Evolutionary divergence of the reptilian and the mammalian brains: considerations on connectivity and development

Glutamatergic pathways in the brains of turtles: A comparative perspective among reptiles, birds, and mammals

Glutamate acts as the main excitatory neurotransmitter in the brain and plays a vital role in physiological and pathological neuronal functions. In mammals, glutamate can cause detrimental excitotoxic effects under anoxic conditions. In contrast, Trachemys scripta, a freshwater turtle, is one of the most anoxia-tolerant animals, being able to survive up to months without oxygen. Therefore, turtles have been investigated to assess the molecular mechanisms of neuroprotective strategies used by them in anoxic conditions, such as maintaining low levels of glutamate, increasing adenosine and GABA, upregulating heat shock proteins, and downregulating KATP channels. These mechanisms of anoxia tolerance of the turtle brain may be applied to finding therapeutics for human glutamatergic neurological disorders such as brain injury or cerebral stroke due to ischemia. Despite the importance of glutamate as a neurotransmitter and of the turtle as an ideal research model, the glutamatergic circuits in the turtle brain remain less described whereas they have been well studied in mammalian and avian brains. In reptiles, particularly in the turtle brain, glutamatergic neurons have been identified by examining the expression of vesicular glutamate transporters (VGLUTs). In certain areas of the brain, some ionotropic glutamate receptors (GluRs) have been immunohistochemically studied, implying that there are glutamatergic target areas. Based on the expression patterns of these glutamate-related molecules and fiber connection data of the turtle brain that is available in the literature, many candidate glutamatergic circuits could be clarified, such as the olfactory circuit, hippocampal–septal pathway, corticostriatal pathway, visual pathway, auditory pathway, and granule cell–Purkinje cell pathway. This review summarizes the probable glutamatergic pathways and the distribution of glutamatergic neurons in the pallium of the turtle brain and compares them with those of avian and mammalian brains. The integrated knowledge of glutamatergic pathways serves as the fundamental basis for further functional studies in the turtle brain, which would provide insights on physiological and pathological mechanisms of glutamate regulation as well as neural circuits in different species.

Signaler's Vasotocin Alters the Relationship between the Responder's Forebrain Catecholamines and Communication Behavior in Lizards (Anolis carolinensis)

Dynamic fluctuations in the distribution of catecholamines across the brain modulate the responsiveness of vertebrates to social stimuli. Previous work demonstrates that green anoles (Anolis carolinensis) increase chemosensory behavior in response to males treated with exogenous arginine vasotocin (AVT), but the neurochemical mechanisms underlying this behavioral shift remains unclear. Since central catecholamine systems, including dopamine, rapidly activate in response to social stimuli, we tested whether exogenous AVT in signalers (stimulus animals) impacts catecholamine concentrations in the forebrain (where olfactory and visual information are integrated and processed) of untreated lizard responders. We also tested whether AVT influences the relationship between forebrain catecholamine concentrations and communication behavior in untreated receivers. We measured global catecholamine (dopamine = DA, epinephrine = Epi, and norepinephrine = NE) concentrations in the forebrain of untreated responders using high-performance liquid chromatography-mass spectrometry following either a 30-min social interaction with a stimulus male or a period of social isolation. Stimulus males were injected with exogenous AVT or vehicle saline (SAL). We found that global DA, but not Epi or NE, concentrations were elevated in lizards responding to SAL-males relative to isolated lizards. Lizards interacting with AVT-males had DA, Epi and NE concentrations that were not significantly different from SAL or isolated groups. For behavior, we found a significant effect of social treatment (AVT vs. SAL) on the relationships between (1) DA concentrations and the motivation to perform a chemical display (latency to tongue flick) and (2) Epi concentrations and time spent displaying mostly green body coloration. We also found a significant negative correlation between DA concentrations and the latency to perform a visual display but found no effect of social treatment on this relationship. These data suggest that catecholamine concentrations in the forebrain of untreated responders are associated with chemical and visual communication in lizards and that signaler AVT alters this relationship for some, but not all, aspects of social communication.

Evolution of behavioural control from chordates to primates

This article outlines a hypothetical sequence of evolutionary innovations, along the lineage that produced humans, which extended behavioural control from simple feedback loops to sophisticated control of diverse species-typical actions. I begin with basic feedback mechanisms of ancient mobile animals and follow the major niche transitions from aquatic to terrestrial life, the retreat into nocturnality in early mammals, the transition to arboreal life and the return to diurnality. Along the way, I propose a sequence of elaboration and diversification of the behavioural repertoire and associated neuroanatomical substrates. This includes midbrain control of approach versus escape actions, telencephalic control of local versus long-range foraging, detection of affordances by the dorsal pallium, diversified control of nocturnal foraging in the mammalian neocortex and expansion of primate frontal, temporal and parietal cortex to support a wide variety of primate-specific behavioural strategies. The result is a proposed functional architecture consisting of parallel control systems, each dedicated to specifying the affordances for guiding particular species-typical actions, which compete against each other through a hierarchy of selection mechanisms. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

