Soybean agglutinin binding by primary olfactory and primary accessory olfactory projections in different frogs

Olfaction across the water-air interface in anuran amphibians

Extant anuran amphibians originate from an evolutionary intersection eventually leading to fully terrestrial tetrapods. In many ways, they have to deal with exposure to both terrestrial and aquatic environments: (i) phylogenetically, as derivatives of the first tetrapod group that conquered the terrestrial environment in evolution; (ii) ontogenetically, with a development that includes aquatic and terrestrial stages connected via metamorphic remodeling; and (iii) individually, with common changes in habitat during the life cycle. Our knowledge about the structural organization and function of the amphibian olfactory system and its relevance still lags behind findings on mammals. It is a formidable challenge to reveal underlying general principles of circuity-related, cellular, and molecular properties that are beneficial for an optimized sense of smell in water and air. Recent findings in structural organization coupled with behavioral observations could help to understand the importance of the sense of smell in this evolutionarily important animal group. We describe the structure of the peripheral olfactory organ, the olfactory bulb, and higher olfactory centers on a tissue, cellular, and molecular levels. Differences and similarities between the olfactory systems of anurans and other vertebrates are reviewed. Special emphasis lies on adaptations that are connected to the distinct demands of olfaction in water and air environment. These particular adaptations are discussed in light of evolutionary trends, ontogenetic development, and ecological demands.

Tract-tracing study of the extrabulbar olfactory projections in the brain of some teleosts

The extrabulbar olfactory projections (EBOP) is a collection of nerve fibers that originate from primary olfactory receptor neurons. These fibers penetrate into the brain, bypassing the olfactory bulbs (OBs). While the presence of an EBOP has been well established in teleosts, here we morphologically characterize the EBOP structure in four species each with a different morphological relationship of OB with the ventral telencephalic area. Tract-tracing methods (carbocyanine DiI/DIA and biocytin) were used. FMRFamide immunoreactive nervus terminalis (NT) components were also visualized to define any neuroanatomical relationship between the NT and EBOP. Unilateral DiI/DiA application to the olfactory chamber stained the entire olfactory epithelium, olfactory nerve fibers, and ipsilateral olfactory bulb. Labeled primary olfactory fibers running ventromedially as extrabulbar primary olfactory projections reached various regions of the secondary prosencephalon. Only in Moenkhausia sanctaefilomenae (no olfactory peduncle) did lipophilic tracer-labeled fibers reach the ipsilateral mesencephalon. The combination of tracing techniques and FMRFamide immunohistochemistry revealed a substantial overlap of the label along the olfactory pathways as well as in the anterior secondary prosencephalon. However, FMRFamide immunoreactivity was never colocalized in the same cellular or fiber component as visualized using tracer molecules. Our results showed a certain uniformity in the neuroanatomy and extension of EBOP in all four species, independent of the pedunculate feature of the OBs. The present study also provided additional evidence to support the view that EBOP and FMRFamide immunoreactive components of the NT are separate anatomical entities.

A putative functional vomeronasal system in anuran tadpoles

We investigated the occurrence and anatomy of the vomeronasal system (VNS) in tadpoles of 13 different anuran species. All of the species possessed a morphologically fully developed VNS with a highly conserved anatomical organisation. We found that a bean-shaped vomeronasal organ (VNO) developed early in the tadpoles, during the final embryonic stages, and was located in the anteromedial nasal region. Histology revealed the presence of bipolar chemosensory neurones in the VNO that were immunoreactive for the Gαo protein. Tract-tracing experiments demonstrated that chemosensory neurones from the VNO reach specific areas in the brain, where a discernible accessory olfactory bulb (AOB) could be observed. The AOB was located in the ventrolateral side of the anterior telencephalon, somewhat caudal to the main olfactory bulb. Synaptophysin-like immunodetection revealed that synaptic contacts between VNO and AOB are established during early larval stages. Moreover, using lectin staining, we identified glomerular structures in the AOB in most of the species that we examined. According to our findings, a significant maturation in the VNS is achieved in anuran larvae. Recent published evidence strongly suggests that the VNS appeared early in vertebrate evolution and was already present in the aquatic last common ancestor of lungfish and tetrapods. In this context, tadpoles may be a good model in which to investigate the anatomical, biochemical and functional aspects of the VNS in an aquatic environment.

