Pain Recognition in Reptiles

Advances in reptile cognitive research would help to (1) better qualify behavioral responses to pain experiences, (2) monitor welfare impacts, and (3) model analgesic studies with ecologically relevant insight to better qualify interventional responses. The focus of future analgesic studies in reptiles require the continued elucidation of the opiate systems and the given variations across taxa in efficacy in nociceptive tests.

Evolution of Pallial Areas and Networks Involved in Sociality: Comparison Between Mammals and Sauropsids

Birds are extremely interesting animals for studying the neurobiological basis of cognition and its evolution. They include species that are highly social and show high cognitive capabilities. Moreover, birds rely more on visual and auditory cues than on olfaction for social behavior and cognition, just like primates. In primates, there are two major brain networks associated to sociality: (1) one related to perception and decision-making, involving the pallial amygdala (with the basolateral complex as a major component), the temporal and temporoparietal neocortex, and the orbitofrontal cortex; (2) another one related to affiliation, including the medial extended amygdala, the ventromedial prefrontal and anterior cingulate cortices, the ventromedial striatum (largely nucleus accumbens), and the ventromedial hypothalamus. In this account, we used an evolutionary developmental neurobiology approach, in combination with published comparative connectivity and functional data, to identify areas and functional networks in the sauropsidian brain comparable to those of mammals that are related to decision-making and affiliation. Both in mammals and sauropsids, there is an important interaction between these networks by way of cross projections between areas of both systems.

Expression of regulatory genes in the embryonic brain of a lizard and implications for understanding pallial organization and evolution

The comparison of gene expression patterns in the embryonic brain of mouse and chicken is being essential for understanding pallial organization. However, the scarcity of gene expression data in reptiles, crucial for understanding evolution, makes it difficult to identify homologues of pallial divisions in different amniotes. We cloned and analyzed the expression of the genes Emx1, Lhx2, Lhx9, and Tbr1 in the embryonic telencephalon of the lacertid lizard Psammodromus algirus. The comparative expression patterns of these genes, critical for pallial development, are better understood when using a recently proposed six‐part model of pallial divisions. The lizard medial pallium, expressing all genes, includes the medial and dorsomedial cortices, and the majority of the dorsal cortex, except the region of the lateral cortical superposition. The latter is rich in Lhx9 expression, being excluded as a candidate of dorsal or lateral pallia, and may belong to a distinct dorsolateral pallium, which extends from rostral to caudal levels. Thus, the neocortex homolog cannot be found in the classical reptilian dorsal cortex, but perhaps in a small Emx1‐expressing/Lhx9‐negative area at the front of the telencephalon, resembling the avian hyperpallium. The ventral pallium, expressing Lhx9, but not Emx1, gives rise to the dorsal ventricular ridge and appears comparable to the avian nidopallium. We also identified a distinct ventrocaudal pallial sector comparable to the avian arcopallium and to part of the mammalian pallial amygdala. These data open new venues for understanding the organization and evolution of the pallium.

Multiple amygdaloid divisions of arcopallium send convergent projections to the nucleus accumbens and neighboring subpallial amygdala regions in the domestic chicken: a selective pathway tracing and reconstruction study