Dopamine Modulation of Motor and Sensory Cortical Plasticity among Vertebrates

Goal-directed learning is a key contributor to evolutionary fitness in animals. The neural mechanisms that mediate learning often involve the neuromodulator dopamine. In higher order cortical regions, most of what is known about dopamine's role is derived from brain regions involved in motivation and decision-making, while significantly less is known about dopamine's potential role in motor and/or sensory brain regions to guide performance. Research on rodents and primates represents over 95% of publications in the field, while little beyond basic anatomy is known in other vertebrate groups. This significantly limits our general understanding of how dopamine signaling systems have evolved as organisms adapt to their environments. This review takes a pan-vertebrate view of the literature on the role of dopamine in motor/sensory cortical regions, highlighting, when available, research on non-mammalian vertebrates. We provide a broad perspective on dopamine function and emphasize that dopamine-induced plasticity mechanisms are widespread across all cortical systems and associated with motor and sensory adaptations. The available evidence illustrates that there is a strong anatomical basis-dopamine fibers and receptor distributions-to hypothesize that pallial dopamine effects are widespread among vertebrates. Continued research progress in non-mammalian species will be crucial to further our understanding of how the dopamine system evolved to shape the diverse array of brain structures and behaviors among the vertebrate lineage.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

The Neurological Asymmetry of Self-Face Recognition

While the desire to uncover the neural correlates of consciousness has taken numerous directions, self-face recognition has been a constant in attempts to isolate aspects of self-awareness. The neuroimaging revolution of the 1990s brought about systematic attempts to isolate the underlying neural basis of self-face recognition. These studies, including some of the first fMRI (functional magnetic resonance imaging) examinations, revealed a right-hemisphere bias for self-face recognition in a diverse set of regions including the insula, the dorsal frontal lobe, the temporal parietal junction, and the medial temporal cortex. In this systematic review, we provide confirmation of these data (which are correlational) which were provided by TMS (transcranial magnetic stimulation) and patients in which direct inhibition or ablation of right-hemisphere regions leads to a disruption or absence of self-face recognition. These data are consistent with a number of theories including a right-hemisphere dominance for self-awareness and/or a right-hemisphere specialization for identifying significant social relationships, including to oneself.

Evolutionary Origin of Distinct NREM and REM Sleep

Sleep is mandatory in most animals that have the nervous system and is universally observed in model organisms ranging from the nematodes, zebrafish, to mammals. However, it is unclear whether different sleep states fulfill common functions and are driven by shared mechanisms in these different animal species. Mammals and birds exhibit two obviously distinct states of sleep, i.e., non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep, but it is unknown why sleep should be so segregated. Studying sleep in other animal models might give us clues that help solve this puzzle. Recent studies suggest that REM sleep, or ancestral forms of REM sleep might be found in non-mammalian or -avian species such as reptiles. These observations suggest that REM sleep and NREM sleep evolved earlier than previously thought. In this review, we discuss the evolutionary origin of the distinct REM/NREM sleep states to gain insight into the mechanistic and functional reason for these two different types of sleep.

Patterns of c-Fos expression in telencephalic areas of Tropidurus hygomi (Iguania: Tropiduridae) exposed to different social contexts

Social behavior in lizards contributes to understanding biological standards and provides models for structuring research about neural mechanisms. Studies have confirmed the effectiveness of comparative models and evidence has contributed to clarifying adult brain plasticity phenomenon when exposed to different stimuli. The expression of c-Fos has been widely used to identify brain areas involved in different behavioral stimuli. The purpose of the present study was to map the expression of c-Fos protein in different telencephalic areas of the lizard Tropidurus hygomi after they were exposed to visual stimuli with another individual of the same species in different social contexts. Lizards were allocated to one of four groups: 1) control group (CTL) - males not exposed to any other animal; 2) exposure to juvenile (EJU) - males exposed to a juvenile; 3) exposure to male (EMA) - males exposed to another adult male; and 4) exposure to females (EFE) -males exposed to female. The EFE group exhibited a greater number of c-Fos + cells in cortical areas (medial cortex - MC and dorsomedial cortex - DMC) and in amygdala (AMY), showing a possible relationship between these structures and behavioral components. Studies like this can contribute significantly to a better understanding of neurophysiological, behavioral, and evolutive aspects.

Toward an Integrative Theory of Thalamic Function

The thalamus has long been suspected to have an important role in cognition, yet recent theories have favored a more corticocentric view. According to this view, the thalamus is an excitatory feedforward relay to or between cortical regions, and cognitively relevant computations are exclusively cortical. Here, we review anatomical, physiological, and behavioral studies along evolutionary and theoretical dimensions, arguing for essential and unique thalamic computations in cognition. Considering their architectural features as well as their ability to initiate, sustain, and switch cortical activity, thalamic circuits appear uniquely suited for computing contextual signals that rapidly reconfigure task-relevant cortical representations. We introduce a framework that formalizes this notion, show its consistency with several findings, and discuss its prediction of thalamic roles in perceptual inference and behavioral flexibility. Overall, our framework emphasizes an expanded view of the thalamus in cognitive computations and provides a roadmap to test several of its theoretical and experimental predictions.

