Chiloscyllium punctatum

There is currently a limited understanding of the morphological and functional organization of the olfactory system in cartilaginous fishes, particularly when compared to bony fishes and terrestrial vertebrates. In this fish group, there is a clear paucity of information on the characterization, density, and distribution of olfactory receptor neurons (ORNs) within the sensory olfactory epithelium lining the paired olfactory rosettes, and their functional implications with respect to the hydrodynamics of incurrent water flow into the nares. This imaging study examines the brownbanded bamboo shark Chiloscyllium punctatum (Elasmobranchii) and combines immunohistochemical labeling using antisera raised against five G-protein α-subunits (Gα_s/olf, Gα_q/11/14, Gα_i–1/2/3, Gα_i–3, Gα_o) with light and electron microscopy, to characterize the morphological ORN types present. Three main ORNs (“long”, “microvillous” and “crypt-like”) are confirmed and up to three additional microvilli-bearing types are also described; “Kappe-like” (potential or homologous “Kappe” as in teleosts), “pear-shaped” and “teardrop-shaped” cells. These morphotypes will need to be confirmed molecularly in the future. Using X-ray diffusible iodine-based contrast-enhanced computed tomography (diceCT), high-resolution scans of the olfactory rosettes, olfactory bulbs (OBs), peduncles, and telencephalon reveal a lateral segregation of primary olfactory inputs within the OBs, with distinct medial and lateral clusters of glomeruli, suggesting a potential somatotopic organization. However, most ORN morphotypes are found to be ubiquitously distributed within the medial and lateral regions of the olfactory rosette, with at least three microvilli-bearing ORNs labeled with anti-Gα_o found in significantly higher densities in lateral lamellae [in lateral lamellae] and on the anterior portion of lamellae (facing the olfactory cavity). These microvilli-bearing ORN morphotypes (microvillous, “Kappe-like,” “pear-shaped,” and “teardrop-shaped”) are the most abundant across the olfactory rosette of this species, while ciliated ORNs are less common and crypt cells are rare. Spatial simulations of the fluid dynamics of the incurrent water flow into the nares and within the olfactory cavities indicate that the high densities of microvilli-bearing ORNs located within the lateral region of the rosette are important for sampling incoming odorants during swimming and may determine subsequent tracking behavior.

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.

Nitrogenous compounds characterized in the deterrent skin extract of migratory adult sea lamprey from the Great Lakes region

The sea lamprey (Petromzons marinus) is a devastating invasive species that represents a significant impediment to restoration of the Laurentian Great Lakes. There is substantial interest in developing environmentally benign control strategies for sea lamprey, and many other aquatic invasive species, that employ the manipulation of semiochemical information (pheromones and chemical cues) to guide the movements of invaders into control opportunities (e.g. traps, locations for safe pesticide application, etc.). A necessary precursor to the use of semiochemicals in conservation activities is the identification of the chemical constituents that compose the odors. Here, we characterize the major nitrogenous substances from the water-soluble fraction of a skin extract that contains the sea lamprey alarm cue, a powerful repellent that has proven effective in guiding the movements of migrating sea lamprey in rivers. Nitrogenous compounds are suspected components of fish alarm cues as the olfactory sensory neurons that mediate alarm responses transduce amino acids and related compounds. A laboratory assay confirmed the behavioral activity contained in the alarm cue resides in the water-soluble fraction of the skin extract. This water-soluble fraction consisted primarily of creatine (70%), heterocyclic nitrogenous compounds (4.3%) and free amino acids (18.4%), respectively. Among the free amino acids characterized in our study, essential amino acids constituted 13% of the water-soluble fraction. Free amino acids isolated from the water-soluble fraction composed of arginine, phenylalanine, threonine, and asparagine 3.9, 2.7, 2.6 and 2.4% of the water-soluble fraction, respectively. We discuss the implications of these findings for understanding the nature and use of the sea lamprey alarm cue in conservation activities.

Differential nickel-induced responses of olfactory sensory neuron populations in zebrafish

The olfactory epithelium of fish includes three main types of olfactory sensory neurons (OSNs). Whereas ciliated (cOSNs) and microvillous olfactory sensory neurons (mOSNs) are common to all vertebrates, a third, smaller group, the crypt cells, is exclusive for fish. Dissolved pollutants reach OSNs, thus resulting in impairment of the olfactory function with possible neurobehavioral damages, and nickel represents a diffuse olfactory toxicant. We studied the effects of three sublethal Ni²⁺ concentrations on the different OSN populations of zebrafish that is a widely used biological model. We applied image analysis with cell count and quantification of histochemically-detected markers of the different types of OSNs. The present study shows clear evidence of a differential responses of OSN populations to treatments. Densitometric values for G_{α olf}, a marker of cOSNs, decreased compared to control and showed a concentration-dependent effect in the ventral half of the olfactory rosette. The densitometric analysis of TRPC2, a marker of mOSNs, revealed a statistically significant reduction compared to control, smaller than the decrease for G_{α olf} and without concentration-dependent effects. After exposure, olfactory epithelium stained with anti-calretinin, a marker of c- and mOSNs, revealed a decrease in thickness while the sensory area appeared unchanged. The thickness reduction together with increased densitometric values for HuC/D, a marker of mature and immature neurons, suggests that the decrements in G_{α olf} and TRPC2 immunostaining may depend on cell death. However, reductions in the number of apical processes and of antigen expression could be a further explanation. We hypothesize that cOSNs are more sensitive than mOSNs to Ni²⁺ exposure. Difference between subpopulations of OSNs or differences in water flux throughout the olfactory cavity could account for the greater susceptibility of the OSNs located in the ventral half of the olfactory rosette. Cell count of anti-TrkA immunopositive cells reveals that Ni²⁺ exposure does not affect crypt cells. The results of this immunohistochemical study are not in line with those obtained by electro-olfactogram.

