Identification of the anterior nucleus of the ansa lenticularis in birds as the homolog of the mammalian subthalamic nucleus

Neurochemistry and circuit organization of the lateral spiriform nucleus of birds: A uniquely nonmammalian direct pathway component of the basal ganglia

We used diverse methods to characterize the role of avian lateral spiriform nucleus (SpL) in basal ganglia motor function. Connectivity analysis showed that SpL receives input from globus pallidus (GP), and the intrapeduncular nucleus (INP) located ventromedial to GP, whose neurons express numerous striatal markers. SpL-projecting GP neurons were large and aspiny, while SpL-projecting INP neurons were medium sized and spiny. Connectivity analysis further showed that SpL receives inputs from subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr), and that the SNr also receives inputs from GP, INP, and STN. Neurochemical analysis showed that SpL neurons express ENK, GAD, and a variety of pallidal neuron markers, and receive GABAergic terminals, some of which also contain DARPP32, consistent with GP pallidal and INP striatal inputs. Connectivity and neurochemical analysis showed that the SpL input to tectum prominently ends on GABAA receptor-enriched tectobulbar neurons. Behavioral studies showed that lesions of SpL impair visuomotor behaviors involving tracking and pecking moving targets. Our results suggest that SpL modulates brainstem-projecting tectobulbar neurons in a manner comparable to the demonstrated influence of GP internus on motor thalamus and of SNr on tectobulbar neurons in mammals. Given published data in amphibians and reptiles, it seems likely the SpL circuit represents a major direct pathway-type circuit by which the basal ganglia exerts its motor influence in nonmammalian tetrapods. The present studies also show that avian striatum is divided into three spatially segregated territories with differing connectivity, a medial striato-nigral territory, a dorsolateral striato-GP territory, and the ventrolateral INP motor territory.

Songbird subthalamic neurons project to dopaminergic midbrain and exhibit singing-related activity

Skill learning requires motor output to be evaluated against internal performance benchmarks. In songbirds, ventral tegmental area (VTA) dopamine neurons (DA) signal performance errors important for learning, but it remains unclear which brain regions project to VTA and how these inputs may contribute to DA error signaling. Here, we find that the songbird subthalamic nucleus (STN) projects to VTA and that STN microstimulation can excite VTA neurons. We also discover that STN receives inputs from motor cortical, auditory cortical, and ventral pallidal brain regions previously implicated in song evaluation. In the first neural recordings from songbird STN, we discover that the activity of most STN neurons is associated with body movements and not singing, but a small fraction of neurons exhibits precise song timing and performance error signals. Our results place the STN in a pathway important for song learning, but not song production, and expand the territories of songbird brain potentially associated with song learning.NEW & NOTEWORTHY Songbird subthalamic (STN) neurons exhibit singing-related signals and are interconnected with the motor cortical nucleus, auditory pallium, ventral pallidum, and ventral tegmental area, areas important for song generation and learning.

Tangential Intrahypothalamic Migration of the Mouse Ventral Premamillary Nucleus and Fgf8 Signaling

The tuberal hypothalamic ventral premamillary nucleus (VPM) described in mammals links olfactory and metabolic cues with mating behavior and is involved in the onset of puberty. We offer here descriptive and experimental evidence on a migratory phase in the development of this structure in mice at E12.5–E13.5. Its cells originate at the retromamillary area (RM) and then migrate tangentially rostralward, eschewing the mamillary body, and crossing the molecularly distinct perimamillary band, until they reach a definitive relatively superficial ventral tuberal location. Corroborating recent transcriptomic studies reporting a variety of adult glutamatergic cell types in the VPM, and different projections in the adult, we found that part of this population heterogeneity emerges already early in development, during tangential migration, in the form of differential gene expression properties of at least 2–3 mixed populations possibly derived from subtly different parts of the RM. These partly distribute differentially in the core and shell parts of the final VPM. Since there is a neighboring acroterminal source of Fgf8, and Fgfr2 is expressed at the early RM, we evaluated a possible influence of Fgf8 signal on VPM development using hypomorphic Fgf8^neo/null embryos. These results suggested a trophic role of Fgf8 on RM and all cells migrating tangentially out of this area (VPM and the subthalamic nucleus), leading in hypomorphs to reduced cellularity after E15.5 without alteration of the migrations proper.

Topological atlas of the hypothalamus in adult rhesus monkey

The prosomeric model explains the embryological development of the central nervous system (CNS) shared by all vertebrates as a Bauplan. As a primary event, the early neural plate is patterned by intersecting longitudinal plates and transverse segments, forming a mosaic of progenitor units. The hypothalamus is specified by three prosomeres (hp1, hp2, and the acroterminal domain) of the secondary prosencephalon with corresponding alar and basal plate parts, which develop apart from the diencephalon. Mounting evidence suggests that progenitor units within alar and basal plate parts of hp1 and hp2 give rise to distinct hypothalamic nuclei, which preserve their relative invariant positioning (topology) in the adult brain. Nonetheless, the principles of the prosomeric model have not been applied so far to the hypothalamus of adult primates. We parcellated hypothalamic nuclei in adult rhesus monkeys (Macaca mulatta) using various stains to view architectonic boundaries. We then analyzed the topological relations of hypothalamic nuclei and adjacent hypothalamic landmarks with homology across rodent and primate species to trace the origin of adult hypothalamic nuclei to the alar or basal plate components of hp1 and hp2. We generated a novel atlas of the hypothalamus of the adult rhesus monkey with developmental ontologies for each hypothalamic nucleus. The result is a systematic reinterpretation of the adult hypothalamus whose prosomeric ontology can be used to study relationships between the hypothalamus and other regions of the CNS. Further, our atlas may serve as a tool to predict causal patterns in physiological and pathological pathways involving the hypothalamus.

