Distribution of CGRP-like immunoreactivity in the chick and quail brain

Functional Analysis and Tissue-Specific Expression of Calcitonin and CGRP with RAMP-Modulated Receptors CTR and CLR in Chickens

Calcitonin (CT) and calcitonin gene-related peptide (CGRP) are critical regulators of calcium balance and have extensive implications for vertebrate physiological processes. This study explores the CT and CGRP signaling systems in chickens through cloning and characterization of the chicken calcitonin receptor (CTR) and calcitonin receptor-like receptor (CLR), together with three receptor activity-modifying proteins (RAMPs). We illuminated the functional roles for chickens between the receptors examined alone and in RAMP-associated complexes using luciferase reporter assays. Chicken CTRs and CLRs stimulated the cAMP/PKA and MAPK/ERK signaling pathways, signifying their functional receptor status, with CT showing appreciable ligand activity at nanomolar concentrations across receptor combinations. Notably, it is revealed that chicken CLR can act as a functional receptor for CT without or with RAMPs. Furthermore, we uncovered a tissue-specific expression profile for CT, CGRP, CTR, CLR, and RAMPs in chickens, indicating the different physiological roles across various tissues. In conclusion, our data establish a clear molecular basis to reveal information on CT, CGRP, CTR, CLR, and RAMPs in chickens and contribute to understanding the conserved or divergent functions of this family in vertebrates.

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.

Heterogeneous organization and connectivity of the chicken auditory thalamus (Gallus gallus)

The auditory ascending system contains parallel pathways in vertebrate brains. In chickens (Gallus gallus), three pathways arise from nucleus laminaris (NL), nucleus angularis (NA), and regio intermedius (RI) in the brainstem, innervating three subdivisions of the nucleus mesencephalicus lateralis pars dorsalis (MLd) in the midbrain. The current study reveals the segregation of these pathways in their subsequent projections to the nucleus ovoidalis (Ov) in the thalamus. Based on cytoarchitecture and myelin distribution, we identified seven Ov subregions, including five neuronal clusters within the Ov proper, the nucleus semilunaris parovoidalis (SPO), and the circum-ovoidalis (cOv). Immunocytochemistry further revealed that a ventromedial cluster of the Ov proper (Ovvm) contains unique cell types expressing α8 subunit nicotinic acetylcholine receptor, while SPO and cOv are characterized with expression of calcitonin-gene-related peptide and cholecystokinin. Tract tracing studies demonstrated that Ovvm is a major target of the NL-recipient zone of MLd, while the RI-recipient zone of MLd predominantly projects to a ventrolateral cluster of the Ov proper. Afferent inputs to the remaining regions of the Ov proper mostly arise from the NA-recipient zone of MLd. SPO and cOv receive a projection from the surrounding areas of MLd, named the nucleus intercollicularis. Importantly, the Ov proper, SPO and cOv all project to the Field L2 in the forebrain, the avian auditory cortex. Taken together, these results demonstrate that the avian auditory thalamus is a structurally and functionally heterogeneous structure, implicating an important role in generating novel representations for specific acoustic features.

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.

Levels of homology and the problem of neocortex

The neocortex is found only in mammals, and the fossil record is silent on how this soft tissue evolved. Understanding neocortex evolution thus devolves to a search for candidate homologous neocortex traits in the extant nonmammalian amniotes. The difficulty is that homology is based on similarity, and the six-layered neocortex structure could hardly be more dissimilar in appearance from the nuclear organization that is so conspicuous in the dorsal telencephalon of birds and other reptiles. Recent molecular data have, however, provided new support for one prominent hypothesis, based on neuronal circuits, that proposes the principal neocortical input and output cell types are a conserved feature of amniote dorsal telencephalon. Many puzzles remain, the greatest being understanding the selective pressures and molecular mechanisms that underlie such tremendous morphological variation in telencephalon structure.

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.

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.

Calcitonin gene-related peptide (CGRP) and its receptor components in human and rat spinal trigeminal nucleus and spinal cord at C1-level

Background

Calcitonin gene-related peptide (CGRP) has a key role in migraine pathophysiology and is associated with activation of the trigeminovascular system. The trigeminal ganglion, storing CGRP and its receptor components, projects peripheral to the intracranial vasculature and central to regions in the brainstem with Aδ- and C-fibers; this constitutes an essential part of the pain pathways activated in migraine attacks. Therefore it is of importance to identify the regions within the brainstem that processes nociceptive information from the trigeminovascular system, such as the spinal trigeminal nucleus (STN) and the C1-level of the spinal cord. Immunohistochemistry was used to study the distribution and relation between CGRP and its receptor components - calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1) - in human and rat STN and at the C1-level, using a set of newly well characterized antibodies. In addition, double-stainings with CGRP and myelin basic protein (MBP, myelin), synaptophysin (synaptic vesicles) or IB4 (C-fibers in general) were performed.

