Subpopulations of corticotropin-releasing factor containing neurons and internal circuits in the chicken central extended amygdala

In mammals, the central extended amygdala is critical for the regulation of the stress response. This regulation is extremely complex, involving multiple subpopulations of GABAergic neurons and complex networks of internal and external connections. Two neuron subpopulations expressing corticotropin-releasing factor (CRF), located in the central amygdala and the lateral bed nucleus of the stria terminalis (BSTL), play a key role in the long-term component of fear learning and in sustained fear responses akin to anxiety. Very little is known about the regulation of stress by the amygdala in nonmammals, hindering efforts for trying to improve animal welfare. In birds, one of the major problems relates to the high evolutionary divergence of the telencephalon, where the amygdala is located. In the present study, we aimed to investigate the presence of CRF neurons of the central extended amygdala in chicken and the local connections within this region. We found two major subpopulations of CRF cells in BSTL and the medial capsular central amygdala of chicken. Based on multiple labeling of CRF mRNA with different developmental transcription factors, all CRF neurons seem to originate within the telencephalon since they express Foxg1, and there are two subtypes with different embryonic origins that express Islet1 or Pax6. In addition, we demonstrated direct projections from Pax6 cells of the capsular central amygdala to BSTL and the oval central amygdala. We also found projections from Islet1 cells of the oval central amygdala to BSTL, which may constitute an indirect pathway for the regulation of BSTL output cells. Part of these projections may be mediated by CRF cells, in agreement with the expression of CRF receptors in both Ceov and BSTL. Our results show a complex organization of the central extended amygdala in chicken and open new venues for studying how different cells and circuits regulate stress in these animals.

Early cellular and synaptic changes in dopaminoceptive forebrain regions of juvenile mice following gestational exposure to valproate

Gestational exposure of mice to valproic acid (VPA) is one currently used experimental model for the investigation of typical failure symptoms associated with autism spectrum disorder (ASD). In the present study we hypothesized that the reduction of dopaminergic source neurons of the VTA, followed by perturbed growth of the mesotelencephalic dopamine pathway (MT), should also modify pattern formation in the dopaminoceptive target regions (particularly its mesoaccumbens/mesolimbic portion). Here, we investigated VPA-evoked cellular morphological (apoptosis-frequency detected by Caspase-3, abundance of Ca-binding proteins, CaBP), as well as synaptic proteomic (western blotting) changes, in selected dopaminoceptive subpallial, as compared to pallial, regions of mice, born to mothers treated with 500 mg/kg VPA on day 13.5 of pregnancy. We observed a surge of apoptosis on VPA treatment in nearly all investigated subpallial and pallial regions; with a non-significant trend of similar increase the nucleus accumbens (NAc) at P7, the age at which the MT pathway reduction has been reported (also supplemented by current findings). Of the CaBPs, calretinin (CR) expression was decreased in pallial regions, most prominently in retrosplenial cortex, but not in the subpallium of P7 mice. Calbindin-D 28K (CB) was selectively reduced in the caudate-putamen (CPu) of VPA exposed animals at P7 but no longer at P60, pointing to a potency of repairment. The VPA-associated overall increase in apoptosis at P7 did not correlate with the abundance and distribution of CaBPs, except in CPu, in which the marked drop of CB was negatively correlated with increased apoptosis. Abundance of parvalbumin (PV) at P60 showed no significant response to VPA treatment in any of the observed regions we did not find colocalization of apoptotic (Casp3+) cells with CaBP-immunoreactive neurons. The proteomic findings suggest reduction of tyrosine hydroxylase in the crude synaptosome fraction of NAc, but not in the CPu, without simultaneous decrease of the synaptic protein, synaptophysin, indicating selective impairment of dopaminergic synapses. The morpho-functional changes found in forebrain regions of VPA-exposed mice may signify dendritic and synaptic reorganization in dopaminergic target regions, with potential translational value to similar impairments in the pathogenesis of human ASD.

