The olfactory amygdala in amniotes: an evo-devo approach

Development and function of the medial amygdala

Across studied vertebrates, the medial amygdala (MeA) is a central hub for relaying sensory information with social and/or survival relevance to downstream nuclei such as the bed nucleus of stria terminalis (BNST) and the hypothalamus. MeA-driven behaviors, such as mating, aggression, parenting, and predator avoidance are processed by different molecularly defined inhibitory and excitatory neuronal output populations. Work over the past two decades has deciphered how diverse MeA neurons arise from embryonic development, revealing contributions from multiple telencephalic and diencephalic progenitor domains. Here, we first provide a brief overview of current findings regarding the role of the MeA in social behaviors, followed by a deeper dive into current knowledge of how this complex structure is specified during development. We outline a conceptual model of MeA formation that has emerged based on these findings. We further postulate how embryonic developmental programming of the MeA may inform later emergence of stereotypical circuitry governing hardwired behaviors.

Neuroendocrinology of Reproduction and Social Behaviors in Reptiles: Advances Made in the Last Decade

Among amniotes, reptiles are ectothermic and are clearly distinguished from mammals and birds. Reptiles show great diversity not only in species numbers, but also in ecological and physiological features. Although their physiological diversity is an interesting research topic, less effort has been made compared to that for mammals and birds, in part due to lack of established experimental models and techniques. However, progress, especially in the field of neuroendocrinology, has been steadily made. With this process, basic data on selected reptilian species have been collected. This review article presents the progress made in the last decade, which includes 1) behavioral regulation by sex steroid hormones, 2) regulation of seasonal reproduction by melatonin and GnRH, and 3) regulation of social interaction by arginine vasotocin. Through these research topics, we provide insights into the physiology of reptiles and the latest findings in the field of amniote neuroendocrinology.

Human cortical amygdala dendrites and spines morphology under open‐source three‐dimensional reconstruction procedures

Visualizing nerve cells has been fundamental for the systematic description of brain structure and function in humans and other species. Different approaches aimed to unravel the morphological features of neuron types and diversity. The inherent complexity of the human nervous tissue and the need for proper histological processing have made studying human dendrites and spines challenging in postmortem samples. In this study, we used Golgi data and open-source software for 3D image reconstruction of human neurons from the cortical amygdaloid nucleus to show different dendrites and pleomorphic spines at different angles. Procedures required minimal equipment and generated high-quality images for differently shaped cells. We used the "single-section" Golgi method adapted for the human brain to engender 3D reconstructed images of the neuronal cell body and the dendritic ramification by adopting a neuronal tracing procedure. In addition, we elaborated 3D reconstructions to visualize heterogeneous dendritic spines using a supervised machine learning-based algorithm for image segmentation. These tools provided an additional upgrade and enhanced visual display of information related to the spatial orientation of dendritic branches and for dendritic spines of varied sizes and shapes in these human subcortical neurons. This same approach can be adapted for other techniques, areas of the central or peripheral nervous system, and comparative analysis between species.

Orthopedia expression during Drosophila melanogaster nervous system development and its regulation by microRNA-252

During brain development of Drosophila melanogaster many transcription factors are involved in regulating neural fate and morphogenesis. In our study we show that the transcription factor Orthopedia (Otp), a member of the 57B homeobox gene cluster, plays an important role in this process. Otp is expressed in a stable pattern in defined lineages from mid-embryonic stages into the adult brain and therefore a very stable marker for these lineages. We determined the abundance of the two different otp transcripts in the brain and hindgut during development using qPCR. CRISPR/Cas9 generated otp mutants of the longer protein form significantly affect the expression of Otp in specific areas. We generated an otp enhancer trap strain by gene targeting and reintegration of Gal4, which mimics the complete expression of otp during development except the embryonic hindgut expression. Since in the embryo, the expression of Otp is posttranscriptionally regulated, we looked for putative miRNAs interacting with the otp 3'UTR, and identified microRNA-252 as a candidate. Further analyses with mutated and deleted forms of the microRNA-252 interacting sequence in the otp 3'UTR demonstrate an in vivo interaction of microRNA-252 with the otp 3'UTR. An effect of this interaction is seen in the adult brain, where Otp expression is partially abolished in a knockout strain of microRNA-252. Our results show that Otp is another important factor for brain development in Drosophila melanogaster.