Retrograde tracing with choleratoxin B, injected into the nucleus accumbens (Ac) and bed nucleus of stria terminalis, lateral part (BSTL), yielded labeled perikarya in a ring-shaped area of arcopallium, including dorsal and hilar subdivisions, with a wedge-shaped node of dense accumulation in the amygdalopiriform area (APir). Also, the position of source neurons for this arcopallio-subpallial pathway was verified by anterograde tracing. Three subregions of arcopallium (amygdalopiriform, dorsal, hilar) were injected with dextran (10 kDa), and fibers and terminal fields were detected in Ac, BSTL and extended amygdala (EA). Most abundant projections to Ac arose from APir. The study enabled precise description of the main output fiber streams: the dorsal stream follows the dorsal border of arcopallium and, continuing in the ventral amygdalofugal tract, it traverses the EA and the BSTL before reaching the Ac. The ventral stream of fibers enters the EA along the ventral subpallial border and terminates in the basal nucleus and ventral pallidum. The course of the pathway was reconstructed in 3D. Retrogradely labeled arcopallial neurons were devoid of DARPP-32. DARPP-32 was present in the Ac but not the BSTL. No colocalization between the calcium binding proteins calbindin, parvalbumin and calretinin, and retrogradely labeled neurons was detected, despite a considerable territorial overlap. This finding further supports the excitatory nature of the arcopallial-accumbens pathway. Conjoint and convergent amygdalar input to EA, including BSTL, as well as to Ac subregions likely transmits fear and aggression related signals to both viscerolimbic (EA) and learned reward- and motivation-related (Ac) ventrobasal forebrain regions.

Genetic and developmental homology in amniote brains. Toward conciliating radical views of brain evolution

The six-layered neocortex is both a unique and a universal character of mammals. Historically, a major concern has been to determine its phylogenetic origins by establishing which structures, if any, correspond to it in the brains of other vertebrates. Two opposing hypotheses have been debated in the last years: (i) the neocortex arises entirely from the dorsal hemisphere of ancestral reptiles, and (ii) a large portion of it originates in the lateral hemisphere, from a structure termed the dorsal ventricular ridge (DVR), which expands significantly in reptiles and especially in birds. While developmental and genetic evidence strongly favors a dorsal origin of the neocortex, there are important similarities in the sensory connectivity to the neocortex and to the DVR, and more recently, in the phenotype of late-produced elements in both structures. It is proposed that, despite originating in different embryonic domains, the proliferative expansion of both the mammalian neocortex and the sauropsidian DVR is partly based on the amplification of similar developmental programs, possibly dependent on Pax6 activity or of related cascades that promote progenitor proliferation. While Pax6 activity is already present in the amphibian pallium, I propose that at some point(s) in amniote evolution it has been upregulated yielding brain expansion in both sauropsids and mammals. However, in the latter there has been an additional dorsalizing influence contributing to the development of the neocortex and restricting the expansion of the lateral hemisphere. Finally, a significant contribution to neocortical origins by anterior signaling centers secreting FGFs is suggested, by virtue of their association to olfactory development and their cortical patterning functions. This perspective fits a dynamical view of brain homology, where instead of searching for a one-to-one correspondence between components, emphasis is placed on changes in the modulation of conserved signaling centers and their corresponding morphogen gradients across species.

Tectorotundal connections in turtles: an electron microscopic tracing and GABA-immunocytochemical study

The nucleus rotundus of the turtles Emys orbicularis and Testudo horsfieldi was analysed by axonal tracing methods and post-embedding GABA immunocytochemistry. After injections of horseradish peroxidase or biotinylated dextran amine into the optic tectum, electron microscopic observations showed that the vast majority of ipsilateral tectorotundal axon terminals were small in size, had smooth contours and contained small, round, densely packed synaptic vesicles. These terminals were GABA-immunonegative, often gathered in clusters, and established asymmetrical synaptic contacts with either small- or medium-sized GABA-negative dendritic profiles and with GABA-immunoreactive (GABA-ir) dendrites, which did not contain synaptic vesicles. Occasional GABA-ir-labelled axon terminals were observed; these may arise from the rare GABAergic neurons in the central tectal layer, or from neurons in the ventral pretectal nucleus, which projects both to the optic tectum and nucleus rotundus. In addition to tracer-labelled axon terminals, we observed both GABA-negative and GABA-ir cell bodies and dendrites also labelled by the tracer. No GABA-ir presynaptic dendritic profiles containing synaptic vesicles were observed. The existence in reptiles of reciprocal connections between the nucleus rotundus and the optic tectum as a phylogenetically ancient feedback system is discussed.

