Comparative Analysis of Nkx2.1 and Islet-1 Expression in Urodele Amphibians and Lungfishes Highlights the Pattern of Forebrain Organization in Early Tetrapods

A detailed 3D MRI brain atlas of the African lungfish Protopterus annectens

The study of the brain by magnetic resonance imaging (MRI) in evolutionary analyses is still in its incipient stage, however, it is particularly useful as it allows us to analyze detailed anatomical images and compare brains of rare or otherwise inaccessible species, evolutionarily contextualizing possible differences, while at the same time being non-invasive. A good example is the lungfishes, sarcopterygians that are the closest living relatives of tetrapods and thus have an interesting phylogenetic position in the evolutionary conquest of the terrestrial environment. In the present study, we have developed a three-dimensional representation of the brain of the lungfish Protopterus annectens together with a rostrocaudal anatomical atlas. This methodological approach provides a clear delineation of the major brain subdivisions of this model and allows to measure both brain and ventricular volumes. Our results confirm that lungfish show neuroanatomical patterns reminiscent of those of extant basal sarcopterygians, with an evaginated telencephalon, and distinctive characters like a small optic tectum. These and additional characters uncover lungfish as a remarkable model to understand the origins of tetrapod diversity, indicating that their brain may contain significant clues to the characters of the brain of ancestral tetrapods.

Critical test of the assumption that the hypothalamic entopeduncular nucleus of rodents is homologous with the primate internal pallidum

The globus pallidus (GP) of primates is divided conventionally into distinct internal and external parts. The literature repeats since 1930 the opinion that the homolog of the primate internal pallidum in rodents is the hypothalamic entopeduncular nucleus (embedded within fiber tracts of the cerebral peduncle). To test this idea, we explored its historic fundaments, checked the development and genoarchitecture of mouse entopeduncular and pallidal neurons, and examined relevant comparative connectivity data. We found that the extratelencephalic mouse entopeduncular structure consists of four different components arrayed along a dorsoventral sequence in the alar hypothalamus. The ventral entopeduncular nucleus (EPV), with GABAergic neurons expressing Dlx5&6 and Nkx2-1, lies within the hypothalamic peduncular subparaventricular area. Three other formations-the dorsal entopeduncular nucleus (EPD), the prereticular entopeduncular nucleus (EPPRt ), and the preeminential entopeduncular nucleus (EPPEm )-lie within the overlying paraventricular area, under the subpallium. EPD contains glutamatergic neurons expressing Tbr1, Otp, and Pax6. The EPPRt has GABAergic cells expressing Isl1 and Meis2, whereas the EPPEm population expresses Foxg1 and may be glutamatergic. Genoarchitectonic observations on relevant areas of the mouse pallidal/diagonal subpallium suggest that the GP of rodents is constituted as in primates by two adjacent but molecularly and hodologically differentiable telencephalic portions (both expressing Foxg1). These and other reported data oppose the notion that the rodent extratelencephalic entopeduncular nucleus is homologous to the primate internal pallidum. We suggest instead that all mammals, including rodents, have dual subpallial GP components, whereas primates probably also have a comparable set of hypothalamic entopeduncular nuclei. Remarkably, there is close similarity in some gene expression properties of the telencephalic internal GP and the hypothalamic EPV. This apparently underlies their notable functional analogy, sharing GABAergic neurons and thalamopetal connectivity.

Cell-type profiling in salamanders identifies innovations in vertebrate forebrain evolution

The evolution of advanced cognition in vertebrates is associated with two independent innovations in the forebrain: the six-layered neocortex in mammals and the dorsal ventricular ridge (DVR) in sauropsids (reptiles and birds). How these innovations arose in vertebrate ancestors remains unclear. To reconstruct forebrain evolution in tetrapods, we built a cell-type atlas of the telencephalon of the salamander Pleurodeles waltl. Our molecular, developmental, and connectivity data indicate that parts of the sauropsid DVR trace back to tetrapod ancestors. By contrast, the salamander dorsal pallium is devoid of cellular and molecular characteristics of the mammalian neocortex yet shares similarities with the entorhinal cortex and subiculum. Our findings chart the series of innovations that resulted in the emergence of the mammalian six-layered neocortex and the sauropsid DVR.