A versatile transcription factor: Multiple roles of orthopedia a (otpa) beyond its restricted localization in dopaminergic systems of developing and adult zebrafish (Danio rerio) brains

Many transcription factors boost neural development and differentiation in specific directions and serve for identifying similar or homologous structures across species. The expression of Orthopedia (Otp) is critical for the development of certain cell groups along the vertebrate neuraxis, for example, the medial amygdala or hypothalamic neurosecretory neurons. Therefore, the primary focus of the present study is the distribution of Orthopedia a (Otpa) in the larval and adult zebrafish (Danio rerio) brain. Since Otpa is also critical for the development of zebrafish basal diencephalic dopaminergic cells, colocalization of Otpa with the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) is studied. Cellular colocalization of Otpa and dopamine is only seen in magnocellular neurons of the periventricular posterior tubercular nucleus and in the posterior tuberal nucleus. Otpa-positive cells occur in many additional structures along the zebrafish neuraxis, from the secondary prosencephalon down to the hindbrain. Furthermore, Otpa expression is studied in shh-GFP and islet1-GFP transgenic zebrafish. Otpa-positive cells only express shh in dopaminergic magnocellular periventricular posterior tubercular cells, and only colocalize with islet1-GFP in the ventral zone and prerecess caudal periventricular hypothalamic zone and the perilemniscal nucleus. The scarcity of cellular colocalization of Otpa in islet1-GFP cells indicates that the Shh-islet1 neurogenetic pathway is not active in most Otpa-expressing domains. Our analysis reveals detailed correspondences between mouse and zebrafish forebrain territories including the zebrafish intermediate nucleus of the ventral telencephalon and the mouse medial amygdala. The zebrafish preoptic Otpa-positive domain represents the neuropeptidergic supraopto-paraventricular region of all tetrapods. Otpa domains in the zebrafish basal plate hypothalamus suggest that the ventral periventricular hypothalamic zone corresponds to the otp-expressing basal hypothalamic tuberal field in the mouse. Furthermore, the mouse otp domain in the mammillary hypothalamus compares partly to our Otpa-positive domain in the prerecess caudal periventricular hypothalamic zone (Hc-a).

Distinct Subdivisions in the Transition Between Telencephalon and Hypothalamus Produce Otp and Sim1 Cells for the Extended Amygdala in Sauropsids

Based on the coexpression of the transcription factors Foxg1 and Otp, we recently identified in the mouse a new radial embryonic division named the telencephalon-opto-hypothalamic (TOH) domain that produces the vast majority of glutamatergic neurons found in the medial extended amygdala. To know whether a similar division exists in other amniotes, we carried out double labeling of Foxg1 and Otp in embryonic brain sections of two species of sauropsids, the domestic chicken (Gallus gallus domesticus), and the long-tailed lacertid lizard (Psammodromus algirus). Since in mice Otp overlaps with the transcription factor Sim1, we also analyzed the coexpression of Foxg1 and Sim1 and compared it to the glutamatergic cell marker VGLUT2. Our results showed that the TOH domain is also present in sauropsids and produces subpopulations of Otp/Foxg1 and Sim1/Foxg1 cells for the medial extended amygdala. In addition, we found Sim1/Foxg1 cells that invade the central extended amygdala, and other Otp and Sim1 cells not coexpressing Foxg1 that invade the extended and the pallial amygdala. These different Otp and Sim1 cell subpopulations, with or without Foxg1, are likely glutamatergic. Our results highlight the complex divisional organization of telencephalon-hypothalamic transition, which contributes to the heterogeneity of amygdalar cells. In addition, our results open new venues to study further the amygdalar cells derived from different divisions around this transition zone and their relationship to other cells derived from the pallium or the subpallium.

Human Primary Olfactory Amygdala Subregions Form Distinct Functional Networks, Suggesting Distinct Olfactory Functions

Three subregions of the amygdala receive monosynaptic projections from the olfactory bulb, making them part of the primary olfactory cortex. These primary olfactory areas are located at the anterior-medial aspect of the amygdala and include the medial amygdala (MeA), cortical amygdala (CoA), and the periamygdaloid complex (PAC). The vast majority of research on the amygdala has focused on the larger basolateral and basomedial subregions, which are known to be involved in implicit learning, threat responses, and emotion. Fewer studies have focused on the MeA, CoA, and PAC, with most conducted in rodents. Therefore, our understanding of the functions of these amygdala subregions is limited, particularly in humans. Here, we first conducted a review of existing literature on the MeA, CoA, and PAC. We then used resting-state fMRI and unbiased k-means clustering techniques to show that the anatomical boundaries of human MeA, CoA, and PAC accurately parcellate based on their whole-brain resting connectivity patterns alone, suggesting that their functional networks are distinct, relative both to each other and to the amygdala subregions that do not receive input from the olfactory bulb. Finally, considering that distinct functional networks are suggestive of distinct functions, we examined the whole-brain resting network of each subregion and speculated on potential roles that each region may play in olfactory processing. Based on these analyses, we speculate that the MeA could potentially be involved in the generation of rapid motor responses to olfactory stimuli (including fight/flight), particularly in approach/avoid contexts. The CoA could potentially be involved in olfactory-related reward processing, including learning and memory of approach/avoid responses. The PAC could potentially be involved in the multisensory integration of olfactory information with other sensory systems. These speculations can be used to form the basis of future studies aimed at clarifying the olfactory functions of these under-studied primary olfactory areas.

