Two interconnected functional systems in the amygdala of amniote vertebrates

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.

Anatomical and molecular features of the amygdalohippocampal transition area and its role in social and emotional behavior processes

The amygdalohippocampal transition area (AHi) has emerged as a critical nucleus of sociosexual behaviors such as mating, parenting, and aggression. The AHi has been overlooked in rodent and human amygdala studies until recently. The AHi is hypothesized to play a role in metabolic and cognitive functions as well as social behaviors based on its connectivity and molecular composition. The AHi is small nucleus rich in neuropeptide and hormone receptors and is contiguous with the ventral subiculum of the hippocampus-hence its designation as a "transition area". Literature focused on the AHi can be difficult to interpret because of changing nomenclature and conflation with neighboring nuclei. Here we summarize what is currently known about AHi structure and development, connections throughout the brain, molecular composition, and functional significance. We aim to delineate current knowledge regarding the AHi, identify potential functions with supporting evidence, and ultimately make clear the importance of the AHi in sociosexual function.

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.

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.

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 radial histogenetic model of the mouse pallial amygdala

Conventional anatomic models of the rodent (mammalian) amygdala are based on section planes oblique to its intrinsic radial glial organization. As a result, we still lack a model of amygdalar histogenesis in terms of radial units (progenitor domains and related radial migration and layering patterns). A radial model of the mouse pallial amygdala is first offered here, based on three logical steps: (1) analysis of amygdalar radial structure in variously discriminative genoarchitectonic material, using an optimal ad hoc section plane; (2) testing preliminary models with experiments labelling at the brain surface single packets of radial glia processes, to be followed into the ventricular surface across intervening predicted elements; (3) selection of 81 differential amygdalar gene markers and checking planar and radial aspects of their distribution across the model elements. This approach shows that subtle changes to the conventional schema of the amygdala allow a radial histogenetic model to be recognized, which is consistent with molecularly coded differential identities of its units and strata. It is expected that this model will help both causal studies of amygdalar developmental patterning and comparative evolutionary studies. It also may have potential impact on hodological and functional studies.

NMDA receptors in the avian amygdala and the premotor arcopallium mediate distinct aspects of appetitive extinction learning

Extinction learning is an essential mechanism that enables constant adaptation to ever-changing environmental conditions. The underlying neural circuit is mostly studied with rodent models using auditory cued fear conditioning. In order to uncover the variant and the invariant neural properties of extinction learning, we adopted pigeons as an animal model in an appetitive sign-tracking paradigm. The animals firstly learned to respond to two conditioned stimuli in two different contexts (CS-1 in context A and CS-2 in context B), before conditioned responses to the stimuli were extinguished in the opposite contexts (CS-1 in context B and CS-2 in context A). Subsequently, responding to both stimuli was tested in both contexts. Prior to extinction training, we locally injected the N-methyl-d-aspartate receptor (NMDAR) antagonist 2-Amino-5-phosphonovaleric acid (APV) in either the amygdala or the (pre)motor arcopallium to investigate their involvement in extinction learning. Our findings suggest that the encoding of extinction memory required the activation of amygdala, as visible by an impairment of extinction acquisition by concurrent inactivation of local NMDARs. In contrast, consolidation and subsequent retrieval of extinction memory recruited the (pre)motor arcopallium. Also, the inactivation of arcopallial NMDARs induced a general motoric slowing during extinction training. Thus, our results reveal a double dissociation between arcopallium and amygdala with respect to acquisition and consolidation of extinction, respectively. Our study therefore provides new insights on the two key components of the avian extinction network and their resemblance to the data obtained from mammals, possibly indicating a shared neural mechanism underlying extinction learning shaped by evolution.