Molecular Markers in the Study of Non-model Vertebrates: Their Significant Contributions to the Current Knowledge of Tetrapod Glial Cells and Fish Olfactory Neurons

The knowledge of the morphological and functional aspects of mammalian glial cells has greatly increased in the last few decades. Glial cells represent the most diffused cell type in the central nervous system, and they play a critical role in the development and function of the brain. Glial cell dysfunction has recently been shown to contribute to various neurological disorders, such as autism, schizophrenia, pain, and neurodegeneration. For this reason, glia constitutes an interesting area of research because of its clinical, diagnostic, and pharmacological relapses. In this chapter, we present and discuss the cytoarchitecture of glial cells in tetrapods from an evolutive perspective. GFAP and vimentin are main components of the intermediate filaments of glial cells and are used as cytoskeletal molecular markers because of their high degree of conservation in the various vertebrate groups. In the anamniotic tetrapods and their progenitors, Rhipidistia (Dipnoi are the only extant rhipidistian fish), the cytoskeletal markers show a model based exclusively on radial glial cells. In the transition from primitive vertebrates to successively evolved forms, the emergence of a new model has been observed which is believed to support the most complex functional aspects of the nervous system in the vertebrates. In reptiles, radial glial cells are prevalent, but star-shaped astrocytes begin to appear in the midbrain. In endothermic amniotes (birds and mammals), star-shaped astrocytes are predominant. In glial cells, vimentin is indicative of immature cells, while GFAP indicates mature ones.Olfactory receptor neurons undergo continuous turnover, so they are an easy model for neurogenesis studies. Moreover, they are useful in neurotoxicity studies because of the exposed position of their apical pole to the external environment. Among vertebrates, fish represent a valid biological model in this field. In particular, zebrafish, already used in laboratories for embryological, neurobiological, genetic, and pathophysiological studies, is the reference organism in olfactory system research. Smell plays an important role in the reproductive behavior of fish, with direct influences also on the numerical consistency of their populations. Taking into account that a lot of species have considerable economic importance, it is necessary to verify if the model of zebrafish olfactory organ is also directly applicable to other fish. In this chapter, we focus on crypt cells, a morphological type of olfactory cells specific of fish. We describe hypothetical function (probably related with social behavior) and evolutive position of these cells (prior to the appearance of the vomeronasal organ in tetrapods). We also offer the first comparison of the molecular characteristics of these receptors between zebrafish and the guppy. Interestingly, the immunohistochemical expression patterns of known crypt cell markers are not overlapping in the two species.

The Chemical Sensitivity and Electrical Activity of Individual Olfactory Sensory Neurons to a Range of Sex Pheromones and Food Odors in the Goldfish

Although it is well established that the olfactory epithelium of teleost fish detects at least 6 classes of biologically relevant odorants using 5 types of olfactory sensory neurons (OSNs), little is understood about the specificity of individual OSNs and thus how they encode identity of natural odors. In this study, we used in vivo extracellular single-unit recording to examine the odor responsiveness and physiological characteristics of 109 individual OSNs in mature male goldfish to a broad range of biological odorants including feeding stimuli (amino acids, polyamines, nucleotides), sex pheromones (sex steroids, prostaglandins [PGs]), and a putative social cue (bile acids). Sixty-one OSNs were chemosensitive, with over half of these (36) responding to amino acids, 7 to polyamines, 7 to nucleotides, 5 to bile acids, 9 to PGs, and 7 to sex steroids. Approximately a quarter of the amino acid-sensitive units also responded to polyamines or nucleotides. Three of 6 amino acid-sensitive units responded to more than 1 amino acid compound, and 5 sex pheromone-sensitive units detected just 1 sex pheromone. While pheromone-sensitive OSNs also responded to the adenylyl cyclase activator, forskolin, amino acid-sensitive OSNs responded to either forskolin or a phospholipase C activator, imipramine. Most OSNs responded to odorants and activators with excitation. Our results suggest that pheromone information is encoded by OSNs specifically tuned to single sex pheromones and employ adenylyl cyclase, suggestive of a labeled-line organization, while food information is encoded by a combination of OSNs that use both adenylyl cyclase and phospholipase C and are often less specifically tuned.

Excreted Steroids in Vertebrate Social Communication

Steroids play vital roles in animal physiology across species, and the production of specific steroids is associated with particular internal biological functions. The internal functions of steroids are, in most cases, quite clear. However, an important feature of many steroids (their chemical stability) allows these molecules to play secondary, external roles as chemical messengers after their excretion via urine, feces, or other shed substances. The presence of steroids in animal excretions has long been appreciated, but their capacity to serve as chemosignals has not received as much attention. In theory, the blend of steroids excreted by an animal contains a readout of its own biological state. Initial mechanistic evidence for external steroid chemosensation arose from studies of many species of fish. In sea lampreys and ray-finned fishes, bile salts were identified as potent olfactory cues and later found to serve as pheromones. Recently, we and others have discovered that neurons in amphibian and mammalian olfactory systems are also highly sensitive to excreted glucocorticoids, sex steroids, and bile acids, and some of these molecules have been confirmed as mammalian pheromones. Steroid chemosensation in olfactory systems, unlike steroid detection in most tissues, is performed by plasma membrane receptors, but the details remain largely unclear. In this review, we present a broad view of steroid detection by vertebrate olfactory systems, focusing on recent research in fishes, amphibians, and mammals. We review confirmed and hypothesized mechanisms of steroid chemosensation in each group and discuss potential impacts on vertebrate social communication.