The conserved core of the nereid brain: Circular CNS, apical nervous system and lhx6-arx-dlx neurons

When bilaterian animals first emerged, an enhanced perception of the Precambrian environment was key to their stunning success. This occurred through the acquisition of an anterior brain, as found in most extant bilaterians. What were the core circuits of the first brain, and how do they relate to today's diversity? With two landmark resources - the full connectome and a multimodal cellular atlas combining gene expression and ultrastructure - the young worm of the marine annelid Platynereis dumerilii takes center stage in comparative bilaterian neuroanatomy. The new data suggest a composite structure of the ancestral bilaterian brain, with the anterior end of a circular CNS fused to a sensory-neurosecretory apical system, and with lhx6-arx-dlx chemosensory circuits giving rise to associative centers in the descending bilaterian lineages.

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.

Chemical Communication in Lizards and a Potential Role for Vasotocin in Modulating Social Interactions

Lizards use chemical communication to mediate many reproductive, competitive, and social behaviors, but the neuroendocrine mechanisms underlying chemical communication in lizards are not well understood and understudied. By implementing a neuroendocrine approach to the study of chemical communication in reptiles, we can address a major gap in our knowledge of the evolutionary mechanisms shaping chemical communication in vertebrates. The neuropeptide arginine vasotocin (AVT) and its mammalian homolog vasopressin are responsible for a broad spectrum of diversity in competitive and reproductive strategies in many vertebrates, mediating social behavior through the chemosensory modality. In this review, we posit that, though limited, the available data on AVT-mediated chemical communication in lizards reveal intriguing patterns that suggest AVT plays a more prominent role in lizard chemosensory behavior than previously appreciated. We argue that these results warrant more research into the mechanisms used by AVT to modify the performance of chemosensory behavior and responses to conspecific chemical signals. We first provide a broad overview of the known social functions of chemical signals in lizards, the glandular sources of chemical signal production in lizards (e.g., epidermal secretory glands), and the chemosensory detection methods and mechanisms used by lizards. Then, we review the locations of vasotocinergic populations and neuronal projections in lizard brains, as well as sites of peripheral receptors for AVT in lizards. Finally, we end with a case study in green anoles (Anolis carolinensis), discussing findings from recently published work on the impact of AVT in adult males on chemosensory communication during social interactions, adding new data from a similar study in which we tested the impact of AVT on chemosensory behavior of adult females. We offer concluding remarks on addressing several fundamental questions regarding the role of AVT in chemosensory communication and social behavior in lizards.

As above, so below: Whole transcriptome profiling demonstrates strong molecular similarities between avian dorsal and ventral pallial subdivisions

Over the last two decades, beginning with the Avian Brain Nomenclature Forum in 2000, major revisions have been made to our understanding of the organization and nomenclature of the avian brain. However, there are still unresolved questions on avian pallial organization, particularly whether the cells above the vestigial ventricle represent distinct populations to those below it or similar populations. To test these two hypotheses, we profiled the transcriptomes of the major avian pallial subdivisions dorsal and ventral to the vestigial ventricle boundary using RNA sequencing and a new zebra finch genome assembly containing about 22,000 annotated, complete genes. We found that the transcriptomes of neural populations above and below the ventricle were remarkably similar. Each subdivision in dorsal pallium (Wulst) had a corresponding molecular counterpart in the ventral pallium (dorsal ventricular ridge). In turn, each corresponding subdivision exhibited shared gene co‐expression modules that contained gene sets enriched in functional specializations, such as anatomical structure development, synaptic transmission, signaling, and neurogenesis. These findings are more in line with the continuum hypothesis of avian brain subdivision organization above and below the vestigial ventricle space, with the pallium as a whole consisting of four major cell populations (intercalated pallium, mesopallium, hyper‐nidopallium, and arcopallium) instead of seven (hyperpallium apicale, interstitial hyperpallium apicale, intercalated hyperpallium, hyperpallium densocellare, mesopallium, nidopallium, and arcopallium). We suggest adopting a more streamlined hierarchical naming system that reflects the robust similarities in gene expression, neural connectivity motifs, and function. These findings have important implications for our understanding of overall vertebrate brain evolution.