The expression of DARPP-32 in adult male zebra finches (Taenopygia guttata)

Although the catecholaminergic circuitry in the zebra finch brain has been well studied, there is little information regarding the postsynaptic targets of dopamine. To answer this question, we looked at overall patterns of immunoreactivity for DARPP-32 (a dopamine and cAMP-regulated phosphoprotein, present mostly in dopaminoceptive neurons) in adult male zebra finches. Our results demonstrated that as in mammals and other avian species, DARPP-32 expression was highest in both medial and lateral striatum. Interestingly, a specific pattern of immunoreactivity was observed in the song control system, with 'core' song control regions, that is, LMANcore (lateral magnocellular nucleus of the anterior nidopallium), RA (nucleus robustus arcopallialis) and HVC being less immunoreactive for DARPP-32 than 'shell' areas such as LMANshell, RAcup, AId (intermediate arcopallium) and HVCshelf. Our results suggest that whereas dopamine may modulate the shell pathways at various levels of the AFP, dopaminergic modulation of the core pathway occurs mainly through Area X, a basal ganglia nucleus. Further, secondary sensory cortices including the perientopallial belt, Fields L1 and L3 had higher DARPP-32-immunoreactivity than primary sensory cortical areas such as the pallial basolateral nucleus, entopallium proper and Field L2, corresponding to somatosensory, visual and auditory systems, respectively. We also found DARPP-32-rich axon terminals surrounding dopaminergic neurons in the ventral tegmental area-substantia nigra complex which in turn project to the striatum, suggesting that there may be a reciprocal modulation between these regions. Overall, DARPP-32 expression appears to be higher in areas involved in integrating sensory information, which further supports the role of this protein as a molecular integrator of different signal processing pathways.

Parvalbumin-immunoreactive cells in the olfactory bulb of the pigeon: Comparison with the rat

The olfactory bulb (OB) shows special characteristics in its phylogenetic cortical structure and synaptic pattern. In the OB, gamma-aminobutyric acid (GABA), as an inhibitory neurotransmitter, is secreted from GABAergic neurons which contain parvalbumin (a calcium-binding protein). Many studies on the distribution of parvalbumin-immunoreactive neurons in the rodent OB have been published but poorly reported in the avian OB. Therefore, in this study, we compared the structure of the OB and distribution of parvalbumin-immunoreactive neurons in the OB between the rat and pigeon using cresyl violet staining and immunohistochemistry for parvalbumin, respectively. Fundamentally, the pigeon OB showed layers like those of the rat OB; however, some layers were not clear like in the rat OB. Parvalbumin-immunoreactive neurons in the pigeon OB were predominantly distributed in the external plexiform layer like that in the rat OB; however, the neurons did not have long processes like those in the rat. Furthermore, parvalbumin-immunoreactive fibres were abundant in some layers; this finding was not shown in the rat OB. In brief, parvalbumin-immunoreactive neurons were found like those in the rat OB; however, parvalbumin-immunoreactive fibres were significantly abundant in the pigeon OB compared to those in the rat OB.

Dopaminergic modulation of basal ganglia output through coupled excitation-inhibition

Learning and maintenance of skilled movements require exploration of motor space and selection of appropriate actions. Vocal learning and social context-dependent plasticity in songbirds depend on a basal ganglia circuit, which actively generates vocal variability. Dopamine in the basal ganglia reduces trial-to-trial neural variability when the bird engages in courtship song. Here, we present evidence for a unique, tonically active, excitatory interneuron in the songbird basal ganglia that makes strong synaptic connections onto output pallidal neurons, often linked in time with inhibitory events. Dopamine receptor activity modulates the coupling of these excitatory and inhibitory events in vitro, which results in a dynamic change in the synchrony of a modeled population of basal ganglia output neurons receiving excitatory and inhibitory inputs. The excitatory interneuron thus serves as one biophysical mechanism for the introduction or modulation of neural variability in this circuit.

The ventromedial hypothalamic nucleus in the zebra finch (Taeniopygia guttata): Afferent and efferent projections in relation to the control of reproductive behavior

Sex-specific mating behaviors occur in a variety of mammals, with the medial preoptic nucleus (POM) and the ventromedial hypothalamic nucleus (VMH) mediating control of male and female sexual behavior, respectively. In birds, likewise, POM is predominantly involved in the control of male reproductive behavior, but the degree to which VMH is involved in female reproductive behavior is unclear. Here, in male and female zebra finches, a combination of aromatase immunohistochemistry and conventional tract tracing facilitated the definition of two separate but adjacent nuclei in the basal hypothalamus: an oblique band of aromatase-positive (AR+) neurons, and ventromedial to this, an ovoid, aromatase-negative (AR-) nucleus. The AR- nucleus, but not the AR+ nucleus, was here shown to receive a projection from rostral parts of the thalamic auditory nucleus ovoidalis and from the nucleus of the tractus ovoidalis. The AR- nucleus also receives an overlapping, major projection from previously uncharted regions of the medial arcopallium and a minor projection from the caudomedial nidopallium. Both the AR- and the AR+ nuclei project to the intercollicular nucleus of the midbrain. No obvious sex differences in either the pattern of AR immunoreactivity or of the afferent projections to the AR- nucleus were observed. The significance of these results in terms of the acoustic control of avian reproductive behavior is discussed, and a comparison with the organization of VMH afferents in lizards suggests a homologous similarity of the caudal telencephalon in sauropsids.