Evidence for convergent evolution of a neocortex-like structure in a late Permian therapsid

The special sensory, motor, and cognitive capabilities of mammals mainly depend upon the neocortex, which is the six-layered cover of the mammalian forebrain. The origin of the neocortex is still controversial and the current view is that larger brains with neocortex first evolved in late Triassic Mammaliaformes. Here, we report the earliest evidence of a structure analogous to the mammalian neocortex in a forerunner of mammals, the fossorial anomodont Kawingasaurus fossilis from the late Permian of Tanzania. The endocranial cavity of Kawingasaurus is almost completely ossified, which allowed a less hypothetical virtual reconstruction of the brain endocast to be generated. A parietal foramen is absent. A small pit between the cerebral hemispheres is interpreted as a pineal body. The inflated cerebral hemispheres are demarcated from each other by a median sulcus and by a possible rhinal fissure from the rest of the endocast. The encephalization quotient estimated by using the method of Eisenberg is 0.52, which is 2-3 times larger than in other nonmammalian synapsids. Another remarkable feature are the extremely ramified infraorbital canals in the snout. The shape of the brain endocast, the extremely ramified maxillary canals as well as the small frontally placed eyes suggest that special sensory adaptations to the subterranean habitat such as a well developed sense of touch and binocular vision may have driven the parallel evolution of an equivalent of the mammalian neocortex and a mammal-like lemnothalamic visual system in Kawingasaurus. The gross anatomy of the brain endocast of Kawingasaurus supports the Outgroup Hypothesis, according to which the neocortex evolved from the dorsal pallium of an amphibian-like ancestor, which receives sensory projections from the lemnothalamic pathway. The enlarged brain as well as the absence of a parietal foramen may be an indication for a higher metabolic rate of Kawingasaurus compared to other nonmammalian synapsids.

Development and Organization of the Evolutionarily Conserved Three-Layered Olfactory Cortex

The olfactory cortex is part of the mammalian cerebral cortex together with the neocortex and the hippocampus. It receives direct input from the olfactory bulbs and participates in odor discrimination, association, and learning (Bekkers and Suzuki, 2013). It is thought to be an evolutionarily conserved paleocortex, which shares common characteristics with the three-layered general cortex of reptiles (Aboitiz et al., 2002). The olfactory cortex has been studied as a “simple model” to address sensory processing, though little is known about its precise cell origin, diversity, and identity. While the development and the cellular diversity of the six-layered neocortex are increasingly understood, the olfactory cortex remains poorly documented in these aspects. Here is a review of current knowledge of the development and organization of the olfactory cortex, keeping the analogy with those of the neocortex. The comparison of olfactory cortex and neocortex will allow the opening of evolutionary perspectives on cortical development.

Molecular evolution of WDR62, a gene that regulates neocorticogenesis

Human brain evolution is characterized by dramatic expansion in cerebral cortex size. WDR62 (WD repeat domain 62) is one of the important gene in controlling human cortical development. Mutations in WDR62 lead to primary microcephaly, a neurodevelopmental disease characterized by three to four fold reduction in cerebral cortex size of affected individuals. This study analyzes comparative protein evolutionary rate to provide a useful insight into the molecular evolution of WDR62 and hence pinpointed human specific amino acid replacements. Comparative analysis of human WDR62 with two archaic humans (Neanderthals and Denisovans) and modern human populations revealed that five hominin specific amino acid residues (human specific amino acids shared with two archaic humans) might have been accumulated in the common ancestor of extinct archaic humans and modern humans about 550,000–765,000 years ago. Collectively, the data demonstrates an acceleration of WDR62 sequence evolution in hominin lineage and suggests that the ability of WDR62 protein to mediate the neurogenesis has been altered in the course of hominin evolution.

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We trace the evolutionary history of WDR62 and its putative paralogs.

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We identify accelerated sequence evolution in human WDR62.

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We pinpoint eight human specific amino acid sites that reside on the C-terminal.

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Out of eight, six sites are shared with archaic humans.

From sauropsids to mammals and back: New approaches to comparative cortical development

Evolution of the mammalian neocortex (isocortex) has been a persisting problem in neurobiology. While recent studies have attempted to understand the evolutionary expansion of the human neocortex from rodents, similar approaches have been used to study the changes between reptiles, birds, and mammals. We review here findings from the past decades on the development, organization, and gene expression patterns in various extant species. This review aims to compare cortical cell numbers and neuronal cell types to the elaboration of progenitor populations and their proliferation in these species. Several progenitors, such as the ventricular radial glia, the subventricular intermediate progenitors, and the subventricular (outer) radial glia, have been identified but the contribution of each to cortical layers and cell types through specific lineages, their possible roles in determining brain size or cortical folding, are not yet understood. Across species, larger, more diverse progenitors relate to cortical size and cell diversity. The challenge is to relate the radial and tangential expansion of the neocortex to the changes in the proliferative compartments during mammalian evolution and with the changes in gene expression and lineages evident in various sectors of the developing brain. We also review the use of recent lineage tracing and transcriptomic approaches to revisit theories and to provide novel understanding of molecular processes involved in specification of cortical regions.

