Mapping the Pattern of Essential Neuroendocrine Cells Related to Puberty and VA Opsin Expression Provides Further Insight in the Photoreceptive Regulation of the Brain-Pituitary-Gonadal Axis in Atlantic Salmon (Salmo salar)

In Atlantic salmon (Salmo salar), seasonal photoperiod is shown to regulate the onset of sexual maturation, yet which brain region(s) is involved, and how light information impacts the neuroendocrine system are still not fully understood in teleosts. Detailed knowledge about the photoperiodic regulation of maturation in fish is still missing. In birds, it is shown that gonadotropin-releasing hormone (Gnrh) is located in the same neurons as vertebrate ancient (VA) opsin, suggesting a direct photoreceptive regulation for the onset of sexual maturity. This study presents a comprehensive topographic mapping of gnrh2, gnrh3, kisspeptin 2 (kiss2), gonadotropin-inhibiting hormone (gnih), and VA opsin using in situ hybridization on mature Atlantic salmon brains. Neurons positive for gnrh3 are expressed in the olfactory bulb and ventral telencephalon, while gnrh2-positive neurons are located dorsally in the midbrain tegmentum. Gonadotropin-inhibiting hormone (Gnih)-expressing cell bodies are present in the ventral thalamus and extend caudally to the hypothalamus with kiss2-expressing cells appearing in a lateral position. VA opsin-positive cells are present in the telencephalon, the rostro-dorsal ring of the left habenula, the ventral thalamus, and the midbrain tegmentum. The results show no similar co-location as found in birds, hypothesizing that the photoreceptive modulation of Gnrh in salmon may interact through neuronal networks. The topography analyses of the essential neuroendocrine cells related to sexual maturation in the Atlantic salmon brain show that diencephalic (thalamus, hypothalamus) and midbrain (tegmentum) regions seem central for controlling sexual maturation.

Light conditions during Atlantic salmon embryogenesis affect key neuropeptides in the melanocortin system during transition from endogenous to exogenous feeding

During the first feeding period, fish will adapt to exogenous feeding as their endogenous source of nutrients is depleted. This requires the development of a functional physiological system to control active search for food, appetite, and food intake. The Atlantic salmon (Salmo salar) melanocortin system, a key player in appetite control, includes neuronal circuits expressing neuropeptide y (npya), agouti-related peptide (agrp1), cocaine- and amphetamine-regulated transcript (cart), and proopiomelanocortin (pomca). Little is known about the ontogeny and function of the melanocortin system during early developmental stages. Atlantic salmon [0-730 day degrees (dd)] were reared under three different light conditions (DD, continuous darkness; LD, 14:10 Light: Dark; LL, continuous light) before the light was switched to LD and the fish fed twice a day. We examined the effects of different light conditions (DD _LD , LD _LD , and LL _LD ) on salmon growth, yolk utilization, and periprandial responses of the neuropeptides npya1, npya2, agrp1, cart2a, cart2b, cart4, pomca1, and pomca2. Fish were collected 1 week (alevins, 830 dd, still containing yolk sac) and 3 weeks (fry, 991 dd, yolk sac fully consumed) into the first feeding period and sampled before (-1 h) and after (0.5, 1.5, 3, and 6 h) the first meal of the day. Atlantic salmon reared under DD _LD , LD _LD , and LL _LD had similar standard lengths and myotome heights at the onset of first feeding. However, salmon kept under a constant light condition during endogenous feeding (DD _LD and LL _LD ) had less yolk at first feeding. At 830 dd none of the neuropeptides analyzed displayed a periprandial response. But 2 weeks later, and with no yolk remaining, significant periprandial changes were observed for npya1, pomca1, and pomca2, but only in the LD _LD fish. This suggests that these key neuropeptides serve an important role in controlling feeding once Atlantic salmon need to rely entirely on active search and ingestion of exogenous food. Moreover, light conditions during early development did not affect the size of salmon at first feeding but did affect the mRNA levels of npya1, pomca1, and pomca2 in the brain indicating that mimicking natural light conditions (LD _LD ) better stimulates appetite control.

Mapping key neuropeptides involved in the melanocortin system in Atlantic salmon (Salmo salar) brain

The melanocortin system is a key regulator of appetite and food intake in vertebrates. This system includes the neuropeptides neuropeptide y (NPY), agouti-related peptide (AGRP), cocaine- and amphetamine-regulated transcript (CART), and pro-opiomelanocortin (POMC). An important center for appetite control in mammals is the hypothalamic arcuate nucleus, with neurons that coexpress either the orexigenic NPY/AGRP or the anorexigenic CART/POMC neuropeptides. In ray-finned fishes, such a center is less characterized. The Atlantic salmon (Salmo salar) has multiple genes of these neuropeptides due to whole-genome duplication events. To better understand the potential involvement of the melanocortin system in appetite and food intake control, we have mapped the mRNA expression of npy, agrp, cart, and pomc in the brain of Atlantic salmon parr using in situ hybridization. After identifying hypothalamic mRNA expression, we investigated the possible intracellular coexpression of npy/agrp and cart/pomc in the tuberal hypothalamus by fluorescent in situ hybridization. The results showed that the neuropeptides were widely distributed, especially in sensory and neuroendocrine brain regions. In the hypothalamic lateral tuberal nucleus, the putative homolog to the mammalian arcuate nucleus, npya, agrp1, cart2b, and pomca were predominantly localized in distinct neurons; however, some neurons coexpressed cart2b/pomca. This is the first demonstration of coexpression of cart2b/pomca in the tuberal hypothalamus of a teleost. Collectively, our data suggest that the lateral tuberal nucleus is the center for appetite control in salmon, similar to that of mammals. Extrahypothalamic brain regions might also be involved in regulating food intake, including the olfactory bulb, telencephalon, midbrain, and hindbrain.