Lectin histochemical study on the olfactory bulb of the newt, Cynops pyrrhogaster

The function and/or morphological features of the vomeronasal olfactory system remain unclear in aquatic animals, although the system appeared first in urodeles based on phylogenic data. We examined the lectin binding patterns in the olfactory bulb of a semi-aquatic urodele, the Japanese red-bellied newt, Cynops pyrrhogaster, using 22 different lectins. Eleven of the lectins showed specific binding to the nerve fibres and glomeruli in the olfactory bulb. Among these, Wheat germ agglutinin, pokeweed and peanut agglutinin preferentially bound the main olfactory bulb, reflecting variation in the expression of glycoconjugates between the main and accessory olfactory bulbs. By contrast, the types of lectins bound to the Cynops olfactory bulb were considerably different from those reported in other urodele families. These results suggest a histochemical distinction between the main and accessory olfactory bulbs, and that glycoconjugate expression may differ significantly among urodele families.

Phylogenic aspects of the amphibian dual olfactory system

The phylogenic significance of the subdivision of dual olfactory system is reviewed mainly on the basis of our findings by electron microscopy and lectin histochemistry in the three amphibian species. The dual olfactory system is present in common in these species and consists of the projection from the olfactory epithelium (OE) to the main olfactory bulb (MOB) and that from the vomeronasal epithelium (VNE) to the accessory olfactory bulb (AOB). The phylogenic significance of subdivisions in the dual olfactory system in the amphibian must differently be interpreted. The subdivision of the MOB into its dorsal region (D-MOB) and ventral region (V-MOB) in Xenopus laevis must be attributed to the primitive features in their olfactory receptors. The middle cavity epithelium lining the middle cavity of this frog possesses both ciliated sensory cells and microvillous sensory cells, reminding the OE in fish. The subdivision of the AOB into the rostral (R-AOB) and caudal part (C-AOB) in Bufo japonicus formosus must be regarded as an advanced characteristic. The lack of subdivisions in both MOB and AOB in Cynops pyrrhogaster may reflect their phylogenic primitiveness. Since our lectin histochemistry to detect glycoconjugates expressed in the olfactory pathway reveals the subdivisions in the dual olfactory system in the amphibian, the glycoconjugates may deeply participate in the organization and function of olfactory pathways in phylogeny.

Subdivision of the accessory olfactory bulb in the Japanese common toad, Bufo japonicus, revealed by lectin histochemical analysis

Lectin binding patterns in the olfactory bulb of the Japanese common toad, Bufo japonicus, were examined using 21 types of lectin. Ten out of 21 lectins, WGA, s-WGA, LEL, STL, DBA, VVA, SJA, RCA-I, PNA, and PHA-L, stained the olfactory nerve, the glomeruli in the main olfactory bulb (MOB), the vomeronasal nerve, and the glomeruli in the accessory olfactory bulb (AOB). The binding patterns of LEL, STL, DBA, and PHA-L subdivided AOB glomeruli into rostral and caudal regions, where LEL, STL, and DBA stained the rostral region more intensely than the caudal region, and PHA-L had the opposite effect. Another lectin, BSL-I, stained both AOB glomeruli and the vomeronasal nerve, but not MOB glomeruli or the olfactory nerve. This is the first report of histological subdivision in the AOB of an amphibian, which suggests that the AOB development in Bufo may be unique.

Distribution of aminopeptidase P like immunoreactivity in the olfactory system and brain of frog, Microhyla ornate

The enzyme aminopeptidase P (AP-P) is encountered in diverse vertebrate and invertebrate phyla and is known to act on proteins and peptides by releasing their N-terminal amino acid when the penultimate amino acid is proline. The present study is the first attempt at visualizing distribution of this polypeptide in the brain of a vertebrate species. The distribution of this enzyme was studied immunocytochemically in the forebrain of frog Microhyla ornata using antisera directed against cytosolic aminopeptidase P (DAP-P) of Drosophila melanogaster. Receptor cells in the olfactory epithelium exhibited strong AP-P like immunoreaction (ir). Immunoreactive fibers arising from the olfactory epithelium as well as vomeronasal organ joined the olfactory nerve, entered into the olfactory bulb, or accessory olfactory bulb and terminated in distinct glomerular formations. Some immunoreactive fibers traveled caudally and terminated in discrete areas in the telencephalon or diencephalon. Strong AP-P-ir was also seen in the cells of pars intermedia and pars distalis of the pituitary. The pattern of immunoreactivity suggests a role for AP-P in the processing of olfactory information and in hypophysial regulation.

