Loreta Medina

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 In particular, the outdated terminology implies that most of the avian telencephalon is a hypertrophied basal ganglia, when it is now clear that most of the avian telencephalon is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.

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.

Structural and functional evolution of the basal ganglia in vertebrates

While a basal ganglia with striatal and pallidal subdivisions is 1 clearly present in many extant anamniote species, this basal ganglia is cell sparse and receives only a relatively modest tegmental dopaminergic input and little if any cortical input. The major basal ganglia influence on motor functions in anamniotes appears to be exerted via output circuits to the tectum. In contrast, in modern mammals, birds, and reptiles (i.e., modern amniotes), the striatal and pallidal parts of the basal ganglia are very neuron-rich, both consist of the same basic populations of neurons in all amniotes, and the striatum receives abundant tegmental dopaminergic and cortical input. The functional circuitry of the basal ganglia also seems very similar in all amniotes, since the major basal ganglia influences on motor functions appear to be exerted via output circuits to both cerebral cortex and tectum in sauropsids (i.e., birds and reptiles) and mammals. The basal ganglia, output circuits to the cortex, however, appear to be considerably more developed in mammals than in birds and reptiles. The basal ganglia, thus, appears to have undergone a major elaboration during the evolutionary transition from amphibians to reptiles. This elaboration may have enabled amniotes to learn and/or execute a more sophisticated repertoire of behaviors and movements, and this ability may have been an important element of the successful adaptation of amniotes to a fully terrestrial habitat. The mammalian lineage appears, however, to have diverged somewhat from the sauropsid lineage with respect to the emergence of the cerebral cortex as the major target of the basal ganglia circuitry devoted to executing the basal ganglia-mediated control of movement.

Do birds possess homologues of mammalian primary visual, somatosensory and motor cortices?

Recent data on the expression of several homeobox genes in the embryonic telencephalon of mammals, birds and reptiles support the homology of a part of the avian pallium, named the Wulst, and at least the more-medial and superior parts of mammalian neocortex. This conclusion is also supported by previous embryological, topological and hodological data. Furthermore, new evidence on the connections and electrophysiological properties of specific subfields within the avian Wulst, and on the thalamic territories that project to these fields, supports the more-specific conclusion that a primary visual area and a primary somatosensory-somatomotor area are present in the avian Wulst; these areas are likely to be homologous to their counterparts in mammals. In spite of this, developmental, morphological and comparative evidence indicate that some structural and physiological traits that appear to be similar in the Wulst and neocortex (such as the lamination or binocularity) evolved independently in birds and mammals.

Pathway tracing using biotinylated dextran amines

Biotinylated dextran amines (BDA) are highly sensitive tools for anterograde and retrograde pathway tracing studies of the nervous system. BDA can be reliably delivered into the nervous system by iontophoretic or pressure injection and visualized with an avidin-biotinylated HRP (ABC) procedure, followed by a standard or metal-enhanced diaminobenzidine (DAB) reaction. High molecular weight BDA (10 k) yields sensitive and exquisitely detailed labeling of axons and terminals, while low molecular weight BDA (3 k) yields sensitive and detailed retrograde labeling of neuronal cell bodies. The detail of neuronal cell body labeling can be Golgi-like. BDA tolerates EM fixation and processing well and can, therefore, be readily used in ultrastructural studies. Additionally, BDA can be combined with other anterograde or retrograde tracers (e.g. PHA-L or cholera toxin B fragment) and visualized either by multi-color DAB multiple-labeling - if permanent labels are desired, or by using multiple simultaneous immunofluorescence - if fluorescence viewing is desired. In the same manner, BDA pathway tracing and neurotransmitter immunolabeling can be combined. Note that BDA pathway tracing can also be combined with anterograde or retrograde labeling with fluorescent dextran amines, if one wishes to exclusively use tracers with the favorable transport properties and sensitivities of dextran amines. In this case, the BDA can be visualized together with the fluorescent dextran amines using fluorescence labeling for the BDA, or the fluorescent dextran amines can be visualized together with the BDA by multicolor DAB labeling via immunolabeling of the fluorescent dextran amines using anti-fluorophore antisera. BDA is, thus, a flexible and valuable pathway tracing tool that has gained widespread popularity in recent years.

