Elevation of gene expression for salmon gonadotropin‐releasing hormone in discrete brain loci of prespawning chum salmon during upstream migration

Our previous studies suggested that salmon gonadotropin‐releasing hormone (sGnRH) neurons regulate both final maturation and migratory behavior in homing salmonids. Activation of sGnRH neurons can occur during upstream migration. We therefore examined expression of genes encoding the precursors of sGnRH, sGnRH‐I, and sGnRH‐II, in discrete forebrain loci of prespawning chum salmon, Oncorhynchus keta. Fish were captured from 1997 through 1999 along their homing pathway: coastal areas, a midway of the river, 4 km downstream of the natal hatchery, and the hatchery. Amounts of sGnRH mRNAs in fresh frozen sections including the olfactory bulb (OB), terminal nerve (TN), ventral telencephalon (VT), nucleus preopticus parvocellularis anterioris (PPa), and nucleus preopticus magnocellularis (PM) were determined by quantitative real‐time polymerase chain reactions. The amounts of sGnRH‐II mRNA were higher than those of sGnRH‐I mRNA, while they showed similar changes during upstream migration. In the OB and TN, the amounts of sGnRH mRNAs elevated from the coast to the natal hatchery. In the VT and PPa, they elevated along with the progress of final maturation. Such elevation was also observed in the rostroventral, middle, and dorsocaudal parts of the PM. The amounts of gonadotropin IIβ and somatolactin mRNAs in the pituitary also increased consistently with the elevation of gene expression for sGnRH. These results, in combination with lines of previous evidence, indicate that sGnRH neurons are activated in almost all the forebrain loci during the last phases of spawning migration, resulting in coordination of final gonadal maturation and migratory behavior to the spawning ground. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2005     

Elevation of gene expression for salmon gonadotropin-releasing hormone in discrete brain loci of prespawning chum salmon during upstream migration

Our previous studies suggested that salmon gonadotropin-releasing hormone (sGnRH) neurons regulate both final maturation and migratory behavior in homing salmonids. Activation of sGnRH neurons can occur during upstream migration. We therefore examined expression of genes encoding the precursors of sGnRH, sGnRH-I, and sGnRH-II, in discrete forebrain loci of prespawning chum salmon, Oncorhynchus keta. Fish were captured from 1997 through 1999 along their homing pathway: coastal areas, a midway of the river, 4 km downstream of the natal hatchery, and the hatchery. Amounts of sGnRH mRNAs in fresh frozen sections including the olfactory bulb (OB), terminal nerve (TN), ventral telencephalon (VT), nucleus preopticus parvocellularis anterioris (PPa), and nucleus preopticus magnocellularis (PM) were determined by quantitative real-time polymerase chain reactions. The amounts of sGnRH-II mRNA were higher than those of sGnRH-I mRNA, while they showed similar changes during upstream migration. In the OB and TN, the amounts of sGnRH mRNAs elevated from the coast to the natal hatchery. In the VT and PPa, they elevated along with the progress of final maturation. Such elevation was also observed in the rostroventral, middle, and dorsocaudal parts of the PM. The amounts of gonadotropin IIbeta and somatolactin mRNAs in the pituitary also increased consistently with the elevation of gene expression for sGnRH. These results, in combination with lines of previous evidence, indicate that sGnRH neurons are activated in almost all the forebrain loci during the last phases of spawning migration, resulting in coordination of final gonadal maturation and migratory behavior to the spawning ground.     

Distribution of salmon gonadotrophin releasing-hormone in the brain and pituitary of the sea bass (Dicentrarchus labrax)