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. To shed light on the functional relevance of these histogenetic domains, this work aims to interpret the location of developed cell groups according to the prosomeric model in the hypothalamus of the catshark Scyliorhinus canicula, a representative of Chondrichthyans (the sister group of Osteichthyes, at the base of the gnathostome lineage). To this end, we review in detail the expression patterns of ScOtp, ScDlx2, and ScPitx2, as well as Pax6-immunoreactivity in embryos at stage 32, when the morphology of the adult catshark hypothalamus is already organized. We also propose homologies with mammals when possible. This study provides a comprehensive tool to better understand previous and novel data on hypothalamic development and evolution.

The Everted Amygdala of Ray-Finned Fish: Zebrafish Makes a Case

The amygdala, a complex array of nuclei in the forebrain, controls emotions and emotion-related behaviors in vertebrates. Current research aims to understand the amygdala's evolution in ray-finned fish such as zebrafish because of the region's relevance for social behavior and human psychiatric disorders. Clear-cut molecular definitions of the amygdala and its evolutionary-developmental relationship to the one of mammals are critical for zebrafish models of affective disorders and autism. In this review, I argue that the prosomeric model and a focus on the olfactory system's organization provide ideal tools for discovering deep ancestral relationships between the emotional systems of zebrafish and mammals. The review's focus is on the "extended amygdala," which refers to subpallial amygdaloid territories including the central (autonomic) and the medial (olfactory) amygdala required for reproductive and social behaviors. Amphibians, sauropsids, and lungfish share many characteristics with the basic amygdala ground plan of mammals, as molecular and hodological studies have shown. Further exploration of the evolution of the amygdala in basally derived fish vertebrates requires researchers to test these "tetrapod-based" concepts. Historically, this has been a daunting task because the forebrains of basally derived fish vertebrates look very different from those of more familiar tetrapod ones. An extreme case are ray-finned fish (Actinopterygii) like zebrafish because their telencephalon develops through a distinct outward-growing process called eversion. To this day, scientists have struggled to determine how the everted telencephalon compares to non-actinopterygian vertebrates. Using the teleost zebrafish as a genetic model, comparative neurologists began to establish quantifiable molecular definitions that allow direct comparisons between ray-finned fish and tetrapods. In this review, I discuss how the most recent discovery of the zebrafish amygdala ground plan offers an opportunity to identify the developmental constraints of amygdala evolution and function. In addition, I explain how the zebrafish prethalamic eminence (PThE) topologically relates to the medial amygdala proper and the nucleus of the lateral olfactory tract (nLOT). In fact, I consider these previously misinterpreted olfactory structures the most critical missing evolutionary links between actinopterygian and tetrapod amygdalae. In this context, I will also explain why recognizing both the PThE and the nLOT is crucial to understanding the telencephalon eversion. Recognizing these anatomical hallmarks allows direct comparisons of the amygdalae of zebrafish and mammals. Ultimately, the new concepts of the zebrafish amygdala will overcome current dogmas and reach a holistic understanding of amygdala circuits of cognition and emotion in actinopterygians.

From retina to motoneurons: A substrate for visuomotor transformation in salamanders