Receptor-driven, multimodal mapping of the human amygdala

The human amygdala consists of subdivisions contributing to various functions. However, principles of structural organization at the cellular and molecular level are not well understood. Thus, we re-analyzed the cytoarchitecture of the amygdala and generated cytoarchitectonic probabilistic maps of ten subdivisions in stereotaxic space based on novel workflows and mapping tools. This parcellation was then used as a basis for analyzing the receptor expression for 15 receptor types. Receptor fingerprints, i.e., the characteristic balance between densities of all receptor types, were generated in each subdivision to comprehensively visualize differences and similarities in receptor architecture between the subdivisions. Fingerprints of the central and medial nuclei and the anterior amygdaloid area were highly similar. Fingerprints of the lateral, basolateral and basomedial nuclei were also similar to each other, while those of the remaining nuclei were distinct in shape. Similarities were further investigated by a hierarchical cluster analysis: a two-cluster solution subdivided the phylogenetically older part (central, medial nuclei, anterior amygdaloid area) from the remaining parts of the amygdala. A more fine-grained three-cluster solution replicated our previous parcellation including a laterobasal, superficial and centromedial group. Furthermore, it helped to better characterize the paralaminar nucleus with a molecular organization in-between the laterobasal and the superficial group. The multimodal cyto- and receptor-architectonic analysis of the human amygdala provides new insights into its microstructural organization, intersubject variability, localization in stereotaxic space and principles of receptor-based neurochemical differences.

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.

Pattern of distribution of serotonergic fibers to the amygdala and extended amygdala in the rat

As is well recognized, serotonergic (5-HT) fibers distribute widely throughout the forebrain, including the amygdala. Although a few reports have examined the 5-HT innervation of select nuclei of the amygdala in the rat, no previous report has described overall 5-HT projections to the amygdala in the rat. Using immunostaining for the serotonin transporter, SERT, we describe the complete pattern of distribution of 5-HT fibers to the amygdala (proper) and to the extended amygdala in the rat. Based on its ontogenetic origins, the amygdala was subdivided into two major parts, pallial and subpallial components, with the pallial component further divided into superficial and deep nuclei (Olucha-Bordonau et al. 2015). SERT⁺ fibers were shown to distributed moderately to densely to the deep and cortical pallial nuclei, but, by contrast, lightly to the subpallial nuclei. Specifically, 1) of the deep pallial nuclei, the lateral, basolateral, and basomedial nuclei contained a very dense concentration of 5-HT fibers; 2) of the cortical pallial nuclei, the anterior cortical and amygdala-cortical transition zone rostrally and the posteromedial and posterolateral nuclei caudally contained a moderate concentration of 5-HT fibers; and 3) of the subpallial nuclei, the anterior nuclei and the rostral part of the medial (Me) nuclei contained a moderate concentration of 5-HT fibers, whereas caudal regions of Me as well as the central nuclei and the intercalated nuclei contained a sparse/light concentration of 5-HT fibers. With regard to the extended amygdala (primarily the bed nucleus of stria terminalis; BST), on the whole, the BST contained moderate numbers of 5-HT fibers, spread fairly uniformly throughout BST. The findings are discussed with respect to a critical serotonergic influence on the amygdala, particularly on the basal complex, and on the extended amygdala in the control of states of fear and anxiety. J. Comp. Neurol. 525:116-139, 2017. © 2016 Wiley Periodicals, Inc.

Toward a systems-oriented approach to the role of the extended amygdala in adaptive responding

Research into the structure and function of the basal forebrain macrostructure called the extended amygdala (EA) has recently seen considerable growth. This paper reviews that work, with the objectives of identifying underlying themes and developing a common goal towards which investigators of EA function might work. The paper begins with a brief review of the structure and the ontological and phylogenetic origins of the EA. It continues with a review of research into the role of the EA in both aversive and appetitive states, noting that these two seemingly disparate avenues of research converge on the concept of reinforcement - either negative or positive - of adaptive responding. These reviews lead to a proposal as to where the EA may fit in the organization of the basal forebrain, and an invitation to investigators to place their findings in a unifying conceptual framework of the EA as a collection of neural ensembles that mediate adaptive responding.

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.

Enhanced c-Fos expression in the central amygdala correlates with increased thigmotaxis in rats with peripheral nerve injury

Background

Pain is associated with affective, cognitive and sensory dysfunction. Animal models can be used to observe ethologically relevant behaviours such as thigmotaxis, giving insight into how ongoing sensory abnormalities influence natural rodent behaviours. The amygdala is a complex group of nuclei implicated in the integration and generation of emotional behavioural responses, including those associated with pain, and a region known as the central amygdala is particularly associated with generation of behavioural responses, due to its links to the descending pain modulation pathways; as such, study of amygdalar c‐Fos immunoreactivity can help identify the neuronal circuits involved.