Differential response of olfactory sensory neuron populations to copper ion exposure in zebrafish

The peripheral olfactory system of fish is in direct contact with the external aqueous environment, so dissolved contaminants can easily impair sensory functions and cause neurobehavioral injuries. The olfactory epithelium of fish is arranged in lamellae forming a rosette in the olfactory cavity and contains three main types of olfactory sensory neurons (OSNs): ciliated (cOSNs) and microvillous olfactory sensory neurons (mOSNs), common to all vertebrates, and a third minor group of olfactory neurons, crypt cells, absent in tetrapods. Since copper is a ubiquitously diffusing olfactory toxicant and a spreading contaminant in urban runoff, we investigated the effect of low copper concentration on the three different OSNs in the olfactory epithelium of zebrafish, a model system widely used in biological research. Image analysis was applied for morphometry and quantification of immunohistochemically detected OSNs. Copper exposure resulted in an evident decrease in olfactory epithelium thickness. Moreover, after exposure, the lamellae of the dorsal and ventral halves of the olfactory rosettes showed a different increase in their sensory areas, suggesting a lateral migration of new cells into non-sensory regions. The results of the present study provide clear evidence of a differential response of the three neural cell populations of zebrafish olfactory mucosa after 96h of exposure to copper ions at the sublethal concentration of 30μgL^-1. Densitometric values of cONS, immunostained with anti-G _αolf, decreased of about 60% compared to the control. When the fish were transferred to water without copper addition and examined after 3, 10 and 30days, we observed a partial restoration of anti-G _αolf staining intensity to normal condition. The recovery of cOSNs appeared sustained by neuronal proliferation, quantified with anti-PCNA immunostaining, in particular in the early days after exposure. The densitometric analysis applied to mOSNs, immunostained with anti-TRPC2, revealed a statistically significant decrease of about 30% compared to the control. For cOSNs and mOSNs, the decrement in staining intensity may be indicative of cell death, but reduction in antigen expression may not be excluded. In the post-exposure period of 1 month we did not find recovery of mOSNs. We hypothesize that cOSNs are more sensitive than mOSNs to copper treatment, but also more prompted to tissue repair. Anti-TrkA-immunopositive crypt cells appeared not to be affected by copper exposure since statistical analysis excluded any significant difference between the control and treated fish. Comparative studies on OSNs would greatly enhance our understanding of the mechanisms of olfaction.

Olfactory Transcriptional Analysis of Salmon Exposed to Mixtures of Chlorpyrifos and Malathion Reveal Novel Molecular Pathways of Neurobehavioral Injury

Pacific salmon exposed to sublethal concentrations of organophosphate pesticides (OP) have impaired olfactory function that can lead to loss of behaviors that are essential for survival. These exposures often involve mixtures and can occur at levels below those which inhibit acetylcholinesterase (AChE). In this study, juvenile Coho salmon were exposed for 24 h to either 0.1, 0.5, or 2.5 ppb chlorpyrifos (CPF), 2, 10, or 50 ppb malathion (MAL), or binary mixtures of 0.1 CPF:2 ppb MAL, 0.5 CPF:10 ppb MAL, or 2.5 CPF:10 ppb MAL to mimic single and binary environmental exposures. Microarray analysis of olfactory rosettes from pesticide-exposed salmon revealed differentially expressed genes involved in nervous system function and signaling, aryl hydrocarbon receptor signaling, xenobiotic metabolism, and mitochondrial dysfunction. Coho exposed to OP mixtures exhibited a more pronounced loss in detection of a predatory olfactory cue relative to those exposed to single compounds, whereas respirometry experiments demonstrated that exposure to OPs, individually and in mixtures, reduced maximum respiratory capacity of olfactory rosette mitochondria. The observed molecular, biochemical, and behavioral effects occurred largely in the absence of effects on brain AChE. In summary, our results provide new insights associated with the sublethal neurotoxic effects of OP mixtures relevant to environmental exposures involving molecular and cellular pathways of injury to the salmon olfactory system that underlie neurobehavioral injury.

Mixed input to olfactory glomeruli from two subsets of ciliated sensory neurons does not impede relay neuron specificity in the crucian carp

The consensus view of olfactory processing is that the axons of receptor-specific primary olfactory sensory neurons (OSNs) converge to a small subset of glomeruli, thus preserving the odour identity before the olfactory information is processed in higher brain centres. In the present study, we show that two different subsets of ciliated OSNs with different odorant specificities converge to the same glomeruli. In order to stain different ciliated OSNs in the crucian carp Carassius carassius we used two different chemical odorants, a bile salt and a purported alarm substance, together with fluorescent dextrans. The dye is transported within the axons and stains glomeruli in the olfactory bulb. Interestingly, the axons from the ciliated OSNs co-converge to the same glomeruli. Despite intermingled innervation of glomeruli, axons and terminal fields from the two different subsets of ciliated OSNs remained mono-coloured. By 4-6 days after staining, the dye was transported trans-synaptically to separately stained axons of relay neurons. These findings demonstrate that specificity of the primary neurons is retained in the olfactory pathways despite mixed innervation of the olfactory glomeruli. The results are discussed in relation to the emerging concepts about non-mammalian glomeruli.

