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

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.

Evolution of the amygdala: new insights from studies in amphibians

The histology of amphibian brains gives an impression of relative simplicity when compared with that of reptiles or mammals. The amphibian telencephalon is small and contains comparatively few and large neurons, which in most parts constitute a dense periventricular cellular layer. However, the view emerging from the last decade is that the brains of all tetrapods, including amphibians, share a general bauplan resulting from common ancestry and the need to perform similar vital functions. To what extent this common organization also applies to higher brain functions is unknown due to a limited knowledge of the neurobiology of early vertebrates. The amygdala is widely recognized as a brain center critical for basic forms of emotional learning (e.g., fear conditioning) and its structure in amphibians could suggest how this capacity evolved. A functional systems approach is used here to synthesize the results of our anatomical investigations of the amphibian amygdala. It is proposed that the connectivity of the amphibian telencephalon portends a capacity for multi-modal association in a limbic system largely similar to that of amniote vertebrates. One remarkable exception is the presence of new sensory-associative regions of the amygdala in amniotes: the posterior dorsal ventricular ridge plus lateral nuclei in reptiles and the basolateral complex in mammals. These presumably homologous regions apparently are capable of modulating the phylogenetically older central amygdala and allow more complex forms of emotional learning.

Ontogeny of tyrosine hydroxylase mRNA expression in mid- and forebrain: neuromeric pattern and novel positive regions

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamines and, thus, critical in determining the catecholaminergic phenotype. In this study, we have examined the expression of TH mRNA by in situ hybridization in the embryonic mouse forebrain and midbrain and have mapped its localization according to the neuromeric pattern. We find that early in embryonic development, 10 to 12 days post coitum (dpc), TH mRNA is expressed in ample continuous regions of the neuroepithelium, extending across several neuromeres. However, from 12.5 dpc onward, the expression becomes restricted to discrete regions, which correspond to the dopaminergic nuclei (A8 to A15). In addition to these nuclei previously described, TH mRNA is also observed in regions that do not express this enzyme according to immunohistochemical studies. This difference in relation to protein expression pattern is consequent with the known posttranscriptional regulation of TH expression. The most representative example of a novel positive region is the conspicuous mRNA expression in both medial and lateral ganglionic eminences. This result agrees with reports describing the capacity of striatal stem cells (that is, located at the lateral ganglionic eminence) to become dopaminergic in vitro. Other regions include the isthmic mantle layer and the early floor plate of the midbrain-caudal forebrain. On the whole, the expression map we have obtained opens new perspectives for evolutionary/comparative studies, as well as for therapeutic approaches looking for potentially dopaminergic cells. Developmental Dynamics 234:709-717, 2005. (c) 2005 Wiley-Liss, Inc.

The septal complex of the fire-bellied toad Bombina orientalis: Chemoarchitecture

In order to investigate whether chemoarchitecture would support the subdivision of the anuran septum based on cytoarchitectonic and hodological studies, we performed enzyme-histochemical detection of NADPH-diaphorase and immunohistological demonstration of choline-acetyl transferase (ChAT), aspartate, calretinin, gamma-aminobutyric acid (GABA), 5-hydroxy-tryptamine, tyrosine hydroxylase, neuropeptide Y (NPY), somatostatin, Leu- and Leu + Met-enkephalin, and substance P in the fire-bellied toad Bombina orientalis. Labeling of cell bodies matched well the previously defined subnuclei: The dorsolateral septal nucleus contains enkephalin-immunoreactive (-ir) and weakly stained GABA-ir neurons; calretinin-ir and weakly labeled GABA-ir neurons are found in the ventrolateral septal nucleus. The medial septal nucleus is characterized by the presence of numerous ChAT-ir and some tyrosine hydroxylase-ir neurons, while the dorsal septal nucleus is outlined by its NPY-ir neurons. Many ChAT-ir and some aspartate-ir and somatostatin-ir neurons are found in the diagonal band of Broca, and the central septal nucleus contains some GABA-ir and ChAT-ir neurons. In contrast, labeled fibers form a pattern which does not match the boundaries of septal subnuclei. Comparing the anuran septal complex with that of other vertebrates reveals that the complexity of the lateral septum has increased during the evolution from anamniote to amniote vertebrates. In spite of this fact, many similarities in chemoarchitecture between anurans and other vertebrates are evident. Some basal septal functions such as involvement in learning and memory formation or inhibition of sexual behavior appear to have persisted during vertebrate evolution.