Dorsal pallidal neurons directly link the nidopallium and midbrain in the zebra finch (Taeniopygia guttata)

The dorsal pallidum in birds is considered similar, if not homologous, to the globus pallidus (GP) of mammals. The dorsal pallidum projects to both thalamic and midbrain targets similar to the direct and indirect pathways arising from the internal and external segments of the GP. In the present study, retrograde and anterograde tracing studies revealed a previously undescribed projection of the avian dorsal pallidum. This arises from a specific dorsomedial component, which terminates in the intercollicular nucleus and partly surrounds the avian equivalent of the central nucleus of the inferior colliculus. The respiratory-vocal dorsomedial nucleus of the intercollicular complex, however, does not receive these projections. The somata of the pallidal neurons retrogradely labeled from injections in the intercollicular nucleus were large and generally multipolar and had extensive, sparsely branching central processes (presumptive dendrites) that together extended up to 2 mm dorsally into the intermediate and caudomedial nidopallium. The size and morphology of these neurons were similar to those of large pallidal neurons labeled by calretinin immunoreactivity, which could be co-localized to the same cells. Thus, rather than being directly involved in the control of movement, the large dorsomedial neurons of the caudal dorsal pallidum may be involved in sensory processing, in that they provide an unusual direct link between sensory (auditory/somatosensory) regions of the nidopallium and sensory regions of the intercollicular nucleus of the midbrain. J. Comp. Neurol. 525:1731-1742, 2017. © 2016 Wiley Periodicals, Inc.

Characterization of the hypothalamus of Xenopus laevis during development. II. The basal regions

The expression patterns of conserved developmental regulatory transcription factors and neuronal markers were analyzed in the basal hypothalamus of Xenopus laevis throughout development by means of combined immunohistochemical and in situ hybridization techniques. The connectivity of the main subdivisions was investigated by in vitro tracing techniques with dextran amines. The basal hypothalamic region is topologically rostral to the basal diencephalon and is composed of the tuberal (rostral) and mammillary (caudal) subdivisions, according to the prosomeric model. It is dorsally bounded by the optic chiasm and the alar hypothalamus, and caudally by the diencephalic prosomere p3. The tuberal hypothalamus is defined by the expression of Nkx2.1, xShh, and Isl1, and rostral and caudal portions can be distinguished by the distinct expression of Otp rostrally and Nkx2.2 caudally. In the mammillary region the xShh/Nkx2.1 combination defined the rostral mammillary area, expressing Nkx2.1, and the caudal retromammillary area, expressing xShh. The expression of xLhx1, xDll4, and Otp in the mammillary area and Isl1 in the tuberal region highlights the boundary between the two basal hypothalamic territories. Both regions are strongly connected with subpallial regions, especially those conveying olfactory/vomeronasal information, and also possess abundant intrahypothalamic connections. They show reciprocal connections with the diencephalon (mainly the thalamus), project to the midbrain tectum, and are bidirectionally related to the rhombencephalon. These results illustrate that the basal hypothalamus of anurans shares many features of specification, regionalization, and hodology with amniotes, reinforcing the idea of a basic bauplan in the organization of this prosencephalic region in all tetrapods.

Evolution of the basal ganglia: dual-output pathways conserved throughout vertebrate phylogeny

The basal ganglia, including the striatum, globus pallidus interna and externa (GPe), subthalamic nucleus (STN), and substantia nigra pars compacta, are conserved throughout vertebrate phylogeny and have been suggested to form a common vertebrate mechanism for action selection. In mammals, this circuitry is further elaborated by the presence of a dual-output nucleus, the substantia nigra pars reticulata (SNr), and the presence of modulatory input from the cholinergic pedunculopontine nucleus (PPN). We sought to determine whether these additional components of the mammalian basal ganglia are also present in one of the phylogenetically oldest vertebrates, the lamprey. We show, by using immunohistochemistry, tract tracing, and whole-cell recordings, that homologs of the SNr and PPN are present in the lamprey. Thus the SNr receives direct projections from inwardly rectifying γ-aminobutyric acid (GABA)-ergic striatal neurons expressing substance P, but it is also influenced by indirect basal ganglia projections from the STN and potentially the GPe. Moreover, GABAergic SNr projection neurons are tonically active and project to the thalamus and brainstem motor areas. The homolog of the PPN contains both cholinergic and GABAergic neurons and is connected with all the nuclei of the basal ganglia, supporting its proposed role as part of an extended basal ganglia. A separate group of cholinergic neurons dorsal to the PPN corresponds to the descending mesencephalic locomotor region. Our results suggest that dual-output nuclei are part of the ancestral basal ganglia and that the PPN appears to have coevolved as part of a mechanism for action selection common to all vertebrates.

Subdivisions of the turtle Pseudemys scripta hypothalamus based on the expression of regulatory genes and neuronal markers

The patterns of distribution of a set of conserved brain developmental regulatory transcription factors and neuronal markers were analyzed in the hypothalamus of the juvenile turtle, Pseudemys scripta. Combined immunohistochemical techniques were used for the identification of the main boundaries and subdivisions in the optic, paraventricular, tuberal, and mammillary hypothalamic regions. The combination of Tbr1 and Pax6 with Nkx2.1 allowed identification of the boundary between the telencephalic preoptic area, rich in Nkx2.1 expression, and the prethalamic eminence, rich in Tbr1 expression. In addition, at this level Nkx2.2 expression defined the boundary between the telencephalon and the hypothalamus. The dorsalmost hypothalamic domain was the supraoptoparaventricular region that was defined by the expression of Otp/Pax6 and the lack of Nkx2.1/Isl1. It is subdivided into rostral, rich in Otp and Nkx2.2, and caudal, only Otp-positive, portions. Ventrally, the suprachiasmatic area was identified by its catecholaminergic groups and the lack of Otp, and could be further divided into a rostral portion, rich in Nkx2.1 and Nkx2.2, and a caudal portion, rich in Isl1 and devoid of Nkx2.1 expression. The expressions of Nkx2.1 and Isl1 defined the tuberal hypothalamus, whereas only the rostral portion expressed Otp. Its caudal boundary was evident by the lack of Isl1 in the adjacent mammillary area, which expressed Nkx2.1 and Otp. All these results provide an important set of data on the interpretation of the hypothalamic organization in a reptile, and hence make a useful contribution to the understanding of hypothalamic evolution.