Photoreception and transcriptomic response to light during early development of a teleost with a life cycle tightly controlled by seasonal changes in photoperiod

Light cues vary along the axis of periodicity, intensity and spectrum and perception of light is dependent on the photoreceptive capacity encoded within the genome and the opsins expressed. A global approach was taken to analyze the photoreceptive capacity and the effect of differing light conditions on a developing teleost prior to first feeding. The transcriptomes of embryos and alevins of Atlantic salmon (Salmo salar) exposed to different light conditions were analyzed, including a developmental series and a circadian profile. The results showed that genes mediating nonvisual photoreception are present prior to hatching when the retina is poorly differentiated. The clock genes were expressed early, but the circadian profile showed that only two clock genes were significantly cycling before first feeding. Few genes were differentially expressed between day and night within a light condition; however, many genes were significantly different between light conditions, indicating that light environment has an impact on the transcriptome during early development. Comparing the transcriptome data from constant conditions to periodicity of white light or different colors revealed overrepresentation of genes related to photoreception, eye development, muscle contraction, degradation of metabolites and cell cycle among others, and in constant light, several clock genes were upregulated. In constant white light and periodicity of green light, genes associated with DNA replication, chromatin remodeling, cell division and DNA repair were downregulated. The study implies a direct influence of light conditions on the transcriptome profile at early developmental stages, by a complex photoreceptive system where few clock genes are cycling.

An EvoDevo Study of Salmonid Visual Opsin Dynamics and Photopigment Spectral Sensitivity

Salmonids are ideal models as many species follow a distinct developmental program from demersal eggs and a large yolk sac to hatching at an advanced developmental stage. Further, these economically important teleosts inhabit both marine- and freshwaters and experience diverse light environments during their life histories. At a genome level, salmonids have undergone a salmonid-specific fourth whole genome duplication event (Ss4R) compared to other teleosts that are already more genetically diverse compared to many non-teleost vertebrates. Thus, salmonids display phenotypically plastic visual systems that appear to be closely related to their anadromous migration patterns. This is most likely due to a complex interplay between their larger, more gene-rich genomes and broad spectrally enriched habitats; however, the molecular basis and functional consequences for such diversity is not fully understood. This study used advances in genome sequencing to identify the repertoire and genome organization of visual opsin genes (those primarily expressed in retinal photoreceptors) from six different salmonids [Atlantic salmon (Salmo salar), brown trout (Salmo trutta), Chinook salmon (Oncorhynchus tshawytcha), coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhynchus mykiss), and sockeye salmon (Oncorhynchus nerka)] compared to the northern pike (Esox lucius), a closely related non-salmonid species. Results identified multiple orthologues for all five visual opsin classes, except for presence of a single short-wavelength-sensitive-2 opsin gene. Several visual opsin genes were not retained after the Ss4R duplication event, which is consistent with the concept of salmonid rediploidization. Developmentally, transcriptomic analyzes of Atlantic salmon revealed differential expression within each opsin class, with two of the long-wavelength-sensitive opsins not being expressed before first feeding. Also, early opsin expression in the retina was located centrally, expanding dorsally and ventrally as eye development progressed, with rod opsin being the dominant visual opsin post-hatching. Modeling by spectral tuning analysis and atomistic molecular simulation, predicted the greatest variation in the spectral peak of absorbance to be within the Rh2 class, with a ∼40 nm difference in λ_max values between the four medium-wavelength-sensitive photopigments. Overall, it appears that opsin duplication and expression, and their respective spectral tuning profiles, evolved to maximize specialist color vision throughout an anadromous lifecycle, with some visual opsin genes being lost to tailor marine-based vision.

Rhythmic Clock Gene Expression in Atlantic Salmon Parr Brain

To better understand the complexity of clock genes in salmonids, a taxon with an additional whole genome duplication, an analysis was performed to identify and classify gene family members (clock, arntl, period, cryptochrome, nr1d, ror, and csnk1). The majority of clock genes, in zebrafish and Northern pike, appeared to be duplicated. In comparison to the 29 clock genes described in zebrafish, 48 clock genes were discovered in salmonid species. There was also evidence of species-specific reciprocal gene losses conserved to the Oncorhynchus sister clade. From the six period genes identified three were highly significantly rhythmic, and circadian in their expression patterns (per1a.1, per1a.2, per1b) and two was significantly rhythmically expressed (per2a, per2b). The transcriptomic study of juvenile Atlantic salmon (parr) brain tissues confirmed gene identification and revealed that there were 2,864 rhythmically expressed genes (p < 0.001), including 1,215 genes with a circadian expression pattern, of which 11 were clock genes. The majority of circadian expressed genes peaked 2 h before and after daylight. These findings provide a foundation for further research into the function of clock genes circadian rhythmicity and the role of an enriched number of clock genes relating to seasonal driven life history in salmonids.