Summary The distribution of salmon gonadotrophin-releasing hormone (sGnRH) was studied in the brain and pituitary of two-year-old immature sea bass (Dicentrarchus labrax) by means of an enzymoimmunoassay (EIA) for sGnRH and immunocytochemistry. The EIA for sGnRH is a competitive assay using a tracer made of sGnRH coupled to acetylcholinesterase from an electric eel. The separation of free and bound tracer is achieved by coating the plates with mouse anti-rabbit IgG monoclonal antibodies. Displacement curves generated by sGnRH and extracts from pituitary and different brain regions showed a good parallelism allowing the assay to be used for sGnRH measurements in this species. Although all parts of the brain contained measurable levels of sGnRH, the highest concentrations were found in the pituitary, the olfactory bulbs and the telencephalon. These data were confirmed by immunocytochemistry. Cell bodies were found in the olfactory bulbs, ventral telencephalon, preoptic region and mediobasal hypothalamus. Immunoreactive fibers could be observed in all parts of the brain including the optic tectum, the cerebellum (corpus and valvula), the vagal lobe, the medulla oblongata and the rostral spinal cord. In most cases, these fibers do not form well defined bundles; however, there was clearly a continuum of immunoreactive fibers, extending from the olfactory bulbs to the pituitary, and along which all the cell bodies described above were located. In the ventral telencephalon and the preoptic region, clear pictures of varicose positive fibers contacting immunoreactive perikarya could be observed. These data indicate that sGnRH is most likely an endogenous peptide in the brain of the sea bass, although the presence of other forms of GnRH cannot be excluded at this point. This study also demonstrates that the general organization of the GnRH systems in the sea bass is highly similar to what has been described in most freshwater teleost species, and provides basis for further studies on the neuroendocrine control of gonadotrophin release in this commercially important species.     

Three GnRH systems in the brain and pituitary of a pleuronectiform fish,the barfin flounder Verasper moseri

To clarify the possible function of gonadotropin-releasing hormone (GnRH) in the brain of a pleuronectiform fish, the barfin flounder Verasper moseri, the distribution of three forms of GnRH in various areas of the brain was examined by radioimmunoassay, and the localization of GnRH-immunoreactive (ir) cell bodies and fibers in the brain and pituitary was determined by immunocytochemistry. The dominant form in the pituitary was seabream GnRH (sbGnRH), levels of which were much higher than those of salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II). In contrast, sbGnRH levels were extremely low in all other brain areas examined. Levels of sGnRH and cGnRH-II were high in the anterior and posterior part of the brain, respectively. sbGnRH-ir cell bodies were located in the preoptic area, whereas sbGnRH-ir fibers were localized mainly in the preoptic area-hypothalamus-pituitary and formed a distinctive bundle of axons projecting to the pituitary. sGnRH-ir cell bodies were located in the ventromedial part of the rostral olfactory bulbs and in the terminal nerve ganglion (the transitional area between the olfactory bulb and the telencephalon). cGnRH-II-ir cell bodies were localized to the midbrain tegmentum. sGnRH-ir and cGnRH-II-ir fibers were observed throughout the brain except in the pituitary gland. These results indicate that sbGnRH is responsible for the neural control of the reproductive endocrinology of the barfin flounder (hypothalamo-hypophysial system), and that sGnRH and cGnRH-II function as neurotransmitters or neuromodulators in the brain.     

Ontogeny of the GnRH systems in zebrafish brain: in situ hybridization and promoter-reporter expression analyses in intact animals

The ontogeny of two gonadotropin-releasing-hormone (GnRH) systems, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), was investigated in zebrafish (Danio rerio). In situ hybridization (ISH) first detected sGnRH mRNA-expressing cells at 1 day post-fertilization (pf) anterior to the developing olfactory organs. Subsequently, cells were seen along the ventral olfactory organs and the olfactory bulbs, reaching the terminal nerve (TN) ganglion at 5–6 days pf. Some cells were detected passing posteriorly through the ventral telencephalon (10–25 days pf), and by 25–30 days pf, sGnRH cells were found in the hypothalamic/preoptic area. Continuous documentation in live zebrafish was achieved by a promoter-reporter expression system. The expression of enhanced green fluorescent protein (EGFP) driven by the sGnRH promoter allowed the earlier detection of cells and projections and the migration of sGnRH neurons. This expression system revealed that long leading processes, presumably axons, preceded the migration of the sGnRH neuron somata. cGnRH-II mRNA expressing cells were initially detected (1 day pf) by ISH analysis at lateral aspects of the midbrain and later on (starting at 5 days pf) at the midline of the midbrain tegmentum. Detection of red fluorescent protein (DsRed) driven by the cGnRH-II promoter confirmed the midbrain expression domain and identified specific hindbrain and forebrain cGnRH-II-cells that were not identified by ISH. The forebrain DsRed-expressing cells seemed to emerge from the same site as the sGnRH-EGFP-expressing cells, as revealed by co-injection of both constructs. These studies indicate that zebrafish TN and hypothalamic sGnRH cell populations share a common embryonic origin and migratory path, and that midbrain cGnRH-II cells originate within the midbrain. This study was supported by the US-Israel Bi-national Agricultural Research and Development (BARD) Foundation (grant 3428-03).     