Extrabulbar olfactory system and nervus terminalis FMRFamide immunoreactive components in Xenopus laevis ontogenesis

The extrabulbar olfactory system (EBOS) is a collection of nerve fibers which originate from primary olfactory receptor-like neurons and penetrate into the brain bypassing the olfactory bulbs. Our description is based upon the application of two neuronal tracers (biocytin, carbocyanine DiI) in the olfactory sac, at the cut end of the olfactory nerve and in the telencephalon of the developing clawed frog. The extrabulbar olfactory system was observed already at stage 45, which is the first developmental stage compatible with our techniques; at this stage, the extrabulbar olfactory system fibers terminated diffusely in the preoptic area. A little later in development, i.e. at stage 50, the extrabulbar olfactory system was maximally developed, extending as far caudally as the rhombencephalon. In the metamorphosing specimens, the extrabulbar olfactory system appeared reduced in extension; caudally, the fiber terminals did not extend beyond the diencephalon. While a substantial overlapping of biocytin/FMRFamide immunoreactivity was observed along the olfactory pathways as well as in the telencephalon, FMRFamide immunoreactivity was never observed to be colocalized in the same cellular or fiber components visualized by tracer molecules. The question whether the extrabulbar olfactory system and the nervus terminalis (NT) are separate anatomical entities or represent an integrated system is discussed.

Differential development of binding sites for four lectins in the vomeronasal system of juvenile mouse: from the sensory transduction site to the first relay stage

Four lectins -the galactose-specific BSI-B(4) (from Bandeiraea simplicifolia), the N-acetyl-galactosamine-specific DBA (from Dolichos biflorus), the L-fucose-specific UEA-I (from Ulex europaeus) and the (oligomeric N-acetylglucosamine)-specific LEA (from Lycopersicum esculentum)- were used to study the vomeronasal organ, vomeronasal nerves and accessory olfactory bulb of the mouse on embryonic days 11, 13, 15, 17 and 19, during the first 3 weeks after birth, at age 25 days, and after reaching maturity. No lectins labelled any structure before the 17th day of gestation, and even on the 19th day staining was sporadic and/or diffuse. During the early postnatal period, the lectin binding patterns differed from those of adults, but the division of the accessory olfactory bulb into anterior, rostral posterior and caudal posterior regions was already present and was shown up by the four lectins in a way that was coherent with the known zone-to-zone correspondence between the apical and basal zones of the sensory epithelium and the anterior and posterior accessory olfactory bulb, respectively. By age 25 days, the staining patterns were essentially those of the adult mouse. BSI-B(4) appears to be specific for the accessory vs. the main olfactory bulb throughout life.

FMRFamide in the amphibian brain: a comprehensive survey

Mapping of FMRFamidergic neural circuitry in the amphibian brain has been done by immunohistochemical methods. Comparative evidence suggests that there are similarities and differences in the overall pattern of distribution of FMRFamide-ir elements in the brain among the three amphibian orders and within each order. FMRFamide is expressed in neurons in some circumscribed areas of the brain. A part of these neurons is concentrated in classical neurosecretory areas of the hypothalamus in a bilaterally symmetrical fashion. Similar neurons occur occasionally in the midbrain, but are virtually absent from the hindbrain. Anurans are unique among amphibians to show FMRFamide neurons in the medial septum and diagonal band of Broca. A viviparous gymnophione is known to possess a small population of such neurons in the dorsal thalamus. Together, the FMRFamide neurons contribute to an extensive fiber network throughout the amphibian brain. Descriptive developmental studies suggest that the rostral forebrain-located FMRFamide neurons originate in the olfactory placode and then migrate into the brain along the route of the vomeronasal-olfactory-terminal nerve complex. Olfactory placodal ablation in an anuran and a urodele provide experimental support to this contention. Other FMRFamide neuronal cell groups, in the hypothalamus and dorsal thalamus, are supposed to arise from non-placodal precursors. The neuroanatomical distribution (projection of immunoreactive processes to areas of the fore-, mid-, and hindbrain as well as to cerebrospinal fluid, co-localization with other neuropeptides, and presence in the median eminence) has furnished morphological correlates of possible functions of FMRFamide in the amphibian CNS. While amphibian FMRFamide-like or structurally related peptides remain to be isolated and characterized, the sum of the distribution pattern of FMRFamide-like immunoreactivity suggests that it may act as a neurotransmitter or a neuromodulator, and also may have endocrine regulatory functions.