Neural activation in photosensitive brain regions of Atlantic salmon (Salmo salar) after light stimulation

Photoreceptive inputs to the teleost brain are perceived as image of the visual world and as photo-modulation of neuroendocrine and neuronal signals. The retina and pineal organ are major receptive organs with projections to various parts of the brain, but in the past decades deep brain photoreceptors have emerged as candidates for photoreceptive inputs, either independent or in combination with projections from light sensory organs. This study aimed to test the effects of narrow bandwidth light using light-emitting diodes technology on brain neural activity through putative opsin stimulation in Atlantic salmon. The expression of c-fos, a known marker of neural activity, was compared in situ between dark-adapted salmon parr and following light stimulation with different wavelengths. c-fos expression increased with duration of light stimulation and the strongest signal was obtained in fish exposed to light for 120 minutes. Distinct and specific brain regions were activated following dark to light stimulation, such as the habenula, suprachiasmatic nucleus, thalamus, and hypothalamus. The c-fos expression was overlapping with photoreceptors expressing melanopsin and/or vertebrate ancient opsin, suggesting a potential direct activation by light. Interestingly in the habenula, a distinct ring of vertebrate ancient opsin and melanopsin expressing cells is overlapping with c-fos expression after neural activation. Salmon exposed to different spectra had neural activation in similar brain regions. The most apparent difference was melanopsin expression in the lateral cells of the lateral tuberal nuclus in the hypothalamus, which appeared to be specifically activated by red light. Light-stimulated neuronal activity in the deep brain was limited to subpopulations of neurons, mainly in regions with neuronal modulation activity, retinal and pineal innervations and known presence of nonvisual photoreceptors. The overlapping expression patterns of c-fos and nonvisual opsins support direct light stimulation of deep brain photoreceptors and the importance of these systems in light induced brain activity.

Heads and tails: The notochord develops differently in the cranium and caudal fin of Atlantic Salmon (Salmo salar, L.)

While it is well known that the notochord of bony fishes changes over developmental time, less is known about how it varies across different body regions. In the development of the Atlantic salmon, Salmo salar L., cranial and caudal ends of the notochord are overlaid by the formation of the bony elements of the neurocranium and caudal fin, respectively. To investigate, we describe how the notochord of the cranium and caudal fin changes from embryo to spawning adult, using light microscopy, SEM, TEM, dissection, and CT scanning. The differences are dramatic. In contrast to the abdominal and caudal regions, at the ends of the notochord vertebrae never develop. While the cranial notochord builds a tapering, unsegmented cone of chordal bone, the urostylic notochordal sheath never ossifies: adjacent, irregular bony elements form from the endoskeleton of the caudal fin. As development progresses, two previously undescribed processes occur. First, the bony cone of the cranial notochord, and its internal chordocytes, are degraded by chordoclasts, an undescribed function of the clastic cell type. Second, the sheath of the urostylic notochord creates transverse septae that partly traverse the lumen in an irregular pattern. By the adult stage, the cranial notochord is gone. In contrast, the urostylic notochord in adults is robust, reinforced with septae, covered by irregularly shaped pieces of cellular bone, and capped with an opistural cartilage that develops from the sheath of the urostylic notochord. A previously undescribed muscle, with its origin on the opistural cartilage, inserts on the lepidotrich ventral to it.

Transient photoreception in the hindbrain is permissive to the life history transition of hatching in Atlantic halibut

In nonmammalian vertebrates, photoreception takes place in the deep brain already early in development, but knowledge is lacking about the functions of these nonvisual photoreceptive systems. Prior to hatching, Atlantic halibut has a transient bilateral cluster of photoreceptive cells in the hindbrain. The cluster is imbedded in a neuronal network projecting to the narrow belt of hatching glands in the yolk sac. In halibut, hatching is inhibited in light and activated by transfer to darkness and c-fos analysis during hatching shows that the hindbrain cluster and hatching glands have neural activation. Unexpectedly, the hindbrain cluster expresses dual photopigments, vertebrate ancient opsin and melanopsin. Evolutionarily, these opsins are believed to belong to different classes of photopigments found in rhabdomeric and ciliary photoreceptors. The concept that an organism develops transient light sensitivity to target critical aspects of life history transitions as hatching provides a fascinating landscape to investigate the timing of other biological events.

Environmental, population and life-stage plasticity in the visual system of Atlantic cod

The visual system is for many fishes essential in guiding behaviors, such as foraging, predator avoidance and mate choice. The marine environment is characterized by large spatio-temporal fluctuations in light intensity and spectral composition. However, visual capabilities are restricted by both space limitations set by eye size and by the genomic content of light-absorbing opsin genes. The rich array of visual opsins in teleosts may be used differentially to tune vision towards specific needs during ontogeny and to changing light. Yet, to what extent visual plasticity is a pre-programmed developmental event, or is triggered by photic environment, is unclear. Our previous studies on Atlantic cod revealed an evolutionary genomic loss of UV-sensitive sws1 and red-sensitive lws opsin families, while blue-sensitive sws2 and green-sensitive rh2 opsins had duplicated. The current study has taken an opsin expression approach to characterize visual plasticity in cod towards different spectral light during the larval stage, to maturation and extreme seasonal changes in the Barents Sea. Our data suggest that opsin plasticity in cod larvae is controlled by developmental programme rather than immediate light environment. The lack of expressional changes during maturation suggests a less important role for visual modulation related to mate choice. Although no seasonal effects on visual opsins were detected in migratory Northeast Arctic cod, the expressed opsin subset differed from the more stationary Norwegian coastal cod described in previous studies. Interestingly, these data provide the first indications of a population difference in actively used visual opsins associated with cod ecotypes.

