Distribution of gamma-aminobutyric acid immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri): Comparison with other fishes.

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. Immunohistochemical techniques with specific antibodies against GABA or against its synthesizing enzyme, glutamic acid decarboxylase (GAD) allowed characterizing GABAergic neurons and fibers in the CNS. However, studies on the CNS distribution of GABAergic neurons and fibers of bony fishes are scant and were done in teleost species.

Developmental genoarchitectonics as a key tool to interpret the mature anatomy of the chondrichthyan hypothalamus according to the prosomeric model.

The hypothalamus is a key vertebrate brain region involved in survival and physiological functions. Understanding hypothalamic organization and evolution is important to deciphering many aspects of vertebrate biology. Recent comparative studies based on gene expression patterns have proposed the existence of hypothalamic histogenetic domains (paraventricular, TPa/PPa; subparaventricular, TSPa/PSPa; tuberal, Tu/RTu; perimamillary, PM/PRM; and mamillary, MM/RM), revealing conserved evolutionary trends.

Identifying Amygdala-Like Territories in Scyliorhinus canicula (Chondrichthyan): Evidence for a Pallial Amygdala.

To identify the putative amygdalar complex in cartilaginous fishes, our first step was to obtain evidence that supports the existence of a pallial amygdala in the catshark Scyliorhinus canicula, at present the prevailing chondrichthyan model in comparative neurobiology and developmental biology. To this end, we analyzed the organization of the lateral walls of the telencephalic hemispheres of adults, juveniles, and early prehatching embryos by immunohistochemistry against tyrosine hydroxylase (TH), somatostatin (SOM), Pax6, serotonin (5HT), substance P (SP), and Met-enkephalin (MetEnk), calbindin-28k (CB), and calretinin (CR), and by in situ hybridization against regulatory genes such as Tbr1, Lhx9, Emx1, and Dlx2. Our data were integrated with those available from the literature related to the secondary olfactory projections in this shark species.

Differential expression of five prosomatostatin genes in the central nervous system of the catshark Scyliorhinus canicula.

Five prosomatostatin genes (PSST1, PSST2, PSST3, PSST5, and PSST6) have been recently identified in elasmobranchs (Tostivint et al., General and Comparative Endocrinology, 2019, 279, 139-147). In order to gain insight into the contribution of each somatostatin to specific nervous systems circuits and behaviors in this important jawed vertebrate group, we studied the distribution of neurons expressing PSST mRNAs in the central nervous system (CNS) of Scyliorhinus canicula using in situ hybridization.

The Shark Basal Hypothalamus: Molecular Prosomeric Subdivisions and Evolutionary Trends.

The hypothalamus is a key integrative center of the vertebrate brain. To better understand its ancestral morphological organization and evolution, we previously analyzed the segmental organization of alar subdivisions in the catshark , a cartilaginous fish and thus a basal representative of gnathostomes (jawed vertebrates). With the same aim, we deepen here in the segmental organization of the catshark basal hypothalamus by revisiting previous data on expression and Shh immunoreactivity jointly with new data on expression and proliferating cell nuclear antigen (PCNA) immunoreactivity.

Neurogenetic asymmetries in the catshark developing habenulae: mechanistic and evolutionary implications.

Analysis of the establishment of epithalamic asymmetry in two non-conventional model organisms, a cartilaginous fish and a lamprey, has suggested that an essential role of Nodal signalling, likely to be ancestral in vertebrates, may have been largely lost in zebrafish. In order to decipher the cellular mechanisms underlying this divergence, we have characterised neurogenetic asymmetries during habenular development in the catshark Scyliorhinus canicula and addressed the mechanism involved in this process. As in zebrafish, neuronal differentiation starts earlier on the left side in the catshark habenulae, suggesting the conservation of a temporal regulation of neurogenesis.

A Developmental Study of the Cerebellar Nucleus in the Catshark, a Basal Gnathostome.

The output of the cerebellar cortex is mainly released via cerebellar nuclei which vary in number and complexity among gnathostomes, extant vertebrates with a cerebellum. Cartilaginous fishes, a basal gnathostome lineage, show a conspicuous, well-organized cerebellar nucleus, unlike ray-finned fishes. To gain insight into the evolution and development of the cerebellar nucleus, we analyzed in the shark Scyliorhinus canicula (a chondrichthyan model species) the developmental expression of several genes coding for transcription factors (ScLhx5,ScLhx9,ScTbr1, and ScEn2) and the distribution of the protein calbindin, since all appear to be involved in cerebellar nuclei patterning in other gnathostomes.

The Shark Alar Hypothalamus: Molecular Characterization of Prosomeric Subdivisions and Evolutionary Trends.