Ontogeny of gonadotropin-releasing hormone (GnRH) neurons in hybrid striped bass Morone sp.: whole-mount in situ hybridization analysis

The ontogeny of gonadotropin-releasing hormone (GnRH) mRNA-producing neurons was studied in developing hybrid striped bass (white bass Morone chrysops female × striped bass Morone saxatilis male), 1–55 days post-fertilization (dpf), by whole-mount in situ hybridization. Neurons that produce salmon (s) GnRH were first detected at 32 h post-fertilization in the olfactory placodes. These neurons migrated posteriorly during development and reached their final position at the olfactory bulbs-telencephalon boundary, possibly the terminal nerve ganglion (TNg) by 11 dpf. First signal of chicken (c) GnRH-II neurons appeared in the midbrain 2 dpf and remained there throughout development. A signal of seabream (sb) GnRH mRNA was first detected at 21 dpf in the preoptic area (POA) and as a bilateral continuum along the ventral telencephalon at 32–55 dpf. The expression of all three forms of GnRH increased throughout development. These results suggest that cGnRH-II neurons originate in the mid-brain, and that sGnRH neurons originate in the olfactory placodes and migrate caudally to the TNg. Neurons that express sbGnRH seem to originate at the preoptic area and then to migrate anteriorly along the ventral telencephalon. An olfactory placodal origin of these neurons, however, cannot be ruled out.     

Environmental rearing conditions produce forebrain differences in wild Chinook salmon Oncorhynchus tshawytscha

Recent studies suggest that hatchery-reared fish can have smaller brain-to-body size ratios than wild fish. It is unclear, however, whether these differences are due to artificial selection or instead reflect differences in rearing environment during development. Here we explore how rearing conditions influence the development of two forebrain structures, the olfactory bulb and the telencephalon, in juvenile Chinook salmon (Oncorhynchus tshawytscha) spawned from wild-caught adults. First, we compared the sizes of the olfactory bulb and telencephalon between salmon reared in a wild stream vs. a conventional hatchery. We next compared the sizes of forebrain structures between fish reared in an enriched NATURES hatchery and fish reared in a conventional hatchery. All fish were size-matched and from the same genetic cohort. We found that olfactory bulb and telencephalon volumes relative to body size were significantly larger in wild fish compared to hatchery-reared fish. However, we found no differences between fish reared in enriched and conventional hatchery treatments. Our results suggest that significant differences in the volume of the olfactory bulb and telencephalon between hatchery and wild-reared fish can occur within a single generation.     

Brain morphology and immunohistochemical localization of the gonadotropin-releasing hormone in the bluefin tuna, Thunnus thynnus