Social experience organizes parallel networks in sensory and limbic forebrain

Successful social behavior can directly influence an individual's reproductive success. Therefore, many organisms readily modify social behavior based on past experience. The neural changes induced by social experience, however, remain to be fully elucidated. We hypothesize that social modulation of neural systems not only occurs at the level of individual nuclei, but also of functional networks, and their relationships with behavior. We used the green anole lizard (Anolis carolinensis), which displays stereotyped, visually triggered social behaviors particularly suitable for comparisons of multiple functional networks in a social context, to test whether repeated aggressive interactions modify behavior and metabolic activity in limbic-hypothalamic and sensory forebrain regions, assessed by quantitative cytochrome oxidase (a slowly accumulating endogenous metabolic marker) histochemistry. We found that aggressive interactions potentiate aggressive behavior, induce changes in activities of individual nuclei, and organize context-specific functional neural networks. Surprisingly, this experiential effect is not only present in a limbic-hypothalamic network, but also extends to a sensory forebrain network directly relevant to the behavioral expression. Our results suggest that social experience modulates organisms' social behavior via modifying sensory and limbic neural systems in parallel both at the levels of individual regions and networks, potentially biasing perceptual as well as limbic processing.

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.

Topography and connections of the telencephalon in a chondrostean,Acipenser baeri: An experimental study

Sturgeons belong to an ancient group of the extant actinopterygian fishes. Accordingly, the study of their brain connections is important to understand brain evolution in the line leading to teleosts. We examined the topography and connections of the various telencephalic regions of the Siberian sturgeon (Acipenser baeri). The telencephalic regions were characterized on the basis of acetylcholinesterase histochemistry and calbindin-D28k and calretinin immunohistochemistry. The telencephalic connections were investigated by using the fluorescent dye DiI (1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate) in fixed brains. Application of DiI to different areas of the pallial (dorsal) regions of the telencephalic lobes showed that they have mostly intratelencephalic connections. A posterior pallial region is characterized by its similar hodology to that of the posterior zone of the teleosts dorsal telencephalon and those described in other ancient groups. Extratelencephalic connections of the pallium are scarce, although a few afferent and efferent connections with the diencephalon, mesencephalon, and rostral rhombencephalon were observed. DiI application to subpallial regions showed both intratelencephalic connections and connections with different brain regions. Afferents to the subpallium originate from the olfactory bulbs, preoptic area, thalamus, posterior tuberculum, hypothalamus, secondary gustatory nucleus, and raphe nuclei. Some of these connections are quite similar to those described for other vertebrates.

Distribution of GABA, calbindin and nitric oxide synthase in the developing chick entopallium

The distribution of GABA, calbindin and neuronal nitric oxide synthase (nNOS) was analyzed in the developing avian entopallium. The study was carried out in chick embryos from embryonic day (E)8 to hatching postnatal day (P)0, using immunohistochemical methods. At E8, GABA-positive cells were observed in pallial regions. Neither calbindin nor nNOS-immunoreactive cells were observed. At E10, the number of GABA neurons in the prospective entopallium increased and also nNOS cells were observed. Lightly stained nNOS neurons predominated over intensely stained ones. Calbindin immunoreactivity was not observed in the entopallium. At E12, the entopallial complex appeared as the pallial region displaying the highest density of GABA neurons. Also the whole entopallium displayed an intensely stained calbindin neuropil with many embedded stained cells. From E12 on, there was a decrease in the expression of nNOS. At E14-16, both GABA and calbindin-immunoreactive neurons were numerous and homogeneously distributed within the entopallium. The whole entopallium displayed a moderately stained neuropil. From E18 to P0, GABA and nNOS immunoreactivities remained similar to previous stages. At these stages, calbindin immunoreactivity within the entopallium consisted of a moderately stained central region bordered dorsally by a pale stained region. These two areas could correspond to the entopallial core and the perientopallial belt, respectively.