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.

Evolutionary conservation of the basal ganglia as a common vertebrate mechanism for action selection

BACKGROUND

Although the basal ganglia are thought to play a key role in action selection in mammals, it is unknown whether this mammalian circuitry is present in lower vertebrates as a conserved selection mechanism. We aim here, using lamprey, to elucidate the basal ganglia circuitry in the phylogenetically oldest group of vertebrates (cyclostomes) and determine how this selection architecture evolved to accommodate the increased behavioral repertoires of advanced vertebrates.

RESULTS

We show, using immunohistochemistry, tract tracing, and whole-cell recordings, that all parts of the mammalian basal ganglia (striatum, globus pallidus interna [GPi] and externa [GPe], and subthalamic nucleus [STN]) are present in the lamprey forebrain. In addition, the circuit features, molecular markers, and physiological activity patterns are conserved. Thus, GABAergic striatal neurons expressing substance P project directly to the pallidal output layer, whereas enkephalin-expressing striatal neurons project indirectly via nuclei homologous to the GPe and STN. Moreover, pallidal output neurons tonically inhibit tectum, mesencephalic, and diencephalic motor regions.

CONCLUSIONS

These results show that the detailed basal ganglia circuitry is present in the phylogenetically oldest vertebrates and has been conserved, most likely as a mechanism for action selection used by all vertebrates, for over 560 million years. Our data also suggest that the mammalian basal ganglia evolved through a process of exaptation, where the ancestral core unit has been co-opted for multiple functions, allowing them to process cognitive, emotional, and motor information in parallel and control a broader range of behaviors.

Distribution and ultrastructural features of the serotonin innervation in rat and squirrel monkey subthalamic nucleus

The main purpose of this light and electron microscopic immunocytochemical study was to characterize and compare the serotonin (5-HT) innervation of the subthalamic nucleus (STN) in rats and squirrel monkeys (Saimiri sciureus) following labeling with an antibody against the 5-HT transporter (SERT). Unbiased counts of SERT+ axon varicosities revealed an average density of 5-HT innervation higher in monkeys (1.52 x 10(6) varicosities/mm3) than rats (1.17 x 10(6)), particularly in the anterior half of the nucleus (1.70 x 10(6)). As measured by electron microscopy, SERT+ axon varicosity profiles in the STN of both species were smaller than unlabeled profiles. The number of SERT+ profiles displaying a synaptic junction indicated that, in both rat and monkey STN, approximately half of 5-HT axon varicosities were asynaptic. In monkeys, all synaptic junctions made by SERT+ varicosities were asymmetrical, as opposed to only 77% in rats. Despite the higher density of 5-HT innervation in the anterior half of monkey STN, the ultrastructural features of its SERT+ varicosities, including synaptic incidence, did not significantly differ from those in its posterior half. These findings suggest that, throughout the rat and monkey STN, 5-HT afferents may exert their influence via both synaptic delivery and diffusion of 5-HT, and that an ambient level of 5-HT maintained in STN by these two modes of transmission might also modulate neuronal activity and influence motor behavior. A better understanding of the factors governing the complex interplay between these signaling processes would greatly improve our knowledge of the physiopathology of the STN.

Anatomy of a songbird basal ganglia circuit essential for vocal learning and plasticity

Vocal learning in songbirds requires an anatomically discrete and functionally dedicated circuit called the anterior forebrain pathway (AFP). The AFP is homologous to cortico-basal ganglia-thalamo-cortical loops in mammals. The basal ganglia portion of this pathway, Area X, shares many features characteristic of the mammalian striatum and pallidum, including cell types and connectivity. The AFP also deviates from mammalian basal ganglia circuits in fundamental ways. In addition, the microcircuitry, role of neuromodulators, and function of Area X are still unclear. Elucidating the mechanisms by which both mammalian-like and unique features of the AFP contribute to vocal learning may help lead to a broad understanding of the sensorimotor functions of basal ganglia circuits.

Fate map of the chick embryo neural tube

Fate-map studies have provided important information in relation to the regional topology of brain areas in different vertebrate species. Moreover, these studies have demonstrated that the distribution of presumptive territories in neural plate and neural tube are highly conserved in vertebrates. The aim of this review is to re-examine and correlate the distribution of presumptive neuroepithelial domains in the chick neural tube with molecular information and discuss recent data. First, we review descriptive fate map studies of neural plate in different vertebrate species that have been studied using diverse fate-mapping methods. Then, we summarize the available data on the localization of neuroepithelial progenitors for the brain subregions in the chick neural tube at stage HH10-11, the most used stage for experimental embryology. This analysis is mainly focused on experimental fate mapping results using quail-chick chimeras.

Distribution of glutamate transporter 1 mRNA in the central nervous system of the pigeon (Columba livia)

Glutamate transporter 1 (GLT1) in glial cells removes glutamate that diffuses from the synaptic cleft into the extracellular space. Previously, we have shown the distribution of glutamatergic neurons in the central nervous system (CNS) of the pigeon. In the present study, we identified cDNA sequence of the pigeon GLT1, and mapped the distribution of the mRNA-expressing cells in CNS to examine whether GLT1 is associated with glutamatergic terminal areas. The cDNA sequence of the pigeon GLT1 consisted of 1889bp nucleotides and the amino acids showed 97% and 87% identity to the chicken and human GLT1, respectively. In situ hybridization autoradiograms revealed GLT1 mRNA expression in glial cells and produced regional differences of GLT1 mRNA distribution in CNS. GLT1 mRNA was expressed preferentially in the pallium than the subpallium. Moderate expression was seen in the hyperpallium, Field L, mesopallium, and hippocampal formation. In the thalamus, moderate expression was found in the ovoidal nucleus, rotundal nucleus, triangular nucleus, and lateral spiriform nucleus, while the dorsal thalamic nuclei were weak. In the brainstem, the isthmic nuclei, optic tectum, vestibular nuclei, and cochlear nuclei expressed moderately, but the cerebellar cortex showed strong expression. Bergmann glial cells expressed GLT1 mRNA very strongly. The results indicate that cDNA sequence of the pigeon GLT1 is comparable with that of the mammalian GLT1, and a large number of GLT1 mRNA-expressing areas correspond with areas where AMPA-type glutamate receptors are located. Avian GLT1 in glial cells probably maintain microenvironment of glutamate concentration around synapses as in mammalian GLT1.