Results

In the STN, the highest density of CGRP immunoreactive fibers were found in a network around fiber bundles in the superficial laminae. CLR and RAMP1 expression were predominately found in fibers in the spinal trigeminal tract region, with some fibers spanning into the superficial laminae. Co-localization between CGRP and its receptor components was not noted. In C1, CGRP was expressed in fibers of laminae I and II. The CGRP staining was similar in rat, except for CGRP positive neurons that were found close to the central canal. In C1, the receptor components were detected in laminae I and II, however these fibers were distinct from fibers expressing CGRP as verified by confocal microscopy.

Conclusions

This study demonstrates the detailed expression of CGRP and its receptor components within STN in the brainstem and in the spinal cord at C1-level, and shows the possibility of CGRP acting postjunctionally in these areas putatively involved in primary headaches.

Core-and-belt organisation of the mesencephalic and forebrain auditory centres in turtles: expression of calcium-binding proteins and metabolic activity

The distribution of immunoreactivity to the calcium-binding proteins parvalbumin, calbindin and calretinin and of cytochrome oxidase activity was studied in the mesencephalic (torus semicircularis), thalamic (nucleus reuniens) and telencephalic (ventromedial part of the anterior dorsal ventricular ridge) auditory centres of two chelonian species Emys orbicularis and Testudo horsfieldi. In the torus semicircularis, the central nucleus (core) showed intense parvalbumin immunoreactivity and high cytochrome oxidase activity, whereas the laminar nucleus (belt) showed low cytochrome oxidase activity and dense calbindin/calretinin immunoreactivity. Within the central nucleus, the central and peripheral areas could be distinguished by a higher density of parvalbumin immunoreactivity and cytochrome oxidase activity in the core than in the peripheral area. In the nucleus reuniens, the dorsal and ventromedial (core) regions showed high cytochrome oxidase activity and immunoreactivity to all three calcium-binding proteins, while its ventrolateral part (belt) was weakly immunoreactive and showed lower cytochrome oxidase activity. In the telencephalic auditory centre, on the other hand, no particular region differed in either immunoreactivity or cytochrome oxidase activity. Our findings provide additional arguments in favour of the hypothesis of a core-and-belt organisation of the auditory sensory centres in non-mammalian amniotes though this organisation is less evident in higher order centres. The data are discussed in terms of the evolution of the auditory system in amniotes.

Insight without cortex: lessons from the avian brain

Insight is a cognitive feature that is usually regarded as being generated by the neocortex and being present only in humans and possibly some closely related primates. In this essay we show that especially corvids display behavioral skills within the domains of object permanence, episodic memory, theory of mind, and tool use/causal reasoning that are insightful. These similarities between humans and corvids at the behavioral level are probably the result of a convergent evolution. Similarly, the telencephalic structures involved in higher cognitive functions in both species show a high degree of similarity, although the forebrain of birds has no cortex-like lamination. The neural substrate for insight-related cognitive functions in mammals and birds is thus not necessarily based on a laminated cortical structure but can be generated by differently organized forebrains. Hence, neither is insight restricted to mammals, as predicted from a "scala naturae", nor is the laminated cortex a prerequisite for the highest cognitive functions.

Thalamic gating of auditory responses in telencephalic song control nuclei

In songbirds, nucleus Uvaeformis (Uva) is the sole thalamic input to the telencephalic nucleus HVC (used as a proper name), a sensorimotor structure essential to learned song production that also exhibits state-dependent responses to auditory presentation of the bird's own song (BOS). The role of Uva in influencing HVC auditory activity is unknown. Using in vivo extracellular and intracellular recordings in urethane-anesthetized zebra finches, we characterized the auditory properties of Uva and examined its influence on auditory activity in HVC and in the telencephalic nucleus interface (NIf), the main auditory afferent of HVC and a corecipient of Uva input. We found robust auditory activity in Uva and determined that Uva is innervated by the ventral nucleus of lateral lemniscus, an auditory brainstem component. Thus, Uva provides a direct linkage between the auditory brainstem and HVC. Although low-frequency electrical stimulation in Uva elicited short-latency depolarizing postsynaptic potentials in HVC neurons, reversibly silencing Uva exerted little effect on BOS-evoked activity in HVC neurons. However, high-frequency stimulation in Uva suppressed auditory-evoked synaptic and suprathreshold activity in all HVC neuron types, a process accompanied by decreased input resistance of individual HVC neurons. Furthermore, high-frequency stimulation in Uva simultaneously suppressed auditory activity in HVC and NIf. These results suggest that Uva can gate auditory responses in HVC through a mechanism that involves inhibition local to HVC as well as withdrawal of auditory-evoked excitatory drive from NIf. Thus, Uva could play an important role in state-dependent gating of auditory activity in telencephalic sensorimotor structures important to learned vocal control.