Expression of Dbx1, Neurogenin 2, Semaphorin 5A, Cadherin 8, and Emx1 Distinguish Ventral and Lateral Pallial Histogenetic Divisions in the Developing Mouse Claustroamygdaloid Complex

We studied the lateral and ventral pallial divisions of the claustroamygdaloid complex by means of analysis of expression patterns of the developmental regulatory genes Tbr1, Dbx1, Neurogenin 2, Emx1, Cadherin 8, and Semaphorin 5A in mouse developing telencephalon, from embryonic day 12.5 until birth. Our results indicate that these genes help to distinguish distinct lateral and ventral pallial histogenetic divisions in the embryonic telencephalon. Tbr1 is broadly expressed in both lateral and ventral pallial histogenetic divisions (the lateroventral migratory stream plus the mantle) during early and intermediate embryonic development; its signal becomes weak in parts of the mantle during late embryonic development. Dbx1 is strongly and specifically expressed in progenitor cells (ventricular zone) of the ventral pallium during early embryonic development, but there is no signal of this gene in the rest of the pallium nor the subpallium. Neurogenin 2 and Semaphorin 5A are both expressed in a ventral subdivision of the lateroventral migratory stream (called by us the ventral migratory stream). Further, specific nuclei of the claustral complex and pallial amygdala show strong expression of Neurogenin 2 and/or Semaphorin 5A, including the ventromedial claustrum and endopiriform nuclei, the lateral and basomedial amygdalar nuclei, the anterior and posteromedial cortical amygdalar areas, plus the amygdalo‐hippocampal area. We interpret these nuclei or areas of the claustroamygdaloid complex as possible derivatives of the ventral pallium. In contrast, during embryonic development the dorsolateral claustrum, the basolateral amygdalar nucleus, and the posterolateral cortical amygdalar area do not express or show weak expression of Neurogenin 2 or Semaphorin 5A, but express selectively and strongly Cadherin 8 plus Emx1, and may be derivatives of the lateral pallium. The lateral pallial and ventral pallial divisions of the claustroamygdaloid complex appear to have some different sets of connections, although this requires further investigation. J. Comp. Neurol. 474:504–523, 2004.

Neurotransmitter Organization and Connectivity of the Basal Ganglia in Vertebrates: Implications for the Evolution of Basal Ganglia (Part 1 of 2)

The basal ganglia in modern mammals, birds and reptiles (i.e. modern amniotes) are very similar in connections and neurotransmitters, suggesting that the evolution of the basal ganglia in amniotes has been very conservative. For example, the basal ganglia in all amniotes possess a dorsal striatum containing two main populations of projection neurons, substance P-containing (SP+) and enkephalin-containing (ENK+) neurons, which have major projections to the dorsal pallidum and the tegmentum (ventral tegmental area and substantia nigra, or VTA/SN). The VTA/SN, in turn, has a major dopaminergic (DA+) projection to the striatum in all amniotes. In this paper, we review these data on the basal ganglia in amniotes and note points of similarity and difference in the functional circuitry of the basal ganglia among amniotes. In addition, we review recent findings on the neurotransmitter organization and connectivity of the basal ganglia in amphibians and fishes, with the goal of assessing whether a basal ganglia showing the same basic features as in amniotes is observed in anamniotes. Published data indicate that in at least two groups of fishes (cartilaginous fishes and lungfishes) and apparently in amphibians, the basal ganglia is present and consists of a distinct striatum and pallidum. The striatum of amphibians, cartilaginous fishes, and lungfishes contain SP+ and ENK+ neurons that seem to project to the pallidum as well as to a brainstem cell group that appears comparable to the VTA/SN of amniotes. Data for ray-finned fishes also suggest the presence of a striatum containing SP+ and ENK+ neurons that projects to VTA/SN-like brainstem cell group. In the basal ganglia of ray-finned fishes, however, a distinct pallidum had not been identified. Finally, the brainstem cell group receiving striatal input in all anamniotes contains DA+ neurons that seem to project to the striatum. The present analysis suggests that a rudimentary basal ganglia was already present in the brain of the ancestral jawed vertebrates. This rudimentary basal ganglia likely consisted of a striatum and a pallidum, and the striatum probably already possessed the same basic connections and some of the same basic cell types as the basal ganglia of modern jawed vertebrates.

Expression of the genes GAD67 and Distal-less-4 in the forebrain of Xenopus laevis confirms a common pattern in tetrapods

We investigated whether γ‐amino butyric acidergic (GABAergic) cell populations correlate positionally with specific Dlx‐expressing histogenetic territories in an anamniote tetrapod, the frog Xenopus laevis. To that end, we cloned a fragment of Xenopus GAD67 gene (xGAD67, expressed in GABAergic neurons) and compared its expression with that of Distal‐less‐4 gene (xDll‐4, ortholog of mouse Dlx2) in the forebrain at late larval and adult stages. In Xenopus, GABAergic neurons were densely concentrated in xDll‐4–positive territories, such as the telencephalic subpallium, part of the hypothalamus, and ventral thalamus, where nearly all neurons expressed both genes. In contrast, the pallium of Xenopus generally contained dispersed neurons expressing xGAD67 or xDll‐4, which may represent local circuit neurons. As in amniotes, these pallial interneurons may have been produced in the subpallium and migrated tangentially into the pallium during development. In Xenopus, the ventral division of the classic lateral pallium contained extremely few GABAergic cells and showed only low signal of the pallial gene Emx1, suggesting that it may represent the amphibian ventral pallium, homologous to that of amniotes. At caudal forebrain levels, a number of GABAergic neurons was observed in several areas (dorsal thalamus, pretectum), but no correlation to xDll‐4 was observed there. The location of GABAergic neurons in the forebrain and their relation to the developmental regulatory genes Dll and Dlx were very similar in Xenopus and in amniotes. The close correlation in the expression of both genes in rostral forebrain regions supported the notion that Dll/Dlx are among the genes involved in the acquisition of the GABAergic phenotype. J. Comp. Neurol. 461:370–393, 2003.