The ascending tectofugal visual system in amniotes: New insights

Ascending tectal axons carrying visual information constitute a fiber pathway linking the mesencephalon with the dorsal thalamus and then with a number of telencephalic centers. The sauropsidian nucleus rotundus and its mammalian homologue(s) occupy a central position in this pathway. The aim of this study was analyzing the rotundic connections in reptiles and birds in relation with comparable connections in mammals, by using biotinylated dextran amines and the lipophilic carbocyanine dye DiI as tracing molecules. In general, rotundic connections in reptiles and birds are quite similar, especially with regards to pretectal and tectal afferences; as a novel finding, we describe varicose fibers arising from nucleus rotundus that reached the developing chick striatum. In addition, this study described the dorsal claustrum as a novel telencephalic target for the suprageniculate nucleus in mammals. Overall, telencephalic projections from the posterior/intralaminar complex of the mammalian thalamus can be compared with the telencephalic projections of the reptilian nucleus rotundus. With the exception of the isocortical connections, the mouse suprageniculate nucleus shares a number of afferent and efferent connections with the sauropsidian nucleus rotundus. Especially significant were the suprageniculate fibers reaching the striatum and then following to reach pallial derivatives such as the lateral amygdala (ventral pallium) and the dorsal claustrum (lateral pallium). These connections can be compared with the rotundic fibers reaching the ventromedial part of the anterior dorsal ventricular ridge in reptiles/entopallium in birds (ventral pallium) and the dorsolateral part of the anterior dorsal ventricular ridge in reptiles (lateral pallium), and probably the mesopallium in birds.

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.

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.

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

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

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.

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.

Tectal mosaic: Organization of the descending tectal projections in comparison to the ascending tectofugal pathway in the pigeon

The optic tectum of vertebrates is an essential relay station for visuomotor behavior and is characterized by a set of connections that comprises topographically ordered input from the eyes and an output that reaches premotor hindbrain regions. In the avian tectofugal system, different ascending cell classes have recently been identified based on their dendritic and axonal projection patterns, although comparable information about the descending cells is missing. By means of retrograde tracing, the present study describes the detailed morphology of tectal output neurons that constitute the descending tectobulbar and tectopontine pathways in pigeons. Descending cells were more numerous in the dorsal tectum and differed in terms of 1) their relative amount of ipsi- vs. contralateral projections, 2) the location of the efferent cell bodies within different tectal layers, and 3) their differential access to visual input via dendritic ramifications within the outer retinorecipient laminae. Thus, the descending tectal system is constituted by different cell classes presumably processing diverse aspects of the visual environment in a visual field-dependent manner. We demonstrate, based on a careful morphological analysis and on double-labeling experiments, that the descending pathways are largely separated from the ascending projections even when they arise from the same layers. These data support the concept that the tectum is arranged as a mosaic of multiple cell types with diverse input functions at the same location of the tectal map. Such an arrangement would enable the tectal projections onto diverse areas to be both retinotopically organized and functionally specific.

Internal structure of the nucleus rotundus revealed by mapping cadherin expression in the embryonic chicken visual system

The nucleus rotundus is the largest nucleus of the avian thalamus. It is an important center of visual information processing and conveys information from the optic tectum to the ectostriatum in the telencephalon. The nucleus rotundus is generally believed to contain internal divisions processing information on color, form, motion, and looming of visual objects. The detailed arrangement of these internal divisions is unclear. Here, we map the expression of four classic cadherins (N-cadherin, R-cadherin, cadherin-6B, and cadherin-7), which are markers for specific functional gray matter divisions and their fiber connections in the vertebrate brain. Results show that each cadherin is expressed by one coherent part of the nucleus rotundus that is connected to other brain structures by fiber tracts expressing the same subtype of cadherin. Overall, the expression of the four cadherins encompasses almost the entire nucleus rotundus. The four cadherin-expressing parts show different degrees of overlap. For example, the cadherin-6B part and the cadherin-7 part overlap extensively, whereas the R-cadherin part and the cadherin-6B part show little overlap and are partially complementary. Regions with shallow gradients of cadherin expression alternate with regions that show relatively abrupt changes in cadherin expression. At some points, changes of cadherin expression are also arranged in a pinwheel-like fashion, alternating between clockwise and counterclockwise orientations. In general, these results are reminiscent of the organization of functional modules in the mammalian visual cortex. It is speculated that each domain of cadherin expression corresponds to a functional domain, which processes a specific stimulus feature.