Evolution of the CT/CGRP family: comparative study with new data from models of teleosts, the eel, and cephalopod molluscs, the cuttlefish and the nautilus

In mammals, alternative splicing of the calcitonin gene generates two distinct peptides: calcitonin (CT), synthesised in the thyroid C cells and involved in the regulation of calcium metabolism, and calcitonin gene-related peptide (CGRP), brain neuromediator synthesised in the peripheral and central nerves. CGRP is well represented and molecularly conserved during evolution whereas CT has not been detected in any of the invertebrates analysed so far. In order to better understand the evolution of this CT/CGRP peptide family we reviewed the major data concerning its evolution from the literature and our recent data obtained in models of teleosts and cephalopod molluscs. The presence of both CGRP-like molecules and its specific bindings sites in the central nervous system of eel, cuttlefish and nautilus, suggests that the brain neurotransmitter role of CGRP could represent an ancient role in metazoa, already present in cephalopods and conserved among vertebrates, as still observed in mammals. In contrast, the presence of CGRP specific binding sites, and not the peptide itself, in the gills suggests an endocrine role for CGRP, in cephalopods and teleosts, that may have been lost during the evolution of the tetrapod lineage. These data, and the absence of CT-like molecules that we observed in cephalopods, support the hypothesis that CGRP represents the ancestral molecule of the CT/CGRP family, appeared in metazoa before the vertebrate emergence. The distinction between CT and CGRP receptors appears to be an event posterior to the emergence of ecdysozoan and lophotrochozoan protostomes, probably in relation to the CT appearance. The evolution of the CT/CGRP peptide family is probably similar to the evolution of the CT/CGRP receptor family. In fact, the genic duplication that induced the appearance of the two separate molecules, CT and CGRP, may constitute an event close to that, which induced the appearance of the two specific receptors. These events remain to be further studied in order to better understand the peptide and receptor evolution of the CT/CGRP family.

Calcitonin is expressed in the chicken pituitary gland: influence of gonadal steroids and sexual maturation

Calcitonin (CT) is primarily produced by the thyroid C cells in mammals or by the ultimobranchial gland in chickens. CT is also expressed by the pituitary gland in rats in which it functions as a paracrine factor causing decreased lactotroph proliferation and prolactin (PRL) secretion. Gonadal steroids influence CT expression in the rat pituitary gland. However, the expression of the CT gene in the pituitary gland of chickens or of any other avian species has not previously been reported. We have tested the hypotheses that CT is expressed in the chicken pituitary gland, and that its expression is influenced by sexual maturation or in response to ovarian steroid administration. We have detected robust expression of CT cDNA in the chicken pituitary gland by reverse transcription/polymerase chain reaction (PCR). The sequence of the pituitary-derived CT cDNA is identical to that of the ultimobranchial gland. CT-immunoreactive (ir) cells have been observed throughout the anterior pituitary gland by confocal microscopy. Many of the PRL-ir cells show co-localization with CT-ir cells. Quantitative real-time PCR analysis has revealed an inverse relationship between the quantities of PRL mRNA and CT mRNA in the pituitary gland: sexually mature hens contain lower amounts of CT mRNA but larger quantities of PRL mRNA compared with sexually immature chickens. Estradiol and/or progesterone treatment of sexually immature chickens leads to a significant decrease in the quantity of pituitary CT mRNA relative to that in the vehicle-treated chickens. We conclude that pituitary CT plays an important paracrine/autocrine role in the control of lactotroph function and PRL secretion in the chicken.

Contact-call driven and tone-driven zenk expression in the nucleus ovoidalis of the budgerigar (Melopsittacus undulatus)

The effectiveness of species-typical contact calls and a 3-kHz pure tone to induce zenk gene protein expression in the primary thalamic auditory relay nucleus ovoidalis was compared in budgerigars (Melopsittacus undulatus), a parrot species capable of lifelong vocal learning. Ovoidalis consists of a core which projects topographically to field L of the telencephalon and a ventromedial shell containing many calcitonin-gene-related peptide neurons that project throughout field L as well as to an adjacent field receiving visual input. Tone-induced and call-induced zenk expression in the ovoidalis core were similar; however, call-induced zenk expression in ventromedial ovoidalis shell was significantly greater than tone-induced expression. These results support the idea that the ovoidalis shell may contain neurons specialized to process complex sounds including species-typical communication sounds.

Organization and evolution of the avian forebrain

Early 20th-century comparative anatomists regarded the avian telencephalon as largely consisting of a hypertrophied basal ganglia, with thalamotelencephalic circuitry thus being taken to be akin to thalamostriatal circuitry in mammals. Although this view has been disproved for more than 40 years, only with the recent replacement of the old telencephalic terminology that perpetuated this view by a new terminology reflecting more accurate understanding of avian brain organization has the modern view of avian forebrain organization begun to become more widely appreciated. The modern view, reviewed in the present article, recognizes that the avian basal ganglia occupies no more of the telencephalon than is typically the case in mammals, and that it plays a role in motor control and motor learning as in mammals. Moreover, the vast majority of the telencephalon in birds is pallial in nature and, as true of cerebral cortex in mammals, provides the substrate for the substantial perceptual and cognitive abilities evident among birds. While the evolutionary relationship of the pallium of the avian telencephalon and its thalamic input to mammalian cerebral cortex and its thalamic input remains a topic of intense interest, the evidence currently favors the view that they had a common origin from forerunners in the stem amniotes ancestral to birds and mammals.