Pushing the limits of photoreception in twilight conditions: The rod-like cone retina of the deep-sea pearlsides

Most vertebrates have a duplex retina comprising two photoreceptor types, rods for dim-light (scotopic) vision and cones for bright-light (photopic) and color vision. However, deep-sea fishes are only active in dim-light conditions; hence, most species have lost their cones in favor of a simplex retina composed exclusively of rods. Although the pearlsides, Maurolicus spp., have such a pure rod retina, their behavior is at odds with this simplex visual system. Contrary to other deep-sea fishes, pearlsides are mostly active during dusk and dawn close to the surface, where light levels are intermediate (twilight or mesopic) and require the use of both rod and cone photoreceptors. This study elucidates this paradox by demonstrating that the pearlside retina does not have rod photoreceptors only; instead, it is composed almost exclusively of transmuted cone photoreceptors. These transmuted cells combine the morphological characteristics of a rod photoreceptor with a cone opsin and a cone phototransduction cascade to form a unique photoreceptor type, a rod-like cone, specifically tuned to the light conditions of the pearlsides' habitat (blue-shifted light at mesopic intensities). Combining properties of both rods and cones into a single cell type, instead of using two photoreceptor types that do not function at their full potential under mesopic conditions, is likely to be the most efficient and economical solution to optimize visual performance. These results challenge the standing paradigm of the function and evolution of the vertebrate duplex retina and emphasize the need for a more comprehensive evaluation of visual systems in general.

The two-step development of a duplex retina involves distinct events of cone and rod neurogenesis and differentiation

Unlike in mammals, persistent postembryonic retinal growth is a characteristic feature of fish, which includes major remodeling events that affect all cell types including photoreceptors. Consequently, visual capabilities change during development, where retinal sensitivity to different wavelengths of light (photopic vision), -and to limited photons (scotopic vision) are central capabilities for survival. Differently from well-established model fish, Atlantic cod has a prolonged larval stage where only cone photoreceptors are present. Rods do not appear until juvenile transition (metamorphosis), a hallmark of indirect developing species. Previously we showed that whole gene families of lws (red-sensitive) and sws1 (UV-sensitive) opsins have been lost in cod, while rh2a (green-sensitive) and sws2 (blue-sensitive) genes have tandem duplicated. Here, we provide a comprehensive characterization of a two-step developing duplex retina in Atlantic cod. The study focuses on cone subtype dynamics and delayed rod neurogenesis and differentiation in all cod life stages. Using transcriptomic and histological approaches we show that different opsins disappear in a topographic manner during development where central to peripheral retina is a key axis of expressional change. Early cone differentiation was initiated in dorso-temporal retina different from previously described in fish. Rods first appeared during initiation of metamorphosis and expression of the nuclear receptor transcription factor nr2e3-1, suggest involvement in rod specification. The indirect developmental strategy thus allows for separate studies of cones and rods development, which in nature correlates with visual changes linked to habitat shifts. The clustering of key retinal genes according to life stage, suggests that Atlantic cod with its sequenced genome may be an important resource for identification of underlying factors required for development and function of photopic and scotopic vision.

Molecular evidence that only two opsin subfamilies, the blue light- (SWS2) and green light-sensitive (RH2), drive color vision in Atlantic cod (Gadus morhua)

Teleosts show a great variety in visual opsin complement, due to both gene duplication and gene loss. The repertoire ranges from one subfamily of visual opsins (scotopic vision) including rod opsin only retinas seen in many deep-sea species to multiple subfamilies of visual opsins in some pelagic species. We have investigated the opsin repertoire of Atlantic cod (Gadus morhua) using information in the recently sequenced cod genome and found that despite cod not being a deep sea species it lacks visual subfamilies sensitive towards the most extreme parts of the light spectra representing UV and red light. Furthermore, we find that Atlantic cod has duplicated paralogs of both blue-sensitive SWS2 and green-sensitive RH2 subfamilies, with members belonging to each subfamily linked in tandem within the genome (two SWS2-, and three RH2A genes, respectively). The presence of multiple cone opsin genes indicates that there have been duplication events in the cod ancestor SWS2 and RH2 opsins producing paralogs that have been retained in Atlantic. Our results are supported by expressional analysis of cone opsins, which further revealed an ontogenetic change in the array of cone opsins expressed. These findings suggest life stage specific programs for opsin regulation which could be linked to habitat changes and available light as the larvae is transformed into an early juvenile. Altogether we provide the first molecular evidence for color vision driven by only two families of cone opsins due to gene loss in a teleost.

Exorhodopsin and melanopsin systems in the pineal complex and brain at early developmental stages of Atlantic halibut (Hippoglossus hippoglossus)

The complexity of the nonvisual photoreception systems in teleosts has just started to be appreciated, with colocalization of multiple photoreceptor types with unresolved functions. Here we describe an intricate expression pattern of melanopsins in early life stages of the marine flat fish Atlantic halibut (Hippoglossus hippoglossus), a period when the unpigmented brain is directly exposed to environmental photons. We show a refined and extensive expression of melanopsins in the halibut brain already at the time of hatching, long before the eyes are functional. We detect melanopsin in the habenula, suprachiasmatic nucleus, dorsal thalamus, and lateral tubular nucleus of first feeding larvae, suggesting conserved functions of the melanopsins in marine teleosts. The complex expression of melanopsins already at larval stages indicates the importance of nonvisual photoreception early in development. Most strikingly, we detect expression of both exorhodopsin and melanopsin in the pineal complex of halibut larvae. Double-fluorescence labeling showed that two clusters of melanopsin-positive cells are located lateral to the central rosette of exorhodopsin-positive cells. The localization of different photopigments in the pineal complex suggests that two parallel photoreceptor systems may be active. Furthermore, the dispersed melanopsin-positive cells in the spinal cord of halibut larvae at the time of hatching may be primary sensory cells or interneurons representing the first example of dispersed high-order photoreceptor cells. The appearance of nonvisual opsins early in the development of halibut provides an alternative model for studying the evolution and functional significance of nonvisual opsins.