Method

This study investigated changes in both nociceptive evoked responses and open field behaviour following spinal nerve transection (SNT) in male Wistar rats, and attempted to correlate these with changes in central amygdala c‐Fos immunoreactivity.

Results

Fourteen days after SNT, mechanical hypersensitivity was present in the hind paw ipsilateral to site of injury. Thigmotactic behaviour was significantly increased in both SNT and sham surgery animals, with c‐Fos immunoreactivity in the central amygdala significantly greater in SNT animals compared to both sham and naive groups. Activation was greatest in the capsular and lateral subnuclei of the central amygdala, and in the caudal‐most regions. There was a strong correlation between thigmotactic behaviour and central amygdala activation following SNT surgery not seen in sham animals suggesting a role for the amygdala in behavioural responses to peripheral nerve injury.

Conclusions

This study provides evidence to support the role of the amygdala in thigmotactic open field behaviour following SNT.

What does this study add?

Thigmotaxis and amygdala activation are positively correlated in rats following spinal nerve transection.

Behavioural changes seen in sham animals did not correlate with amygdala activation, suggesting amygdala activation is related to nociceptive input.

Evoked measures, such as hindpaw withdrawal, are not correlated with either thigmotaxis or amygdala activation, emphasizing the importance of complex behaviours when studying pain.

Arborization patterns of amygdalopetal axons from the rat ventral pallidum

We previously analyzed the arborization patterns of rat ventral pallidal (VP) axons that coursed caudally to innervate the thalamus and brainstem (Tripathi et al. in Brain Struct Funct 218:1133-1157, 2013). Here, we have reconstructed 16 previously undetected axons from the same tracer deposits that follow a more lateral trajectory. Virtually all 16 axons emanating from the different VP compartments collateralized in the extended amygdala system (EAS) and amygdaloid complex. The most frequent targets of axons from the lateral and medial (VPm) VP compartments were the rostral sublenticular extended amygdala, the extended amygdala (EA), the central nucleus of the amygdala and the posterior part of the basolateral amygdaloid nucleus. In contrast, axons from the rostral extension of the VP preferentially innervated the anterior amygdaloid area, the magnocellular preoptic nucleus, and the anterior part of the basomedial amygdaloid nucleus. We additionally found and reconstructed a single corticopetal axon arising from the VPm. The new results show that both direct and indirect projections from the basolateral complex and EAS to the ventral striatopallidal system are reciprocated by VP projections, and suggest that the systems can be activated simultaneously. The results additionally suggest that the amygdaloid complex and cortex are innervated separately from the VP. Finally, the combination of new and previous data indicate that approximately 84 % of VP axons (88/105) participate in basal ganglia circuits, 15 % (16/105) target the amygdaloid complex, and less than 1 % innervate the cortex.

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.

What's in a name? Considerations of homologies and nomenclature for vertebrate social behavior networks

Behavioral neuroendocrinology is an integrative discipline that spans a wide range of taxa and neural systems, and thus the appropriate designation of homology (sameness) across taxa is critical for clear communication and extrapolation of findings from one taxon to another. In the present review we address issues of homology that relate to neural circuits of social behavior and associated systems that mediate reward and aversion. We first address a variety of issues related to the so-called "social behavior network" (SBN), including homologies that are only partial (e.g., whereas the preoptic area of fish and amphibians contains the major vasopressin-oxytocin cell groups, these populations lie in the hypothalamus of other vertebrates). We also discuss recent evidence that clarifies anterior hypothalamus and periaqueductal gray homologies in birds. Finally, we discuss an expanded network model, the "social decision-making network" (SDM) which includes the mesolimbic dopamine system and other structures that provide an interface between the mesolimbic system and the SBN. This expanded model is strongly supported in mammals, based on a wide variety of evidence. However, it is not yet clear how readily the SDM can be applied as a pan-vertebrate model, given insufficient data on numerous proposed homologies and a lack of social behavior data for SDM components (beyond the SBN nodes) for amphibians, reptiles or fish. Functions of SDM components are also poorly known for birds. Nonetheless, we contend that the SDM model provides a very sound and important framework for the testing of many hypotheses in nonmammalian vertebrates.