The transformation of visual input into motor output is essential to approach a target or avoid a predator. In salamanders, visually guided orientation behaviors have been extensively studied during prey capture. However, the neural circuitry involved is not resolved. Using salamander brain preparations, calcium imaging and tracing experiments, we describe a neural substrate through which retinal input is transformed into spinal motor output. We found that retina stimulation evoked responses in reticulospinal neurons of the middle reticular nucleus, known to control steering movements in salamanders. Microinjection of glutamatergic antagonists in the optic tectum (superior colliculus in mammals) decreased the reticulospinal responses. Using tracing, we found that retina projected to the dorsal layers of the contralateral tectum, where the dendrites of neurons projecting to the middle reticular nucleus were located. In slices, stimulation of the tectal dorsal layers evoked glutamatergic responses in deep tectal neurons retrogradely labeled from the middle reticular nucleus. We then examined how tectum activation translated into spinal motor output. Tectum stimulation evoked motoneuronal responses, which were decreased by microinjections of glutamatergic antagonists in the contralateral middle reticular nucleus. Reticulospinal fibers anterogradely labeled from tracer injection in the middle reticular nucleus were preferentially distributed in proximity with the dendrites of ipsilateral motoneurons. Our work establishes a neural substrate linking visual and motor centers in salamanders. This retino‐tecto‐reticulo‐spinal circuitry is well positioned to control orienting behaviors. Our study bridges the gap between the behavioral studies and the neural mechanisms involved in the transformation of visual input into motor output in salamanders.

Analysis of the Pallial Amygdala in Anurans: Derivatives and Cellular Components

The amygdaloid complex plays a crucial role in socio-emotional conduct, learning, survival, and reproductive behaviors. It is constituted by a set of nuclei presenting a great cellular heterogeneity and embryonic origin diversity (pallial, subpallial, and even extra-telencephalic). In the last two decades, the tetrapartite pallial paradigm defined the pallial portion of the amygdala as a derivative of the lateroventral pallium. However, the pallial conception is currently being reanalyzed and one of these new proposals is to consider the mouse pallial amygdala as a radial histogenetic domain independent from the rest of the pallial subdomains. In anamniotes, and particularly in amphibian anurans, the amygdaloid complex was described as a region with pallial and subpallial components similar to those described in amniotes. In the present study carried out in Xenopus laevis, after a detailed analysis of the orientation of the amygdalar radial glia, we propose an additional amygdala derived from the pallial region. It is independent of the vomeronasal/olfactory amygdaloid nuclei described in anurans, expresses markers such as Lhx9 present in the mammalian pallial amygdala, and lacks Otp-expressing cells, detected in the adjacent medial amygdala. Further studies are needed to clarify the functional involvement of this area, and whether it is a derivative of the adjacent ventral pallium or an independent pallial domain.

Developmental-Based Classification of Enkephalin and Somatostatin Containing Neurons of the Chicken Central Extended Amygdala

The central extended amygdala, including the lateral bed nucleus of the stria terminalis and the central amygdala, plays a key role in stress response. To understand how the central extended amygdala regulates stress it is essential to dissect this structure at molecular, cellular and circuit levels. In mammals, the central amygdala contains two distinct cell populations that become active (on cells) or inactive (off cells) during the conditioned fear response. These two cell types inhibit each other and project mainly unidirectionally to output cells, thus providing a sophisticated regulation of stress. These two cell types express either protein kinase C-delta/enkephalin or somatostatin, and were suggested to originate in different embryonic domains of the subpallium that respectively express the transcription factors Pax6 or Nkx2.1 during development. The regulation of the stress response by the central extended amygdala is poorly studied in non-mammals. Using an evolutionary developmental neurobiology approach, we previously identified several subdivisions in the central extended amygdala of chicken. These contain Pax6, Islet1 and Nkx2.1 cells that originate in dorsal striatal, ventral striatal or pallidopreoptic embryonic divisions, and also contain neurons expressing enkephalin and somatostatin. To know the origin of these cells, in this study we carried out multiple fluorescent labeling to analyze coexpression of different transcription factors with enkephalin or somatostatin. We found that many enkephalin cells coexpress Pax6 and likely derive from the dorsal striatal division, resembling the off cells of the mouse central amygdala. In contrast, most somatostatin cells coexpress Nkx2.1 and derive from the pallidal division, resembling the on cells. We also found coexpression of enkephalin and somatostatin with other transcription factors. Our results show the existence of multiple cell types in the central extended amygdala of chicken, perhaps including on/off cell systems, and set the basis for studying the role of these cells in stress regulation.