Contaminant-specific targeting of olfactory sensory neuron classes: connecting neuron class impairment with behavioural deficits

The olfactory system of fish comprises several classes of olfactory sensory neurons (OSNs). The odourants L-alanine and taurocholic acid (TCA) specifically activate microvillous or ciliated OSNs, respectively, in fish. We recorded electro-olfactograms (EOG) in fathead minnows (Pimephales promelas; a laboratory-reared model species) and wild yellow perch (Perca flavescens) whose olfactory chambers were perfused with either L-alanine or TCA to determine if OSN classes were differentially vulnerable to contaminants, in this case copper or nickel. Results were consistent in both species and demonstrated that nickel targeted and impaired microvillous OSN function, while copper targeted and impaired ciliated OSN function. This result suggests that contaminant-specific effects observed in model laboratory species extrapolate to wild fish populations. Moreover, fathead minnows exposed to copper failed to perceive a conspecific alarm cue in a choice maze, whereas those exposed to nickel could respond to the same conspecific cue. These results demonstrate that fathead minnows perceive conspecific, damage-released alarm cue by ciliated, but not microvillous, OSNs. Fish living in copper-contaminated environments may be more vulnerable to predation than those in clean lakes owing to targeted effects on ciliated OSNs.

Combinatorial analysis of calcium-binding proteins in larval and adult zebrafish primary olfactory system identifies differential olfactory bulb glomerular projection fields

In the zebrafish (Danio rerio) olfactory epithelium, the calcium-binding proteins (CBPs) calretinin and S100/S100-like protein are mainly expressed in ciliated or crypt olfactory sensory neurons (OSNs), respectively. In contrast parvalbumin and calbindin1 have not been investigated. We present a combinatorial immunohistological analysis of all four CBPs, including their expression in OSNs and their axonal projections to the olfactory bulb in larval and adult zebrafish. A major expression of calretinin and S100 in ciliated and crypt cells, respectively, with some expression of S100 in microvillous cells is confirmed. Parvalbumin and calbindin1 are strongly expressed in ciliated and microvillous cells, but not in crypt cells. Moreover, detailed combinatorial double-label experiments indicate that there are eight subpopulations of zebrafish OSNs: S100-positive crypt cells (negative for all other three CBPs), parvalbumin only, S100 and parvalbumin, parvalbumin and calbindin1, and parvalbumin and calbindin1 and calretinin-positive microvillous OSNs, as well as a major parvalbumin and calbindin1 and calretinin, and minor parvalbumin and calbindin1 and calretinin-only-positive ciliated OSN populations. CBP-positive projections to olfactory bulb are consistent with previous reports of ciliated OSNs projecting to dorsal and ventromedial glomerular fields and microvillous OSNs to ventrolateral glomerular fields. We newly describe parvalbumin-positive fibers to the mediodorsal field which is calretinin free, with its anterior part showing additionally calbindin1-positive fibers, but absence thereof in the posterior part, indicating an origin from microvillous OSNs in both parts. One singular glomerulus (mdG2) exhibits S100 and parvalbumin-positive fibers, apparently originating from all crypt cells plus some microvillous OSNs. Arguments for various olfactory labeled lines are discussed.

Immunohistochemical characterization of the crypt neurons in the olfactory epithelium of adult zebrafish

The fish sensory epithelium contains three types of sensory cells denominated ciliated, microvillous, and crypt neurons. Each one differs from the other in its morphological, ultrastructural and molecular features, as well as in their projections to the central nervous system. Crypt neurons are present in both bony and cartilaginous fish and can be identified on the basis of their morphology and the expression of some specific proteins and genes. In this study we have investigated the morphology of crypt neurons, as well as the occurrence and co-localization of S100 protein, calretinin and TRPV4, three proposed markers for crypt cells, in the olfactory epithelium of adult zebrafish (Danio rerio) using double immunofluorescence associated to laser confocal microscopy. A sparse population of superficial S100 protein positive cells was detected being identified as crypt neurons. The calretinin immunoreactive cells were more abundant, occasionally resembling the morphology of the crypt cells but never displaying co-localization of both proteins. The TRPV4 positive cells differed in morphology from crypt cells, thus excluding the occurrence of TRPV4 in those cells. These results demonstrate that only S100 protein immunoreactivity can be used to identify crypt cells. Because some calretinin positive cells showed localization and morphology similar to the crypt cells of the sensory epithelium, the occurrence of two subtypes of crypt cells, S100 protein and calretinin positive, cannot be excluded. The significance of these findings remains to be elucidated.

Mechanisms underlying odorant-induced and spontaneous calcium signals in olfactory receptor neurons of spiny lobsters, Panulirus argus

We determined if a newly developed antennule slice preparation allows studying chemosensory properties of spiny lobster olfactory receptor neurons under in situ conditions with Ca(2+) imaging. We show that chemical stimuli reach the dendrites of olfactory receptor neurons but not their somata, and that odorant-induced Ca(2+) signals in the somata are sufficiently stable over time to allow stimulation with a substantial number of odorants. Pharmacological manipulations served to elucidate the source of odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations in the somata of olfactory receptor neurons. Both Ca(2+) signals are primarily mediated by an influx of extracellular Ca(2+) through voltage-activated Ca(2+) channels that can be blocked by CoCl2 and the L-type Ca(2+) channel blocker verapamil. Intracellular Ca(2+) stores contribute little to odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations. The odorant-induced Ca(2+) transients as well as the spontaneous Ca(2+) oscillations depend on action potentials mediated by Na(+) channels that are largely TTX-insensitive but blocked by the local anesthetics tetracaine and lidocaine. Collectively, these results corroborate the conclusion that odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations in the somata of olfactory receptor neurons closely reflect action potential activity associated with odorant-induced phasic-tonic responses and spontaneous bursting, respectively. Therefore, both types of Ca(2+) signals represent experimentally accessible proxies of spiking.