Organization of the songbird basal ganglia, including area X

Area X is a songbird basal ganglia nucleus that is required for vocal learning. Both Area X and its immediate surround, the medial striatum (MSt), contain cells displaying either striatal or pallidal characteristics. We used pathway-tracing techniques to compare directly the targets of Area X and MSt with those of the lateral striatum (LSt) and globus pallidus (GP). We found that the zebra finch LSt projects to the GP, substantia nigra pars reticulata (SNr) and pars compacta (SNc), but not the thalamus. The GP is reciprocally connected with the subthalamic nucleus (STN) and projects to the SNr and motor thalamus analog, the ventral intermediate area (VIA). In contrast to the LSt, Area X and surrounding MSt project to the ventral pallidum (VP) and dorsal thalamus via pallidal-like neurons. A dorsal strip of the MSt contains spiny neurons that project to the VP. The MSt, but not Area X, projects to the ventral tegmental area (VTA) and SNc, but neither MSt nor Area X projects to the SNr. Largely distinct populations of SNc and VTA dopaminergic neurons innervate Area X and surrounding the MSt. Finally, we provide evidence consistent with an indirect pathway from the cerebellum to the basal ganglia, including Area X. Area X projections thus differ from those of the GP and LSt, but are similar to those of the MSt. These data clarify the relationships among different portions of the oscine basal ganglia as well as among the basal ganglia of birds and mammals.

The presence and absence of prosencephalic cell groups relaying striatal information to the medial and lateral thalamus in tenrec

Although there are remarkable differences regarding the output organization of basal ganglia between mammals and non-mammals, mammalian species with poorly differentiated brain have scarcely been investigated in this respect. The aim of the present study was to identify the pallidal neurons giving rise to thalamic projections in the Madagascar lesser hedgehog tenrec (Afrotheria). Following tracer injections into the thalamus, retrogradely labelled neurons were found in the depth of the olfactory tubercle (particularly the hilus of the Callejal islands and the insula magna), in subdivisions of the diagonal band complex, the peripeduncular region and the thalamic reticular nucleus. No labelled cells were seen in the globus pallidus. Pallidal neurons were tentatively identified on the basis of their striatal afferents revealed hodologically using anterograde axonal tracer substances and immunohistochemically with antibodies against enkephalin and substance P. The data showed that the tenrec's medial thalamus received prominent projections from ventral pallidal cells as well as from a few neurons within and ventral to the cerebral peduncle. The only regions projecting to the lateral thalamus appeared to be the thalamic reticular nucleus (RTh) and the dorsal peripeduncular nucleus (PpD). On the basis of immunohistochemical data and the topography of its thalamic projections, the PpD was considered to be an equivalent to the pregeniculate nucleus in other mammals. There was no evidence of entopeduncular (internal pallidal) neurons being present within the RTh/PpD complex, neuropils of which did not stain for enkephalin and substance P. The ventrolateral portion of RTh, the only region eventually receiving a striatal input, projected to the caudolateral rather than the rostrolateral thalamus. Thus, the striatopallidal output organization in the tenrec appeared similar, in many respects, to the output organization in non-mammals. This paper considers the failure to identify entopeduncular neurons projecting to the rostrolateral thalamus in a mammal with a little differentiated cerebral cortex, and also stresses the discrepancy between this absence and the presence of a distinct external pallidal segment (globus pallidus).

Early mammillary pouch specification in the course of prechordal ventralization of the forebrain tegmentum

The mammillary body, a ventral specialization of the caudal hypothalamus, lies close to the transition between epichordal and prechordal parts of the forebrain (Puelles and Rubenstein, 2003). This report examines its presumed causal connection with either prechordal or notochordal mesodermal induction, as well as the timing of its specification, in the context of early ventral forebrain patterning. It was recently found that the ephrin receptor gene EphA7 is selectively expressed in the mammillary pouch from early stages of development (HH14: García-Calero et al., 2006). We used mammillary EphA7 expression as well as ventral hypothalamic expression of the gene markers Nkx2.1 and Shh to analyze experimental effects on mammillary specification and morphogenesis after axial mesoderm ablation at stages HH4+ to HH6. Progressively delayed ablation of the prechordal plate revealed its sequential implication in molecular specification of the entire ventral forebrain, including the mammillary and tuberal regions of the hypothalamus. We observed differential contact requirements for induction by the prechordal plate of all the forebrain regions expressing Shh and Nkx2.1, including distant subpallial ones. In contrast, ablation of the anterior notochordal tip at these stages did not elicit significant patterning changes, particularly no effects on mammillary EphA7 expression or mammillary pouch development.

Mammalian and avian neuroanatomy and the question of consciousness in birds

Some birds display behavior reminiscent of the sophisticated cognition and higher levels of consciousness usually associated with mammals, including the ability to fashion tools and to learn vocal sequences. It is thus important to ask what neuroanatomical attributes these taxonomic classes have in common and whether there are nevertheless significant differences. While the underlying brain structures of birds and mammals are remarkably similar in many respects, including high brain-body ratios and many aspects of brain circuitry, the architectural arrangements of neurons, particularly in the pallium, show marked dissimilarity. The neural substrate for complex cognitive functions that are associated with higher-level consciousness in mammals and birds alike may thus be based on patterns of circuitry rather than on local architectural constraints. In contrast, the corresponding circuits in reptiles are substantially less elaborated, with some components actually lacking, and in amphibian brains, the major thalamopallial circuits involving sensory relay nuclei are conspicuously absent. On the basis of these criteria, the potential for higher-level consciousness in these taxa appears to be lower than in birds and mammals.