Pallial patterning and the origin of the isocortex

Together with a complex variety of behavioral, physiological, morphological, and neurobiological innovations, mammals are characterized by the development of an extensive isocortex (also called neocortex) that is both laminated and radially organized, as opposed to the brain of birds and reptiles. In this article, we will advance a developmental hypothesis in which the mechanisms of evolutionary brain growth remain partly conserved across amniotes (mammals, reptiles and birds), all based on Pax6 signaling or related morphogens. Despite this conservatism, only in mammals there is an additional upregulation of dorsal and anterior signaling centers (the cortical hem and the anterior forebrain, respectively) that promoted a laminar and a columnar structure into the neocortex. It is possible that independently, some birds also developed an upregulated dorsal pallium.

A common developmental plan for neocortical gene-expressing neurons in the pallium of the domestic chicken Gallus gallus domesticus and the Chinese softshell turtle Pelodiscus sinensis

The six-layered neocortex is a unique characteristic of mammals and likely provides the neural basis of their sophisticated cognitive abilities. Although all mammalian species share the layered structure of the neocortex, the sauropsids exhibit an entirely different cytoarchitecture of the corresponding pallial region. Our previous gene expression study revealed that the chicken pallium possesses neural subtypes that express orthologs of layer-specific genes of the mammalian neocortex. To understand the evolutionary steps leading toward animal group-specific neuronal arrangements in the pallium in the course of amniote diversification, we examined expression patterns of the same orthologs and a few additional genes in the pallial development of the Chinese softshell turtle Pelodiscus sinensis, and compared these patterns to those of the chicken. Our analyses highlighted similarities in neuronal arrangements between the two species; the mammalian layer 5 marker orthologs are expressed in the medial domain and the layer 2/3 marker orthologs are expressed in the lateral domain in the pallia of both species. We hypothesize that the mediolateral arrangement of the neocortical layer-specific gene-expressing neurons originated in their common ancestor and is conserved among all sauropsid groups, whereas the neuronal arrangement within the pallium could have highly diversified independently in the mammalian lineage.

Bringing dicynodonts back to life: paleobiology and anatomy of a new emydopoid genus from the Upper Permian of Mozambique

Dicynodontia represent the most diverse tetrapod group during the Late Permian. They survived the Permo-Triassic extinction and are central to understanding Permo-Triassic terrestrial ecosystems. Although extensively studied, several aspects of dicynodont paleobiology such as, neuroanatomy, inner ear morphology and internal cranial anatomy remain obscure. Here we describe a new dicynodont (Therapsida, Anomodontia) from northern Mozambique: Niassodon mfumukasi gen. et sp. nov. The holotype ML1620 was collected from the Late Permian K5 formation, Metangula Graben, Niassa Province northern Mozambique, an almost completely unexplored basin and country for vertebrate paleontology. Synchrotron radiation based micro-computed tomography (SRµCT), combined with a phylogenetic analysis, demonstrates a set of characters shared with Emydopoidea. All individual bones were digitally segmented allowing a 3D visualization of each element. In addition, we reconstructed the osseous labyrinth, endocast, cranial nerves and vasculature. The brain is narrow and the cerebellum is broader than the forebrain, resembling the conservative, "reptilian-grade" morphology of other non-mammalian therapsids, but the enlarged paraflocculi occupy the same relative volume as in birds. The orientation of the horizontal semicircular canals indicates a slightly more dorsally tilted head posture than previously assumed in other dicynodonts. In addition, synchrotron data shows a secondary center of ossification in the femur. Thus ML1620 represents, to our knowledge, the oldest fossil evidence of a secondary center of ossification, pushing back the evolutionary origins of this feature. The fact that the specimen represents a new species indicates that the Late Permian tetrapod fauna of east Africa is still incompletely known.

The vertebrate diencephalic MCH system: a versatile neuronal population in an evolving brain

Neurons synthesizing melanin-concentrating hormone (MCH) are described in the posterior hypothalamus of all vertebrates investigated so far. However, their anatomy is very different according to species: they are small and periventricular in lampreys, cartilaginous fishes or anurans, large and neuroendocrine in bony fishes, or distributed over large regions of the lateral hypothalamus in many mammals. An analysis of their comparative anatomy alongside recent data about the development of the forebrain, suggests that although very different, MCH neurons of the caudal hypothalamus are homologous. We further hypothesize that their divergent anatomy is linked to divergence in the forebrain - in particular telencephalic evolution.