Neural pathways of olfactory kin imprinting and kin recognition in zebrafish

Teleost fish exhibit extraordinary cognitive skills that are comparable to those of mammals and birds. Kin recognition based on olfactory and visual imprinting requires neuronal circuits that were assumed to be necessarily dependent on the interaction of mammalian amygdala, hippocampus, and isocortex, the latter being a structure that teleost fish are lacking. We show that teleosts—beyond having a hippocampus and pallial amygdala homolog—also have subpallial amygdalar structures. In particular, we identify the medial amygdala and neural olfactory central circuits related to kin imprinting and kin recognition corresponding to an accessory olfactory system despite the absence of a separate vomeronasal organ.

Development of the mouse anterior amygdalar radial unit marked by Lhx9-expression

The amygdala in mammals plays a key role in emotional processing and learning, being subdivided in pallial and subpallial derivatives. Recently, the cortical ring model and the pallial amygdalar radial model (Puelles et al. 2019; Garcia-Calero et al. 2020) described the pallial amygdala as an histogenetic field external to the allocortical ring, and subdivided it in five major radial domains called lateral, basal, anterior, posterior and retroendopiriform units. The anterior radial unit, whose cells typically express the Lhx9 gene (see molecular profile in Garcia-Calero et al. 2020), is located next to the pallial/subpallial boundary. This radial domain shows massive radial translocation and accumulation of its derivatives into its intermediate and superficial strata, with only a glial palisade representing its final periventricular domain. To better understand the development of this singular radial domain, not described previously, we followed the expression of Lhx9 during mouse amygdalar development in the context of the postulated radial subdivisions of the pallial amygdala and other telencephalic developmental features.

A novel telencephalon-opto-hypothalamic morphogenetic domain coexpressing Foxg1 and Otp produces most of the glutamatergic neurons of the medial extended amygdala

Deficits in social cognition and behavior are a hallmark of many psychiatric disorders. The medial extended amygdala, including the medial amygdala and the medial bed nucleus of the stria terminalis, is a key component of functional networks involved in sociality. However, this nuclear complex is highly heterogeneous and contains numerous GABAergic and glutamatergic neuron subpopulations. Deciphering the connections of different neurons is essential in order to understand how this structure regulates different aspects of sociality, and it is necessary to evaluate their differential implication in distinct mental disorders. Developmental studies in different vertebrates are offering new venues to understand neuronal diversity of the medial extended amygdala and are helping to establish a relation between the embryonic origin and molecular signature of distinct neurons with the functional subcircuits in which they are engaged. These studies have provided many details on the distinct GABAergic neurons of the medial extended amygdala, but information on the glutamatergic neurons is still scarce. Using an Otp-eGFP transgenic mouse and multiple fluorescent labeling, we show that most glutamatergic neurons of the medial extended amygdala originate in a distinct telencephalon-opto-hypothalamic embryonic domain (TOH), located at the transition between telencephalon and hypothalamus, which produces Otp-lineage neurons expressing the telencephalic marker Foxg1 but not Nkx2.1 during development. These glutamatergic cells include a subpopulation of projection neurons of the medial amygdala, which activation has been previously shown to promote autistic-like behavior. Our data open new venues for studying the implication of this neuron subtype in neurodevelopmental disorders producing social deficits.

Olfaction of aquatic amniotes

Amniotes originated on land, but aquatic/amphibious groups emerged multiple times independently in amniotes. On becoming aquatic, species with different phylogenetic backgrounds and body plans have to adapt themselves to handle similar problems inflicted by their new environment, and this makes aquatic adaptation of amniotes one of the greatest natural experiments. Particularly, evolution of the sense of smell upon aquatic adaptation is of great interest because receptors required for underwater olfaction differ remarkably from those for terrestrial olfaction. Here, I review the olfactory capabilities of aquatic/amphibious amniotes, especially those of cetaceans and sea snakes. Most aquatic/amphibious amniotes show reduced olfactory organs, receptor gene repertoires, and olfactory capabilities. Remarkably, cetaceans and sea snakes show extreme examples: cetaceans have lost the vomeronasal system, and furthermore, toothed whales have lost all of their olfactory nervous systems. Baleen whales can smell in the air, but their olfactory capability is limited. Fully aquatic sea snakes have lost the main olfactory system but they retain the vomeronasal system for sensing underwater. Amphibious species show an intermediate status between terrestrial and aquatic species, implying their importance on understanding the process of aquatic adaptation. The olfactory capabilities of aquatic amniotes are diverse, reflecting their diverse phylogenetic backgrounds and ecology.