Isolation and characterization of melanopsin photoreceptors of Atlantic salmon (Salmo salar)

Melanopsins constitute a recently described group of vertebrate opsin photoreceptors that are involved in nonvisual photoreception. Here we describe the identification of six melanopsin genes of Atlantic salmon (Salmo salar), a valuable teleost model for studying nonvisual photoreception and the basis of photoperiodism. The results show that genes belonging to two different groups, the mammalian-like (Opn4m) and the Xenopus-like (Opn4x) melanopsins have been duplicated in teleosts. In addition, two pairs of salmon duplicates were identified, presumably originating from the salmon lineage whole genome duplication event. The expression pattern of melanopsins was studied by in situ hybridization. The results show that Opn4m and Opn4x melanopsins are differentially expressed in the brain and retina, indicating a functional divergence. In the retina, Opn4m and Opn4x melanopsin are differentially expressed in ganglion, amacrine, and horizontal cells. In the brain, Opn4m is expressed in the dorsal thalamus and in the nucleus lateralis tuberis of the hypothalamus, which is closely connected to and involved in the regulation of pituitary function. Opn4x melanopsins are expressed in the dopaminergic, hypophysiotrophic cell population of the suporaoptic/chiasmatic nucleus and in the serotonergic cell population of the left habenula. The results suggest that melanopsin photoreceptors can be involved in signaling of photoperiodic information through multiple pathways, involving both the retina and possibly as deep-brain photoreceptors directly transmitting photoperiodic information to the hypothalamus-pituitary axis.

Corticotropin-releasing factor neurogenesis during midlife development in salmon: genetic, environmental and thyroid hormone regulation

Salmon parr-smolt transformation (smoltification) is a mid-life transitional stage between life in freshwater and seawater that entails a wide range of neural, endocrine and physiological modifications. In salmon, the neuroendocrine corticotropin-releasing factor (CRF) system regulates pituitary adrenocorticotrophic hormone and thyrotrophin release. Four experimental groups of Atlantic salmon, Salmo salar, were used to investigated CRF neurogenesis and its regulation during smoltification. We compared: (i) developmental stages (parr and early-smolt) in anadromous controls; (ii) a developmentally arrested model: anadromous reared under continuous light (LL) with anadromous controls; (iii) a natural hypoendocrine/incomplete smolt development salmon model (landlocked) with anadromous controls; and (iv) landlocked treated with thyroxine to anadromous control smolt levels. CRF neurogenesis between groups was studied with bromodeoxyuradine (BrdU) incorporation followed by double-labelling CRF and BrdU immunhistochemistry. The rate of CRF neurogenesis in the preoptic area (POA) increased from parr to early-smolts in anadromous salmon. By contrast, neurogenesis was inhibited in the LL group and reduced in the landlocked salmon. The administration of thyroxine in landlocked salmon to match anadromous levels increased the rate of CRF neurogenesis to anadromous levels. In conclusion, newly-formed CRF cells in the POA during smoltification are associated with increased retinal innervation to the POA and endocrine responsiveness to increased photoperiod. Both genetic and environmental factors influence the degree of salmon brain development. Thyroid hormones increase CRF neurogenesis during this critical period of development in salmon. We hypothesise that a positive-feedback of thyroid hormones on CRF neurogenesis may be an important event in reaching the developmental climax during critical periods.

Identification and characterization of two zebrafish nectin-1 genes that are differentially expressed in the developing eye and brain

Nectins are cell adhesion molecules of the immunoglobulin type that play important roles in the development of the nervous system. We have characterized two paralogous zebrafish nectin-1 genes, nectin-1a and nectin-1b, that differ in expression. Nectin-1a expression is first found in the anterior neural keel and later in the optic cup. In the retina, nectin-1a appears in the outer part and extends inwards, while nectin-1b starts in the inner part and spreads outwards. Only nectin-1a was detected in the cornea, the lens, and in the region of photoreceptor cell differentiation in the retina. Both genes were expressed in ganglion cells and inner nuclear neurons. In the brain, nectin-1a was restricted to the epiphysis and a cluster of cells in the posterior hindbrain, whereas nectin-1b was found in several brain areas. Zebrafish may, therefore, be a useful model for identifying different functions of nectin-1 in the developing eye and nervous system.

Expression of the eight AMPA receptor subunit genes in the developing central nervous system and sensory organs of zebrafish

The AMPA type glutamate receptors mediate the majority of fast synaptic transmission in the vertebrate nervous system. Whereas mammals have four subunit genes, Gria1-4, zebrafish has retained a duplicated set of eight genes named gria1-4a and b. We give here a detailed overview of the expression patterns of all eight zebrafish subunits within the developing central nervous system and sensory organs at 24, 48, and 72 hr after fertilization. Expression domains include distinct neuronal subsets in the developing forebrain, midbrain, hindbrain, and spinal cord, as well as in the ganglion- and inner nuclear layers of the retina. As a general rule, each pair of duplicated gria genes is differentially expressed, indicating subfunctionalization of AMPA receptor subunit expression in the teleost lineage. Our findings suggest that zebrafish can serve as a useful model system to investigate the role of AMPA receptors and their differential expression in the vertebrate nervous system.