Lead roles for supporting actors: critical functions of inner ear supporting cells

Many studies that aim to investigate the underlying mechanisms of hearing loss or balance disorders focus on the hair cells and spiral ganglion neurons of the inner ear. Fewer studies have examined the supporting cells that contact both of these cell types in the cochlea and vestibular end organs. While the roles of supporting cells are still being elucidated, emerging evidence indicates that they serve many functions vital to maintaining healthy populations of hair cells and spiral ganglion neurons. Here we review recent studies that highlight the critical roles supporting cells play in the development, function, survival, death, phagocytosis, and regeneration of other cell types within the inner ear. Many of these roles have also been described for glial cells in other parts of the nervous system, and lessons from these other systems continue to inform our understanding of supporting cell functions. This article is part of a Special Issue entitled "Annual Reviews 2013".

Sexually dimorphic expression of receptor-alpha in the cerebral cortex of neonatal Caiman latirostris (Crocodylia: Alligatoridae)

In mammals, estrogens have been described as endocrine and paracrine modulators of neuronal differentiation and synapse formation. However, the functional role of circulating estrogens and the distribution of estrogen receptors (ERs) in the cerebral cortex of reptiles have not been clearly established. Caiman latirostris (C. latirostris) is a South American species that presents temperature-dependent sex determination (TSD). By using immunohistochemistry, we have studied the distribution of ERα in the cerebral cortex of neonatal caimans. We studied brain samples from ten-day-old TSD-females and TSD-males and from female caimans that were administered estradiol during embryonic development (hormone-dependent sex determination, HSD-females). ERα was detected in the medial (MC), dorsal (DC) and lateral (LC) cortices. ERα expression in the MC showed sex-associated differences, being significantly greater in TSD-females compared to TSD-males. Interestingly, the highest ERα expression in the MC was exhibited by HSD-females. In addition, the circulating levels of estradiol were significantly higher in females (both TSD and HSD) than in TSD-males. Double immunostaining showed that ERα is expressed by neural precursor cells (as detected by ERα/doublecortin or ERα/glial fibrillary acidic protein) and mature neurons (ERα/neuron-specific nuclear protein). Our results demonstrate that the expression of ERα in the neonatal caiman cortex is sexually dimorphic and is present in the early stages of neuronal differentiation.

The vertebrate mesolimbic reward system and social behavior network: a comparative synthesis

All animals evaluate the salience of external stimuli and integrate them with internal physiological information into adaptive behavior. Natural and sexual selection impinge on these processes, yet our understanding of behavioral decision-making mechanisms and their evolution is still very limited. Insights from mammals indicate that two neural circuits are of crucial importance in this context: the social behavior network and the mesolimbic reward system. Here we review evidence from neurochemical, tract-tracing, developmental, and functional lesion/stimulation studies that delineates homology relationships for most of the nodes of these two circuits across the five major vertebrate lineages: mammals, birds, reptiles, amphibians, and teleost fish. We provide for the first time a comprehensive comparative analysis of the two neural circuits and conclude that they were already present in early vertebrates. We also propose that these circuits form a larger social decision-making (SDM) network that regulates adaptive behavior. Our synthesis thus provides an important foundation for understanding the evolution of the neural mechanisms underlying reward processing and behavioral regulation.

Reptilian-transcriptome v1.0, a glimpse in the brain transcriptome of five divergent Sauropsida lineages and the phylogenetic position of turtles

Background

Reptiles are largely under-represented in comparative genomics despite the fact that they are substantially more diverse in many respects than mammals. Given the high divergence of reptiles from classical model species, next-generation sequencing of their transcriptomes is an approach of choice for gene identification and annotation.

Results

Here, we use 454 technology to sequence the brain transcriptome of four divergent reptilian and one reference avian species: the Nile crocodile, the corn snake, the bearded dragon, the red-eared turtle, and the chicken. Using an in-house pipeline for recursive similarity searches of >3,000,000 reads against multiple databases from 7 reference vertebrates, we compile a reptilian comparative transcriptomics dataset, with homology assignment for 20,000 to 31,000 transcripts per species and a cumulated non-redundant sequence length of 248.6 Mbases. Our approach identifies the majority (87%) of chicken brain transcripts and about 50% of de novo assembled reptilian transcripts. In addition to 57,502 microsatellite loci, we identify thousands of SNP and indel polymorphisms for population genetic and linkage analyses. We also build very large multiple alignments for Sauropsida and mammals (two million residues per species) and perform extensive phylogenetic analyses suggesting that turtles are not basal living reptiles but are rather associated with Archosaurians, hence, potentially answering a long-standing question in the phylogeny of Amniotes.

Conclusions

The reptilian transcriptome (freely available at http://www.reptilian-transcriptomes.org) should prove a useful new resource as reptiles are becoming important new models for comparative genomics, ecology, and evolutionary developmental genetics.