Mapping of corticotropin-releasing factor, receptors, and binding protein mRNA in the chicken telencephalon throughout development

Understanding the neural mechanisms that regulate the stress response is critical to know how animals adapt to a changing world and is one of the key factors to be considered for improving animal welfare. Corticotropin-releasing factor (CRF) is crucial for regulating physiological and endocrine responses, triggering the activation of the sympathetic nervous system and the hypothalamo-pituitary-adrenal axis (HPA) during stress. In mammals, several telencephalic areas, such as the amygdala and the hippocampus, regulate the autonomic system and the HPA responses. These centers include subpopulations of CRF containing neurons that, by way of CRF receptors, play modulatory roles in the emotional and cognitive aspects of stress. CRF binding protein also plays a role, buffering extracellular CRF and regulating its availability. CRF role in activation of the HPA is evolutionary conserved in vertebrates, highlighting the relevance of this system to help animals cope with adversity. However, knowledge on CRF systems in the avian telencephalon is very limited, and no information exists on detailed expression of CRF receptors and binding protein. Knowing that the stress response changes with age, with important variations during the first week posthatching, the aim of this study was to analyze mRNA expression of CRF, CRF receptors 1 and 2, and CRF binding protein in chicken telencephalon throughout embryonic and early posthatching development, using in situ hybridization. Our results demonstrate an early expression of CRF and its receptors in pallial areas regulating sensory processing, sensorimotor integration and cognition, and a late expression in subpallial areas regulating the stress response. However, CRF buffering system develops earlier in the subpallium than in the pallium. These results help to understand the mechanisms underlying the negative effects of noise and light during prehatching stages in chicken, and suggest that stress regulation becomes more sophisticated with age.

Molecular and cellular evolution of the amygdala across species analyzed by single-nucleus transcriptome profiling

The amygdala, or an amygdala-like structure, is found in the brains of all vertebrates and plays a critical role in survival and reproduction. However, the cellular architecture of the amygdala and how it has evolved remain elusive. Here, we generated single-nucleus RNA-sequencing data for more than 200,000 cells in the amygdala of humans, macaques, mice, and chickens. Abundant neuronal cell types from different amygdala subnuclei were identified in all datasets. Cross-species analysis revealed that inhibitory neurons and inhibitory neuron-enriched subnuclei of the amygdala were well-conserved in cellular composition and marker gene expression, whereas excitatory neuron-enriched subnuclei were relatively divergent. Furthermore, LAMP5⁺ interneurons were much more abundant in primates, while DRD2⁺ inhibitory neurons and LAMP5⁺SATB2⁺ excitatory neurons were dominant in the human central amygdalar nucleus (CEA) and basolateral amygdalar complex (BLA), respectively. We also identified CEA-like neurons and their species-specific distribution patterns in chickens. This study highlights the extreme cell-type diversity in the amygdala and reveals the conservation and divergence of cell types and gene expression patterns across species that may contribute to species-specific adaptations.

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

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

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.

Telencephalic regulation of the HPA axis in birds

The hypothalamo-pituitary-adrenal (HPA) axis is one of the major output systems of the vertebrate stress response. It controls the release of cortisol or corticosterone from the adrenal gland. These hormones regulate a range of processes throughout the brain and body, with the main function of mobilizing energy reserves to improve coping with a stressful situation. This axis is regulated in response to both physical (e.g., osmotic) and psychological (e.g., social) stressors. In mammals, the telencephalon plays an important role in the regulation of the HPA axis response in particular to psychological stressors, with the amygdala and part of prefrontal cortex stimulating the stress response, and the hippocampus and another part of prefrontal cortex inhibiting the response to return it to baseline. Birds also mount HPA axis responses to psychological stressors, but much less is known about the telencephalic areas that control this response. This review summarizes which telencephalic areas in birds are connected to the HPA axis and are known to respond to stressful situations. The conclusion is that the telencephalic control of the HPA axis is probably an ancient system that dates from before the split between sauropsid and synapsid reptiles, but more research is needed into the functional relationships between the brain areas reviewed in birds if we want to understand the level of this conservation.