Cholecystokinin mRNA in Atlantic herring, Clupea harengus--molecular cloning, characterization, and distribution in the digestive tract during the early life stages

The mRNA of the peptide hormone cholecystokinin (CCK) was isolated from juvenile Atlantic herring, Clupea harengus, by RT-PCR. The open reading frame encodes a 137 amino acid-long precursor protein. The peptide sequence of herring CCK-8, DYMGWMDF, is identical to that of higher vertebrates and elasmobranchs, and contains methionine in the sixth position from the C-terminus, which has not been reported previously in teleosts. Expression analysis by in situ hybridization shows that positive endocrine-like cells were mainly located in the pyloric caeca and to a less extent in the rectum of the juvenile. A few positive cells were also found in the pyloric portion of the stomach and the intestine. CCK cells were present in all the larvae examined from the day of hatching onwards. Although the CCK cells were scattered throughout the whole midgut, no signals were detected in either the foregut or the hindgut. Since herring larvae have a straight gut, the distribution pattern of CCK cells seems to be reflected in the anatomy of the gut.

Ontogeny of ultraviolet-sensitive cones in the retina of rainbow trout (Oncorhynchus mykiss)

In order to facilitate emerging models of retinal development, we developed electroretinogram and in situ hybridization protocols to examine the ontogeny of photoreceptors in the retina of a land-locked salmonid, the rainbow trout (Oncorhynchus mykiss). We cloned cDNA fragments corresponding to the rod opsin and each of the four cone opsin gene families, which we utilized to produce riboprobes. We established the specificity of the in situ hybridization protocol by examining subcellular signal localization and through double-labeling experiments. We confirm the assumption that the accessory corner cones in the square mosaic are the ultraviolet wavelength-sensitive (UVS) cone photoreceptor (i.e., they express an SWS1 opsin) and observed UVS cones throughout the retina of small trout. Larger fish have a decrease in sensitivity to short wavelength light stimuli and the distribution of UVS cones in the mature retina is limited to the dorsal-temporal quadrant. These larger fish also possess differentiated UVS cones in the peripheral germinal zone (PGZ), including within areas peripheral to mature retina lacking UVS cones. These data are consistent with the loss of putative UVS cones from the PGZ of a migratory salmonid of another genus, and thus the disappearance of UVS cones appears to be general to the Family Salmonidae, regardless of life history strategy. The generation, differentiation, and subsequent loss of UVS cones in the smolt PGZ is a dramatic example of the supposition that the mechanisms of PGZ development recapitulate the retinal embryogenesis of that species.

Identification and localization of eight distinct hormone-producing cell types in the pituitary of male Atlantic halibut (Hippoglossus hippoglossus L.)

The eight distinct hormone-producing cell types in the adenohypophysis of male Atlantic halibut (Hippoglossus hippoglossus L.) were identified and localized using immunohistochemistry and in situ hybridization. Lactotropes either occupied most of the rostral pars distalis (RPD) or they were arranged in follicular structures located along the periphery of the RPD. Corticotropes were confined to a thin layer of RPD cells bordering the pars nervosa (PN). The somatotropes were arranged in multicellular layers bordering the highly convoluted PN penetrating the proximal pars distalis (PPD), while thyrotropes, scattered in small islets in between the somatotropes, were located in the centro-dorsal part of the PPD. Gonadotropes were found throughout the PPD. Immunoreactivity to glycoprotein-alpha and luteinizing hormone beta-subunit was also observed along the periphery of the pars intermedia (PI), indicating that a thin extension of the PPD surrounded the PI. In situ hybridization showed that follicle-stimulating hormone and luteinizing hormone were produced in distinct cells of the PPD. PI contained somatolactotropes bordering the highly convoluted PN, and melanotropes that showed positive immunostaining against both anti-alpha-melanocyte-stimulating hormone and anti-beta-endorphin. The general cellular organization was similar to that of other teleost fish. These results lay the basis for future investigations on Atlantic halibut pituitary physiology.

Isolation and characterization of two teleost melanopsin genes and their differential expression within the inner retina and brain

Melanopsin is a newly discovered photopigment that is believed to be involved in the regulation of circadian rhythms in tetrapods. Here we describe the characterization of the first two teleost melanopsins (opn4a and opn4b) isolated from Atlantic cod (Gadus morhua). These two teleost genes belong to a subgroup of melanopsins that also include members from Xenopus, chicken, and Takifugu. In situ hybridization revealed that opn4a and opn4b are differentially expressed within the retina and brain. In the larval and adult retina, both melanopsins are expressed in a subset of cells in the inner retina, resembling amacrine and ganglion cells. In addition, opn4a is expressed in the horizontal cells, indicating a separate task for this gene. In the brain, the two melanopsins are separately expressed in two major retinal and extraretinal photosensitive integration centers, namely, the suprachiasmatic nucleus (opn4a) and the habenula (opn4b). The expression of opn4a in the suprachiasmatic nucleus in cod is similar to the melanopsin expression found in Xenopus. This suggests a conserved role for this opsin and an involvement in mediation of nonvisual photoreceptive tasks, such as entraining circadian rhythms and/or hypophysiotrophic systems. The differential expression of opn4b in the habenula suggests that this gene plays a role similar to that of opn4a, in that it is also situated in an area that integrates photic inputs from the pineal as well as other brain regions. Thus, the habenula may be an additional region that mediates photic cues in teleosts.