Cadherin expression delineates the divisions of the postnatal and adult mouse amygdala

The amygdaloid complex represents a group of telencephalic nuclei and cortical areas that control emotional and social behavior. Amygdalar development is poorly understood. It is generally accepted that the structures of the amygdala originate from the neuroepithelium at both sides of the pallial-subpallial boundary. In the present study, we mapped the expression of 13 members of the cadherin superfamily of cell adhesion molecules, which provide an adhesive code for the development and maintenance of functional structures in the central nervous system (CNS). Five classic cadherins (Cdh4, Cdh6, Cdh7, Cdh8, Cdh11) and eight delta-protocadherins (Pcdh1, Pcdh7, Pcdh8, Pcdh9, Pcdh10, Pcdh11, PCdh17, PCdh19) were studied by in situ hybridization in the postnatal (P5) and adult mouse amygdala. In the different parts of the amygdala, each of these (proto-) cadherins shows a distinct and spatially restricted expression pattern that is highly similar at postnatal and adult stages. The combinatorial expression of (proto-) cadherins allows the distinction of multiple molecular subdivisions within the amygdala that partially coincide with previously described morphological divisions. Beyond these expected results, a number of novel molecular subdivisions and subpopulations of cells were identified; for example, additional molecular subdomains, patches, or cell aggregates with distinct (proto-) cadherin expression in several nuclei/areas of the amygdala. We also show that several cadherins are molecular markers for particular functional subsystems within the amygdala, such as in the olfactory projections. In summary, (proto-) cadherins provide a code of potentially adhesive cues that can aid the understanding of functional organization in the amygdala.

Birdsong: is it music to their ears?

Since the time of Darwin, biologists have wondered whether birdsong and music may serve similar purposes or have the same evolutionary precursors. Most attempts to compare song with music have focused on the qualities of the sounds themselves, such as melody and rhythm. Song is a signal, however, and as such its meaning is tied inextricably to the response of the receiver. Imaging studies in humans have revealed that hearing music induces neural responses in the mesolimbic reward pathway. In this study, we tested whether the homologous pathway responds in songbirds exposed to conspecific song. We played male song to laboratory-housed white-throated sparrows, and immunolabeled the immediate early gene product Egr-1 in each region of the reward pathway that has a clear or putative homologue in humans. We found that the responses, and how well they mirrored those of humans listening to music, depended on sex and endocrine state. In females with breeding-typical plasma levels of estradiol, all of the regions of the mesolimbic reward pathway that respond to music in humans responded to song. In males, we saw responses in the amygdala but not the nucleus accumbens – similar to the pattern reported in humans listening to unpleasant music. The shared responses in the evolutionarily ancient mesolimbic reward system suggest that birdsong and music engage the same neuroaffective mechanisms in the intended listeners.

One nose, one brain: contribution of the main and accessory olfactory system to chemosensation

The accessory olfactory system is present in most tetrapods. It is involved in the perception of chemical stimuli, being implicated also in the detection of pheromones. However, it is sensitive also to some common odorant molecules, which have no clear implication in intraspecific chemical communication. The accessory olfactory system may complement the main olfactory system and may contribute different perceptual features to the construction of a unitary representation, which merges the different chemosensory qualities. Crosstalk between the main and accessory olfactory systems occurs at different levels of central processing, in brain areas where the inputs from the two systems converge. Interestingly, centrifugal projections from more caudal brain areas are deeply involved in modulating both main and accessory sensory processing. A high degree of interaction between the two systems may be conceived and partial overlapping appears to occur in many functions. Therefore, the central chemosensory projections merge inputs from different organs to obtain a complex chemosensory picture.

