Afferent connections of the striatum and the nucleus accumbens in the lizard Gekko gecko

Neural control of reproduction in reptiles

Reptiles display considerable diversity in reproductive behavior, making them great models to study the neuroendocrine control of reproductive behavior. Many reptile species are seasonally breeding, such that they become reproductively active during their breeding season and regress to a nonreproductive state during their nonbreeding season, with this transition often prompted by environmental cues. In this review, we will focus on summarizing the neural and neuroendocrine mechanisms controlling reproductive behavior. Three major areas of the brain are involved in reproductive behavior: the preoptic area (POA), amygdala, and ventromedial hypothalamus (VMH). The POA and VMH are sexually dimorphic areas, regulating behaviors in males and females respectively, and all three areas display seasonal plasticity. Lesions to these areas disrupt the onset and maintenance of reproductive behaviors, but the exact roles of these regions vary between sexes and species. Different hormones influence these regions to elicit seasonal transitions. Circulating testosterone (T) and estradiol (E2) peak during the breeding season and their influence on reproduction is well-documented across vertebrates. The conversion of T into E2 and 5α-dihydrotestosterone can also affect behavior. Melatonin and corticosterone have generally inhibitory effects on reproductive behavior, while serotonin and other neurohormones seem to stimulate it. In general, there is relatively little information on the neuroendocrine control of reproduction in reptiles compared to other vertebrate groups. This review highlights areas that should be considered for future areas of research.

Glutamatergic pathways in the brains of turtles: A comparative perspective among reptiles, birds, and mammals

Glutamate acts as the main excitatory neurotransmitter in the brain and plays a vital role in physiological and pathological neuronal functions. In mammals, glutamate can cause detrimental excitotoxic effects under anoxic conditions. In contrast, Trachemys scripta, a freshwater turtle, is one of the most anoxia-tolerant animals, being able to survive up to months without oxygen. Therefore, turtles have been investigated to assess the molecular mechanisms of neuroprotective strategies used by them in anoxic conditions, such as maintaining low levels of glutamate, increasing adenosine and GABA, upregulating heat shock proteins, and downregulating KATP channels. These mechanisms of anoxia tolerance of the turtle brain may be applied to finding therapeutics for human glutamatergic neurological disorders such as brain injury or cerebral stroke due to ischemia. Despite the importance of glutamate as a neurotransmitter and of the turtle as an ideal research model, the glutamatergic circuits in the turtle brain remain less described whereas they have been well studied in mammalian and avian brains. In reptiles, particularly in the turtle brain, glutamatergic neurons have been identified by examining the expression of vesicular glutamate transporters (VGLUTs). In certain areas of the brain, some ionotropic glutamate receptors (GluRs) have been immunohistochemically studied, implying that there are glutamatergic target areas. Based on the expression patterns of these glutamate-related molecules and fiber connection data of the turtle brain that is available in the literature, many candidate glutamatergic circuits could be clarified, such as the olfactory circuit, hippocampal–septal pathway, corticostriatal pathway, visual pathway, auditory pathway, and granule cell–Purkinje cell pathway. This review summarizes the probable glutamatergic pathways and the distribution of glutamatergic neurons in the pallium of the turtle brain and compares them with those of avian and mammalian brains. The integrated knowledge of glutamatergic pathways serves as the fundamental basis for further functional studies in the turtle brain, which would provide insights on physiological and pathological mechanisms of glutamate regulation as well as neural circuits in different species.

Neural architecture of the vertebrate brain: implications for the interaction between emotion and cognition

Cognition is considered a hallmark of the primate brain that requires a high degree of signal integration, such as achieved in the prefrontal cortex. Moreover, it is often assumed that cognitive capabilities imply "superior" computational mechanisms compared to those involved in emotion or motivation. In contrast to these ideas, we review data on the neural architecture across vertebrates that support the concept that association and integration are basic features of the vertebrate brain, which are needed to successfully adapt to a changing world. This property is not restricted to a few isolated brain centers, but rather resides in neuronal networks working collectively in a context-dependent manner. In different vertebrates, we identify shared large-scale connectional systems involving the midbrain, hypothalamus, thalamus, basal ganglia, and amygdala. The high degree of crosstalk and association between these systems at different levels supports the notion that cognition, emotion, and motivation cannot be separated - all of them involve a high degree of signal integration.

Thalamic reticular nucleus in Caiman crocodilus: forebrain connections

Forebrain connections of the thalamic reticular nucleus associated with the lateral forebrain bundle were analyzed in Caiman crocodilus. Both the compact portion, the dorsal peduncular nucleus, and the diffuse part, the perireticular region, associated with the lateral forebrain bundle, were studied. A small tracer injection into the dorsal peduncular nucleus demonstrated reciprocal connections with a restricted portion of the dorsal thalamus. Tracer placements into this nucleus retrogradely labeled cells in a caudal portion of the ventrolateral area of the telencephalon. These results are compared with similar studies in other amniotes.