Molecular cloning and characterization of five opsin genes from the marine flatfish Atlantic halibut (Hippoglossus hippoglossus)

Most molecular studies on the visual system in fish have been performed on freshwater teleosts such as goldfish and zebrafish where cones and rods appear simultaneously. Many marine fishes have long larval phase in the upper pelagic zone before transformation into a juvenile and a benthic life style. The retina at the larval stages consists of only single cone cells; later during metamorphosis double cones and rods develop. The flatfish Atlantic halibut (Hippoglossus hippoglossus) is a typical example of a marine species with such a two-step retina development. In this study, we have cloned five different opsins from Atlantic halibut larvae and juvenile retinas. Sequence comparisons with other opsins and phylogenetic analysis show that the five genes belong to the opsins of long-wavelength sensitive (L); middle-wavelength sensitive, M(Cone) and M(Rod); and short-wavelength sensitive, S(Blue) and S(Ultraviolet), respectively. In situ hybridization analysis reveals expression in double cone (L and M(Cone)), single cone (S(Blue) and S(Ultraviolet)), and rod (M(Rod)) types of photoreceptor cells in juvenile halibut retina. The visual system in Atlantic halibut seems therefore to have all four types of cone photoreceptors in addition to rod photoreceptors. This work shows for the first time molecular isolation of a complete set of retinal visual pigment genes from a marine teleost and describes the first cloning of an ultraviolet-sensitive opsin type from a marine teleost.

Topography of different photoreceptor cell types in the larval retina of Atlantic halibut (Hippoglossus hippoglossus)

The identities of single cone cells in the retina of Atlantic halibut (Hippoglossus hippoglossus) larvae were studied by in situ hybridisation using RNA probes for the five different halibut opsins. Four different cone opsins (ultraviolet-, blue-, green- and red-sensitive) are expressed in Atlantic halibut at the end of the yolk-sac period, whereas rod opsin is expressed later in development. Photoreceptor cells expressing ultraviolet-sensitive opsin are found only in the ventral retina, presumably to optimise detection of the downwelling ultraviolet light. The majority of the photoreceptors (approximately 90%) in the retina express green-sensitive opsin and its distribution shows no regional differences. In contrast, blue- and red-sensitive opsins are expressed much less frequently (in approximately 10% of photoreceptors), although these two opsins are also found over the entire retina. The expression patterns of the different visual pigments indicate some form of mosaic expression in the single-coned larval retina, and this is reminiscent of the square mosaic expression found in post-metamorphic Atlantic halibut. These findings suggest plasticity in green-opsin-expressing cells during development, resulting in a square mosaic expression pattern.

Emergence of axonal tracts in the developing brain of the turbot (Psetta maxima)

In this study we have investigated the pattern of morphogenesis and axogenesis in the turbot brain during embryonic and early larval stages with immunohistochemistry using an antibody against acetylated tubulin. The first immunoreactive elements were detected at 74 h post-fertilization in fibers running in the medial and lateral longitudinal fascicles. Newly positive axonal bundles are progressively added during development forming rostrocaudally directed tracts. The tract of the postoptic commissure appears at 86 h post-fertilization located rostrally to the medial longitudinal fascicle. Together, the medial longitudinal fascicle and the tract of the postoptic commissure constitute a major longitudinal axonal pathway, which is extended rostrally in embryos of 98 h post-fertilization by the supraoptic tract. In the forebrain, two vertical tracts, the tract of the posterior commissure (appearing around 98 h post-fertilization) and the tract of the anterior commissure (detected at 110 h post-fertilization) project descending axons to the pre-existing axonal longitudinal pathway. These early tracts are connected by four associated commissures (ventral tegmental, postoptic, posterior and anterior commissure). Some groups of labeled cell bodies are identified either as the origin of the embryonic tracts or contributing axons to the axonal pathways. Additionally, a conspicuous cluster of large cells, not clearly associated with any axonal bundle, was observed from 98 h post-fertilization lining the caudal floor of the presumptive hypothalamus. Several hypotheses are proposed to determine the nature of these cells. A comparison of the emergence of the axonal circuitry in turbot and that of other teleosts reveals significant analogies, suggesting that a common pattern underlies the establishment of the embryonic tracts in this vertebrate group. The minor differences observed between different teleost species, associated with the absence of some axonal fascicles, is also considered.

Role of the pineal organ in the photoregulated hatching of the Atlantic halibut

The timing of hatching in the Atlantic halibut (Hippoglossus hippoglossus) has been suggested to be regulated by environmental light conditions. However, the photosensory organ that perceives the triggering light has not been identified. In the present study, we investigated the morphogenesis of the pineal organ and the neurochemical differentiation of photoreceptors in the pineal organ and the retina of the Atlantic halibut during embryonic development. Immunocytochemical techniques were used for detection of integral protein components of the phototransduction process: opsins, arrestin (S-antigen) and alpha-transducin. We also studied the expression of serotonin (5-HT), a precursor of the neurohormone melatonin known to be synthesized by pineal photoreceptors. In the pineal anlage, opsin immunoreactive (ir) cells appear at 11 days post-fertilization (pf), arrestin, alpha-transducin and serotonin ir cells appear at 14 days pf; hatching took place 15 days pf. The retina contained no immunoreactive cells in embryos or in newly hatched larva. During this period, the pineal anlage is morphologically discernible only as a wedge-shaped region in the diencephalic roof, where elongated cells are aligned with their long axes converging toward a centrally located presumptive pineal lumen. The results show that the pineal photoreceptors contain serotonin and molecules involved in the phototransduction cascade before hatching. We suggest that the pineal organ has the capacity to perceive and mediate photic information before hatching in halibut embryos, and may thereby influence the timing of hatching.