Brief exposure to copper induces apoptosis and alters mediators of olfactory signal transduction in coho salmon

Pacific salmon are particularly susceptible to copper (Cu)-induced olfactory injuries that can ultimately inhibit neurobehaviors critical to survival. However, the molecular mechanisms underlying Cu-mediated olfactory impairment remain poorly understood. In the present study, we conducted a short-term Cu exposure at levels relevant to urban runoff (5, 25 and 50 ppb) , and investigated the roles of impaired olfactory signal transduction and induced apoptosis as underlying mechanisms of olfactory injury. Increased cell death in the olfactory epithelium was evident in coho receiving 4h exposures to 25 and 50 ppb Cu. Expression of olfactory marker protein (omp), a marker of mature olfactory sensory neurons, also decreased at 50 ppb Cu. Immunohistochemical analysis of coho olfactory epithelium demonstrated a loss of type 3 adenylate cyclase (ACIII) in the apical olfactory epithelium cilia at all levels of Cu exposure, suggesting an inhibitory effect of Cu in olfactory signaling. Accompanying the loss of ACIII in Cu-exposed coho were reduced intracellular cyclic guanosine monophosphate (cGMP) levels in the olfactory rosettes. Collectively, these results support a linkage among the initial steps of olfactory signaling in Cu-induced salmon olfactory injury, and suggesting that monitoring olfactory cGMP levels may aid in the assessment of salmon olfactory injury.

Zebrafish crypt neurons project to a single, identified mediodorsal glomerulus

Crypt neurons are a third type of olfactory receptor neurons with a highly unusual "one cell type--one receptor" mode of expression, the same receptor being expressed by the entire population of crypt neurons. Attempts to identify the target region(s) of crypt neurons have been inconclusive so far. We report that TrkA-like immunoreactivity specifically labeled somata, axons, and terminals of zebrafish crypt neurons and reveal a single glomerulus, mdg2 of the dorsomedial group, as target glomerulus of crypt neurons. Injection of a fluorescent tracing dye into the mdg2 glomerulus retrogradely labeled mostly crypt neurons, as assessed by quantitative morphometry, whereas no crypt neurons were found after injections in neighboring glomeruli. Our data provide strong evidence that crypt neurons converge onto a single glomerulus, and thus form a labeled line consisting of a single sensory cell type, a single olfactory receptor and a single target glomerulus.

Aversive cues fail to activate fos expression in the asymmetric olfactory-habenula pathway of zebrafish

The dorsal habenular nuclei of the zebrafish epithalamus have become a valuable model for studying the development of left-right (L-R) asymmetry and its function in the vertebrate brain. The bilaterally paired dorsal habenulae exhibit striking differences in size, neuroanatomical organization, and molecular properties. They also display differences in their efferent connections with the interpeduncular nucleus (IPN) and in their afferent input, with a subset of mitral cells distributed on both sides of the olfactory bulb innervating only the right habenula. Previous studies have implicated the dorsal habenulae in modulating fear/anxiety responses in juvenile and adult zebrafish. It has been suggested that the asymmetric olfactory-habenula pathway (OB-Ha), revealed by selective labeling from an lhx2a:YFP transgene, mediates fear behaviors elicited by alarm pheromone. Here we show that expression of the fam84b gene demarcates a unique region of the right habenula that is the site of innervation by lhx2a:YFP-labeled olfactory axons. Upon ablation of the parapineal, which normally promotes left habenular identity; the fam84b domain is present in both dorsal habenulae and lhx2a:YFP-labeled olfactory bulb neurons form synapses on the left and the right side. To explore the relevance of the asymmetric olfactory projection and how it might influence habenular function, we tested activation of this pathway using odorants known to evoke behaviors. We find that alarm substance or other aversive odors, and attractive cues, activate fos expression in subsets of cells in the olfactory bulb but not in the lhx2a:YFP expressing population. Moreover, neither alarm pheromone nor chondroitin sulfate elicited fos activation in the dorsal habenulae. The results indicate that L-R asymmetry of the epithalamus sets the directionality of olfactory innervation, however, the lhx2a:YFP OB-Ha pathway does not appear to mediate fear responses to aversive odorants.

Neural circuits mediating olfactory-driven behavior in fish

The fish olfactory system processes odor signals and mediates behaviors that are crucial for survival such as foraging, courtship, and alarm response. Although the upstream olfactory brain areas (olfactory epithelium and olfactory bulb) are well-studied, less is known about their target brain areas and the role they play in generating odor-driven behaviors. Here we review a broad range of literature on the anatomy, physiology, and behavioral output of the olfactory system and its target areas in a wide range of teleost fish. Additionally, we discuss how applying recent technological advancements to the zebrafish (Danio rerio) could help in understanding the function of these target areas. We hope to provide a framework for elucidating the neural circuit computations underlying the odor-driven behaviors in this small, transparent, and genetically amenable vertebrate.

Properties, projections, and tuning of teleost olfactory receptor neurons

In many fishes, the olfactory sense participates in such vital processes as feeding, reproduction, orientation, and predator avoidance. In teleosts, these tasks are fulfilled by a single type of olfactory organ for odorant and pheromone detection, containing ciliated and microvillus receptor neurons, and olfactory crypt cells. Recently, progress was made in understanding crypt cell function with the discovery of a V1R-like odorant receptor expressed in this neuron, an analysis of crypt cell odorant tuning properties, and the dissection of crypt cell connectivity within the telecephalon. Here, we review recent findings on the molecular properties, functions, and associated neural pathways of the three types of teleost olfactory receptor neurons with special emphasis on the crypt cell, and evaluate their roles in the detection of food, social and sexual odorants.

Neural mechanisms of alarm pheromone signaling

Alarm pheromones are important semiochemicals used by many animal species to alert conspecifics or other related species of impending danger. In this review, we describe recent developments in our understanding of the neural mechanisms underlying the ability of fruit flies, zebrafish and mice to mediate the detection of alarm pheromones. Specifically, alarm pheromones are detected in these species through specialized olfactory subsystems that are unique to the chemosensitive receptors, second messenger-signaling and physiology. Thus, the alarm pheromones appears to be detected by signaling mechanisms that are distinct from those seen in the canonical olfactory system.