Histogenetic Compartments of the Mouse Centromedial and Extended Amygdala Based on Gene Expression Patterns during Development

The amygdala controls emotional and social behavior and regulates instinctive reflexes such as defense and reproduction by way of descending projections to the hypothalamus and brainstem. The descending amygdalar projections are suggested to show a cortico‐striato‐pallidal organization similar to that of the basal ganglia (Swanson [2000] Brain Res 886:113–164). To test this model we investigated the embryological origin and molecular properties of the mouse centromedial and extended amygdalar subdivisions, which constitute major sources of descending projections. We analyzed the distribution of key regulatory genes that show restricted expression patterns within the subpallium (Dlx5, Nkx2.1, Lhx6, Lhx7/8, Lhx9, Shh, and Gbx1), as well as genes considered markers for specific subpallial neuronal subpopulations. Our results indicate that most of the centromedial and extended amygdala is formed by cells derived from multiple subpallial subdivisions. Contrary to a previous suggestion, only the central—but not the medial—amygdala derives from the lateral ganglionic eminence and has striatal‐like features. The medial amygdala and a large part of the extended amygdala (including the bed nucleus of the stria terminalis) consist of subdivisions or cell groups that derive from subpallial, pallial (ventral pallium), or extratelencephalic progenitor domains. The subpallial part includes derivatives from the medial ganglionic eminence, the anterior peduncular area, and possibly a novel subdivision, called here commissural preoptic area, located at the base of the septum and related to the anterior commissure. Our study provides a molecular and morphological foundation for understanding the complex embryonic origins and adult organization of the centromedial and extended amygdala. J. Comp. Neurol. 506:46–74, 2008.

Expression of the Genes Emx1, Tbr1, and Eomes (Tbr2) in the Telencephalon of Xenopus laevis Confirms the Existence of a Ventral Pallial Division in All Tetrapods

To investigate the pallial organization and the exact location and extension of the ventral pallium in amphibians, we cloned a fragment of the homeobox XenopusTbr1 (xTbr1) gene and analyzed its expression compared with that of the genes xEomes (Tbr2) and xEmx1 in the telencephalon of the frog Xenopus laevis during embryonic and larval development. The expression of xEmx1 was also analyzed in the adult frog. We compared the expression patterns of these pallial marker genes with that of the subpallial gene xDistal‐less‐4 (xDll4). Our results indicate that the whole pallium of Xenopus expresses the T‐box genes xTbr1 and xEomes (in proliferating cells and/or mantle) during embryonic and larval development, and the expression of these genes is topographically complementary to that of xDll4 in the subpallium. In addition to their massive expression in the pallium, both xTbr1 and xEomes are expressed in a few dispersed cells in the subpallium, which may represent immigrant cells of pallial origin, because these genes are not found in the subpallial proliferating cells. On the other hand, during development xEmx1 is expressed in a large part of the pallium (proliferating and postmitotic cells) except for an area adjacent to the pallio‐subpallial boundary, where xEmx1 is observed only in some mantle cells. This pallial area poor in xEmx1 expression and poor in expression of the subpallial gene xDll4, but expressing the pallial marker genes xTbr1 and xEomes, appears to represent the amphibian ventral pallium, comparable to that described in other vertebrates (Puelles et al. [2000] J. Comp. Neurol. 424:409–438). In the adult frog, the ventral pallium appears to include the rostral part of the lateral amygdalar nucleus as well as a large part of the medial amygdalar nucleus (as defined by Marín et al. [1998] J. Comp. Neurol. 392:285–312). In contrast, the caudal part of the previously termed lateral amygdalar nucleus shows strong xEmx1 expression and may be a lateral pallial derivative. The possible homology of these amphibian amygdalar nuclei is discussed. Finally, expression of xTbr1, xEomes, and xEmx1 is observed in the mitral cell layer of the olfactory bulb from early developmental stages, further supporting that this structure is a pallial derivative. J. Comp. Neurol. 474:562–577, 2004.