Emotional responses in monkeys differ depending on the stimulus type, sex, and neonatal amygdala lesion status

The amygdala plays an essential role in evaluating social information, threat detection, and learning fear associations. Yet, most of that knowledge comes from studies in adult humans and animals with a fully developed amygdala. Given the considerable protracted postnatal development of the amygdala, it is important to understand how early damage to this structure may impact the long-term development of behavior. The current study examined behavioral responses toward social, innate, or learned aversive stimuli among neonatal amygdala lesion (Neo-Aibo; males = 3, females = 3) or sham-operated control (Neo-C; males = 3, females = 4) rhesus macaques. Compared with controls, Neo-Aibo animals exhibited less emotional reactivity toward aversive objects, including faster retrieval of food reward, fewer fearful responses, and more manipulation of objects. This lower reactivity was only seen in response to social and innate aversive stimuli, whereas Neo-Aibo animals had similar responses to controls for learned aversive stimuli. The current study also detected sex differences in behavioral response to aversive stimuli, such that, as compared with males, females took longer to retrieve the food reward across all aversive stimuli types, but only expressed more hostility and more coo vocalizations during learned aversive trials. Early amygdala damage impacted the expression of some, but not all, sex differences. For example, neonatal amygdala damage eliminated the sex difference in object manipulation. These findings add important information that broaden our understanding of the role of the amygdala in the expression of sexually dimorphic behaviors, as well as its role in learning fear associations and threat detection. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

The Zebrafish Amygdaloid Complex - Functional Ground Plan, Molecular Delineation, and Everted Topology

In mammals and other tetrapods, a multinuclear forebrain structure, called the amygdala, forms the neuroregulatory core essential for emotion, cognition, and social behavior. Currently, higher circuits of affective behavior in anamniote non-tetrapod vertebrates (“fishes”) are poorly understood, preventing a comprehensive understanding of amygdala evolution. Through molecular characterization and evolutionary-developmental considerations, we delineated the complex amygdala ground plan of zebrafish, whose everted telencephalon has made comparisons to the evaginated forebrains of tetrapods challenging. In this radical paradigm, thirteen telencephalic territories constitute the zebrafish amygdaloid complex and each territory is distinguished by conserved molecular properties and structure-functional relationships with other amygdaloid structures. Central to our paradigm, the study identifies the teleostean amygdaloid nucleus of the lateral olfactory tract (nLOT), an olfactory integrative structure that links dopaminergic telencephalic groups to the amygdala alongside redefining the putative zebrafish olfactory pallium (“Dp”). Molecular characteristics such as the distribution of substance P and the calcium-binding proteins parvalbumin (PV) and calretinin (CR) indicate, that the zebrafish extended centromedial (autonomic and reproductive) amygdala is predominantly located in the GABAergic and isl1-negative territory. Like in tetrapods, medial amygdaloid (MeA) nuclei are defined by the presence of substance P immunoreactive fibers and calretinin-positive neurons, whereas central amygdaloid (CeA) nuclei lack these characteristics. A detailed comparison of lhx5-driven and vGLut2a-driven GFP in transgenic reporter lines revealed ancestral topological relationships between the thalamic eminence (EmT), the medial amygdala (MeA), the nLOT, and the integrative olfactory pallium. Thus, the study explains how the zebrafish amygdala and the complexly everted telencephalon topologically relate to the corresponding structures in mammals indicating that an elaborate amygdala ground plan evolved early in vertebrates, in a common ancestor of teleosts and tetrapods.

Signatures of sex: Sex differences in gene expression in the vertebrate brain

Women and men differ in disease prevalence, symptoms, and progression rates for many psychiatric and neurological disorders. As more preclinical studies include both sexes in experimental design, an increasing number of sex differences in physiology and behavior have been reported. In the brain, sex-typical behaviors are thought to result from sex-specific patterns of neural activity in response to the same sensory stimulus or context. These differential firing patterns likely arise as a consequence of underlying anatomic or molecular sex differences. Accordingly, gene expression in the brains of females and males has been extensively investigated, with the goal of identifying biological pathways that specify or modulate sex differences in brain function. However, there is surprisingly little consensus on sex-biased genes across studies and only a handful of robust candidates have been pursued in the follow-up experiments. Furthermore, it is not known how or when sex-biased gene expression originates, as few studies have been performed in the developing brain. Here we integrate molecular genetic and neural circuit perspectives to provide a conceptual framework of how sex differences in gene expression can arise in the brain. We detail mechanisms of gene regulation by steroid hormones, highlight landmark studies in rodents and humans, identify emerging themes, and offer recommendations for future research. This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Sex Determination.