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.

Cellular Localization of AMPA Type Glutamate Receptor Subunits in the Basal Ganglia of Pigeons (Columba livia)

Corticostriatal and thalamostriatal projections utilize glutamate as a neurotransmitter in mammals and birds. The influence on striatum is mediated, in part, by ionotropic AMPA-type glutamate receptors, which are heteromers composed of GluR1-4 subunits. Although the cellular localization of AMPA-type subunits has been well characterized in mammalian basal ganglia, their localization in avian basal ganglia has not. We thus carried out light microscopic single- and double-label and electron microscopic single-label immunohistochemical studies of GluR1-4 distribution and cellular localization in pigeon basal ganglia. Single-label studies showed that the striatal neuropil is rich in GluR1, GluR2, and GluR2/3 immunolabeling, suggesting the localization of GluR1, GluR2 and/or GluR3 to the dendrites and spines of striatal projection neurons. Double-label studies and perikaryal size distribution determined from single-label material indicated that about 25% of enkephalinergic and 25% of substance P-containing striatal projection neuron perikarya contained GluR1, whereas GluR2 was present in about 75% of enkephalinergic neurons and all substance-P -containing neurons. The perikaryal size distribution for GluR2 compared to GluR2/3 suggested that enkephalinergic neurons might more commonly contain GluR3 than do substance P neurons. Parvalbuminergic and calretininergic striatal interneurons were rich in GluR1 and GluR4, a few cholinergic striatal interneurons possessed GluR2, but somatostatinergic striatal interneurons were devoid of all subunits. The projection neurons of globus pallidus all possessed GluR1, GluR2, GluR2/3 and GluR4 immunolabeling. Ultrastructural analysis of striatum revealed that GluR1 was preferentially localized to dendritic spines, whereas GluR2/3 was found in spines, dendrites, and perikarya. GluR2/3-rich spines were generally larger than GluR1 spines and more frequently possessed perforated post-synaptic densities. These results show that the diverse basal ganglia neuron types each display different combinations of AMPA subunit localization that shape their responses to excitatory input. For striatal projection neurons and parvalbuminergic interneurons, the combinations resemble those for the corresponding cell types in mammals, and thus their AMPA responses to glutamate are likely to be similar.

Differential distribution of L-aspartate- and L-glutamate-immunoreactive structures in the arcopallium and medial striatum of the domestic chick (Gallus domesticus)

The role of amino acid neurotransmitters in learning and memory is well established. We investigated the putative role of L-aspartate as a neurotransmitter in the arcopallial-medial striatal pathway, which is known to be involved in passive avoidance learning in domestic chicks. Double immunocytochemistry against L-aspartate and L-glutamate was performed at both light and electron microscopic levels. L-aspartate- and L-glutamate-immunoreactive neurons in the arcopallium and posterior amygdaloid pallium were identified and counted by using fluorescence microscopy and confocal laser scanning microscopy. Most labeled neurons of arcopallium were enriched in glutamate as well as aspartate. However, the arcopallium and posterior amygdaloid pallium differed from a neighboring telencephalic region (nidopallium; formerly neostriatum) by containing a substantial proportion of cells singly labeled for L-aspartate (15%, vs. 5.3% in the nidopallium). Aspartate-labeled neurons constitute approximately 20%, 25%, 42%, and 28% of total in the posterior amygdaloid pallium and the medial, dorsal, and anterior arcopallia, respectively. Immunoelectron microscopy showed that L-aspartate was enriched in terminals of the medial striatum. The labeled terminals had clear and round vesicles and asymmetric junctions; similar to those immunoreactive to L-glutamate. Axon terminals singly labeled for L-aspartate made up 17% of the total. In addition, 7% of neuronal perikarya and 26% of all dendritic profiles appeared to be labeled specifically with L-aspartate but not L-glutamate. The results indicate that L-aspartate may play a specific role (as distinct from that of L-glutamate) in the intrinsic and extrinsic circuits instrumental in avian learning and memory.

Cognitive effects of dopaminergic and glutamatergic blockade in nucleus accumbens in pigeons

In earlier studies it was found that glutamatergic transmission within the nucleus accumbens septi is involved in the performance of a learned visual shape discrimination in pigeons. This study examines what effects several kinds of glutamate and dopamine antagonists have on the same task. Pigeons were trained with the relevant discrimination, bilaterally implanted with cannulas into the nucleus accumbens and tested after various transmission blockers had been administered intracerebrally. SCH-23390, a D1 dopamine antagonist, at the dose used, had no effect, and Spiperone, a D2-dopamine and 5HT2a-serotonine antagonist, significantly decreased the error repeat trials. CNQX, a non-NMDA glutamate receptor antagonist, and Cycloleucine, an antagonist of the glycine allosteric site of NMDA receptors, had no effect. CGS-19755, a selective competitive NMDA antagonist, significantly impaired performance by significantly decreasing the percent correct trials and increasing the error repeat trials. CPPG, a II/III metabotropic glutamate antagonist, remarkably improved performance. MMPG, a III/II metabotropic glutamate antagonist, at the dose used, did not have any significant effect. The preparation employed may be a useful animal model of perceptual disturbances in schizophrenia.