The common organization of the amygdaloid complex in tetrapods: new concepts based on developmental, hodological and neurochemical data in anuran amphibians

Research over the last few years has demonstrated that the amygdaloid complex in amniotes shares basic developmental, hodological and neurochemical features. Furthermore, homolog territories of all main amygdaloid subdivisions have been recognized among amniotes, primarily highlighted by the common expression patterns for numerous developmental genes. With the achievement of new technical approaches, the study of the precise neuroanatomy of the telencephalon of the anuran amphibians has been possible, revealing that most of the structures present in amniotes are recognizable in these anamniotes. Thus, recent investigations have yielded enough results to support the notion that the organization of the anuran amygdaloid complex includes subdivisions with origin in ventral pallial and subpallial territories, a strong relationship with the vomeronasal and olfactory systems, abundant intra-amygdaloid connections, a main output center involved in the autonomic system, profuse amygdaloid fiber systems, and distinct chemoarchitecture. When all these new data about the development, connectivity and neurochemistry of the amygdaloid complex in anurans are taken into account, it becomes patent that a basic organization pattern is shared by both amniotic and anamniotic tetrapods.

The avian 'prefrontal cortex' and cognition

Both mammals and birds can flexibly organize their behavior over time. In mammals, the mental operations generating this ability are called executive functions and are associated with the prefrontal cortex. The corresponding structure in birds is the nidopallium caudolaterale. Anatomical, neurochemical, electrophysiological and behavioral studies show these structures to be highly similar. The avian forebrain displays no lamination that corresponds to the mammalian neocortex, hence lamination does not seem to be a requirement for higher cognitive functions. Because all other aspects of the neural architecture of the mammalian and the avian prefrontal areas are extremely comparable, the freedom to create different neural architectures that generate prefrontal functions seems to be very limited.

Subpallial amygdala and nucleus taeniae in birds resemble extended amygdala and medial amygdala in mammals in their expression of markers of regional identity

Two regions were recently recognized as subpallial amygdaloid nuclei in birds, the nucleus taeniae of the amygdala (TnA) and the newly identified subpallial amygdala (SpA). Here we further confirm these nuclei to be subpallial and amygdaloid and show similarity to specific mammalian subpallial amygdaloid nuclei. By its topological, connectional and neurochemical traits, avian TnA has been suggested to be comparable to mammalian medial amygdala (MeA) and SpA to be comparable to the sublenticular part of mammalian extended amygdala (ExA). We examined molecular traits of these areas using immunohistochemistry for limbic system-associated membrane protein (LAMP) and in situ hybridization for glutamic acid decarboxylase-65 (GAD65) and chicken ovalbumin upstream promoter-transcription factor II (COUP-TF II). Mammalian GAD65 is a subpallial marker and was enriched in ExA and MeA. Chick GAD65 was enriched in SpA and TnA, indicating that they are subpallial. LAMP, which is enriched in limbic regions such as mammalian ExA and MeA, was enriched in avian SpA and TnA. COUP-TF II was enriched in mammalian amygdala including MeA and ExA to a lesser extent. In birds, COUP-TF II was enriched in TnA and moderate in SpA. Overlap of these markers confirms avian TnA resembles mammalian MeA and SpA resembles ExA.

Amygdalostriatal projections in reptiles: A tract-tracing study in the lizardPodarcis hispanica

Whereas the lacertilian anterior dorsal ventricular ridge contains unimodal sensory areas, its posterior part (PDVR) is an associative center that projects to the hypothalamus, thus being comparable to the amygdaloid formation. To further understand the organization of the reptilian cerebral hemispheres, we have used anterograde and retrograde tracing techniques to study the projections from the PDVR and adjoining areas (dorsolateral amygdala, DLA; deep lateral cortex, dLC; nucleus sphericus, NS) to the striatum in the lizard Podarcis hispanica. This information is complemented with a detailed description of the organization of the basal telencephalon of Podarcis. The caudal aspect of the dorsal ventricular ridge projects nontopographically mainly (but not exclusively) to the ventral striatum. The NS projects bilaterally (with strong ipsilateral dominance) to the nucleus accumbens, thus recalling the posteromedial cortical amygdala of mammals. The PDVR (especially its lateral aspect) and the dLC project massively to a continuum of structures connecting the striatoamygdaloid transition area (SAT) and the nucleus accumbens (rostrally), the projection arising from the dLC being probably bilateral. Finally, the DLA projects massively and bilaterally to both the ventral and dorsal striatum, including the SAT. Our findings lend further support to the view that the PDVR and neighboring structures constitute the reptilian basolateral amygdala and indicate that an emotional brain was already present in the ancestral amniote. These results are important to understand the comparative significance of the caudal aspect of the amniote cerebral hemispheres, and specifically challenge current views on the nature of the avian caudal neostriatum.