Germinal sites and migrating routes of cells in the mesencephalic and diencephalic auditory areas in the African clawed frog (Xenopus laevis)

There is a clear core-shell organization in the auditory nuclei of amniotes. However, such organization only exists in the mesencephalic, but not in the diencephalic auditory regions of amphibians. To gain insights into how this core-shell organization developed and evolved, we injected a small dose of [(3)H]-thymidine into tadpoles of Xenopus laevis at peak stages of neurogenesis in the mesencephalic and diencephalic auditory areas. Following different survival times, the germinal sites and migrating routes of cells were examined in the shell (laminar nucleus, Tl; magnocellular nucleus, Tmc) and core (principal nucleus, Tp) regions of the mesencephalic auditory nucleus, torus semicircularis (Ts), as well as in the diencephalic auditory areas (posterior thalamic nucleus, P; central thalamic nucleus, C). Double labeling for [(3)H]-thymidine autoradiography and immunohistochemistry for vimentin was also performed to help determine the routes of cell migration. We found three major results. First, the germinal sites of Tp were intercalated between Tl and Tmc, arising from those of the shell regions. Second, although the germinal sites of Tl, Tmc, and Tp were located in the same brain levels (at rostromedial or caudomedial levels of Ts), neurogenesis in Tl or Tmc started earlier than that in Tp. Finally, the P and C were also generated in different ventricle sites. However, unlike Ts their neurogenesis showed no obvious temporal differences. These data demonstrate that a highly differentiated auditory region, such as Tp in Ts, is lacking in the diencephalon of amphibian. Our data are discussed from the view of the constitution and evolutionary origins of auditory nuclei in vertebrates.

Lhx2 specifies regional fate in Emx1 lineage of telencephalic progenitors generating cerebral cortex

Cerebral cortex is comprised of regions including six-layer neocortex and three-layer olfactory cortex generated by telencephalic progenitors of an Emx1 lineage. The mechanism specifying region-specific subpopulations within this lineage is unknown. We show in mouse that the LIM homeodomain transcription factor Lhx2, expressed in graded levels by progenitors, determines their regional identity and fate decisions to generate neocortex or olfactory cortex. Emx1-Cre deletion of Lhx2 at E10.5 refates progenitors to generate three-layer cortex phenocopying olfactory cortex rather than lateral neocortex. Progenitors do not generate ectopic olfactory cortex following Lhx2 deletion at E11.5. Thus, Lhx2 regulates a regional-fate decision by telencephalic progenitors during a critical period that closes as they differentiate from neuroepithelial cells to neuronogenic radial glia. “Exposure” of progenitors to Lhx2 may dictate their regional-fate decisions. These findings establish a genetic mechanism determining regional fate in the Emx1 lineage of telencephalic progenitors that generate cerebral cortex.

Neurogenic development of the visual areas in the Chinese softshell turtle (Pelodiscus sinensis) and evolutionary implications

To characterize the neurogenic development of the visual areas of the turtle (Pelodiscus sinensis) during embryogenesis, a single dose of [(3)H]-thymidine (10 microCi) was injected into egg yolks from stages S11-12 to S21. At hatching, localization of [(3)H]-thymidine incorporation was examined, and led to three main observations. (1) Neurogenesis occurred in the stratum griseum centrale of the tectum opticum from S11-12 to S16 with a peak at S12. No obvious gradients of neurogenesis were observed. (2) Neurogenesis in the nucleus rotundus (Rot) and in the dorsal lateral geniculate nucleus (GLd) occurred from S11-12 to S15. Gradients of neurogenesis were detected along ventral-dorsal and lateral-medial axes in the Rot, but only the latter neurogenic gradient occurred in the GLd. (3) In the visual region of the dorsal ventricular ridge, neurogenesis lasted from S11-12 to S16. Similarly, neurogenesis occurred from S11-12 to S16-17 in the dorsal cortex, with a peak at S12 for both telencephalic visual regions. Neurogenesis followed a ventrolateral to dorsomedial gradient in the visual region of the dorsal ventricular ridge, and a superficial to deep gradient in the caudal dorsal cortex. A significant number of neurons in the rostral dorsal cortex followed a deep (earlier arising) to superficial (later arising) pattern of neurogenesis, similar to that in the avian Wulst or in the mammalian isocortex. Finally, we compared the timing and development of neurogenesis in the turtle with birds and mammals to understand the evolutionary implications of these processes.

A stream of cells migrating from the caudal telencephalon reveals a link between the amygdala and neocortex

The amygdaloid complex consists of diverse nuclei that belong to distinct functional systems, yet many issues about its development are poorly understood. Here, we identify a stream of migrating cells that form specific amygdaloid nuclei in mice. In utero electroporation showed that this caudal amygdaloid stream (CAS) originated in a unique domain at the caudal telencephalic pole that is contiguous with the dorsal pallium, which was previously thought to generate only neocortical cells. The CAS and the neocortex share mechanisms for specification (transcription factors Tbr1, Lhx2 and Emx1/2) and migration (reelin and Cdk5). Reelin, a critical cue for migration in the neocortex, and Cdk5, which is specifically required for migration along radial glia in the neocortex, were both selectively required for the normal migration of the CAS, but not for that of other amygdaloid nuclei. This is first evidence of a dorsal pallial contribution to the amygdala, demonstrating a developmental and mechanistic link between the amygdala and the neocortex.