A new avian brain nomenclature: Why, how and what

Many of the assumptions of homology on which the standard nomenclature for the cell groups and fiber tracts of the avian brain have been based are in error, and consequently that terminology promotes misunderstanding of the functional organization of avian brain and its evolutionary relationship to mammalian brain. Recognizing this problem, a number of avian brain researchers began an effort to revise the terminology, which culminated in the Avian Brain Nomenclature Forum, held at Duke University from 18 to 20 July 2002. In the new terminology approved at this Forum, the flawed conception that the telencephalon of birds consists nearly entirely of a hypertrophied basal ganglia has been purged, and the actual parts of the basal ganglia and its brainstem afferent cell groups given names reflecting their now-evident mammalian homologues. The pallial telencephalic regions that were erroneously named to reflect presumed homology to mammalian basal ganglia were renamed as parts of pallium, using prefixes in most cases that retained established abbreviations (for continuity with the replaced nomenclature). The new nomenclature should lead to better communication among neuroscientists, especially between avian brain specialists and those not specialized in avian neurobiology. More information is available at the Avian Brain Nomenclature Exchange website ().

Genetics of subthalamic nucleus in development and disease

The subthalamic nucleus (STN) is a crucial node in the basal ganglia. Clinical success in targeting the STN for deep brain stimulation in Parkinson's disease patients has prompted increased interest in understanding STN biology. In this report, we discuss recent evidence for transcription factor mediated regulation of STN development. We also review STN developmental neurobiology and known patterns of gene expression in the developing and mature STN.

The distribution and cellular localization of glutamic acid decarboxylase-65 (GAD65) mRNA in the forebrain and midbrain of domestic chick

The distribution and cellular localization of GAD65 mRNA in the forebrain and midbrain of domestic chick were examined by in situ hybridization histochemistry with (35)[S]-UTP labeled cRNA probes, using film and emulsion autoradiography. Film autoradiograms showed intense GAD65 labeling in many structures of the basal telencephalon, such as the medial and lateral striatum, the septum, the olfactory tubercle, the lateral bed nucleus of the stria terminalis, and the intrapeduncular nucleus, while the pallial telencephalon showed only a low level of labeling. Emulsion-coated sections revealed that GAD65 mRNA-containing neurons were at least six times more abundant in striatum than pallium, with only a uniformly scattered subpopulation labeled in pallium, and that the vast majority of the large scattered projection neurons of globus pallidus were heavily labeled for GAD65. Prominent labeling was also evident in the nucleus taeniae and subpallial amygdala, but not in the arcopallium in film autoradiograms. Within the diencephalon, the hypothalamus was more GAD65-rich than the thalamus. Additional subtelencephalic cell groups showing prominent labeling included the thalamic reticular nucleus and ventral lateral geniculate nucleus of the diencephalon, the nucleus pretectalis, subpretectalis and spiriformis lateralis of the pretectum, and the magnocellular isthmic nucleus of the optic lobe. Tectal layers 9-10 were also rich in GAD65. These results further clarify GABAergic circuits of the avian forebrain and midbrain, and show them to closely resemble those in mammals.

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain - in particular the neocortex-like cognitive functions of the avian pallium - requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

Electrophysiological properties of neurons in the basal ganglia of the domestic chick: conservation and divergence in the evolution of the avian basal ganglia

Although the basal ganglia of birds and mammals share an enormous number of anatomical, histochemical, and electrophysiological characteristics, studies in songbirds have revealed some important differences. Specifically, a specialized region of songbird striatum (the input structure of the basal ganglia) has an anatomical projection and a physiologically defined cell type that are characteristic of the globus pallidus. At present, it is not clear if these differences result from adaptations specific to songbirds and perhaps a few other avian taxa or are common to all birds. We shed some light on this issue by characterizing the morphology and electrophysiological properties of basal ganglia neurons in an avian species that is only distantly related to songbirds: the domestic chick. We recorded neurons in chick basal ganglia in a brain slice preparation, using the whole cell technique. We found that chick striatum, like songbird striatum, contains a pallidum-like cell type never reported in mammalian striatum, supporting the hypothesis that this feature is common to all birds. We also discovered that spiny neurons, the most common cell type in the striatum of all amniotes, possess a diverse set of physiological properties in chicks that distinguish them from both mammals and songbirds. This study revealed an unexpectedly complex pattern of conservation and divergence in the properties of neurons recorded in avian striatum.

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain - in particular the neocortex-like cognitive functions of the avian pallium - requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

Evidence for ?direct? and ?indirect? pathways through the song system basal ganglia

Song learning in oscine birds relies on a circuit known as the "anterior forebrain pathway," which includes a specialized region of the avian basal ganglia. This region, area X, is embedded within a telencephalic structure considered homologous to the striatum, the input structure of the mammalian basal ganglia. Area X has many features in common with the mammalian striatum, yet has distinctive traits, including largely aspiny projection neurons that directly innervate the thalamus and a cell type that physiologically resembles neurons recorded in the mammalian globus pallidus. We have proposed that area X is a mixture of striatum and globus pallidus and has the same functional organization as circuits in the mammalian basal ganglia. Using electrophysiological and anatomical approaches, we found that area X contains a functional analog of the "direct" striatopallidothalamic pathway of mammals: axons of the striatal spiny neurons make close contacts on the somata and dendrites of pallidal cells. A subset of pallidal neurons project directly to the thalamus. Surprisingly, we found evidence that many pallidal cells may not project to the thalamus, but rather participate in a functional analog of the mammalian "indirect" pathway, which may oppose the effects of the direct pathway. Our results deepen our understanding of how information flows through area X and provide more support for the notion that song learning in oscines employs physiological mechanisms similar to basal ganglia-dependent forms of motor learning in mammals.

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.