Hodological characterization of the medial amygdala in anuran amphibians

Early studies in anuran amphibians defined the amygdala as a single unit that only later could be subdivided into medial and lateral parts with the achievement of sensitive immunohistochemical and tracing techniques. However, the terminology used was often misleading when comparing with "homologous" amygdaloid nuclei in amniotes. Recently, the basal telencephalon of anurans has been demonstrated to be more complex than previously thought, and distinct amygdaloid nuclei were proposed on the basis of immunohistochemistry. Moreover, developmental data are increasing that support this notion. In the present study, we analyzed the patterns of afferent and efferent connections of the medial amygdala (MeA; formerly amygdala pars lateralis), considered as the main target of the vomeronasal information from the accessory olfactory bulb, as in other vertebrates. By means of axonal transport of dextran amines, the afferent and efferent connections of the MeA were traced in Rana perezi and Xenopus laevis under in vivo and in vitro conditions. Largely similar results were found in both species. The results showed abundant intratelencephalic and extratelencephalic connections that were readily comparable to those of other tetrapods. Most of these connections were reciprocal and, in particular, the strong relation of the MeA with the hypothalamus, via the stria terminalis, was demonstrated. Immunohistochemical techniques showed staining patterns that revealed abundant peptidergic afferents to the MeA, as well as minor inputs containing other neurotransmitters such as catecholamines. Double-labeling experiments demonstrated that the peptidergic fibers that reach the MeA originate in the ventral hypothalamus, whereas the catecholaminergic innervation of the MeA arises in the caudal extent of the posterior tubercle. Taken together, the results about connectivity in our study support the comparison of the MeA in anurans with its counterparts (and similarly named) amygdaloid nuclei in amniotes. Most of the hodological features of the medial amygdala seem to be shared by those tetrapods with well-developed vomeronasal systems.

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

We investigated whether gamma-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.

Critical steps in the early evolution of the isocortex: insights from developmental biology

This article proposes a comprehensive view of the origin of the mammalian brain. We discuss i) from which region in the brain of a reptilian-like ancestor did the isocortex originate, and ii) the origin of the multilayered structure of the isocortex from a simple-layered structure like that observed in the cortex of present-day reptiles. Regarding question i there have been two alternative hypotheses, one suggesting that most or all the isocortex originated from the dorsal pallium, and the other suggesting that part of the isocortex originated from a ventral pallial component. The latter implies that a massive tangential migration of cells from the ventral pallium to the dorsal pallium takes place in isocortical development, something that has not been shown. Question ii refers to the origin of the six-layered isocortex from a primitive three-layered cortex. It is argued that the superficial isocortical layers can be considered to be an evolutionary acquisition of the mammalian brain, since no equivalent structures can be found in the reptilian brain. Furthermore, a characteristic of the isocortex is that it develops according to an inside-out neurogenetic gradient, in which late-produced cells migrate past layers of early-produced cells. It is proposed that the inside-out neurogenetic gradient was partly achieved by the activation of a signaling pathway associated with the Cdk5 kinase and its activator p35, while an extracellular protein called reelin (secreted in the marginal zone during development) may have prevented migrating cells from penetrating into the developing marginal zone (future layer I).