Analysis of the Expression Pattern of Cajal-Retzius Cell Markers in the Xenopus laevis Forebrain

Cajal-Retzius cells are essential for cortical development in mammals, and their involvement in the evolution of this structure has been widely postulated, but very little is known about their progenitor domains in non-mammalian vertebrates. Using in situhybridization and immunofluorescence techniques we analyzed the expression of some of the main Cajal-Retzius cell markers such as Dbx1, Ebf3, ER81, Lhx1, Lhx5, p73, Reelin, Wnt3a, Zic1, and Zic2 in the forebrain of the anuran Xenopus laevis, because amphibians are the only class of anamniote tetrapods and show a tetrapartite evaginated pallium, but no layered or nuclear organization. Our results suggested that the Cajal-Retzius cell progenitor domains were comparable to those previously described in amniotes. Thus, at dorsomedial telencephalic portions a region comparable to the cortical hem was defined in Xenopus based on the expression of Wnt3a, p73, Reelin, Zic1, and Zic2. In the septum, two different domains were observed: a periventricular dorsal septum, at the limit between the pallium and the subpallium, expressing Reelin, Zic1, and Zic2, and a related septal domain, expressing Ebf3, Zic1, and Zic2. In the lateral telencephalon, the ventral pallium next to the pallio-subpallial boundary, the lack of Dbx1 and the unique expression of Reelin during development defined this territory as the most divergent with respect to mammals. Finally, we also analyzed the expression of these markers at the prethalamic eminence region, suggested as Cajal-Retzius progenitor domain in amniotes, observing there Zic1, Zic2, ER81, and Lhx1 expression. Our data show that in anurans there are different subtypes and progenitor domains of Cajal-Retzius cells, which probably contribute to the cortical regional specification and territory-specific properties. This supports the notion that the basic organization of pallial derivatives in vertebrates follows a comparable fundamental arrangement, even in those that do not have a sophisticated stratified cortical structure like the mammalian cerebral cortex.

Neural architecture of the vertebrate brain: implications for the interaction between emotion and cognition

Cognition is considered a hallmark of the primate brain that requires a high degree of signal integration, such as achieved in the prefrontal cortex. Moreover, it is often assumed that cognitive capabilities imply "superior" computational mechanisms compared to those involved in emotion or motivation. In contrast to these ideas, we review data on the neural architecture across vertebrates that support the concept that association and integration are basic features of the vertebrate brain, which are needed to successfully adapt to a changing world. This property is not restricted to a few isolated brain centers, but rather resides in neuronal networks working collectively in a context-dependent manner. In different vertebrates, we identify shared large-scale connectional systems involving the midbrain, hypothalamus, thalamus, basal ganglia, and amygdala. The high degree of crosstalk and association between these systems at different levels supports the notion that cognition, emotion, and motivation cannot be separated - all of them involve a high degree of signal integration.

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.

Transmitter receptors reveal segregation of the arcopallium/amygdala complex in pigeons (Columba livia)

At the beginning of the 20th century it was suggested that a complex group of nuclei in the avian posterior ventral telencephalon is comparable to the mammalian amygdala. Subsequent findings, however, revealed that most of these structures share premotor characteristics, while some indeed constitute the avian amygdala. These developments resulted in 2004 in a change of nomenclature of these nuclei, which from then on were named arcopallial or amygdala nuclei and referred to as the arcopallium/amygdala complex. The structural basis for the similarities between avian and mammalian arcopallial and amygdala subregions is poorly understood. Therefore, we analyzed binding site densities for glutamatergic AMPA, NMDA and kainate, GABAergic GABA_A , muscarinic M₁ , M₂ and nicotinic acetylcholine (nACh; α₄ β₂ subtype), noradrenergic α₁ and α₂ , serotonergic 5-HT_1A and dopaminergic D_1/5 receptors using quantitative in vitro receptor autoradiography combined with a detailed analysis of the cyto- and myelo-architecture. Our approach supports a segregation of the pigeon's arcopallium/amygdala complex into the following subregions: the arcopallium anterius (AA), the arcopallium ventrale (AV), the arcopallium dorsale (AD), the arcopallium intermedium (AI), the arcopallium mediale (AM), the arcopallium posterius (AP), the nucleus posterioris amygdalopallii pars basalis (PoAb) and pars compacta (PoAc), the nucleus taeniae amgygdalae (TnA) and the area subpallialis amygdalae (SpA). Some of these subregions showed further subnuclei and each region of the arcopallium/amygdala complex are characterized by a distinct multi-receptor density expression. Here we provide a new detailed map of the pigeon's arcopallium/amygdala complex and compare the receptor architecture of the subregions to their possible mammalian counterparts.