Origin of the anterior forebrain pathway

The brain nuclei and pathways comprising the song system of oscine songbirds bear many similarities with circuits in other bird species and in mammals. This suggests that the song system evolved as a specialization of pre-existing circuits and may retain fundamental properties in common with those of other taxa. Here we review evidence for these similarities, including electrophysiological, morphological, and neurochemical data for identifying specific cell types. In addition, we discuss connectional data, addressing similarities in axonal projections among nuclei across taxa. We focus primarily on the anterior forebrain pathway, a circuit essential for song learning and vocal plasticity, because the evidence is strongest that this circuit is homologous to mammalian circuits. These fundamental similarities highlight the importance of comparative approaches; for example, understanding the role the anterior forebrain pathway plays in song plasticity may shed light on general principles of basal ganglia function. In addition, understanding specializations of such circuits in songbirds may illuminate specific innovations critical for vocal learning.

The avian song system in comparative perspective

The song system of oscine birds has become a versatile model system that is used to study diverse problems in neurobiology. Because the song system is often studied with the intention of applying the results to mammalian systems, it is important to place song system brain nuclei in a broader context and to understand the relationships between these avian structures and regions of the mammalian brain. This task has been impeded by the distinctiveness of the song system and the vast apparent differences between the forebrains of birds and mammals. Fortunately, accumulating data on the development, histochemistry, and anatomical organization of avian and mammalian brains has begun to shed light on this issue. We now know that the forebrains of birds and mammals are more alike than they first appeared, even though many questions remain unanswered. Furthermore, the song system is not as singular as it seemed-it has much in common with other neural systems in birds and mammals. These data provide a firmer foundation for extrapolating knowledge of the song system to mammalian systems and suggest how the song system might have evolved.

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

Identification and morphogenesis of the eminentia thalami in the zebrafish

This study documents early zebrafish brain expression patterns (2-5 days postfertilization) of proliferating neural (PCNA) as well as early-determined (Pax6, Zash-1a, Zash-1b, neurogenin1, neuroD) and differentiating (Hu-proteins) neuronal cells. These patterns are used to outline the spatiotemporal local dynamics of secondary neurogenesis as well as neuronal migration and differentiation in the region of the eminentia thalami. The analysis presented not only allows identification for the first time of the eminentia thalami in the zebrafish model system (because it forms a neurogenin1/neuroD-guided locus of neurogenesis in contrast to adjacent preoptic region and ventral thalamus) but furthermore shows that the entopeduncular complex is a derivative of the embryonic zebrafish eminentia thalami, which has never been reported for a teleost before. An analysis of the relevant literature shows that the mammalian entopeduncular nucleus/avian paleostriatum primitivum/reptilian globus pallidus clearly are part of the basal ganglia (i.e., the pallidum). In amniote embryos, an anterior entopeduncular area is recognized at the base of the medial ganglionic eminence (i.e., the future pallidum; part of alar plate of prosomere 5), separate from the more posterior eminentia thalami (alar prosomere 4). There is a comparable periventricular eminentia thalami in (young and adult) amphibians and teleosts. However, the migrated anterior entopeduncular nucleus of anuran amphibians likely is homologous to part of the pallidum of other vertebrates and has no developmental relationship to the eminentia thalami. In contrast, the migrated teleostean entopeduncular complex does not correspond to a pallidal division but is indeed the adult derivative of the early-recognized eminentia thalami as shown in this study.

Fate map of the diencephalon and the zona limitans at the 10-somites stage in chick embryos

The diencephalon is a central area of the vertebrate developing brain, where the thalamic nuclear complex, the pretectum and the anterior tegmental structures are generated. It has been subdivided into prosomeres, which are transversal domains defined by morphological and molecular criteria. The zona limitans intrathalamica is a central boundary in the diencephalon that separates the posterior diencephalon (prosomeres 1 and 2), from the anterior diencephalon (prosomere 3). This intrathalamic limit appears early on in neural tube development, and the molecular pattern that it reveals suggests an important role in the diencephalic histogenesis. We hereby present a fate map of the presumptive territories in the diencephalon of a chick embryo at the 10-11 somite stages (HH9-10), by homotopic and isochronic quail-chick grafts. The anatomical interpretation of chimeric brains was aided by correlative whole-mount in situ hybridization with RNA probes for chicken genes expressed in specific diencephalic territories. The resulting fate map describes the distribution of the presumptive diencephalic prosomeres in the neural tube, and demonstrates their topologically conserved relationships throughout the neural development. Moreover, we show that the presumptive epithelium of ZLI can be localized at early developmental stages in the diencephalic alar plate at the anterior limit of the Wnt8b gene expression domain.

An immunohistochemical and pathway tracing study of the striatopallidal organization of area X in the male zebra finch

Area X is a nucleus within songbird basal ganglia that is part of the anterior forebrain song learning circuit. It receives cortical song-related input and projects to the dorsolateral medial nucleus of thalamus (DLM). We carried out single- and double-labeled immunohistochemical and pathway tracing studies in male zebra finch to characterize the cellular organization and circuitry of area X. We found that 5.4% of area X neuronal perikarya are relatively large, possess aspiny dendrites, and are rich in the pallidal neuron/striatal interneuron marker Lys8-Asn9-neurotensin8-13 (LANT6). Many of these perikarya were found to project to the DLM, and their traits suggest that they are pallidal. Area X also contained several neuron types characteristic of the striatum, including interneurons co-containing LANT6 and the striatal interneuron marker parvalbumin (2% of area X neurons), interneurons containing parvalbumin but not LANT6 (4.8%), cholinergic interneurons (1.4%), and neurons containing the striatal spiny projection neuron marker dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein (DARPP-32) (30%). Area X was rich in substance P (SP)-containing terminals, and many ended on area X neurons projecting to the DLM with the woolly fiber morphology characteristic of striatopallidal terminals. Although SP+ perikarya were not detected in area X, prior studies suggest it is likely that SP-synthesizing neurons are present and the source of the SP+ input to area X neurons projecting to the DLM. Area X was poor in enkephalinergic fibers and perikarya. The present data support the premise that area X contains both striatal and pallidal neurons, with the striatal neurons likely to include SP+ neurons that project to the pallidal neurons.