Development of hatchability in halibut (Hippoglossus hippoglossus) embryos

Halibut (Hippoglossus hippoglossus) eggs raised in darkness hatched between days 14.5 and 16 after fertilization. Eggs incubated in white light (3.2 microE/sr1/m2) remained unhatched, so that time of intra ovo development could be doubled. Photo-arrest of hatching was non-diapausal since embryonic growth continued. Transfer of photo-arrested eggs to darkness induced rapid and synchronous hatching. This procedure allowed analysis of development of hatchability. Hatching was not observed prior to day 14. Nonsynchronous hatching over three days was seen when eggs were induced on day 14 + 1 h, or on day 14 + 9 h. However, darkness-induction on day 14 + 22 h produced synchronous hatching within 140 min. This high rate of inducibility persisted until day 18, before declining slowly. Hatching-induction was not observed beyond day 22. Low hatchability in long-term photo-arrested embryos apparently reflects a loss of the anatomical prerequisites for the rim-hatching mechanism. Altered hatchability and morphogenesis after prolonged intra ovo development indicate that hatching in halibut is possible only at an early, defined ontogenetic stage.

Eggshell zona radiata-proteins from cod (Gadus morhua): extra-ovarian origin and induction by estradiol-17 beta

Using Atlantic cod (Gadus morhua) as a model organism, the aim of this report was to delineate whether teleostean eggshell zona radiata proteins have their origin, i.e., site of synthesis, in gonadal or somatic tissues. Estradiol-17 beta was administered intraperitoneally to one-year-old cod (Gadus morhua) with either undeveloped gonads or with differentiated gonads. By immunoblotting procedures estradiol-dependent protein induction was investigated using specific rabbit antisera directed against cod eggshell proteins and brown trout vitellogenin. No immunological cross-reactions were observed between the two antisera, and eggshell proteins and vitellogenin were detected in blood plasma and somatic tissues only in estradiol-treated cod. Three plasma-components were immunoreactive to antiserum directed against eggshell proteins, and these proteins possessed molecular weights of 78, 54 and 47 kDa, identical to the molecular weights of the cod eggshell alpha, beta and gamma zona radiata-proteins. These three immunoreactive plasma-components were observed after administration of estradiol-17 beta to both sexes, also in males having reached spermiation, and in juveniles of either sex without developed gonads. The data are interpreted to signify that cod eggshell zona radiata-proteins originate in an extra-ovarian tissue and are transported in the blood for deposition in the ovaries. We propose that oogenesis involves estradiol-17 beta regulation of both eggshell zona radiata-proteins and vitellogenin synthesis.

Oestradiol-17 beta induces the major vitelline envelope proteins in both sexes in teleosts

During growth of the ovarian follicle, the teleost oocyte becomes surrounded by an acellular coat, the vitelline envelope. The nature, origin and number of the vitelline envelope proteins in fish appear to vary with species. In this work, polyclonal antibodies directed against vitelline envelope proteins from rainbow trout, brown trout and turbot were used to show that oestradiol-17 beta induces the major vitelline envelope proteins in juveniles, both males and females, from different species. The fact that males can synthesize vitelline envelope constituents shows that the origin of these proteins is not confined to the ovary. The vitelline envelope of rainbow trout eggs consists of three major proteins, designated alpha (60 kDa), beta (55 kDa) and gamma (50 kDa). The amino acid composition of each of the three proteins indicated that the three proteins are alike and the suggestion that these proteins represent a separate class of structural proteins is sustained.

The hatching mechanism in Atlantic halibut (Hippoglossus hippoglossus)

In general fish larvae emerge from the protective egg after secreting a hatching enzyme (HE) from diffusely located hatching gland cells (HGCs). This proteolytic enzyme is distributed over the entire inner part of the eggshell (zona radiata). In a marine flatfish halibut, (Hippoglossus hippoglossus), we have found a more specialized hatching process. A strategic location of the HGCs in a narrow belt on the anterior part of the yolk sac leads to restricted degradation of the eggshell resulting in cleavage of the eggshell into two distinct rigid parts. This hatching process--termed "rim-hatching"--results in an empty eggshell with a lid approximately 1/4 the size of the bottom shell. During the hatching process the yolk sac is reshaped. The posterior part of the yolk sac contracts and the yolk mass is squeezed forward before hatching. This mechanism ensures close contact between the HGCs and the eggshell during the release of the hatching enzyme, which is a prerequisite for restricted degradation of the eggshell.

The major structural proteins of cod (Gadus morhua) eggshells and protein crosslinking during teleost egg hardening

The highly hydrophobic protein aggregate which constitutes the fish eggshell has for the first time been quantitatively solubilized. This study shows that the nonactivated eggshell from cod is composed primarily of only three protein monomers, designated alpha (74 kDa) beta (54 kDa) and gamma (47 kDa). Protein extraction studies of the eggshells before and after egg activation demonstrate that egg hardening is accompanied by a 10-fold decline in total protein solubility, which is due to nonextraction of the alpha, beta, and gamma chains. When present during the egg activation process monodansylcadaverine (MDC-a fluorescent lysine analog) inhibits eggshell hardening and at the same time becomes covalently incorporated into the eggshell. This MDC incorporation is calcium-dependent and suggests the induction of a perivitelline transglutaminase activity after egg activation. (Transglutaminases catalyze the formation of an amide bond (isopeptide bond) between the gamma-carbonyl group of glutamine and the epsilon-amino group of lysine with release of ammonia. Crosslinks between proteins are generated when the two amino acid residues are located on different proteins.) Protein solubilization studies and NaDodSO4 gel analysis of the eggshell proteins from eggs subjected to 5 mM MDC during egg activation, reveal that when eggshell hardening is blocked by MDC, the three main eggshell proteins remain extractable even after egg activation. Simultaneously we observed a covalent incorporation of MDC into the gamma protein.