Songbirds and the revised avian brain nomenclature

It has become increasingly clear that the standard nomenclature for many telencephalic and related brainstem structures of the avian brain is based on flawed once-held assumptions of homology to mammalian brain structures, greatly hindering functional comparisons between avian and mammalian brains. This has become especially problematic for those researchers studying the neurobiology of birdsong, the largest single group within the avian neuroscience community. To deal with the many communication problems this has caused among researchers specializing in different vertebrate classes, the Avian Brain Nomenclature Forum, held at Duke University from July 18-20, 2002, set out to develop a new terminology for the avian telencephalon and some allied brainstem cell groups. In one major step, the erroneous conception that the avian telencephalon consists mainly of a hypertrophied basal ganglia has been purged from the telencephalic terminology, and the actual parts of the basal ganglia and its brainstem afferent cell groups have been given new names to reflect their now-evident homologies. The telencephalic regions that were incorrectly named to reflect presumed homology to mammalian basal ganglia have been renamed as parts of the pallium. The prefixes used for the new names for the pallial subdivisions have retained most established abbreviations, in an effort to maintain continuity with the pre-existing nomenclature. Here we present a brief synopsis of the inaccuracies in the old nomenclature, a summary of the nomenclature changes, and details of changes for specific songbird vocal and auditory nuclei. We believe this new terminology will promote more accurate understanding of the broader neurobiological implications of song control mechanisms and facilitate the productive exchange of information between researchers studying avian and mammalian systems.

Chemoarchitectonic subdivisions of the songbird septum and a comparative overview of septum chemical anatomy in jawed vertebrates

Available data demonstrate that the avian septal region shares a number of social behavior functions and neurochemical features in common with mammals. However, the structural and functional subdivisions of the avian septum remain largely unexplored. In order to delineate chemoarchitectural zones of the avian septum, we prepared a large dataset of double-, triple-, and quadruple-labeled material in a variety of songbird species (finches and waxbills of the family Estrildidae and a limited number of emberizid sparrows) using antibodies against 10 neuropeptides and enzymes. Ten septal zones were identified that were placed into lateral, medial, caudocentral, and septohippocampal divisions, with the lateral and medial divisions each containing multiple zones. The distributions of numerous immunoreactive substances in the lateral septum closely match those of mammals (i.e., distributions of met-enkephalin, vasotocin, galanin, calcitonin gene-related peptide, tyrosine hydroxylase, vasoactive intestinal polypeptide, substance P, corticotropin-releasing factor, and neuropeptide Y), enabling detailed comparisons with numerous chemoarchitectonic zones of the mammalian lateral septum. Our septohippocampal and caudocentral divisions are topographically comparable to the mammalian septohippocampal and septofimbrial nuclei, respectively, although additional data will be required to establish homology. The present data also demonstrate the presence of a medial septal nucleus that is histochemically comparable to the medial septum of mammals. The avian medial septum is clearly defined by peptidergic markers and choline acetyltransferase immunoreactivity. These findings should provide a useful framework for functional and comparative studies, as they suggest that many features of the septum are highly conserved across vertebrate taxa.

Revised nomenclature for avian telencephalon and some related brainstem nuclei

The standard nomenclature that has been used for many telencephalic and related brainstem structures in birds is based on flawed assumptions of homology to mammals. In particular, the outdated terminology implies that most of the avian telencephalon is a hypertrophied basal ganglia, when it is now clear that most of the avian telencephalon is neurochemically, hodologically, and functionally comparable to the mammalian neocortex, claustrum, and pallial amygdala (all of which derive from the pallial sector of the developing telencephalon). Recognizing that this promotes misunderstanding of the functional organization of avian brains and their evolutionary relationship to mammalian brains, avian brain specialists began discussions to rectify this problem, culminating in the Avian Brain Nomenclature Forum held at Duke University in July 2002, which approved a new terminology for avian telencephalon and some allied brainstem cell groups. Details of this new terminology are presented here, as is a rationale for each name change and evidence for any homologies implied by the new names. Revisions for the brainstem focused on vocal control, catecholaminergic, cholinergic, and basal ganglia-related nuclei. For example, the Forum recognized that the hypoglossal nucleus had been incorrectly identified as the nucleus intermedius in the Karten and Hodos (1967) pigeon brain atlas, and what was identified as the hypoglossal nucleus in that atlas should instead be called the supraspinal nucleus. The locus ceruleus of this and other avian atlases was noted to consist of a caudal noradrenergic part homologous to the mammalian locus coeruleus and a rostral region corresponding to the mammalian A8 dopaminergic cell group. The midbrain dopaminergic cell group in birds known as the nucleus tegmenti pedunculopontinus pars compacta was recognized as homologous to the mammalian substantia nigra pars compacta and was renamed accordingly; a group of gamma-aminobutyric acid (GABA)ergic neurons at the lateral edge of this region was identified as homologous to the mammalian substantia nigra pars reticulata and was also renamed accordingly. A field of cholinergic neurons in the rostral avian hindbrain was named the nucleus pedunculopontinus tegmenti, whereas the anterior nucleus of the ansa lenticularis in the avian diencephalon was renamed the subthalamic nucleus, both for their evident mammalian homologues. For the basal (i.e., subpallial) telencephalon, the actual parts of the basal ganglia were given names reflecting their now evident homologues. For example, the lobus parolfactorius and paleostriatum augmentatum were acknowledged to make up the dorsal subdivision of the striatal part of the basal ganglia and were renamed as the medial and lateral striatum. The paleostriatum primitivum was recognized as homologous to the mammalian globus pallidus and renamed as such. Additionally, the rostroventral part of what was called the lobus parolfactorius was acknowledged as comparable to the mammalian nucleus accumbens, which, together with the olfactory tubercle, was noted to be part of the ventral striatum in birds. A ventral pallidum, a basal cholinergic cell group, and medial and lateral bed nuclei of the stria terminalis were also recognized. The dorsal (i.e., pallial) telencephalic regions that had been erroneously named to reflect presumed homology to striatal parts of mammalian basal ganglia were renamed as part of the pallium, using prefixes that retain most established abbreviations, to maintain continuity with the outdated nomenclature. We concluded, however, that one-to-one (i.e., discrete) homologies with mammals are still uncertain for most of the telencephalic pallium in birds and thus the new pallial terminology is largely devoid of assumptions of one-to-one homologies with mammals. The sectors of the hyperstriatum composing the Wulst (i.e., the hyperstriatum accessorium intermedium, and dorsale), the hyperstriatum ventrale, the neostriatum, and the archistriatum have been renamed (respectively) the hyperpallium (hypertrophied pallium), the mesopallium (middle pallium), the nidopallium (nest pallium), and the arcopallium (arched pallium). The posterior part of the archistriatum has been renamed the posterior pallial amygdala, the nucleus taeniae recognized as part of the avian amygdala, and a region inferior to the posterior paleostriatum primitivum included as a subpallial part of the avian amygdala. The names of some of the laminae and fiber tracts were also changed to reflect current understanding of the location of pallial and subpallial sectors of the avian telencephalon. Notably, the lamina medularis dorsalis has been renamed the pallial-subpallial lamina. We urge all to use this new terminology, because we believe it will promote better communication among neuroscientists. Further information is available at http://avianbrain.org