Forebrain projections to the hypothalamus are topographically organized in anurans: conservative traits as compared with amniotes

The organization of the forebrain in amphibians (anamniotes) is currently being re-evaluated in terms of evolution and several evidences have corroborated numerous traits shared by amphibians and amniotes, such as the organization of the basal ganglia and the amygdaloid complex. In the present study we have analysed the organization of forebrain afferent systems to the hypothalamus of the frog Rana perezi. In vivo and in vitro tract-tracing techniques with dextran amines and immunohistochemistry for localizing nitric oxide synthase (NOS) in a series of single or combined experiments were used as NOS labelling reveals hypothalamic afferents arising from the lateral amygdala and the combination allowed analysis of the relationship between fibers of different origins in the same section. The results showed a large segregation of afferents in the hypothalamic region depending on their site of origin in the forebrain. Four highly topographically organized prosencephalic tracts reaching the anuran hypothalamus were observed: (i) the medial forebrain bundle, from the medial pallium and septal complex; (ii) the caudal branch of the stria terminalis formed by fibers arising in the lateral and medial amygdala; (iii) part of the lateral forebrain bundle with fibers from the central amygdala and (iv) the dorsal thalamo-hypothalamic tract. Fibers coursing in each tract reach the hypothalamus and terminate in distinct fields. The resemblance in pattern of forebrain-hypothalamic organization between amphibians and amniotes suggests that this feature represents an important trait conserved in the evolution of all tetrapods and therefore essential for the hypothalamic function.

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.

Expression patterns of developmental regulatory genes show comparable divisions in the telencephalon of Xenopus and mouse: insights into the evolution of the forebrain

In this study, we review data on the existence of comparable divisions and subdivisions in the telencephalon of different groups of tetrapods based on expression of some developmental regulatory genes, having a particular focus in the comparison of the anuran amphibian Xenopus and the mouse. The available data on Xenopus, mouse, chick and turtle indicate that apparently all tetrapod groups possess the same molecularly distinct divisions and subdivisions in the telencephalon. This basic organization was likely present in the telencephalon of stem tetrapods. Each division/subdivision is characterized by expression of a unique combination of developmental regulatory genes, and appears to represent a self-regulated and topologically constant histogenetic brain compartment that gives rise to specific groups of cells. This interpretation has an important consequence for searching homologies, since a basic condition for cell groups in different vertebrates to be considered homologous is that they originate in the same compartment. However, evolution may allow individual cell groups derived from comparable (field homologous) subdivisions to be either similar or dissimilar across the vertebrate groups, giving rise to several possible scenarios of evolution, which include both the evolutionary conservation of similar (homologous) cells or the production of novel cell groups. Finally, available data in the lamprey, a jawless fish, suggest that not all telencephalic subdivisions were present at the origin of vertebrates, raising important questions about their evolution.