The somatotopic organization of the olfactory bulb in elasmobranchs

The olfactory bulbs (OBs) are bilaterally paired structures in the vertebrate forebrain that receive and process odor information from the olfactory receptor neurons (ORNs) in the periphery. Virtually all vertebrate OBs are arranged chemotopically, with different regions of the OB processing different types of odorants. However, there is some evidence that elasmobranch fishes (sharks, rays, and skates) may possess a gross somatotopic organization instead. To test this hypothesis, we used histological staining and retrograde tracing techniques to examine the morphology and organization of ORN projections from the olfactory epithelium (OE) to the OB in three elasmobranch species with varying OB morphologies. In all three species, glomeruli in the OB received projections from ORNs located on only the three to five lamellae situated immediately anterior within the OE. These results support that the gross arrangement of the elasmobranch OB is somatotopic, an organization unique among fishes and most other vertebrates. In addition, certain elasmobranch species possess a unique OB morphology in which each OB is physically subdivided into two or more "hemi-olfactory bulbs." Somatotopy could provide a preadaptation which facilitated the evolution of olfactory hemibulbs in these species.

Sensitivity and specificity of the olfactory epithelia of two elasmobranch species to bile salts

Odor detection in vertebrates occurs when odorants enter the nose and bind to molecular olfactory receptors on the cilia or microvilli of olfactory receptor neurons (ORNs). Several vertebrate groups possess multiple, morphologically distinct types of ORNs. In teleost fishes, these different ORN types detect specific classes of biologically relevant odorants, such as amino acids, nucleotides and bile salts. For example, bile salts are reported to be detected exclusively by ciliated ORNs. The olfactory epithelium of elasmobranch fishes (sharks, rays and skates) is comprised of microvillous and crypt ORNs, but lacks ciliated ORNs; thus, it was questioned whether the olfactory system of this group of fishes is capable of detecting bile salts. The present investigation clearly indicates that the olfactory system of representative shark and stingray species does detect and respond to bile salts. Additionally, these species detect glycine-conjugated, taurine-conjugated and non-conjugated bile salts, as do teleosts. These elasmobranchs are less sensitive to the tested bile salts than reported for both agnathans and teleosts, but this may be due to the particular bile salts selected in this study, as elasmobranch-produced bile salts are commercially unavailable. Cross-adaptation experiments indicate further that the responses to bile salts are independent of those to amino acids, a major class of odorant molecules for all tested fishes.

Odorant tuning of olfactory crypt cells from juvenile and adult rainbow trout

Teleost fish lack independent olfactory organs for odorant and pheromone detection. Instead, they have a single sensory epithelium with two populations of receptor neurons, ciliated and microvillous, that are conserved among vertebrates, and a unique receptor cell type named the olfactory crypt cell. Crypt cells were shown to be chemosensory neurons that project to specific areas in the olfactory bulb, but their odorant tuning and overall function remain unclear. Reproduction in fish is generally synchronized by sex pheromonal signaling between males and females, but the sensors responsible for pheromone detection remain unknown. In crucian carp, a seasonal variation in the population of olfactory crypt cells and their brain projections pathways, involved in reproduction, led to the hypothesis of a role as sex pheromone detectors. In the present study, morphology and localization of olfactory crypt cells were compared between juvenile and mature rainbow trout of both sexes, and calcium imaging was used to visualize responses of crypt cells from the three groups to common social and food-related odorants, sex hormones and conspecific tissue extracts. Crypt cells from mature trout were found to be larger than those of juvenile specimens, and preferentially localized to the apical surface of the olfactory epithelium. Although a fraction of crypt cells of all groups responded to common odorants such as amino acids and bile salts, cells from mature trout showed a characteristic preference for gonadal extracts and hormones from the opposite sex. These results support an involvement of olfactory crypt cells in reproduction-related olfactory signaling in fishes.

Quantitative analysis of crypt cell population during postnatal development of the olfactory organ of the guppy, Poecilia reticulata (Teleostei, Poecilidae), from birth to sexual maturity

Crypt cells are one of three types of olfactory sensory neuron, differing from ciliated and microvillar cells in shape, localization and number, and found only in fish. Although crypt cells are morphologically well characterized, their function remains unclear. They were hypothesized to be involved in reproductive behaviours by detecting sex pheromones, but electrophysiological investigations revealed sensitivity to only amino acids. However, the number of crypt cells in adult guppies is not the same in the two sexes. In this study, we compared the size of the crypt cell population in juvenile guppies during the first 90 days after birth. The purpose of our study was to clarify whether a correlation exists between sex and the number of these olfactory neurons. The data show that guppies reach adult crypt cell density when they become sexually mature. Despite a constant increment in volume during development of the olfactory organ, the minimum density of crypt neurons occurs at ∼45 days. Moreover, in the early weeks, the density of crypt neurons is greater in males than in females because in females the total number of cells decreases significantly after just 7 days. In adults, however, crypt neurons are found in higher density in females than in males. These findings suggest that the number of crypt cells is sex specific, with independent developmental dynamics between males and females. A role in pheromone detection could explain such a difference, but the early appearance of crypt cells in the first days of life is suggestive of other, not sexually related, functions.