Expression patterns of glycoconjugates in the three distinctive olfactory pathways of the clawed frog, Xenopus laevis

Xenopus laevis has three distinctive olfactory neuroepithelia. We examined the axonal projection from each of these epithelia to the olfactory bulb by Di-I labeling, and confirmed that the Xenopus primary olfactory pathways involve the dorsal pathway from the olfactory epithelium to the dorsal region of the main olfactory bulb, the ventral pathway from the middle chamber epithelium to the ventral region of the main olfactory bulb, and the vomeronasal pathway from the vomeronasal epithelium to the accessory olfactory bulb. We next examined expression patterns of glycoconjugates in the three olfactory pathways by lectin-histochemistry using 21 biotinylated lectins. Fourteen out of 21 lectins stained the Xenopus primary olfactory system. RCA-I stained the three olfactory pathways uniformly. PHA-E stained only the dorsal pathway. LEL, STL, PNA, ECL and UEA-I stained the dorsal pathway more intensely than the ventral pathway, and among them, only UEA-I stained the vomeronasal pathway. In contrast, s-WGA, DBA, SBA, BSL-I VVA, SJA and PHA-L showed intense stainings in the ventral pathway and moderate stainings in the vomeronasal pathway, but faint or weak stainings in the dorsal pathway. These observations suggest that the ventral pathway expresses glycoconjugates shared commonly with either the dorsal or the vomeronasal pathway. In addition, from the binding patterns of the lectins with a binding specificity for N-acetylgalactosamine, glycoconjugates containing this saccharide seem to play an important role for the organization of the olfactory pathways.

Transduction mechanisms in vertebrate olfactory receptor cells

Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein Golf, an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca2+ has the highest permeability through this channel, and the increase in intracellular Ca2+ concentration activates a Cl- current which, owing to an elevated reversal potential for Cl-, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca2+ and/or inositol polyphosphate -activated cation channels. In addition, there is evidence for odorant-modulated K+ and Cl- conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.

Subdivisions of the olfactory system in the sterlet Acipenser ruthenus

Subdivisions of the olfactory system of the sterlet Acipenser ruthenus were investigated by means of horseradish peroxidase (HRP) injections into the nose, and by soybean agglutinin binding studies. With both methods primary olfactory fibers were labeled which projected to the ventral part of the glomerular layer of the olfactory bulb. The dorsal part of the olfactory bulb did not bind soybean agglutinin, however, even though HRP tracing showed primary olfactory fibers in that area. This confirms earlier morphological studies which claim the existence of distinct subdivisions of the olfactory system in the sturgeon. The lack of soybean agglutinin binding in the dorsal part of the olfactory bulb suggests, however, that this part is not homologous with the accessory olfactory system of tetrapods.

Differential labelling of primary olfactory system subcomponents by SBA (lectin) and NADPH-d histochemistry in the frog Pipa

SBA and NADPH-d histochemistries allow identification of functionally distinct components of the amphibian primary olfactory system. In Pipa, a secondarily aquatic frog, combination of both methodologies, using alternate sets of histological sections, reveals that, apart from Jacobson's organ, this species has a "water-nose" and an "air-nose". The epithelia occupy separate chambers of the olfactory organ and give rise to olfactory nerve fiber bundles that are identified by the dual staining procedure.