The present study was focused on the morphology of the diencephalic nuclei (likely involved in reproductive functions) as well as on the distribution of GnRH (gonadotropin-releasing hormone) in the rhinencephalon, telencephalon and the diencephalon of the brain of bluefin tuna (Thunnus thynnus) by means of immunohistochemistry. Bluefin tuna has an encephalization quotient (QE) similar to that of other large pelagic fish. Its brain exhibits well-developed optic tecta and corpus cerebelli. The diencephalic neuron cell bodies involved in reproductive functions are grouped in two main nuclei: the nucleus preopticus-periventricularis and the nucleus lateralis tuberis. The nucleus preopticus-periventricularis consists of the nucleus periventricularis and the nucleus preopticus consisting of a few sparse multipolar neurons in the rostral part and numerous cells closely packed and arranged in several layers in its aboral part. The nucleus lateralis tuberis is located in the ventral-lateral area of the diencephalon and is made up of a number of large multipolar neurones. Four different polyclonal primary antibodies against salmon (s)GnRH, chicken (c)GnRH-II (cGnRH-II 675, cGnRH-II 6) and sea bream (sb)GnRH were employed in the immunohistochemical experiments. No immunoreactive structures were found with anti sbGnRH serum. sGnRH and cGnRH-II antisera revealed immunoreactivity in the perikarya of the olfactory bulbs, preopticus-periventricular nucleus, oculomotor nucleus and midbrain tegmentum. The nucleus lateralis tuberis showed immunostaining only with anti-sGnRH serum. Nerve fibres immunoreactive to cGnRH and sGnRH sera were found in the olfactory bulbs, olfactory nerve and neurohypophysis. The significance of the distribution of the GnRH-immunoreactive neuronal structures is discussed.     

Ontogenic development of three GnRH systems in the brain of a pleuronectiform fish, barfin flounder

A pleuronectiform fish, the barfin flounder Verasper moseri, has three molecular forms of gonadotropin-releasing hormone (GnRH) in the brain, salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II) and seabream GnRH (sbGnRH). To elucidate the ontogenic origin of the neurons that produce these GnRH molecules, the development of three GnRH systems was examined by in situ hybridization and immunocytochemistry. Neuronal somata that express sGnRH mRNA were detected first in the vicinity of the olfactory epithelium 21 days after hatching (Day 21), and then in the transitional area between the olfactory nerve and olfactory bulb and the terminal nerve ganglion on Day 28. cGnRH-II mRNA-expressing neuronal somata were first identified in the midbrain tegmentum near the ventricle on Day 7. cGnRH-II-immunoreactive (ir) fibers were first found in the brain on Day 7. sbGnRH mRNA-expressing neuronal somata were first detected in the preoptic area on Day 42. sbGnRH-ir fibers were localized in the preoptic area-hypothalamus, and formed a distinctive bundle of axons projecting to the pituitary on Day 70. These results indicate that three forms of GnRH neurons have separate embryonic origins in the barfin flounder as in other perciform fish such as tilapia Oreochromis niloticus and red seabream Pagrus major: sGnRH, cGnRH-II and sbGnRH neurons derive from the olfactory placode, the midbrain tegmentum near the ventricle and the preoptic area, respectively.     

Triploidy alters brain morphology in pre‐smolt Atlantic salmon Salmo salar: possible implications for behaviour

Total brain mass and the volumes of five specific brain regions in diploid and triploid Atlantic salmon Salmo salar pre‐smolts were measured using digital images. There were no significant differences (P > 0·05) in total brain mass when corrected for fork length, or the volumes of the optic tecta or hypothalamus when corrected for brain mass, between diploids and triploids. There was a significant effect (P < 0·01) of ploidy on the volume of the olfactory bulb, with it being 9·0% larger in diploids compared with triploids. The cerebellum and telencephalon, however, were significantly larger, 17 and 8% respectively, in triploids compared with diploids. Sex had no significant effect (P > 0·05) on total brain mass or the volumes of any measured brain region. As the olfactory bulbs, cerebellum and telencephalon are implicated in a number of functions, including foraging ability, aggression and spatial cognition, these results may explain some of the behavioural differences previously reported between diploids and triploids.     