Integrated Behavioral, Genetic and Brain Circuit Visualization Methods to Unravel Functional Anatomy of Zebrafish Amygdala

The mammalian amygdala is a complex forebrain structure consisting of a heterogeneous group of nuclei derived from the pallial and subpallial telencephalon. It plays a critical role in a broad range of behaviors such as emotion, cognition, and social behavior; within the amygdala each nucleus has a distinct role in these behavioral processes. Topological, hodological, molecular, and functional studies suggest the presence of an amygdala-like structure in the zebrafish brain. It has been suggested that the pallial amygdala homolog corresponds to the medial zone of the dorsal telencephalon (Dm) and the subpallial amygdala homolog corresponds to the nuclei in the ventral telencephalon located close to and topographically basal to Dm. However, these brain regions are broad and understanding the functional anatomy of the zebrafish amygdala requires investigating the role of specific populations of neurons in brain function and behavior. In zebrafish, the highly efficient Tol2 transposon-mediated transgenesis method together with the targeted gene expression by the Gal4-UAS system has been a powerful tool in labeling, visualizing, and manipulating the function of specific cell types in the brain. The transgenic resource combined with neuronal activity imaging, optogenetics, pharmacology, and quantitative behavioral analyses enables functional analyses of neuronal circuits. Here, we review earlier studies focused on teleost amygdala anatomy and function and discuss how the transgenic resource and tools can help unravel the functional anatomy of the zebrafish amygdala.

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. We have characterized two possible amygdalar territories. One, which may represent a ventropallial component, was identified by its chemical signature (moderate density of Pax6-ir cells, scarce TH-ir and SOM-ir cells, and absence of CR-ir and CB-ir cells) and gene expressions (Tbr1 and Lhx9 expressions in an Emx1 negative domain, as the ventral pallium of amniotes). It is perhaps comparable to the lateral amygdala of amphibians and the pallial amygdala of teleosts. The second was a territory related to the pallial-subpallial boundary with abundant Pax6-ir and CR-ir cells, and 5HT-ir, SP-ir, and MetEnk-ir fibers capping dorsally the area superficialis basalis. This olfactory-related region at the neighborhood of the pallial-subpallial boundary may represent a subpallial amygdala subdivision that possibly contains migrated cells of ventropallial origin.

Analysis of Islet-1, Nkx2.1, Pax6, and Orthopedia in the forebrain of the sturgeon Acipenser ruthenus identifies conserved prosomeric characteristics

The distribution patterns of a set of conserved brain developmental regulatory transcription factors were analyzed in the forebrain of the basal actinopterygian fish Acipenser ruthenus, consistent with the prosomeric model. In the telencephalon, the pallium was characterized by ventricular expression of Pax6. In the subpallium, the combined expression of Nkx2.1/Islet-1 (Isl1) allowed to propose ventral and dorsal areas, as the septo-pallidal (Nkx2.1/Isl1+) and striatal derivatives (Isl1+), respectively, and a dorsal portion of the striatal derivatives, ventricularly rich in Pax6 and devoid of Isl1 expression. Dispersed Orthopedia (Otp) cells were found in the supracommissural and posterior nuclei of the ventral telencephalon, related to the medial portion of the amygdaloid complex. The preoptic area was identified by the Nkx2.1/Isl1 expression. In the alar hypothalamus, an Otp-expressing territory, lacking Nkx2.1/Isl1, was identified as the paraventricular domain. The adjacent subparaventricular domain (Spa) was subdivided in a rostral territory expressing Nkx2.1 and an Isl1+ caudal one. In the basal hypothalamus, the tuberal region was defined by the Nkx2.1/Isl1 expression and a rostral Otp-expressing domain was identified. Moreover, the Otp/Nkx2.1 combination showed an additional zone lacking Isl1, tentatively identified as the mamillary area. In the diencephalon, both Pax6 and Isl1 defined the prethalamic domain, and within the basal prosomere 3, scattered Pax6- and Isl1-expressing cells were observed in the posterior tubercle. Finally, a small group of Pax6 cells was observed in the pretectal area. These results improve the understanding of the forebrain evolution and demonstrate that its basic bauplan is present very early in the vertebrate lineage.