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 Pallial Areas and Networks Involved in Sociality: Comparison Between Mammals and Sauropsids

Birds are extremely interesting animals for studying the neurobiological basis of cognition and its evolution. They include species that are highly social and show high cognitive capabilities. Moreover, birds rely more on visual and auditory cues than on olfaction for social behavior and cognition, just like primates. In primates, there are two major brain networks associated to sociality: (1) one related to perception and decision-making, involving the pallial amygdala (with the basolateral complex as a major component), the temporal and temporoparietal neocortex, and the orbitofrontal cortex; (2) another one related to affiliation, including the medial extended amygdala, the ventromedial prefrontal and anterior cingulate cortices, the ventromedial striatum (largely nucleus accumbens), and the ventromedial hypothalamus. In this account, we used an evolutionary developmental neurobiology approach, in combination with published comparative connectivity and functional data, to identify areas and functional networks in the sauropsidian brain comparable to those of mammals that are related to decision-making and affiliation. Both in mammals and sauropsids, there is an important interaction between these networks by way of cross projections between areas of both systems.

The caudo-ventral pallium is a novel pallial domain expressing Gdf10 and generating Ebf3-positive neurons of the medial amygdala

In rodents, the medial nucleus of the amygdala receives direct inputs from the accessory olfactory bulbs and is mainly implicated in pheromone-mediated reproductive and defensive behaviors. The principal neurons of the medial amygdala are GABAergic neurons generated principally in the caudo-ventral medial ganglionic eminence and preoptic area. Beside GABAergic neurons, the medial amygdala also contains glutamatergic Otp-expressing neurons cells generated in the lateral hypothalamic neuroepithelium and a non-well characterized Pax6-positive population. In the present work, we describe a novel glutamatergic Ebf3-expressing neuronal subpopulation distributed within the periphery of the postero-ventral medial amygdala. These neurons are generated in a pallial domain characterized by high expression of Gdf10. This territory is topologically the most caudal tier of the ventral pallium and accordingly, we named it Caudo-Ventral Pallium (CVP). In the absence of Pax6, the CVP is disrupted and Ebf3-expressing neurons fail to be generated. Overall, this work proposes a novel model of the neuronal composition of the medial amygdala and unravels for the first time a new novel pallial subpopulation originating from the CVP and expressing the transcription factor Ebf3.

Identification of accessory olfactory system and medial amygdala in the zebrafish

Zebrafish larvae imprint on visual and olfactory cues of their kin on day 5 and 6 postfertilization, respectively. Only imprinted (but not non-imprinted) larvae show strongly activated crypt (and some microvillous) cells demonstrated by pERK levels after subsequent exposure to kin odor. Here, we investigate the olfactory bulb of zebrafish larvae for activated neurons located at the sole glomerulus mdG2 which receives crypt cell input. Imprinted larvae show a significantly increased activation of olfactory bulb cells compared to non-imprinted larvae after exposure to kin odor. Surprisingly, pERK activated Orthopedia-positive cell numbers in the intermediate ventral telencephalic nucleus were higher in non-imprinted, kin odor stimulated larvae compared to control and to kin-odor stimulated imprinted larvae and control. Moreover, DiI tracing experiments in adult zebrafish show a neuronal circuit from crypt/microvillous olfactory sensory neurons via dorsomedial olfactory bulb and intermediate ventral telencephalic nucleus (thus, arguably the teleostean medial amygdala) to tuberal hypothalamus, demonstrating for the first time an accessory olfactory system in teleosts.

Cell migration in the developing rodent olfactory system

The components of the nervous system are assembled in development by the process of cell migration. Although the principles of cell migration are conserved throughout the brain, different subsystems may predominantly utilize specific migratory mechanisms, or may display unusual features during migration. Examining these subsystems offers not only the potential for insights into the development of the system, but may also help in understanding disorders arising from aberrant cell migration. The olfactory system is an ancient sensory circuit that is essential for the survival and reproduction of a species. The organization of this circuit displays many evolutionarily conserved features in vertebrates, including molecular mechanisms and complex migratory pathways. In this review, we describe the elaborate migrations that populate each component of the olfactory system in rodents and compare them with those described in the well-studied neocortex. Understanding how the components of the olfactory system are assembled will not only shed light on the etiology of olfactory and sexual disorders, but will also offer insights into how conserved migratory mechanisms may have shaped the evolution of the brain.

Genoarchitecture of the extended amygdala in zebra finch, and expression of FoxP2 in cell corridors of different genetic profile

We used a battery of genes encoding transcription factors (Pax6, Islet1, Nkx2.1, Lhx6, Lhx5, Lhx9, FoxP2) and neuropeptides to study the extended amygdala in developing zebra finches. We identified different components of the central extended amygdala comparable to those found in mice and chickens, including the intercalated amygdalar cells, the central amygdala, and the lateral bed nucleus of the stria terminalis. Many cells likely originate in the dorsal striatal domain, ventral striatal domain, or the pallidal domain, as is the case in mice and chickens. Moreover, a cell subpopulation of the central extended amygdala appears to originate in the prethalamic eminence. As a general principle, these different cells with specific genetic profiles and embryonic origin form separate or partially intermingled cell corridors along the extended amygdala, which may be involved in different functional pathways. In addition, we identified the medial amygdala of the zebra finch. Like in the chickens and mice, it is located in the subpallium and is rich in cells of pallido-preoptic origin, containing minor subpopulations of immigrant cells from the ventral pallium, alar hypothalamus and prethalamic eminence. We also proposed that the medial bed nucleus of the stria terminalis is composed of several parallel cell corridors with different genetic profile and embryonic origin: preoptic, pallidal, hypothalamic, and prethalamic. Several of these cell corridors with distinct origin express FoxP2, a transcription factor implicated in synaptic plasticity. Our results pave the way for studies using zebra finches to understand the neural basis of social behavior, in which the extended amygdala is involved.