The hypothalamus is an important physiologic center of the vertebrate brain involved in the elaboration of individual and species survival responses. To better understand the ancestral organization of the alar hypothalamus we revisit previous data on , , , 1 expression and Pax6 immunoreactivity jointly with new data on , , , and expression, in addition to immunoreactivity to serotonin (5-HT) and doublecortin (DCX) in the catshark , a key species for this purpose since cartilaginous fishes are basal representatives of gnathostomes (jawed vertebrates). Our study revealed a complex genoarchitecture for the chondrichthyan alar hypothalamus.

Prosomeric organization of the hypothalamus in an elasmobranch, the catshark Scyliorhinus canicula.

The hypothalamus has been a central topic in neuroanatomy because of its important physiological functions, but its mature organization remains elusive. Deciphering its embryonic and adult organization is crucial in an evolutionary approach of the organization of the vertebrate forebrain. Here we studied the molecular organization of the hypothalamus and neighboring telencephalic domains in a cartilaginous fish, the catshark, Scyliorhinus canicula, focusing on ScFoxg1a, ScShh, ScNkx2.

The ancestral role of nodal signalling in breaking L/R symmetry in the vertebrate forebrain.

Left-right asymmetries in the epithalamic region of the brain are widespread across vertebrates, but their magnitude and laterality varies among species. Whether these differences reflect independent origins of forebrain asymmetries or taxa-specific diversifications of an ancient vertebrate feature remains unknown. Here we show that the catshark Scyliorhinus canicula and the lampreys Petromyzon marinus and Lampetra planeri exhibit conserved molecular asymmetries between the left and right developing habenulae.

Morphogenesis of the cerebellum and cerebellum-related structures in the shark Scyliorhinus canicula: insights on the ground pattern of the cerebellar ontogeny.

Because the cerebellum emerged at the agnathan-gnathostome transition and cartilaginous fishes are at the base of the gnathostome lineage, this group is crucial to determine the basic developmental pattern of the cerebellum and to gain insights into its origin. We have systematically analyzed key events in the development of cerebellum and cerebellum-related structures of the shark Scyliorhinus canicula. Three developmental periods are distinguished based on anatomical observations combined with molecular analysis.

Genoarchitecture of the rostral hindbrain of a shark: basis for understanding the emergence of the cerebellum at the agnathan-gnathostome transition.

The cerebellum is present in all extant gnathostomes or jawed vertebrates, of which cartilaginous fishes represent the most ancient radiation. Since the isthmic organizer induces the formation of the cerebellum, comparative genoarchitectonic analysis on the meso-isthmo-cerebellar region of cartilaginous fishes with respect to that of jawless vertebrates could reveal why the isthmic organizer acquires the ability to induce the formation of the cerebellum in gnathostomes. In the present work we analyzed the expression pattern of a variety of genes related to the cerebellar formation and patterning (ScOtx2, ScGbx2, ScFgf8, ScLmx1b, ScIrx1, ScIrx3, ScEn2, ScPax6 and ScLhx9) by in situ hybridization, and the distribution of Pax6 protein in the developing hindbrain of the shark Scyliorhinus canicula.

Development of the terminal nerve system in the shark Scyliorhinus canicula.

The nervus terminalis (or terminal nerve) system was discovered in an elasmobranch species more than a century ago. Over the past century, it has also been recognized in other vertebrate groups, from agnathans to mammals. However, its origin, functions or relationship with the olfactory system are still under debate.

Tangential migratory pathways of subpallial origin in the embryonic telencephalon of sharks: evolutionary implications.

Tangential neuronal migration occurs along different axes from the axis demarcated by radial glia and it is thought to have evolved as a mechanism to increase the diversity of cell types in brain areas, which in turn resulted in increased complexity of functional networks. In the telencephalon of amniotes, different embryonic tangential pathways have been characterized. However, little is known about the exact routes of migrations in basal vertebrates.

Development of the cerebellar afferent system in the shark Scyliorhinus canicula: insights into the basal organization of precerebellar nuclei in gnathostomes.

The cerebellum is recognized as an evolutionary innovation of jawed vertebrates, whose most primitive group is represented by the chondrichthyans, or cartilaginous fishes. A comprehensive knowledge of cerebellar connections in these fishes might shed light on the basal organization of the cerebellar system. Although the organization of the precerebellar system is known in adults, developmental studies are essential for understanding the origin and evolution of precerebellar nuclei.

Glycine-immunoreactive neurons in the brain of a shark (Scyliorhinus canicula L.).

The glycinergic cell populations in the brain of the lesser spotted dogfish were studied by a glycine immunofluorescence method. Numerous glycine-immunoreactive (Gly-ir) neurons were observed in different brain nuclei. In the telencephalon, Gly-ir cells were observed in the olfactory bulb, telencephalic hemispheres, and preoptic region.

Developmental, tract-tracing and immunohistochemical study of the peripheral olfactory system in a basal vertebrate: insights on Pax6 neurons migrating along the olfactory nerve.