Visual subdivisions of the dorsal ventricular ridge of the iguana (Iguana iguana) as determined by electrophysiologic mapping

The dorsal ventricular ridge (DVR) of reptiles is one of two regions of the reptilian telencephalon that receives input from the dorsal thalamus. Although studies demonstrate that two visual thalamic nuclei, the dorsal lateral geniculate and rotundus, send afferents to the dorsal cortex and DVR, respectively, relatively little is known about physiologic representations. The present study determined the organization of the visual recipient region of the iguana DVR. Microelectrode mapping techniques were used to determine the extent, number of subdivisions, and retinotopy within the visually responsive region of the anterior DVR (ADVR). Visually responsive neurons were restricted to the anterior two thirds of the ADVR. Within this region, two topographically organized subdivisions were determined. Each subdivision contained a full representation of the visual field and could be distinguished from the other by differences in receptive field properties and reversals in receptive field progressions across their mutual border. A third subdivision of the ADVR, in which neurons are responsive to visual stimulation is also described; however, a distinct visuotopic representation could not be determined for this region. This third region forms a shell surrounding the lateral, dorsal, and medial aspects of the topographically organized subdivisions. These results demonstrate that there are multiple physiologic subdivisions in the thalamic recipient zone of the ADVR of the iguana. Comparisons to the ADVR of other reptiles are made, homologies to ectostriatial regions of the bird are proposed, and the findings are discussed in relation to telencephalic organization of other vertebrates.

Evolutionary divergence of the reptilian and the mammalian brains: considerations on connectivity and development

The isocortex is a distinctive feature of the mammalian brain, with no clear counterpart in other amniotes. There have been long controversies regarding possible homologues of this structure in reptiles and birds. The brains of the latter are characterized by the presence of a structure termed dorsal ventricular ridge (DVR), which receives ascending auditory and visual projections, and has been postulated to be homologous to parts of the mammalian isocortex (i.e., the auditory and the extrastriate visual cortices). Dissenting views, now supported by molecular evidence, claim that the DVR originates from a region termed ventral pallium, while the isocortex may arise mostly from the dorsal pallium (in mammals, the ventral pallium relates to the claustroamygdaloid complex). Although it is possible that in mammals the embryonic ventral pallium contributes cells to the developing isocortex, there is no evidence yet supporting this alternative. The possibility is raised that the expansion of the cerebral cortex in the origin of mammals was product of a generalized dorsalizing influence in pallial development, at the expense of growth in ventral pallial regions. Importantly, the evidence suggests that organization of sensory projections is significantly different between mammals and sauropsids. In reptiles and birds, some sensory pathways project to the ventral pallium and others project to the dorsal pallium, while in mammals sensory projections end mainly in the dorsal pallium. We suggest a scenario for the origin of the mammalian isocortex which relies on the development of associative circuits between the olfactory, the dorsal and the hippocampal cortices in the earliest mammals.

Mesencephalic and diencephalic afferent connections to the thalamic nucleus rotundus in the lizard, Psammodromus algirus

The present work is an analysis of the afferent projections to the thalamic nucleus rotundus in a lizard, both at the light- and electron-microscopic level, using biotinylated dextran amine (BDA) as a neuroanatomical tracer. This study has confirmed previously reported afferent projections to nucleus rotundus in reptiles and has also identified a number of new cellular aggregates projecting to this dorsal thalamic nucleus. After BDA injections into nucleus rotundus, retrogradely labelled neurons were observed consistently within the following neuronal groups in the midbrain and the diencephalon: (i) the stratum griseum centrale of the optic tectum; (ii) the nucleus subpretectalis in the pretectum; (iii) the nucleus ansa lenticularis posterior, the posterior nucleus of the ventral supraoptic commissure, and the posteroventral nucleus, in the dorsal thalamus and (iv) the lateral suprachiasmatic nucleus and part of the reticular complex in the ventral thalamus. Tectal axons entering nucleus rotundus were fine and varicose and formed exclusively asymmetric synaptic contacts, mainly on small dendritic profiles. Rotundal neurons had symmetric synapses made by large boutons probably of nontectal origin. After comparing our results with those in other reptiles, birds and mammals, we propose that the sauropsidian nucleus rotundus forms part of a visual tectofugal pathway that conveys mesencephalic visual information to the striatum and dorsal ventricular ridge, and is similar to the mammalian colliculo-posterior/intralaminar-striatoamygdaloid pathway, the function of which may be to participate in visually guided behaviour.