Crypt cells of the zebrafish Danio rerio mainly project to the dorsomedial glomerular field of the olfactory bulb

The olfactory mucosa of the zebrafish consists of 3 morphological types of olfactory receptor neurons (ORNs): ciliated, microvillous, and crypt cells. Previous studies in the zebrafish have revealed differential projections of ciliated and microvillous ORNs, which project to different glomerular fields. However, the bulbar targets of zebrafish crypt cells were not identified. Here, we analyze the relationship between crypt cells of the olfactory epithelium and dorsal glomerular fields of the zebrafish olfactory bulbs, as wells as the connections between these bulbar regions and forebrain regions. For this purpose, a lipophilic carbocyanine tracer (DiI) was used in fixed tissue. Application of DiI to the dorsomedial glomerular field mainly labeled crypt cells in the zebrafish olfactory epithelium. By contrast, application of DiI to the dorsolateral glomerular fields mainly labeled bipolar ORNs and only occasionally crypt cells. Bulbar efferent cells (mitral cells) contacting these dorsal glomerular fields project to different telencephalic areas, with the posterior zone of the dorsal telencephalic area (Dp) as the common target. However, dorsomedial and dorsolateral glomerular fields projected differentially to the ventral telencephalon, the former projecting to the ventrolateral supracommissural region. Retrograde labeling from the ventrolateral supracommissural region revealed mitral cells associated with 2 large glomeruli in the bulbar dorsomedial region, which putatively receives inputs from the crypt cells, indicating the existence of a crypt cell olfactory subsystem with separate projections, in the zebrafish. The comparative significance of the secondary olfactory pathways of zebrafish that convey information from crypt cells is discussed.

Chemosensory-induced motor behaviors in fish

Chemical sensory signals play a crucial role in eliciting motor behaviors. We now review the different motor behaviors induced by chemosensory stimuli in fish as well as their neural substrate. A great deal of research has focused on migratory, reproductive, foraging, and escape behaviors but it is only recently that the molecules mediating these chemotactic responses have become well-characterized. Chemotactic responses are mediated by three sensory systems: olfactory, gustatory, and diffuse chemosensory. The olfactory sensory neuron responses to chemicals are now better understood. In addition, the olfactory projections to the central nervous system were recently shown to display an odotopic organization in the forebrain. Moreover, a specific downward projection underlying motor responses to olfactory inputs was recently described.

Crypt neurons express a single V1R-related ora gene

Both ciliated and microvillous olfactory sensory neuron populations express large families of olfactory receptor genes. However, individual neurons generally express only a single receptor gene according to the "one neuron-one receptor" rule. We report here that crypt neurons, the third type of olfactory neurons in fish species, use an even more restricted mode of expression. We recently identified a novel olfactory receptor family of 6 highly conserved G protein-coupled receptors, the v1r-like ora genes. We show now that a single member of this family, ora4 is expressed in nearly all crypt neurons, whereas the other 5 ora genes are not found in this cell type. Consistent with these findings, ora4 is never coexpressed with any of the remaining 5 ora genes. Furthermore, several lines of evidence indicate the absence of any other olfactory receptor families in crypt neurons. These results suggest that the vast majority of the crypt neuron population may select one and the same olfactory receptor gene, a "one cell type-one receptor" mode of expression. Such an expression pattern is familiar in the visual system, with rhodopsin as the sole light receptor of rod photoreceptor cells, but unexpected in the sense of smell.

Responses of round goby (Neogobius melanostomus) olfactory epithelium to steroids released by reproductive males

The wild perciform teleost Neogobius melanostomus (the round goby) originated from the Ponto-Caspian region and is now a highly successful invasive species in the Laurentian Great Lakes. Males may attract females into their nests for spawning by releasing reproductive pheromones, and it has been previously shown that reproductive males synthesize and release the 5β-reduced and 3α-hydroxyl steroids 3α-hydroxy-5β-androstane-11,17-dione (11-oxo-etiocholanolone; 11-O-ETIO) and 3α-hydroxy-5β-androstane-11,17-dione 3-sulfate (11-oxo-etiocholanolone-3-sulfate; 11-O-ETIO-3-s) and 3α,17β-dihydroxy-5β-androstan-11-one 17-sulfate. In this study, we investigated properties of these released steroids by recording field potential responses from the olfactory epithelium (electro-olfactogram, EOG). The steroid 3α,17β-dihydroxy-5β-androstan-11-one 17-sulfate did not elicit olfactory responses while both 11-O-ETIO and 11-O-ETIO-3-s stimulated olfactory field potentials in the round goby, but not in the goldfish. Cross-adaptation analysis demonstrated that round gobies discriminated between11-O-ETIO and 11-O-ETIO-3-s (as well as etiocholanolone, ETIO) at the sensory level. Second messenger cascades depending on both cAMP and IP(3) were inferred for steroids from pharmacological inhibition studies, while the canonical teleost odors taurocholic acid (a bile acid) and L: -alanine (an amino acid) used only cAMP and IP(3), respectively. The round goby presents itself as an excellent species for the study of olfactory function of fish in the wild, given its possible use of these released steroids as pheromones.

Differential bulbar and extrabulbar projections of diverse olfactory receptor neuron populations in the adult zebrafish (Danio rerio)

Immunohistochemical methods were used to characterize the expression of two calcium-binding proteins, calretinin (CR) and S100, in the olfactory rosette of the adult zebrafish. These proteins are expressed in different sets of sensory neurons, and together represent a large proportion of these cells. Double immunofluorescence for CR and Gα(olf) protein, and CR immunoelectron microscopy, indicated that most CR-immunoreactive (ir) cells were ciliary neurons. Differential S100- and CR-ir projections to glomerular fields of the olfactory bulb were also observed, although these projections overlap in some glomeruli. Application of the carbocyanine dye DiI to either S100-ir or CR-ir glomerular regions led to labeling of cells mostly similar to S100-ir and CR-ir neurons, respectively. Instead, these bulbar regions project to similar telencephalic targets. On the other hand, antibodies against keyhole limpet hemocyanin (KLH)-stained numerous sensory cells in the olfactory rosette, including cells that were CR- and S100-negative. This antiserum also stained most primary bulbar projections and revealed extrabulbar olfactory primary projections coursing to the ventral area of the telencephalon through the medial olfactory tract. This extrabulbar projection was confirmed by tract-tracing with DiI. A loose association of this extrabulbar primary olfactory projection and the catecholaminergic populations of the ventral area was also observed with double tyrosine hydroxylase/KLH-like immunohistochemistry. Comparison between KLH-like-ir pathways and the structures revealed by FMRFamide immunohistochemistry (a marker of terminal ganglion cells and fibers) indicated that the KLH-like-ir extrabulbar projection was different from the terminal nerve system. The significance of the extrabulbar olfactory projection of zebrafish is discussed.