Identification of neuronal loci involved with displays of affective aggression in NC900 mice

Aggression is a complex behavior that is essential for survival. Of the various forms of aggression, impulsive violent displays without prior planning or deliberation are referred to as affective aggression. Affective aggression is thought to be caused by aberrant perceptions of, and consequent responses to, threat. Understanding the neuronal networks that regulate affective aggression is pivotal to development of novel approaches to treat chronic affective aggression. Here, we provide a detailed anatomical map of neuronal activity in the forebrain of two inbred lines of mice that were selected for low (NC100) and high (NC900) affective aggression. Attack behavior was induced in male NC900 mice by exposure to an unfamiliar male in a novel environment. Forebrain maps of c-Fos+ nuclei, which are surrogates for neuronal activity during behavior, were then generated and analyzed. NC100 males rarely exhibited affective aggression in response to the same stimulus, thus their forebrain c-Fos maps were utilized to identify unique patterns of neuronal activity in NC900s. Quantitative results indicated robust differences in the distribution patterns and densities of c-Fos+ nuclei in distinct thalamic, subthalamic, and amygdaloid nuclei, together with unique patterns of neuronal activity in the nucleus accumbens and the frontal cortices. Our findings implicate these areas as foci regulating differential behavioral responses to an unfamiliar male in NC900 mice when expressing affective aggression. Based on the highly conserved patterns of connections and organization of neuronal limbic structures from mice to humans, we speculate that neuronal activities in analogous networks may be disrupted in humans prone to maladaptive affective aggression.

Characterization of the bed nucleus of the stria terminalis in the forebrain of anuran amphibians

Major common features have been reported for the organization of the basal telencephalon in amniotes, and most characteristics were thought to be acquired in the transition from anamniotes to amniotes. However, gene expression, neurochemical, and hodological data obtained for the basal ganglia and septal and amygdaloid complexes in amphibians (anamniotic tetrapods) have strengthened the idea of a conserved organization in tetrapods. A poorly characterized region in the forebrain of amniotes has been the bed nucleus of the stria terminalis (BST), but numerous recent investigations have characterized it as a member of the extended amygdala. Our study analyzes the main features of the BST in anuran amphibians to establish putative homologies with amniotes. Gene expression patterns during development identified the anuran BST as a subpallial, nonstriatal territory. The BST shows Nkx2.1 and Lhx7 expression and contains an Islet1-positive cell subpopulation derived from the lateral ganglionic eminence. Immunohistochemistry for diverse peptides and neurotransmitters revealed that the distinct chemoarchitecture of the BST is strongly conserved among tetrapods. In vitro tracing techniques with dextran amines revealed important connections between the BST and the central and medial amygdala, septal territories, medial pallium, preoptic area, lateral hypothalamus, thalamus, and prethalamus. The BST receives dopaminergic projections from the ventral tegmental area and is connected with the laterodorsal tegmental nucleus and the rostral raphe in the brainstem. All these data suggest that the anuran BST shares many features with its counterpart in amniotes and belongs to a basal continuum, likely controlling similar reflexes, reponses, and behaviors in tetrapods.

The avian subpallium: new insights into structural and functional subdivisions occupying the lateral subpallial wall and their embryological origins

The subpallial region of the avian telencephalon contains neural systems whose functions are critical to the survival of individual vertebrates and their species. The subpallial neural structures can be grouped into five major functional systems, namely the dorsal somatomotor basal ganglia; ventral viscerolimbic basal ganglia; subpallial extended amygdala including the central and medial extended amygdala and bed nuclei of the stria terminalis; basal telencephalic cholinergic and non-cholinergic corticopetal systems; and septum. The paper provides an overview of the major developmental, neuroanatomical and functional characteristics of the first four of these neural systems, all of which belong to the lateral telencephalic wall. The review particularly focuses on new findings that have emerged since the identity, extent and terminology for the regions were considered by the Avian Brain Nomenclature Forum. New terminology is introduced as appropriate based on the new findings. The paper also addresses regional similarities and differences between birds and mammals, and notes areas where gaps in knowledge occur for birds.

Aggregates of cAMP-dependent kinase isoforms characterize different areas in the developing central nervous system of the chicken, Gallus gallus