The serotonin reuptake blocker citalopram destabilizes fictive locomotor activity in salamander axial circuits through 5-HT1A receptors

Serotoninergic (5-HT) neurons are powerful modulators of spinal locomotor circuits. Most studies on 5-HT modulation focused on the effect of exogenous 5-HT and these studies provided key information about the cellular mechanisms involved. Less is known about the effects of increased release of endogenous 5-HT with selective serotonin reuptake inhibitors. In mammals, such molecules were shown to destabilize the fictive locomotor output of spinal limb networks through 5-HT_1A receptors. However, in tetrapods little is known about the effects of increased 5-HT release on the locomotor output of axial networks, which are coordinated with limb circuits during locomotion from basal vertebrates to mammals. Here, we examined the effect of citalopram on fictive locomotion generated in axial segments of isolated spinal cords in salamanders, a tetrapod where raphe 5-HT reticulospinal neurons and intraspinal 5-HT neurons are present as in other vertebrates. Using electrophysiological recordings of ventral roots, we show that fictive locomotion generated by bath-applied glutamatergic agonists is destabilized by citalopram. Citalopram-induced destabilization was prevented by a 5-HT_1A receptor antagonist, whereas a 5-HT_1A receptor agonist destabilized fictive locomotion. Using immunofluorescence experiments, we found 5-HT-positive fibers and varicosities in proximity with motoneurons and glutamatergic interneurons that are likely involved in rhythmogenesis. Our results show that increasing 5-HT release has a deleterious effect on axial locomotor activity through 5-HT_1A receptors. This is consistent with studies in limb networks of turtle and mouse, suggesting that this part of the complex 5-HT modulation of spinal locomotor circuits is common to limb and axial networks in limbed vertebrates.NEW & NOTEWORTHY Little is known about the modulation exerted by endogenous serotonin on axial locomotor circuits in tetrapods. Using axial ventral root recordings in salamanders, we found that a serotonin reuptake blocker destabilized fictive locomotor activity through 5-HT_1A receptors. Our anatomical results suggest that serotonin is released on motoneurons and glutamatergic interneurons possibly involved in rhythmogenesis. Our study suggests that common serotoninergic mechanisms modulate axial motor circuits in amphibians and limb motor circuits in reptiles and mammals.

Sonic hedgehog expression in zebrafish forebrain identifies the teleostean pallidal signaling center and shows preglomerular complex and posterior tubercular dopamine cells to arise from shh cells

Ventralization, a major patterning process in the developing vertebrate neural tube (central nervous system, CNS), depends on Sonic hedgehog (SHH) as a main signaling morphogen. We studied the CNS of late larval and young adult zebrafish in a transgenic shh-GFP line revealing increased neuroanatomical detail due to the progressed differentiation state compared to earlier stages. Some major findings emerge from the present study. (a) shh -GFP is still expressed along the adult zebrafish CNS neuraxis in most locations seen in larvae. (b) We newly identify a ventroposterior shh pallidal domain representing the basal telencephalic signaling center important for basal ganglia development known in other vertebrates (i.e., the anterior entopeduncular area-basal medial ganglionic eminence of mammals). (c) We further show late-emerging shh-GFP positive radial glia cells in the medial zone of the dorsal telencephalon (i.e., the teleostan pallial amygdala). (d) Immunostains for tyrosine hydroxylase demonstrate that there is selective colocalization in adult dopamine cells with shh-GFP in the posterior tuberculum, including in projection cells to striatum, which represents a striking parallel to amniote mesodiencephalic dopamine cell origin from shh expressing floor plate cells. (e) There is no colocalization of shh and islet1 as shown by respective shh-GFP and islet1-GFP lines. (f) The only radially far migrated shh-GFP cells are located in the preglomerular area. (g) There are no adult cerebellar and tectal shh-GFP cells confirming their exclusive role during early development as previously reported by our laboratory.