Radial derivatives of the mouse ventral pallium traced with Dbx1-LacZ reporters

The progeny of Dbx1-expressing progenitors was studied in the developing mouse pallium, using two transgenic mouse lines: (1) Dbx1(nlslacZ) mice, in which the gene of the β-galactosidase reporter (LacZ) is inserted directly under the control of the Dbx1 promoter, allowing short-term lineage tracing of Dbx1-derived cells; and (2) Dbx1(CRE) mice crossed with a Cre-dependent reporter strain (ROSA26(loxP-stop-loxP-LacZ)), in which the Dbx1-derived cells result permanently labeled (Bielle et al., 2005). We thus examined in detail the derivatives of the postulated longitudinal ventral pallium (VPall) sector, which has been defined among other features by its selective ventricular zone expression of Dbx1 (the recent ascription by Puelles, 2014 of the whole olfactory cortex primordium to the VPall was tested). Earlier notions about a gradiental caudorostral reduction of Dbx1 signal were corroborated, so that virtually no signal was found at the olfactory bulb and the anterior olfactory area. The piriform cortex was increasingly labeled caudalwards. The only endopiriform grisea labeled were the ventral endopiriform nucleus and the bed nucleus of the external capsule. Anterior and basolateral parts of the whole pallial amygdala also were densely marked, in contrast to the negative posterior parts of these pallial amygdalar nuclei (leaving apart medial amygdalar parts ascribed to subpallial or extratelencephalic sources of Dbx1-derived GABAergic and non-GABAergic neurons). Alternative tentative interpretations are discussed to explain the partial labeling obtained of both olfactory and amygdaloid structures. This includes the hypothesis of an as yet undefined part of the pallium, potentially responsible for the posterior amygdala, or the hypothesis that the VPall may not be wholly characterized by Dbx1 expression (this gene not being necessary for VPall molecular distinctness and histogenetic potency), which would leave a dorsal Dbx1-negative VPall subdomain of variable size that might contribute partially to olfactory and posterior amygdalar structures.

Embryonic Origin of the Islet1 and Pax6 Neurons of the Chicken Central Extended Amygdala Using Cell Migration Assays and Relation to Different Neuropeptide-Containing Cells

In a recent study, we tentatively identified different subdivisions of the central extended amygdala (EAce) in chicken based on the expression of region-specific transcription factors (including Pax6 and Islet1) and several phenotypic markers during embryonic development. Such a proposal was partially based on the suggestion that, similarly to the subdivisions of the EAce of mammals, the Pax6 and Islet1 neurons of the comparable chicken subdivisions derive from the dorsal (Std) or ventral striatal embryonic domains (Stv), respectively. To investigate whether this is true, in the present study, we carried out cell migration assays from chicken Std or Stv combined with immunofluorescence for Pax6 or Islet1. Our results showed that the cells of the proposed chicken EAce truly originate in either Std (expressing Pax6) or Stv (expressing Islet1). This includes lateral subdivisions previously compared to the intercalated amygdalar cells and the central amygdala of mammals, also rich in Std-derived Pax6 cells and/or Stv-derived Islet1 cells. In the medial region of the chicken EAce, the dorsal part of the lateral bed nucleus of the stria terminalis (BSTL) contains numerous cells expressing Nkx2.1 (mostly derived from the pallidal domain), but our migration assays showed that it also contains neuron subpopulations from the Stv (expressing Islet1) and Std (expressing Pax6), resembling the mouse BSTL. These findings, together with those previously published in different species of mammals, birds and reptiles, support the homology of the chicken EAce to that of other vertebrates, and reinforce the existence of several cell subcorridors inside the EAce. In addition, together with previously published data on neuropeptidergic cells, these results led us to propose the existence of at least seventeen neuron subtypes in the EAce in rodents and/or some birds (chicken and pigeon). The functional significance and the evolutionary origin of each subtype needs to be analyzed separately, and such studies are mandatory in order to understand the multifaceted modulation by the EAce of fear responses, ingestion, motivation and pain in different vertebrates.