The intracellular second messenger adenosine 3',5'-cyclic monophosphate (cAMP) acts mainly through cAMP-dependent protein kinases (PKA). In mammals and reptiles, the PKA regulatory isoforms (RI and RII) are differentially distributed among the various brain areas and cell types, according to the age of the animal. Since PKA distribution may be an additional marker for homologous areas, PKA regulatory subunit types RI and RII were examined in the chicken brain, a species not yet investigated. Chicken brains were examined from prehatching to adult age, by means of immunohistochemistry and biochemical characterization. Most PKA regulatory subunits were segregated in discrete non-soluble clusters that contained either RI or RII. While RII aggregates were present also in non-neuronal cells, RI aggregates were detected only in neurons of some brain areas that are mainly related to the telencephalon. They appeared later than RII aggregates; their presence and location varied during development. RI aggregates were detected first in the olfactory bulb, around embryonic day 14; within 3 days they appeared in the hyperpallium and nidopallium, where the most intense labeling was observed in the perihatching period. Fainter RI aggregates persisted up to 3 years in the olfactory bulb and nidopallium caudale. Less intense RI aggregates were present for a shorter time, from 2 weeks to 3 months, in the septal nuclei, thalamic medial nuclei, periventricular hypothalamus, optic tectum periventricular area, brainstem reticular formation and spinal cord substantia gelatinosa. RI aggregates were not detected in many brain areas including the arcopallium, striatum and cranial nerve nuclei. RII distribution showed less variation during development. From embryonic day 12, some insoluble RII aggregates were detected in the brain; however, only minor modifications were observed in positive structures once they started to harbor insoluble RII aggregates. The present results suggest that the distribution of PKA aggregates may assist in characterizing phylogenetically homologous structures of the vertebrate central nervous system and may also unravel biochemical differences among areas considered homologous.

Nonapeptides and the evolution of social group sizes in birds

Species-typical patterns of grouping have profound impacts on many aspects of physiology and behavior. However, prior to our recent studies in estrildid finches, neural mechanisms that titrate species-typical group-size preferences, independent of other aspects of social organization (e.g., mating system and parental care), have been wholly unexplored, likely because species-typical group size is typically confounded with other aspects of behavior and biology. An additional complication is that components of social organization are evolutionarily labile and prone to repeated divergence and convergence. Hence, we cannot assume that convergence in social structure has been produced by convergent modifications to the same neural characters, and thus any comparative approach to grouping must include not only species that differ in their species-typical group sizes, but also species that exhibit convergent evolution in this aspect of social organization. Using five estrildid finch species that differ selectively in grouping (all biparental and monogamous) we have demonstrated that neural motivational systems evolve in predictable ways in relation to species-typical group sizes, including convergence in two highly gregarious species and convergence in two relatively asocial, territorial species. These systems include nonapeptide (vasotocin and mesotocin) circuits that encode the valence of social stimuli (positive–negative), titrate group-size preferences, and modulate anxiety-like behaviors. Nonapeptide systems exhibit functional and anatomical properties that are biased toward gregarious species, and experimental reductions of nonapeptide signaling by receptor antagonism and antisense oligonucleotides significantly decrease preferred group sizes in the gregarious zebra finch. Combined, these findings suggest that selection on species-typical group size may reliably target the same neural motivation systems when a given social structure evolves independently.

Distribution of 5-HT2A receptor immunoreactivity in the rat amygdaloid complex and colocalization with γ-aminobutyric acid

The 5-HT2A receptor (5-HT2Ar) is located in a variety of excitatory and inhibitory neurons in many regions of the central nervous system and is a major target for atypical antipsychotic drugs. In the present study, an immunoperoxidase experiment was used to investigate the distribution of 5-HT2Ar immunoreactivity in the rat amygdaloid complex. In the basolateral amygdala, the colocalization of 5-HT2Ar with inhibitory transmitter γ-aminobutyric acid (GABA) was studied using double-immunofluorescence confocal microscopy. The staining pattern obtained was colchicine-sensitive. In fact, pretreatment with colchicine increased the number of 5-HT2Ar-immunoreactive somata. Accordingly, with the exception of the intercalated nuclei, the amygdaloid complex of colchicine-injected rats exhibited a high density of 5-HT2Ar-IR somata. Morphological analyses indicated that 5-HT2Ar was located on both excitatory and inhibitory neurons in the rat amygdaloid complex. In addition, double-immunofluorescence observations revealed that the great majority of GABA-immunoreactive neurons in the basolateral amygdala exhibited 5-HT2Ar immunoreactivity (66.3%-70.6% depending on the nucleus). These data help to clarify the complex role of the 5-HT2Ar in the amygdaloid complex suggesting that this receptor can regulate amygdaloid activity by acting on different neuronal populations.