The olfactory system represents an excellent model for studying different aspects of the development of the nervous system ranging from neurogenesis to mechanisms of axon growth and guidance. Important findings in this field come from comparative studies. We have analyzed key events in the development of the olfactory system of the shark Scyliorhinus canicula by combining immunohistochemical and tract-tracing methods.

Contributions of developmental studies in the dogfish Scyliorhinus canicula to the brain anatomy of elasmobranchs: insights on the basal ganglia.

The basic anatomy of the elasmobranch brain has been previously established after studying the organization of the different subdivisions in the adult brain. However, despite the relatively abundant immunohistochemical and hodologic studies performed in different species of sharks and skates, the organization of some brain subdivisions remains unclear. The present study focuses on some brain regions in which subdivisions established on the basis of anatomical data in adults remain controversial, such as the subpallium, mainly the striatal subdivision.

Pax6 expression during retinogenesis in sharks: comparison with markers of cell proliferation and neuronal differentiation.

Pax6 is a highly conserved transcription factor that appears involved in the entire process of retinogenesis, including maintenance of proliferation of retinal progenitors and differentiation of particular neuron fates. To gain insight into the retinogenesis in fish, we study the dynamics of Pax6 expression in the developing and mature retina of two sharks that inhabit in particular environments, and compare it with the dynamics of a marker of cell proliferation (proliferating cell nuclear antigen, PCNA) and markers of neuronal differentiation, such as glutamic acid decarboxylase (GAD), calretinin (CR), tyrosine-hydroxylase, and serotonin (5-HT). Our results reveal that Pax6 is expressed in PCNA-immunoreactive cells within the nonlayered retina, suggesting a role for Pax6 in proliferating progenitors.

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway.

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position.

Development of tyrosine hydroxylase-immunoreactive cell populations and fiber pathways in the brain of the dogfish Scyliorhinus canicula: new perspectives on the evolution of the vertebrate catecholaminergic system.

Developmental studies of the central catecholaminergic (CA) system are essential for understanding its evolution. To obtain knowledge about the CA system in chondrichthyans, an ancient gnathostome group, we used immunohistochemical techniques for detecting tyrosine hydroxylase (TH), the initial rate-limiting enzyme of the CA synthesis, to study: 1) the neuromery of developing TH-immunoreactive (ir) neuronal populations, 2) the development of TH-ir innervation, and 3) the organization of TH-ir cells and fibers in the brain of postembryonic stages of the shark Scyliorhinus canicula. The first TH-ir cells appeared in the hypothalamus and rostral diencephalon (suprachiasmatic, posterior recess and posterior tubercle nuclei at embryonic stage 26, and dorsomedial hypothalamus at stage 28); then in more caudal basal regions of the diencephalon and rostral mesencephalon (substantia nigra/ventral tegmental area); and later on in the anterior (locus coeruleus/nucleus subcoeruleus) and posterior (vagal lobe and reticular formation) rhombencephalon.

Regionalization of the shark hindbrain: a survey of an ancestral organization.

Cartilaginous fishes (chondrichthyans) represent an ancient radiation of vertebrates currently considered the sister group of the group of gnathostomes with a bony skeleton that gave rise to land vertebrates. This out-group position makes chondrichthyans essential in assessing the ancestral organization of the brain of jawed vertebrates. To gain knowledge about hindbrain evolution we have studied its development in a shark, the lesser spotted dogfish Scyliorhinus canicula by analyzing the expression of some developmental genes and the origin and distribution of specific neuronal populations, which may help to identify hindbrain subdivisions and boundaries and the topology of specific cell groups.

Distribution of glycine immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri): comparison with γ-aminobutyric acid.

Glycine and γ-aminobutyric acid (GABA) are the main inhibitory neurotransmitters in the central nervous system (CNS) of vertebrates. Studies on the distribution of glycinergic neurons and fibers have been carried out mainly in rodents and lampreys. With the aim of discovering more about the early evolution of this system in vertebrates, we analyzed the distribution of glycine-immunoreactive (Gly-ir) neurons and fibers in the CNS of a basal ray-finned fish, the Siberian sturgeon (Chondrostei, Acipenseriformes), by use of immunohistochemical techniques.

Calretinin immunoreactivity in the developing retina of sharks: comparison with cell proliferation and GABAergic system markers.

The calcium-binding protein calretinin (CR) has been widely used as a marker of neuronal differentiation. In the present study we analyzed the distribution of CR-immunoreactive (CR-ir) elements in the embryonic and postembryonic retina of two elasmobranchs, the lesser spotted dogfish (Scyliorhinus canicula) and the brown shyshark (Haploblepharus fuscus). We compared the distribution of CR with that of a proliferation marker (the proliferating cell nuclear antigen, PCNA) in order to investigate the time course of CR expression during retinogenesis and explored the relationship between CR and glutamic acid decarboxylase (GAD), the synthesizing enzyme of the gamma-aminobutyric acid (GABA), which has been reported to play a role in shark retinogenesis.