Comparative Analysis of the Organization of the Catecholaminergic Systems in the Brain of Holostean Fishes (Actinopterygii/Neopterygii)

Living holosteans, comprising 8 species of bowfins and gars, form a small monophyletic group of actinopterygian fishes, which are currently considered as the sister group to the enormously numerous teleosts and have largely been neglected in neuroanatomical studies. We have studied the catecholaminergic (CAergic) systems by means of antibodies against tyrosine hydroxylase (TH) and dopamine (DA) in the brain of representative species of the 3 genera included in the 2 orders of holostean fishes: Amia calva (Amiiformes) and Lepisosteus platyrhincus, Lepisosteus oculatus, and Atractosteus spatula (Lepisosteiformes). Different groups of TH/DA-immunoreactive (ir) cells were observed in the olfactory bulb, subpallium, and preoptic area of the telencephalon. Hypothalamic groups were labeled in the suprachiasmatic nucleus, tuberal (only in A. calva), retrotuberal, and retromamillary areas; specifically, the paraventricular organ showed only DA immunoreactivity. In the diencephalon, TH/DA-ir groups were detected in the prethalamus, posterior tubercle, and pretectum. In the caudal hindbrain, the solitary tract nucleus and area postrema presented TH/DA-ir cell groups, and also the spinal cord and the retina. Only in A. calva, particular CAergic cell groups were observed in the habenula, the mesencephalic tegmentum, and in the locus coeruleus. Following a neuromeric analysis, the comparison of these results with those obtained in other classes of fishes and tetrapods shows many common traits of CAergic systems shared by most vertebrates and in addition highlights unique features of actinopterygian fishes.

Pattern of nitrergic cells and fibers organization in the central nervous system of the Australian lungfish, Neoceratodus forsteri (Sarcopterygii: Dipnoi)

The Australian lungfish Neoceratodus forsteri is the only extant species of the order Ceratodontiformes, which retained most of the primitive features of ancient lobe finned-fishes. Lungfishes are the closest living relatives of land vertebrates and their study is important for deducing the neural traits that were conserved, modified, or lost with the transition from fishes to land vertebrates. We have investigated the nitrergic system with neural nitric oxide synthase (NOS) immunohistochemistry and NADPH-diaphorase (NADPH-d) histochemistry, which yielded almost identical results except for the primary olfactory projections and the terminal and preoptic nerve fibers labeled only for NADPH-d. Combined immunohistochemistry was used for simultaneous detection of NOS with catecholaminergic, cholinergic, and serotonergic structures, aiming to establish accurately the localization of the nitrergic elements and to assess possible interactions between these neurotransmitter systems. The results demonstrated abundant nitrergic cells in the basal ganglia, amygdaloid complex, preoptic area, basal hypothalamus, mesencephalic tectum and tegmentum, laterodorsal tegmental nucleus, reticular formation, spinal cord, and retina. In addition, low numbers of nitrergic cells were observed in the olfactory bulb, all pallial divisions, lateral septum, suprachiasmatic nucleus, prethalamic and thalamic areas, posterior tubercle, pretectum, torus semicircularis, cerebellar nucleus, interpeduncular nucleus, the medial octavolateral nucleus, nucleus of the solitary tract, and the dorsal column nucleus. Colocalization of NOS and tyrosine hydroxylase was observed in numerous cells of the ventral tegmental area/substantia nigra complex. Comparison with other vertebrates, using a neuromeric analysis, reveals that the nitrergic system of Neoceratodus shares many neuroanatomical features with tetrapods and particularly with amphibians.