Central amygdala in anuran amphibians: Neurochemical organization and connectivity

The evolution of the amygdaloid complex in tetrapods is currently under debate on the basis of new neurochemical, hodological, and gene expression data. The anuran amygdaloid complex, in particular, is being examined in an effort to establish putative homologies with amniotes. The lateral and medial amygdala, comparable to their counterparts in amniotes, have recently been identified in anurans. In the present study we characterized the autonomic portion of the anuran amygdala, the central amygdala (CeA). First, the distribution of several neuronal markers (substance P, neuropeptide Y, somatostatin, tyrosine hydroxylase, and nitric oxide synthase) was analyzed. The localization of immunoreactive cells, primarily nitrergic cells, and the topographically arranged fiber labeling for all markers characteristically identified the CeA. Subsequently, the afferent and efferent connections of the CeA were investigated by means of in vivo and in vitro tracing techniques with dextran amines. The anuran CeA was revealed as the main component of the amygdaloid autonomic system, showing important connections with brainstem centers such as the parabrachial nucleus and the nucleus of the solitary tract. Only scarce CeA-hypothalamic projections were observed, whereas bidirectional connections between the CeA and the lateral and medial amygdala were abundant. The present neurochemical and hodological results support the homology of the anuran CeA with its counterpart in amniotes and strengthen the idea of a conserved amygdaloid organization in the evolution of tetrapods.

Immunohistological characterization of striatal and amygdalar structures in the telencephalon of the fire-bellied toad bombina orientalis

The subpallium of the fire-bellied toad Bombina orientalis was studied by means of enzyme-histological detection of NADPH-diaphorase and immunohistological demonstration of aspartate, GABA, calretinin, choline-acetyl transferase, Leu-and Met-enkephalin, neuropeptide Y, 5-hydroxy-tryptamine (serotonin), somatostatin, substance P and tyrosine-hydroxylase. As in other vertebrates, the striato-pallidum is characterized by GABA-, substance P- and enkephalin-immunoreactivity. Neurons and fibers differing in immunoreactivity are arranged in layers. Choline-acetyl transferase-immunoreactive neurons were found in a position corresponding to the mammalian cholinergic cell-group (Ch4-group), which therefore may be homologous to the nucleus basalis of Meynert. Within the amygdaloid complex, the cortical and lateral (vomeronasal) nuclei are similar in calretinin-, GABA-, NADPH-diaphorase-, enkephalin, substance P- and neuropeptide Y-(immuno)histology. The medial and central amygdaloid nuclei reveal a dense peptidergic innervation, and the medial amygdala additionally exhibits serotonergic fibers and cell bodies staining for neuropeptides and tyrosine-hydroxylase. Differences between Bombina and other anuran species exist, such as the absence of cholinergic neurons in the striatum. Our findings corroborate the view based on recent studies on the hodology and cytoarchitecture of the anuran telencephalon that the anuran ventral telencephalon contains most of the structures found in the mammalian brain. This concerns a septal region, a dorsal and ventral striato-pallidum including a nucleus accumbens and an amygdaloid complex consisting of a central, cortical and vomeronasal amygdala. The only major difference appears to concern the lack of a basolateral amygdala.

Localization and connectivity of the lateral amygdala in anuran amphibians

On the basis of chemoarchitecture and gene expression patterns in the amphibian amygdaloid complex, new subdivisions have been proposed and compared with their counterparts in amniotes. Thus, a portion of the ventral pallium of anurans has been tentatively named "lateral amygdala" (LA) and compared with the basolateral complex of mammals. To strengthen the putative homology, we have analyzed the pattern of afferent and efferent connections of the LA in the anurans Rana perezi and Xenopus laevis. Tract-tracing techniques with dextran amines were used under in vivo and in vitro conditions. The results showed important connections with the main olfactory bulb, via the lateral olfactory tract. In addition, abundant intratelencephalic connections, via the rostral branch of the stria terminalis, were revealed, involving mainly the basal ganglia, septal nuclei, bed nucleus of the stria terminalis, and especially other amygdaloid nuclei. Nontelencephalic connections were found from the dorsal thalamus and parabrachial area and, in particular, from the hypothalamus through the caudal branch of the stria terminalis. All these results strongly suggest that the LA in anurans is a multimodal area in the ventral pallium that shares many hodological features with the amygdaloid ventropallial derivatives of the basolateral complex of amniotes.

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.