Expression of regulatory genes in the embryonic brain of a lizard and implications for understanding pallial organization and evolution

The comparison of gene expression patterns in the embryonic brain of mouse and chicken is being essential for understanding pallial organization. However, the scarcity of gene expression data in reptiles, crucial for understanding evolution, makes it difficult to identify homologues of pallial divisions in different amniotes. We cloned and analyzed the expression of the genes Emx1, Lhx2, Lhx9, and Tbr1 in the embryonic telencephalon of the lacertid lizard Psammodromus algirus. The comparative expression patterns of these genes, critical for pallial development, are better understood when using a recently proposed six‐part model of pallial divisions. The lizard medial pallium, expressing all genes, includes the medial and dorsomedial cortices, and the majority of the dorsal cortex, except the region of the lateral cortical superposition. The latter is rich in Lhx9 expression, being excluded as a candidate of dorsal or lateral pallia, and may belong to a distinct dorsolateral pallium, which extends from rostral to caudal levels. Thus, the neocortex homolog cannot be found in the classical reptilian dorsal cortex, but perhaps in a small Emx1‐expressing/Lhx9‐negative area at the front of the telencephalon, resembling the avian hyperpallium. The ventral pallium, expressing Lhx9, but not Emx1, gives rise to the dorsal ventricular ridge and appears comparable to the avian nidopallium. We also identified a distinct ventrocaudal pallial sector comparable to the avian arcopallium and to part of the mammalian pallial amygdala. These data open new venues for understanding the organization and evolution of the pallium.

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.

Multiple amygdaloid divisions of arcopallium send convergent projections to the nucleus accumbens and neighboring subpallial amygdala regions in the domestic chicken: a selective pathway tracing and reconstruction study

Retrograde tracing with choleratoxin B, injected into the nucleus accumbens (Ac) and bed nucleus of stria terminalis, lateral part (BSTL), yielded labeled perikarya in a ring-shaped area of arcopallium, including dorsal and hilar subdivisions, with a wedge-shaped node of dense accumulation in the amygdalopiriform area (APir). Also, the position of source neurons for this arcopallio-subpallial pathway was verified by anterograde tracing. Three subregions of arcopallium (amygdalopiriform, dorsal, hilar) were injected with dextran (10 kDa), and fibers and terminal fields were detected in Ac, BSTL and extended amygdala (EA). Most abundant projections to Ac arose from APir. The study enabled precise description of the main output fiber streams: the dorsal stream follows the dorsal border of arcopallium and, continuing in the ventral amygdalofugal tract, it traverses the EA and the BSTL before reaching the Ac. The ventral stream of fibers enters the EA along the ventral subpallial border and terminates in the basal nucleus and ventral pallidum. The course of the pathway was reconstructed in 3D. Retrogradely labeled arcopallial neurons were devoid of DARPP-32. DARPP-32 was present in the Ac but not the BSTL. No colocalization between the calcium binding proteins calbindin, parvalbumin and calretinin, and retrogradely labeled neurons was detected, despite a considerable territorial overlap. This finding further supports the excitatory nature of the arcopallial-accumbens pathway. Conjoint and convergent amygdalar input to EA, including BSTL, as well as to Ac subregions likely transmits fear and aggression related signals to both viscerolimbic (EA) and learned reward- and motivation-related (Ac) ventrobasal forebrain regions.