Pax6 expression highlights regional organization in the adult brain of lungfishes, the closest living relatives of land vertebrates

The Pax6 gene encodes a regulatory transcription factor that is key in brain development. The molecular structure of Pax6, the roles it plays and its patterns of expression in the brain have been highly conserved during vertebrate evolution. As neurodevelopment proceeds, the Pax6 expression changes from the mitotic germinal zone in the ventricular zone to become distributed in cell groups in the adult brain. Studies in various vertebrates, from fish to mammals, found that the Pax6 expression is maintained in adults in most regions that express it during development. Specifically, in amphibians, Pax6 is widely expressed in the adult brain and its distribution pattern serves to highlight regional organization of the brain. In the present study, we analyzed the detailed distribution of Pax6 cells in the adult central nervous system of lungfishes, the closest living relatives of all tetrapods. Immunohistochemistry performed using double labeling techniques with several neuronal markers of known distribution patterns served to evaluate the actual location of Pax6 cells. Our results show that the Pax6 expression is maintained in the adult brain of lungfishes, in distinct regions of the telencephalon (pallium and subpallium), diencephalon, mesencephalon, hindbrain, spinal cord, and retina. The pattern of Pax6 expression is largely shared with amphibians and helps to understand the primitive condition that would have characterized the common ancestors to all sarcopterygians (lobe-finned fishes and tetrapods), in which Pax6 would be needed to maintain specific entities of subpopulations of neurons.

Evolution of the Chordate Telencephalon

The dramatic evolutionary expansion of the neocortex, together with a proliferation of specialized cortical areas, is believed to underlie the emergence of human cognitive abilities. In a broader phylogenetic context, however, neocortex evolution in mammals, including humans, is remarkably conservative, characterized largely by size variations on a shared six-layered neuronal architecture. By contrast, the telencephalon in non-mammalian vertebrates, including reptiles, amphibians, bony and cartilaginous fishes, and cyclostomes, features a great variety of very different tissue structures. Our understanding of the evolutionary relationships of these telencephalic structures, especially those of basally branching vertebrates and invertebrate chordates, remains fragmentary and is impeded by conceptual obstacles. To make sense of highly divergent anatomies requires a hierarchical view of biological organization, one that permits the recognition of homologies at multiple levels beyond neuroanatomical structure. Here we review the origin and diversification of the telencephalon with a focus on key evolutionary innovations shaping the neocortex at multiple levels of organization.

Adult islet1 Expression Outlines Ventralized Derivatives Along Zebrafish Neuraxis

Signals issued by dorsal roof and ventral floor plates, respectively, underlie the major patterning process of dorsalization and ventralization during vertebrate neural tube development. The ventrally produced morphogen Sonic hedgehog (SHH) is crucial for vertebrate hindbrain and spinal motor neuron development. One diagnostic gene for motor neurons is the LIM/homeodomain gene islet1, which has additional ventral expression domains extending into mid- and forebrain. In order to corroborate motor neuron development and, in particular, to improve on the identification of poorly documented zebrafish forebrain islet1 populations, we studied adult brains of transgenic islet1-GFP zebrafish (3 and 6 months). This molecular neuroanatomical analysis was supported by immunostaining these brains for tyrosine hydroxylase (TH) or choline acetyltransferase (ChAT), respectively, revealing zebrafish catecholaminergic and cholinergic neurons. The present analysis of ChAT and islet1-GFP label confirms ongoing adult expression of islet1 in zebrafish (basal plate) midbrain, hindbrain, and spinal motor neurons. In contrast, non-motor cholinergic systems lack islet1 expression. Additional presumed basal plate islet1 positive systems are described in detail, aided by TH staining which is particularly informative in the diencephalon. Finally, alar plate zebrafish forebrain systems with islet1 expression are described (i.e., thalamus, preoptic region, and subpallium). We conclude that adult zebrafish continue to express islet1 in the same brain systems as in the larva. Further, pending functional confirmation we hypothesize that the larval expression of sonic hedgehog (shh) might causally underlie much of adult islet1 expression because it explains findings beyond ventrally located systems, for example regarding shh expression in the zona limitans intrathalamica and correlated islet1-GFP expression in the thalamus.