A putative striato-dorsal thalamic pathway in lizards

Striatal targets related to the dorsal thalamus were studied in reptiles. The lateral striatum projects to globus pallidus and to three cellular groups associated to the lateral forebrain bundle: the anterior entopeduncular nucleus, the suprapeduncular nucleus, and the ventromedial thalamic nucleus. The projection is heavier on the suprapeduncular nucleus, which in turn projects on nucleus rotundus in the dorsal thalamus. Nucleus rotundus is the origin of a prominent projection to the lateral striatum among other forebrain areas. The intermediomedial striatum projects also to globus pallidus and to the three cellular groups associated with the lateral forebrain bundle, but in this case, the projection is heavier on the ventromedial thalamic nucleus. The latter nucleus targets a number of nuclear aggregates in the ventral tier of the dorsal thalamus, which in turn project to the intermediomedial striatum. As in mammals, the striatum in reptiles may influence through these pathways its input from the dorsal thalamus.

Organization of the mouse dorsal thalamus based on topology, calretinin immnunostaining, and gene expression

To better understand the organization and evolution of the dorsal thalamus, we have made a first approach to analyze the possible histogenetic compartments of the mammalian dorsal thalamus using mouse embryos. For that, we have analyzed the expression of the proneural gene Math4a and the protein calretinin. Our results suggest the existence of rostrodorsal, caudoventral, and ventral compartments in the embryonic dorsal thalamus of the mouse, which partly parallel the dorsoventral histogenetic tiers postulated in the dorsal thalamus of sauropsids. The rostrodorsal compartment of the mouse dorsal thalamus is characterized by expression of Math4a, and it appears to include sensory and motor thalamic nuclei projecting to the dorsal pallium (isocortex). This compartment appears equivalent to the lemnothalamus proposed by Butler in tetrapods based on hodological grounds. The caudoventral and ventral compartments of the mouse dorsal thalamus lack expression of Math4a in the mantle, but they are characterized by several populations of calretinin-immunorective neurons that show projections to the claustroamygdaloid region in the ventrolateral pallium. More studies will be needed to analyze if the compartments proposed in this study represent true histogenetic units, and to find homologous developmental fields in all vertebrates.

Development and evolution of the collopallium in amniotes: a new hypothesis of field homology

Embryological development is uniquely positioned to illuminate both hodology in adult brains and its inherited genetic bases. The lateral corner of the lateral ventricle in mammals is a particularly crucial region where cell migration patterns, transiently formed connections, axonal growth kinetics, and fasciculation patterns are complex and interactive. Based on hodology, the sauropsid anterior dorsal ventricular ridge (ADVR) has been proposed as the homologue on a one-to-one basis of the mammalian lateral neocortex (LNC), the basolateral amygdalar complex (BLA), or the claustrum-endopiriform nucleus (CE). Data on gene expression patterns during development have indicated ADVR homology with parts of the latter two structures rather than with LNC. Collothalamic nuclei (the set of dorsal thalamic nuclei that receive their predominant input from the midbrain roof) project to part of BLA and to LNC. Recent findings demonstrate a complex pattern of mutually overlapping but noncongruent gene expression territories and collothalamic projections, which suggests a new, collopallial field hypothesis that the ADVR is homologous as a field to all three structures LNC, BLA, and CE. This hypothesis accounts for current hodological and developmental data as well as for lack of a CE in monotremes and for an abnormal subcortical lamina of gray matter that results from a genetic abnormality in humans.