Genetic identification of the central nucleus and other components of the central extended amygdala in chicken during development

In mammals, the central extended amygdala shows a highly complex organization, and is essential for animal survival due to its implication in fear responses. However, many aspects of its evolution are still unknown, and this structure is especially poorly understood in birds. The aim of this study was to define the central extended amygdala in chicken, by means of a battery of region-specific transcription factors (Pax6, Islet1, Nkx2.1) and phenotypic markers that characterize these different subdivisions in mammals. Our results allowed the identification of at least six distinct subdivisions in the lateral part of the avian central extended amygdala: (1) capsular central subdivision; (2) a group of intercalated-like cell patches; (3) oval central nucleus; (4) peri-intrapeduncular (peri-INP) island field; (5) perioval zone; and (6) a rostral part of the subpallial extended amygdala. In addition, we identified three subdivisions of the laterodorsal bed nucleus of the stria terminalis (BSTLd) belonging to the medial region of the chicken central extended amygdala complex. Based on their genetic profile, cellular composition and apparent embryonic origin of the cells, we discuss the similarity of these different subdivisions of chicken with different parts of the mouse central amygdala and surrounding cell masses, including the intercalated amygdalar masses and the sublenticular part of the central extended amygdala. Most of the subdivisions include various subpopulations of cells that apparently originate in the dorsal striatal, ventral striatal, pallidal, and preoptic embryonic domains, reaching their final location by either radial or tangential migrations. Similarly to mammals, the central amygdala and BSTLd of chicken project to the hypothalamus, and include different neurons expressing proenkephalin, corticotropin-releasing factor, somatostatin or tyrosine hydroxylase, which may be involved in the control of different aspects of fear/anxiety-related behavior.

Combinatorial expression of Lef1, Lhx2, Lhx5, Lhx9, Lmo3, Lmo4, and Prox1 helps to identify comparable subdivisions in the developing hippocampal formation of mouse and chicken

We carried out a study of the expression patterns of seven developmental regulatory genes (Lef1, Lhx2, Lhx9, Lhx5, Lmo3, Lmo4, and Prox1), in combination with topological position, to identify the medial pallial derivatives, define its major subdivisions, and compare them between mouse and chicken. In both species, the medial pallium is defined as a pallial sector adjacent to the cortical hem and roof plate/choroid tela, showing moderate to strong ventricular zone expression of Lef1, Lhx2, and Lhx9, but not Lhx5. Based on this, the hippocampal formation (indusium griseum, dentate gyrus, Ammon's horn fields, and subiculum), the medial entorhinal cortex, and part of the amygdalo-hippocampal transition area of mouse appeared to derive from the medial pallium. In the chicken, based on the same position and gene expression profile, we propose that the hippocampus (including the V-shaped area), the parahippocampal area (including its caudolateral part), the entorhinal cortex, and the amygdalo-hippocampal transition area are medial pallial derivatives. Moreover, the combinatorial expression of Lef1, Prox1, Lmo4, and Lmo3 allowed the identification of dentate gyrus/CA3-like, CA1/subicular-like, and medial entorhinal-like comparable sectors in mouse and chicken, and point to the existence of mostly conserved molecular networks involved in hippocampal complex development. Notably, while the mouse medial entorhinal cortex derives from the medial pallium (similarly to the hippocampal formation, both being involved in spatial navigation and spatial memory), the lateral entorhinal cortex (involved in processing non-spatial, contextual information) appears to derive from a distinct dorsolateral caudal pallial sector.

Who would have thought that 'Jaws' also has brains? Cognitive functions in elasmobranchs

Adaptation of brain structures, function and higher cognitive abilities most likely have contributed significantly to the evolutionary success of elasmobranchs, but these traits remain poorly studied when compared to other vertebrates, specifically mammals. While the pallium of non-mammalian vertebrates lacks the mammalian neocortical organization responsible for all cognitive abilities of mammals, several behavioural and neuroanatomical studies in recent years have clearly demonstrated that elasmobranchs, just like teleosts and other non-mammalian vertebrates, can nonetheless solve a multitude of cognitive tasks. Sharks and rays can learn and habituate, possess spatial memory; can orient according to different orientation strategies, remember spatial and discrimination tasks for extended periods of time, use tools; can imitate and learn from others, distinguish between conspecifics and heterospecifics, discriminate between either visual objects or electrical fields; can categorize visual objects and perceive illusory contours as well as bilateral symmetry. At least some neural correlates seem to be located in the telencephalon, with some pallial regions matching potentially homologous areas in other vertebrates where similar functions are being processed. Results of these studies indicate that the assessed cognitive abilities in elasmobranchs are as well developed as in teleosts or other vertebrates, aiding them in fundamental activities such as food retrieval, predator avoidance, mate choice and habitat selection.