Olfactory bulb axonal outgrowth is inhibited by draxin

Olfactory bulb (OB) projection neurons receive sensory input from olfactory receptor neurons and precisely relay it through their axons to the olfactory cortex. Thus, olfactory bulb axonal tracts play an important role in relaying information to the higher order of olfactory structures in the brain. Several classes of axon guidance molecules influence the pathfinding of the olfactory bulb axons. Draxin, a recently identified novel class of repulsive axon guidance protein, is essential for the formation of forebrain commissures and can mediate repulsion of diverse classes of neurons from chickens and mice. In this study, we have investigated the draxin expression pattern in the mouse telencephalon and its guidance functions for OB axonal projection to the telencephalon. We have found that draxin is expressed in the neocortex and septum at E13 and E17.5 when OB projection neurons form the lateral olfactory tract (LOT) rostrocaudally along the ventrolateral side of the telencephalon. Draxin inhibits axonal outgrowth from olfactory bulb explants in vitro and draxin-binding activity in the LOT axons in vivo is detected. The LOT develops normally in draxin−/− mice despite subtle defasciculation in the tract of these mutants. These results suggest that draxin functions as an inhibitory guidance cue for OB axons and indicate its contribution to the formation of the LOT.     

Functional regeneration of the olfactory bulb requires reconnection to the olfactory nerve in Xenopus larvae

Larvae of the South African clawed frog (Xenopus laevis) can regenerate the telencephalon, which consists of the olfactory bulb and the cerebrum, after it has been partially removed. Some authors have argued that the telencephalon, once removed, must be reconnected to the olfactory nerve in order to regenerate. However, considerable regeneration has been observed before reconnection. Therefore, we have conducted several experiments to learn whether or not reconnection is a prerequisite for regeneration. We found that the olfactory bulb did not regenerate without reconnection, while the cerebrum regenerated by itself. On the other hand, when the brain was reconnected by the olfactory nerve, both the cerebrum and the olfactory bulb regenerated. Morphological and histological investigation showed that the regenerated telencephalon was identical to the intact one in morphology, types and distributions of cells, and connections between neurons. Froglets with a regenerated telencephalon also recovered olfaction, the primary function of the frog telencephalon. These results suggest that the Xenopus larva requires reconnection of the regenerating brain to the olfactory nerve in order to regenerate the olfactory bulb, and thus the regenerated brain functions, in order to process olfactory information.     

Expression of extracellular matrix molecules in the embryonic rat olfactory pathway

Primary olfactory neurons arise from placodal neuroepithelium that is separate from the neuroepithelial plate that forms the neural tube and crest. The axons of these neurons course along a stereotypical pathway and invade the rostral telencephalic vesicle where they induce the formation of the olfactory bulb. In the present study we examined the expression of several extracellular matrix constituents during formation of the olfactory nerve pathway in order to identify putative developmentally significant molecules. Double-label immunofluorescence was used to simultaneously map the trajectory of growing primary olfactory axons by expression of growth associated protein 43 (GAP-43) and the distribution of either laminin, heparan sulfate proteoglycans (HSPG), or chondroitin sulfate proteoglycans (CSPG). At embryonic day 12.5 (E12.5) primary olfactory axons have exited the olfactory neuroepithelium of the nasal pit and formed a rudimentary olfactory nerve. These axons together with migrating neural cells form a large mass outside the rostral surface of the telencephalon. This nerve pathway is clearly defined by a punctate distribution of laminin and HSPG. CSPG is selectively present in the mesenchyme between the olfactory nerve pathway and the nasal pit and in the marginal zone of the telencephalon. At E14.5 primary olfactory axons pierce the telencephalon through gaps that have emerged in the basement membrane. At this age both laminin and HSPG are colocalized with the primary olfactory axons that have entered the marginal zone of the telencephalon. CSPG expression becomes downregulated in this same region while it remains highly expressed in the marginal zone adjacent to the presumptive olfactory bulb. By E16.5 most of the basement membrane separating the olfactory nerve from the telencephalon has degraded, and there is direct continuity between the olfactory nerve pathway and the central nervous system. This strict spatiotemporal regulation of extracellular matrix constituents in the olfactory nerve pathway supports an important role of these molecules in axon guidance. We propose that laminin and HSPG are expressed by migrating olfactory Schwann cells in the developing olfactory nerve pathway and that these molecules provide a conducive substrate for axon growth between the olfactory neuroepithelium and the brain. CSPG in the surrounding mesenchyme may act to restrict axon growth to within this pathway. The regional degradation of the basement membrane of the telencephalon and the downregulation of CSPG within the marginal zone probably facilitates the passage of primary olfactory axons into the brain to form the presumptive nerve fiber layer of the olfactory bulb. © 1996 John Wiley & Sons, Inc.     