A proposed homology between the reptilian dorsomedial thalamic nucleus and the mammalian paraventricular thalamic nucleus

We have compared the reptilian dorsomedial thalamic nucleus with the mammalian paraventricular thalamic nucleus from a topographic, chemoarchitectonic, and hodological point of view. Both nuclei are localized to a similar position in the dorsalmost aspect of the dorsal thalamus (midline nuclei). They also are uniformly calretinin-immunoreactive, both their cells and neuropil are strongly immunostained for calretinin, and the dorsomedial nucleus presents a strong calbindin immunoreactivity as well. Finally, the reptilian and the mammalian nuclei share a set of afferent and efferent connections with a number of forebrain structures. On the whole, this set of data allows us to propose that the dorsomedial nucleus and the paraventricular thalamic nucleus are homologous. Both represent an important relay station in pathways connecting the hypothalamus with telencephalic areas involved in visceral and motivational aspects of behavior, such as nucleus accumbens and the central amygdala.

Thalamo-telencephalic connections: new insights on the cortical organization in reptiles

Tracer injections into the dorsal tier of the lacertilian dorsal thalamus revealed an extensive innervation of the cerebral cortex. The medial cortex, the dorsomedial cortex, and the medial part of the dorsal cortex received a bilateral projection, whereas the lateral part of dorsal cortex and the dorsal part of the lateral cortex received only an ipsilateral thalamic projection. Thalamocortical fibers were found superficially in all cortical regions, but in the dorsal part of the lateral cortex, varicose axons within the cellular layer were also observed. The bilateral thalamocortical projection originates from a cell population located throughout the dorsolateral anterior nucleus, whereas the ipsilateral input originates mainly from a rostral neuronal subpopulation of the nucleus. This feature suggests that the dorsolateral anterior nucleus consists of various parts with different projections. The dorsal subdivision of the lateral cortex displayed hodological and topological (radial glia processes) features of a dorsal pallium derivative. After tracer injections into the dorsal cortex of lizards, we found long descending projections that reached the striatum, the diencephalic basal plate, and the mesencephalic tegmentum, which suggests that it may represent a sensorimotor cortex.

Expression of calcium-binding proteins in the diencephalon of the lizard Psammodromus algirus

This work is a study of the distribution pattern of calbindin-D28k, calretinin, and parvalbumin in the diencephalic alar plate of a reptile, the lizard Psammodromus algirus, by using the prosomeric model (Puelles [1995] Brain Behav Evol 46:319-337), which divides the alar plate of the diencephalon into the caudorostrally arranged pretectum (p1), dorsal thalamus plus epithalamus (p2), and ventral thalamus (p3). Calbindin and calretinin are more extensively expressed in the dorsal thalamus than in the neighboring alar regions, and therefore these calcium-binding proteins are particularly suitable markers for delimiting the dorsal thalamus/epithalamus complex from the ventral thalamus and the pretectum. Conversely, parvalbumin is more intensely expressed in the pretectum and ventral thalamus than in the dorsal thalamus/epithalamus complex. Within the dorsal thalamus, calcium-binding protein immunoreactivity reveals a three-tiered division. The pretectum displays the most intense expression of parvalbumin within the diencephalon. Virtually all nuclei in the three sectors of the pretectum (commissural, juxtacommissural, and precommissural) present strong to moderate expression of parvalbumin. We compare the distribution of calcium-binding proteins in the diencephalon of Psammodromus with other vertebrates, with mammals in particular, and suggest that the middle and ventral tiers of the reptilian dorsal thalamus may be comparable to nonspecific or plurimodal posterior/intralaminar thalamic nuclei in mammals, on the basis of the calcium-binding protein expression patterns, as well as the hodological and embryological data in the literature.