The two sides of beauty: laterality and the duality of facial attractiveness

We hypothesized that facial attractiveness represents a dual judgment, a combination of reward-based, sexual processes, and aesthetic, cognitive processes. Herein we describe a study that demonstrates that sexual and nonsexual processes both contribute to attractiveness judgments and that these processes can be dissociated. Female participants rated the general attractiveness of faces presented in either their left or right visual field. In order to examine sexual and nonsexual components of these judgments, general attractiveness ratings were correlated with ratings of these same faces made by two independent groups of raters in two specific contexts, one sexual and one nonsexual. Based on an items analysis, partial correlation coefficients were computed for each individual and used as the dependent variable of interest in a 2 (laterality: right, left) by 2 (context: sexual, nonsexual) ANOVA. This analysis revealed an interaction such that faces rated in a sexual context better predicted attractiveness ratings of faces shown in the left than right visual field, whereas faces rated in a nonsexual context better predicted attractiveness of faces shown in the right than left visual field. This finding is consistent with the assertion that sexual and nonsexual preferences involve predominantly lateralized processing routes that independently contribute to what is perceived to be attractive.

On the organization of olfactory and vomeronasal cortices

Classically, the olfactory and vomeronasal pathways are thought to run in parallel non-overlapping axes in the forebrain subserving different functions. The olfactory and vomeronasal epithelia project to the main and accessory olfactory bulbs (primary projections), which in turn project to different areas of the telencephalon in a non-topographic fashion (secondary projections) and so on (tertiary projections). New data indicate that projections arising from the main and accessory olfactory bulbs converge widely in the rostral basal telencephalon. In contrast, in the vomeronasal system, cloning two classes of vomeronasal receptors (V1R and V2R) has led to the distinction of two anatomically and functionally independent pathways that reach some common, but also some different, targets in the amygdala. Tertiary projections from the olfactory and vomeronasal amygdalae are directed to the ventral striatum, which thus becomes a site for processing and potential convergence of chemosensory stimuli. Functional data indicate that the olfactory and vomeronasal systems are able to detect and process volatiles (presumptive olfactory cues) as well as pheromones in both epithelia and bulbs. Collectively, these data indicate that the anatomical and functional distinction between the olfactory and vomeronasal systems should be re-evaluated. Specifically, the recipient cortex should be reorganized to include olfactory, vomeronasal (convergent and V1R and V2R specific areas) and mixed (olfactory and vomeronasal) chemosensory cortices. This new perspective could help to unravel olfactory and vomeronasal interactions in behavioral paradigms.

Vomeronasal inputs to the rodent ventral striatum

Vertebrates sense chemical signals through the olfactory and vomeronasal systems. In squamate reptiles, which possess the largest vomeronasal system of all vertebrates, the accessory olfactory bulb projects to the nucleus sphericus, which in turn projects to a portion of the ventral striatum known as olfactostriatum. Characteristically, the olfactostriatum is innervated by neuropeptide Y, tyrosine hydroxylase and serotonin immunoreactive fibers. In this study, the possibility that a structure similar to the reptilian olfactostriatum might be present in the mammalian brain has been investigated. Injections of dextran-amines have been aimed at the posteromedial cortical amygdaloid nucleus (the putative mammalian homologue of the reptilian nucleus sphericus) of rats and mice. The resulting anterograde labeling includes the olfactory tubercle, the islands of Calleja and sparse terminal fields in the shell of the nucleus accumbens and ventral pallidum. This projection has been confirmed by injections of retrograde tracers into the ventral striato-pallidum that render retrograde labeling in the posteromedial cortical amygdaloid nucleus. The analysis of the distribution of neuropeptide Y, tyrosine hydroxylase, serotonin and substance P in the ventral striato-pallidum of rats, and the anterograde tracing of the vomeronasal amygdaloid input in the same material confirm that, similar to reptiles, the ventral striatum of mammals includes a specialized vomeronasal structure (olfactory tubercle and islands of Calleja) displaying dense neuropeptide Y-, tyrosine hydroxylase- and serotonin-immunoreactive innervations. The possibility that parts of the accumbens shell and/or ventral pallidum could be included in the mammalian olfactostriatum cannot be discarded.