The trivial function of sleep

Rest in poikilothermic animals is an adaptation of the organism to adjust to the geophysical cycles, a doubtless valuable function for all animals. In this review, we argue that the function of sleep could be trivial for mammals and birds because sleep does not provide additional advantages over simple rest. This conclusion can be reached by using the null hypothesis and parsimony arguments. First, we develop some theoretical and empirical considerations supporting the absence of specific effects after sleep deprivation. Then, we question the adaptive value of sleep traits by using non-coding DNA as a metaphor that shows that the complexity in the design is not a definitive proof of adaptation. We then propose that few, if any, phenotypic selectable traits do exist in sleep. Instead, the selection of efficient waking has been the major determinant of the most significant aspects in sleep structure. In addition, we suggest that the regulation of sleep is only a mechanism to enforce rest, a state that was challenged after the development of homeothermy. As a general conclusion, there is no direct answer to the problem of why we sleep; only an explanation of why such a complex set of mechanisms is used to perform what seems to be a simple function. This explanation should be reached by following the evolution of wakefulness rather than that of sleep. Sleep could have additional functions secondarily added to the trivial one, although, in this case, the necessity and sufficiency of these sleep functions should be demonstrated.

Distribution of insoluble cAMP-dependent kinase type RI and RII in the lizard and turtle central nervous system

cAMP is a universal second messenger. In eucaryotes it acts mainly via protein kinases composed of regulatory (R) and catalytic subunits; their subcellular distribution may differ according to the cell type. In rodent brain, peculiar detergent-insoluble RIalpha aggregates were previously described in neurons of areas related to the limbic system, while RIIbeta is more evenly distributed also in non-nervous cells. It is unclear whether the regional distribution of regulatory subunits is typical of mammalian brain. Western blots and immunohistochemistry showed that in lizard brains a large fraction of the cAMP-dependent protein kinase regulatory isoforms is insoluble, as in mammals. Insoluble RIalpha and RII regulatory isoforms were not evenly distributed but organized in clearly separated aggregates. Numerous RII aggregates were present in almost all brain regions and were found also in non-nervous cells. As shown by immunohistochemistry and equilibrium binding of fluorescently tagged cAMP, RIalpha aggregates were restricted to neurons of some brain regions: telencephalon, particularly medial cortical areas, dorsal ventricular ridge, olfactory pathways, medial hypothalamus and cerebellar granular layer were intensely labelled. A very weak RIalpha labelling was detected in the brainstem reticular formation, in the periaqueductal gray and in the spinal cord dorsal horn. A similar distribution of RIalpha aggregates was also found in turtle brains. Their distribution is reminiscent of that observed in mammals, although with some differences in relative intensity and persistence. The supramolecular organization of the RIalpha isoform may help in establishing homologies and differences between brain areas involved in visceroemotional control.

The reach-to-grasp-food task for rats: a rare case of modularity in animal behavior?

Humans and non-human animals make use of sensory hierarchies in "selecting" strategies for solving many cognitive and behavioral tasks. Often, if a preferred type of sensory information is unavailable or is not useful for solving a given task, the animal can switch to a lower-priority strategy, making use of a different class of sensory information. In the case of rats performing a classic reach-to-grasp-food task, however, prior studies indicate that the reaching maneuver may be a fixed action pattern that is guided exclusively by the food's odor plume until the point of contact with the food morsel [Whishaw IQ, Tomie JA. Olfaction directs skilled forelimb reaching in the rat. Behav Brain Res 1989;32(1):11-21; Metz GA, Whishaw IQ. Skilled reaching an action pattern: stability in rat (Rattus norvegicus) grasping movements as a function of changing food pellet size. Behav Brain Res 2000;116(2):111-22; Whishaw IQ. Did a change in sensory control of skilled movements stimulate the evolution of the primate frontal cortex? Behav Brain Res 2003;146(1/2):31-41]. We sought to confirm and extend these findings in several ways. In Experiment 1, using a GO/NO-GO variant of the classic task, we demonstrated that rats used the GO target's odor both to trigger and guide their reaches. In Experiment 2, we showed that rats deprived of (a) vision, (b) object-recognizing rostral whiskers and forearm sinus hairs, or (c) both, displayed no deficits in triggering and guiding their reaches. Finally, in a third experiment in which the GO target's location varied randomly across trials and only olfactory cues were available, we demonstrated that rats could determine the spatial endpoint of their reach without any loss of accuracy. Combined with results from a prior study in which bulbectomized rats never developed a new, successful reaching strategy despite extensive post-operative training [Whishaw IQ, Tomie JA. Olfaction directs skilled forelimb reaching in the rat. Behav Brain Res 1989;32(1):11-21], these results indicate that rats do not have a sensory hierarchy for solving the reach-to-grasp-food task, but rather, are guided by olfaction alone until their paw contacts the food morsel.