Molecular cloning and brain distribution of three types of gonadotropin‐releasing hormone from mummichog Fundulus heteroclitus

Complementary DNAs encoding gonadotropin‐releasing hormone (GnRH) precursors were cloned from the mummichog Fundulus heteroclitus brain, showing that this species has three GnRH forms, i.e. medaka Oryzias latipes GnRH (mdGnRH), chicken GnRH‐II (cGnRH‐II) and Atlantic salmon Salmo salar GnRH (sGnRH). The F. heteroclitus prepro GnRHs have common structural architectures of vertebrate GnRHs, consisting of the signal peptide, 10 amino acids of mature peptide, GKR sequence and GnRH‐associated peptide (GAP). Phylogenetic analysis of fish prepro GnRHs showed that F. heteroclitus mdGnRH is a homologue of sbGnRHs and mdGnRHs of other acanthopterygian. Quantitative real‐time PCR revealed that mdGnRH was abundantly expressed in the olfactory bulb and in olfactory lobe areas and is expressed in the pituitary. The cGnRH‐II was mainly expressed in the midbrain and interbrain areas, and the sGnRH was expressed not only in the olfactory bulb but also in other regions of the brain. These results suggest that the mdGnRH is involved in the stimulation of gonadotrophs in the pituitary, whereas cGnRH‐II and sGnRH are involved in neurotransmission and neuromodulation.     

Gonadotropin releasing hormone (GnRH) neurons project to growth hormone and somatolactin cells in the steelhead trout

Analysis of gene expression using gonadotropin-releasing hormone (GnRH) antisense oligonucleotide confirmed by immunocytochemical localization the occurrence of GnRH neurons along the nervus terminalis in the steelhead trout (Oncorhynchus mykiss). Double-label immunocytochemistry revealed the distribution of mammalian (m), salmon (s) and chicken II (cII)-type GnRHs and various pituitary hormones. Both sGnRH and mGnRH appeared to be colocalized in the same cells of the nervus terminalis. Chicken GnRH II-immunoreactivity was found only in fibers and terminals. In the younger fish [73 and 186 days after fertilization (DAF)] GnRH neurons were seen rostral to the olfactory bulb. A novel GnRH ganglion, along the nervus terminalis, was found at the cribriform bone (gCB). A few non-immunoreactive rounded cells were seen among the GnRH neurons. A second smaller ganglion was seen at the most rostrally located part of the ventromedial olfactory bulb (gROB). In the older fish (850 DAF) GnRH neurons were also observed in the basal forebrain. A small group of neurons (2–3 cells), at the caudoventromedial border of the olfactory bulb, formed the ganglion terminale. Occasionally isolated GnRH-immunoreactive cells were seen at the base of the olfactory epithelium, along the ventromedial margins of the olfactory nerve. GnRH-immunoreactive and GnRH mRNA expressing neurons were absent from midbrain regions at the ages observed. GnRH-immunoreactive fibers were present only in older fish. The pattern of distribution of fibers that were immunoreactive to all three forms of GnRH was identical. Fibers were seen along the medial side of the olfactory nerve, throughout the brain and in the pituitary, associated with growth hormone and somatolactin cells. This morphological study shows that molecular forms of GnRHs might have multiple functions.     

Pax6-dependent boundary defines alignment of migrating olfactory cortex neurons via the repulsive activity of ephrin A5

Neuronal migration is a prerequisite event for the establishment of highly ordered neuronal circuits in the developing brain. Here, we report Pax6-dependent alignment of the olfactory cortex neurons in the developing telencephalon. These neurons were generated in the dorsal part of telencephalon, migrated ventrally and stopped at the pallium-subpallium boundary (PSB). In Pax6 mutant rat embryos, however, these neurons invaded the ventral part of the telencephalon by crossing the PSB. Ephrin A5, one of the ligands for EphA receptors, was specifically expressed in the ventral part of the telencephalon, and its expression level was markedly reduced in the Pax6 mutant. Gain- and loss-of-function studies of ephrin A5 indicated that ephrin A5 plays an important role in the alignment of olfactory cortex neurons at the PSB. Our results suggest that Pax6-regulated ephrin A5 acts as a repulsive molecule for olfactory cortex neurons in the developing telencephalon.     