Distribution of corticotropin-releasing factor-immunoreactive neurons in the central nervous system of the domestic chicken and Japanese quail

In birds, as in mammals, corticotropin-releasing factor (CRF) is present in a number of extrahypothalamic brain regions, indicating that CRF may play a role in physiological and behavioral responses other than the control of adrenocorticotropin hormone release by the pituitary. To provide a foundation for investigation of the roles of CRF in the control of avian behavior, the distribution of CRF immunoreactivity was determined throughout the central nervous system of the domestic chicken (Gallus domesticus) and Japanese quail (Coturnix japonica). The distribution of CRF-immunoreactive (-ir) perikarya and fibers in the chicken and quail brain was found to be more extensive than previously reported, notably in the telencephalon. Numerous CRF-ir perikarya and fibers were present in the hyperstriatum, hippocampus, neostriatum, lobus parolfactorius, and archistriatum, as well as in the nucleus taeniae, nucleus accumbens, and bed nucleus of the stria terminalis, which exhibited the strongest immunolabeling in the telencephalon. The presence of dense populations of CRF-ir perikarya in the medial lobus parolfactorius, nucleus of the stria terminalis, and paleostriatum ventrale, apparently giving rise to CRF-ir projections to the mesencephalic reticular formation, the parabrachial/pericerulear region, and the dorsal vagal complex, suggests that these telencephalic areas may constitute part of the avian "central extended amygdala." These results have important implications for understanding the role of extrahypothalamic CRF systems in emotional responses in birds.

Organization of the avian basal forebrain: chemical anatomy in the parrot (Melopsittacus undulatus)

Hodological, electrophysiological, and ablation studies indicate a role for the basal forebrain in telencephalic vocal control; however, to date the organization of the basal forebrain has not been extensively studied in any nonmammal or nonhuman vocal learning species. To this end the chemical anatomy of the avian basal forebrain was investigated in a vocal learning parrot, the budgerigar (Melopsittacus undulatus). Immunological and histological stains, including choline acetyltransferase, acetylcholinesterase, tyrosine hydroxylase, dopamine and cAMP-regulated phosphoprotein (DARPP)-32, the calcium binding proteins calbindin D-28k and parvalbumin, calcitonin gene-related peptide, iron, substance P, methionine enkephalin, nicotinamide adenine dinucleotide phosphotase diaphorase, and arginine vasotocin were used in the present study. We conclude that the ventral paleostriatum (cf. Kitt and Brauth [1981] Neuroscience 6:1551-1566) and adjacent archistriatal regions can be subdivided into several distinct subareas that are chemically comparable to mammalian basal forebrain structures. The nucleus accumbens is histochemically separable into core and shell regions. The nucleus taeniae (TN) is theorized to be homologous to the medial amygdaloid nucleus. The archistriatum pars ventrolateralis (Avl; comparable to the pigeon archistriatum pars dorsalis) is theorized to be a possible homologue of the central amygdaloid nucleus. The TN and Avl are histochemically continuous with the medial aspects of the bed nucleus of the stria terminalis and the ventromedial striatum, forming an avian analogue of the extended amygdala. The apparent counterpart in budgerigars of the mammalian nucleus basalis of Meynert consists of a field of cholinergic neurons spanning the basal forebrain. The budgerigar septal region is theorized to be homologous as a field to the mammalian septum. Our results are discussed with regard to both the evolution of the basal forebrain and its role in vocal learning processes.