Onset and dynamic expression of S100 proteins in the olfactory organ and the lateral line system in zebrafish development

In the zebrafish olfactory epithelium, three morphologically distinct olfactory neurons express different marker proteins. We utilize this feature to access developmental dynamics of one of the neuron types, the crypt cells, to determine whether they are differentiated at a stage similar to other olfactory neurons. Immunohistochemical studies using an S100 antibody that specifically recognizes crypt cells showed that S100-positive cells appear in olfactory rosettes as early as at 2day postfertilization (dpf). However, some of the rosettes did not have any S100-positive cells until 4 dpf. The number of S100-positive cells in individual rosettes increased steadily over the next 3days before it decreased significantly. There were 7.8 S100-positive cells per rosettes on average in larvae at 7 dpf. The number reduced to 2.2 at 9 dpf. A recovery to a pre-reduction level was detected in 12 dpf larvae. We also observed S100-positive cells in neuromasts of the lateral line system in 2 dpf larvae, suggesting that the crypt cells and sensory cells in the neuromasts have similar onsets of differentiation. Our data have provided a time line of differentiation of crypt cells in development of the olfactory system and demonstrated that this type of cell is differentiated at a stage similar to ciliated and microvillous olfactory neurons. A nonlinear growth trajectory of the crypt cell population in the first nine days of zebrafish development implicates a possible functional significance of crypt cells in early life stages of zebrafish.

Immunolocalization of G-protein alpha subunits in the olfactory system of the cartilaginous fish Scyliorhinus canicula

In the olfactory and vomeronasal systems of vertebrates, the morphology of the receptor neurons, the receptor gene family they express, the G-protein coupled with the receptor (in particular the G-protein alpha subunit), and their projection to the olfactory bulb are correlated. Much information about this complicated system have been collected in different groups, but nothing is known about Chondrichthyes. In this work, the presence and distribution of immunoreactivity for different types of G-protein alpha subunit (Galpha(o), Galpha(q) and Galpha(s/olf)) were investigated in the olfactory mucosa and olfactory bulb of the shark Scyliorhinus canicula. Only Galpha(o)-like immunoreactivity was detected in the olfactory mucosa and bulb, both in tissues and homogenates. Its distribution was partially similar to that found in other vertebrates: it was localized in the microvillous receptor neurons, in numerous axon bundles of the fila olfactoria, in the stratum nervosum and in the most of glomeruli in the stratum glomerulosum. No immunoreactivity was instead observed in the crypt neurons, the second type of olfactory neurons present in cartilaginous fish. The projections of crypt neurons to olfactory bulb probably correspond to the few ventrally-located glomeruli which were negative to the antiserum against Galpha(o). These data suggest, in S. canicula, different olfactory neuron types send projections to the olfactory bulb with a segregated distribution, as observed in other vertebrates.

Molecular biological research on olfactory chemoreception in fishes

This review describes recent molecular biological research on olfactory chemoreception in fishes. The recent rapid development of molecular biological techniques has provided new valuable information on the main and vomeronasal olfactory receptor (OR) genes, the axonal projection from ciliated, microvillous and crypt-olfactory receptor cells to the olfactory bulb, properties of odorant substances and olfactory imprinting and homing in salmon. Many important questions, however, remain unanswered on functional differences among OR genes, on ligand binding to each OR and on the molecular biological mechanisms underlying olfactory imprinting and homing in salmon. Olfactory chemoreception is believed to be the oldest sensory cue for both animal survival and adaptation to various different environments. Further intensive molecular biological research on olfactory memory formation and remembrance should be carried out to clarify the fundamental process of olfactory chemoreception in fishes.

The molecular evolution of teleost olfactory receptor gene families

Four olfactory receptor gene families, all of them G protein-coupled receptors, have been identified and characterized in mammals--the odorant (OR), vomeronasal (V1R and V2R) and trace amine-associated (TAARs) receptors. Much less attention has been directed towards non-mammalian members of these families. Since a hallmark of mammalian olfactory receptors is their remarkable species specificity, an evaluation of the non-mammalian olfactory receptors is instructive both for comparative purposes and in its own right. In this review I have compiled the results currently available for all four olfactory gene families and discuss their phylogenomic properties in relation to their mammalian counterparts. Representatives of all four families are found in cartilaginous fish and/or jawless fish, allowing a minimal estimate for the evolutionary origin as preceding the segregation between cartilaginous and bony fish or cartilaginous and jawless fish, respectively. Gene repertoires of teleost olfactory receptors are smaller in size (OR, ORA), comparable (olfC), or even larger (TAAR) than the corresponding mammalian gene repertoires. Despite their smaller repertoire size, the teleost OR and ORA families show much larger divergence than their mammalian counterparts. Evolutionary rates vary greatly between families, with evidence for positive selection in teleost OR genes, whereas the ora genes are subject to strong negative selection, and in fact are being conserved among all teleost species investigated. With one exception, ligands are unknown for any of the four teleost olfactory receptor gene families, and so the considerable knowledge about the odor responses of the olfactory epithelium and the olfactory bulb can only be linked indirectly to the receptor repertoires.