Brain morphology in large pelagic fishes: a comparison between sharks and teleosts

A quantitative comparison was made of both relative brain size (encephalization) and the relative development of five brain area of pelagic sharks and teleosts. Two integration areas (the telencephalon and the corpus cerebellum) and three sensory brain areas (the olfactory bulbs, optic tectum and octavolateralis area, which receive primary projections from the olfactory epithelium, eye and octavolateralis senses, respectively), in four species of pelagic shark and six species of pelagic teleost were investigated. The relative proportions of the three sensory brain areas were assessed as a proportion of the total 'sensory brain', while the two integration areas were assessed relative to the sensory brain. The allometric analysis of relative brain size revealed that pelagic sharks had larger brains than pelagic teleosts. The volume of the telencephalon was significantly larger in the sharks, while the corpus cerebellum was also larger and more heavily foliated in these animals. There were also significant differences in the relative development of the sensory brain areas between the two groups, with the sharks having larger olfactory bulbs and octavolateralis areas, whilst the teleosts had larger optic tecta. Cluster analysis performed on the sensory brain areas data confirmed the differences in the composition of the sensory brain in sharks and teleosts and indicated that these two groups of pelagic fishes had evolved different sensory strategies to cope with the demands of life in the open ocean.     

Cxcl12/Cxcr4 chemokine signaling is required for placode assembly and sensory axon pathfinding in the zebrafish olfactory system

Positioning neurons in the right places and wiring axons to the appropriate targets are essential events for establishment of neural circuits. In the zebrafish olfactory system, precursors of olfactory sensory neurons (OSNs) assemble into a compact cluster to form the olfactory placode. Subsequently, OSNs differentiate and extend their axons to the presumptive olfactory bulb with high precision. In this study, we aim to elucidate the molecular mechanism underlying these two developmental processes. cxcr4b, encoding a chemokine receptor, is expressed in the migrating olfactory placodal precursors, and cxcl12a (SDF-1a), encoding a ligand for Cxcr4b, is expressed in the abutting anterior neural plate. The expression of cxcr4b persists in the olfactory placode at the initial phase of OSN axon pathfinding. At this time, cxcl12a is expressed along the placode-telencephalon border and at the anterior tip of the telencephalon, prefiguring the route and target of OSN axons, respectively. Interfering with Cxcl12a/Cxcr4b signaling perturbs the assembly of the olfactory placode, resulting in the appearance of ventrally displaced olfactory neurons. Moreover, OSN axons frequently fail to exit the olfactory placode and accumulate near the placode-telencephalon border in the absence of Cxcr4b-mediated signaling. These data indicate that chemokine signaling contributes to both the olfactory placode assembly and the OSN axon pathfinding in zebrafish.     

Electrophoretic changes in olfactory system proteins in masu salmon during parr-smolt transformation

Changes in cytosolic proteins of the olfactory system (olfactory epithelium, olfactory nerve and olfactory bulb) and the telencephalon of masu salmon Oncorhynchus masou were analysed by two-dimensional polyacrylamide gel electrophoresis during parr-smolt transformation; parr, pre-smolt and full-smolt stages. In the olfactory system, several protein spots appeared and disappeared in the course of smolting. One protein spot in particular with an estimated molecular weight of 27 kDa and isoelectric point of 5.6 (M27) disappeared in common with the olfactory system during smolting. The disappearance of M27 was also observed in the telencephalon. These proteins, which appeared and disappeared, may reflect the changes in olfactory function during smolting. Simultaneously, the present study confirmed a salmonid olfactory specific protein of 24 kDa (N24), which existed in the olfactory system but not in the telencephalon, as a single neutral protein spot in masu salmon.