Calcitonin gene-related peptide immunoreactive cells and fibers in forebrain vocal and auditory nuclei of the budgerigar (Melopsittacus undulatus)

The distributions of calcitonin gene-related peptide (CGRP) immunoreactive neurons and fibers were mapped within forebrain vocal control and auditory nuclei of a vocal learning psittacine species, the budgerigar (Melopsittacus undulatus). Immunoreactivity was exhibited by telencephalic nuclei previously associated with vocal control pathways on the basis of both tract tracing studies and gene mapping: the central nucleus of the anterior archistriatum (AAc), central nucleus of the lateral neostriatum (NLc), magnocellular nucleus the lobus parolfactorius (LPOm), the oval nucleus of the ventral hyperstiratum (HVo) and the medial division of the oval nucleus of the anterior neostriatum (NAom). The main body of NAo also contained an exceptionally high density of immunoreactive fibers. In contrast to the condition in oscine songbirds, CGRP-positive neuronal somata were not present in any telencephalic vocal control nucleus. CGRP-positive somata were present, however, in diencephalic cell groups that included the shell region of the nucleus ovoidalis (Ov), the nucleus dorsolateralis posterior (DLP) and a region of the ventral thalamus that was retrogradely labeled by tracer deposits into HVo and AAc. CGRP immunoreactive fibers were observed within auditory areas of the telencephalon including Field L and the neostriatum intermedium pars dorsolateralis. The likely sources of these fibers are CGRP-positive neurons within the Ov shell and DLP.

Distribution of calcitonin gene-related peptide-like immunoreactivity in the brain of the lizard Podarcis hispanica

The present work studies the distribution of calcitonin gene-related peptide-immunoreactive (CGRP-li) neurons and fibers in the brain of a reptile, the lizard Podarcis hispanica. CGRP-li perikarya were not present in the telencephalon. In the thalamus, CGRP-li perikarya were restricted to the posteromedial and posterolateral nuclei. In the hypothalamus, CGRP-li cells were found mainly in the supramammillary and mammillary nuclei. In the midbrain and brainstem, CGRP-li cells appeared in the ventral tegmental area, the parabrachial nucleus, and the motor nuclei of the III-VII, IX, X, and XII cranial nerves. Motoneurons of the ventral horn of the spinal cord were also immunoreactive for CGRP. CGRP-li fibers were seen in the telencephalic hemispheres, where a dense plexus of reactive fibers appeared in the septum and in the lateral striatoamygdaloid transition area. From the latter, CGRP-li fibers entered the posterior dorsal ventricular ridge, the cell layer and deep stratum of the ventral lateral cortex, and various amygdaloid nuclei. Parts of the striatum (nucleus accumbens) and pallidum also displayed CGRP-li innervation. In the diencephalon, CGRP-li innervation was observed in parts of the dorsal thalamus and in the periventricular and medial hypothalamus. The pretectum and deep layers of the optic tectum also showed CGRP-li fibers, and numerous CGRP-li fibers were observed in the midbrain central gray, tegmentum, and pons. Some of the sensory fibers of the trigeminal, vagal, and spinal nerves were also CGRP-li. These results show that the distribution of CGRP-li structures in the reptilian brain is similar to that described for other vertebrates and suggest that the thalamotelencephalic CGRPergic projections appear to be conserved among amniote vertebrates.

Functional aspects of dopamine metabolism in the putative prefrontal cortex analogue and striatum of pigeons (Columba livia)

Dopamine (DA) in mammalian associative structures, such as the prefrontal cortex (PFC), plays a prominent role in learning and memory processes, and its homeostasis differs from that of DA in the striatum, a sensorimotor region. The neostriatum caudolaterale (NCL) of birds resembles the mammalian PFC according to connectional, electrophysiological, and behavioral data. In the present study, DA regulation in the associative NCL and the striatal lobus parolfactorius (LPO) of pigeons was compared to uncover possible differences corresponding to those between mammalian PFC and striatum. Extracellular levels of DA and its metabolites (homovanillic acid [HVA], dihydroxyphenylacetic acid [DOPAC]) and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) were investigated by in vivo microdialysis of urethane-anesthetized pigeons under basal conditions and after systemic administration of D-amphetamine. DA was reliably determined only in LPO dialysates, and DA metabolite levels were significantly higher in LPO than in NCL. The HVA/DOPAC ratio, indicating extracellular lifetime of DA, was more than twice as high in NCL than in LPO dialysates. After amphetamine, DA increased in LPO while still being undetectable in NCL, and DA metabolites decreased in both regions. 5-HIAA slightly decreased in NCL dialysates. Amphetamine effects were delayed in NCL compared with the striatum. In conclusion, effects of amphetamine on the pigeon's ascending monoamine systems resemble those found in mammals, suggesting similar regulatory properties. The neurochemical differences between NCL and LPO parallel those between associative regions, such as PFC and dorsal striatum in mammals. They may reflect weaker regulation of extracellular DA, favoring DAergic volume transmission, in associative than striatal forebrain regions.