A selective serotonin reuptake inhibitor reduces REM sleep in the homing pigeon

Avian and mammalian 'rapid eye movement' sleep (REM sleep) resemble each other in several aspects. However, the question of whether REM sleep has a shared evolutionary ancestry in birds and mammals has yet to be thoroughly explored. The brain regions and neurotransmitter systems involved in the generation of mammalian REM sleep are phylogenetically ancient, and are also found in extant birds and reptiles. Several pharmacological experiments in birds indicate that similar neural substrates are involved in the regulation of avian and mammalian sleep. However, because the drugs used in these studies generally resulted in non-specific sleep loss, the neurochemical regulation of avian REM sleep in particular remains uncertain. The selective serotonin reuptake inhibitor (SSRI) zimelidine is known to reduce REM sleep in mammals. If avian REM sleep is similarly regulated by serotonin, it would be expected that an acute dose of a SSRI should also reduce avian REM sleep. To investigate a putative role of serotonin in the regulation of avian REM sleep, changes in sleep electroencephalogram (EEG) and behavior were recorded in five pigeons (Columba livia) after the administration of an acute dose of zimelidine. Our results demonstrate that the effects of zimelidine on avian REM sleep are comparable to those observed in mammals, indicating that serotonin may serve a similar function in the control of avian and mammalian REM sleep.

Thalamotelencephalic organization in birds

Investigation of thalamo-telencephalic connections reveals correspondences between the avian and mammalian thalamic subdivisions (which may or may not mean true homologies). Based mainly on hodological comparisons, the avian thalamus possesses the principal anatomical and functional subdivisions characteristic for mammals. The current review is focused on a comparative analysis of intralaminar, midline and mediodorsal nuclei. There is evidence for matching subdivisions in the case of midline thalamic and mediodorsal nuclei within the avian dorsal thalamic zone, whereas such correspondence is evident, if less complete, in the case of the intralaminar nuclei. Thalamic connections are also relevant to the debated issue of the avian 'prefrontal' cortex. From the current study it is suggested that the prefrontal analogue regions of the bird may spread across the rostrocaudal extent of telencephalon, the rostral nidopallial/mesopallial region (formerly known as medial neostriatum/hyperstriatum) being one subdivision, receiving direct input from the paraventricular thalamic nucleus homologue of midline thalamic region (the medial juxtaventricular region of the nucleus dorsomedialis posterior). Hodological evidence from the current study and other reports argues for the possibility that the area corticoidea dorsolateralis might be hodologically comparable to the cingulate cortex, receiving input from a mediodorsal thalamic-relevant subdivision (lateral subdivision of nucleus dorsomedialis anterior, and medial aspect of nucleus dorsolateralis pars medialis), which also projects on the caudal nidopallium close to (but not coextensive with) the nidopallium caudolaterale, another potential analogue of avian prefrontal cortex. The rostral dorsolateral aspect of nucleus dorsomedialis anterior thalami and the dorsal aspect of nucleus dorsolateralis pars medialis are partially comparable to the mammalian intralaminar nuclei, sharing connections to non-limbic 'corticoid' areas (the Wulst), and the reticular thalamic nuclei.

Reconstructing the behaviors of extinct species: An excursion into comparative paleoneurology

How can the behavior of an extinct species be reconstructed-say a dinosaur such as Allosaurus? Despite the relatively abundant fossilized remains of this and other dinosaurs, the incompleteness of the fossil record has permitted room for considerable speculation, mythology, and perhaps a bit of unsettling reflection on what factors contributed to the eventual fate of these remarkably successful animals. Among the speculations is how these 'bigger-than-life' creatures behaved, a topic that itself can attain equal diversity and grandeur. With recent advancements in measuring the relatedness of living organisms, how genetics contribute to brain development and how this relates to behavior, combined with the availability of newly discovered high quality fossils and imaging methods to exploit their secrets, novel insights into how extinct creatures such as Allosaurus intermingled with its many relatives over 100 million years ago are beginning to emerge.

What, if anything, is the medial temporal lobe, and how can the amygdala be part of it if there is no such thing?

Should the medial temporal lobe (MTL) of primates--which includes allocortical structures such as the hippocampus, neocortical structures such as the parahippocampal cortex, and nuclear structures such as the basolateral amygdala--be considered a single "thing"? According to the prevailing view, here termed the reification theory, the answer is yes. According to this theory, the MTL functions as an amalgamated entity that provides the neuronal mechanisms for declarative memory; the greater the damage to the MTL or any of its components, the greater the deleterious effects on declarative memory. A countervailing view, here called the balkanization theory, holds that the various components of the MTL process and store different kinds of information. According to this theory, damage to each part of the MTL causes a unique set of behavioral deficits-some involving memory, others involving perception, and yet others involving response selection. The empirical neuropsychological evidence favors the balkanization theory, as do some new concepts in theoretical neuroanatomy.