The pallial amygdala of amniote vertebrates: evolution of the concept, evolution of the structure

Embryological studies indicate that the amygdala includes pallial structures, namely the cortical amygdala (olfactory and vomeronasal) and the basolateral complex deep to it. In squamate reptiles, the cortical amygdala includes secondary olfactory (the ventral anterior amygdala) and vomeronasal centres (the nucleus sphericus). In birds, the situation is far less clear, due to the relative underdevelopment of the chemosensory systems. The basolateral amygdala of squamate reptiles includes two ventropallial structures: the posterior dorsal ventricular ridge and the lateral amygdala. Like their mammalian counterparts, these centres give rise to glutamatergic projections to the striatal (centromedial) amygdala and the ventromedial hypothalamus. Using the same criteria, the caudal neostriatum and the ventral intermediate archistriatum may represent the ventral pallial amygdala of birds. The basal nucleus of the mammalian amygdala is a lateropallial territory. In reptiles, the lateral pallium includes the dorsolateral amygdala, which, like the mammalian basal nucleus, projects bilaterally to the striatum/accumbens and receives distinct cholinergic and dopaminergic innervations. In the avian brain, the same embryological, hodological, and histochemical criteria are met by the area temporo-parieto-occipitalis, the caudolateral neostriatum and the dorsal intermediate archistriatum. Therefore, the projections from these structures to the paleostriatum and the lobus paraolfactorius are amygdalostriatal, rather than corticostriatal connections.

Comparison of thalamic populations in mammals and birds: expression of ErbB4 mRNA

The expression of ErbB4 mRNA was examined in dorsal thalamic regions of chicks and rats. In rats, ErbB4 expression was observed in the habenular nuclei, the paraventricular nucleus, intermediodorsal nucleus, the central medial thalamic nucleus, the posterior nucleus, the parafascicular nucleus, the subparafascicular nucleus, the suprageniculate nucleus, the posterior limitans nucleus, the medial part of the medial geniculate nucleus, the peripeduncular nucleus, the posterior intralaminar nucleus, the lateral subparafascicular nucleus, the lateral posterior nucleus, and all ventral thalamic nuclei. In chicks, expression was observed in the subhabenular nucleus, the dorsomedialis posterior nucleus, the dorsointermedius posterior nucleus, the nucleus of the septomesencephalic tract, and areas surrounding the rotundus and ovoidalis nuclei, including the subrotundal and suprarotundal areas, and all ventral thalamic nuclei. Most cells within ovoidalis and rotundus were not labeled. The similar pattern of afferent and efferent projections originating from ErbB4-expressing regions of the mammalian and bird dorsal thalamus suggests that ErbB4 hybridizing cells may be derived from a single anlage that migrates into multiple thalamic regions. Most neurons in the rotundus and ovoidalis nuclei of chick may be homologous to unlabeled clusters of neurons intermingled with ErbB4-expressing cells of the mammalian posterior/intralaminar region.

Thoughts on the development, structure and evolution of the mammalian and avian telencephalic pallium

Various lines of evidence suggest that the development and evolution of the mammalian isocortex cannot be easily explained without an understanding of correlative changes in surrounding areas of the telencephalic pallium and subpallium. These are close neighbours in a common morphogenetic field and are postulated as sources of some cortical neuron types (and even of whole cortical areas). There is equal need to explain relevant developmental evolutionary changes in the dorsal thalamus, the major source of afferent inputs to the telencephalon (to both the pallium and subpallium). The mammalian isocortex evolved within an initially small dorsal part of the pallium of vertebrates, surrounded by other pallial parts, including some with a non-cortical, nuclear structure. Nuclear pallial elements are markedly voluminous in reptiles and birds, where they build the dorsal ventricular ridge, or hypopallium, which has been recently divided molecularly and structurally into a lateral pallium and a ventral pallium. Afferent pallial connections are often simplified as consisting of thalamic fibres that project either to focal cell aggregates in the ventral pallium (predominant in reptiles and birds) or to corticoid areas in the dorsal pallium (predominant in mammals). Karten's hypothesis, put forward in 1969, on the formation of some isocortical areas postulates an embryonic translocation into the nascent isocortex of the ventropallial thalamorecipient foci and respective downstream ventropallial target populations, as specific layer IV, layers II- III, or layers V-VI neuron populations. This view is considered critically in the light of various recent data, contrasting with the alternative possibility of a parallel, separate evolution of the different pallial parts. The new scenario reveals as well a separately evolving tiered structure of the dorsal thalamus, some of whose parts receive input from midbrain sensory centres (collothalamic nuclei), whereas other parts receive oligosynaptic 'lemniscal' connections bypassing the midbrain (lemnothalamic nuclei). An ampler look into known hodological patterns from this viewpoint suggests that ancient collothalamic pathways, which target ventropallial foci, are largely conserved in mammals, while some emergent cortical connections can be established by means of new collaterals in some of these pathways. The lemnothalamic pathways, which typically target ancestrally the dorsopallial isocortex, show parallel increments of relative size and structural diversification of both the thalamic cell populations and the cortical recipient areas. The evolving lemnothalamic pathways may interact developmentally with collothalamic corticopetal collaterals in the modality-specific invasion of the emergent new areas of isocortex.