Thalamic reticular nucleus in Caiman crocodilus: Relationship with the dorsal thalamus

The thalamic reticular nucleus was investigated in one group of crocodilians, Caiman crocodilus. This neuronal aggregate is composed of two parts: a compact portion and a diffuse region made up of scattered cells within the forebrain bundles. In Caiman, both the lateral and medial forebrain bundles project to the telencephalon and the thalamic reticular nucleus is associated with each fiber tract. In the lateral forebrain bundle, the compact area is termed the nucleus of the dorsal peduncle (dorsal peduncular nucleus) while the diffuse part is called the perireticular area. In the medial forebrain bundle, the interstitial nucleus comprises one part of the compact area while another region without a specific neuronal label is also present. Similar to the perireticular cells of the lateral forebrain bundle, scattered cells are also present in the medial forebrain bundle. Morphological features of the thalamic reticular nucleus are revealed with stains for the following: fibers; cells; succinic acid dehydrogenase; and acetylcholinesterase. Regardless of which dorsal thalamic nucleus was injected, a localized region of the thalamic reticular nucleus contained retrogradely labeled cells and anterogradely labeled axons and terminals. This grouping was termed clusters and was felt to represent the densest interconnection between the dorsal thalamus and the reticular nucleus. Using clusters as an index of interconnections, the reticular nucleus was divided into sectors, each of which was associated with a specific dorsal thalamic nucleus. An organization similar to that found in Caiman is present in other sauropsids as well as in mammals. These data suggest that a thalamic reticular nucleus is present in all amniotes and has morphological properties similar to those described in this analysis. Lastly, a hypothesis is presented to explain how the external shape of the reticular nucleus in Caiman might be transformed into the homologous area in a representative bird and mammal.

Peramorphic traits in the tokay gecko skull

Traditionally, geckos have been conceived to exhibit paedomorphic features relative to other lizards (e.g., large eyes, less extensively ossified skulls, and amphicoelous and notochordal vertebrae). In contrast, peramorphosis has not been considered an important process in shaping their morphology. Here, we studied different sized specimens of Gekko gecko to document ontogenetic changes in cranial anatomy, especially near maturity. Comparison of this species with available descriptions of other geckos resulted in the identification of 14 cranial characteristics that are expressed more strongly with size increase. These characteristics become move evident in later stages of post-hatching development, especially near maturation, and are, therefore, attributed to peramorphosis (hyperossification). ACCTRAN and DELTRAN character optimizations were applied to these characters using a tree of 11 genera derived from a gekkotan molecular phylogeny. This analysis revealed that G. gecko expresses the majority of these putative peramorphic features near maturity, and that some of these features are also expressed in species closely related to G. gecko. The characters studied have the potential to be applied in future phylogenetic and taxonomic studies of this group of lizards.

Sociosexual investigation in sexually experienced, hormonally manipulated male leopard geckos: relation with phosphorylated DARPP-32 in dopaminergic pathways

Dopaminergic activity is both associated with sociosexual exposure and modulated by sexual experience and hormonal state across vertebrate taxa. Mature leopard geckos, a reptile with temperature-dependent sex determination, have dopaminoceptive nuclei that are influenced by their embryonic environment and sensitive to adult hormonal manipulation. In this study, we exposed hormonally manipulated male leopard geckos from different incubation temperatures to conspecifics and measured their sociosexual investigation, as well as phosphorylated DARPP-32 at Threonine 34 (pDARPP-32) immunoreactivity as a marker for D1 dopamine receptor activity in the nucleus accumbens, striatum, and preoptic area. Social investigation time by males of different incubation temperatures was modulated in opposite directions by exogenous androgen treatment. Males exposed to novel stimuli spent a greater proportion of time investigating females of different incubation temperatures. The time spent investigating females was positively correlated to pDARPP-32 immunoreactivity in the preoptic area. This is the first study quantifying pDARPP-32 in a lizard species, and suggests the protein as a potential marker to measure differences in the dopaminergic pathway in a social setting with consideration of embryonic environment and hormonal state.

Amygdalostriatal projections in the neurocircuitry for motivation: a neuroanatomical thread through the career of Ann Kelley

In MacLean's triune brain, the amygdala putatively subserves motivated behavior by modulating the "reptilian" basal ganglia. Accordingly, Ann Kelley, with Domesick and Nauta, influentially showed that amygdalostriatal projections are much more extensive than were appreciated. They highlighted that amygdalar projections to the rostral ventromedial striatum converged with projections from the ventral tegmental area and cingulate cortex, forming a "limbic striatum". Caudal of the anterior commissure, the entire striatum receives afferents from deep basal nuclei of the amygdala. Orthologous topographic projections subsequently were observed in fish, amphibians, and reptiles. Subsequent functional studies linked acquired value to action via this neuroanatomical substrate. From Dr. Kelley's work evolved insights into components of the distributed, interconnected network that subserves motivated behavior, including the nucleus accumbens shell and core and the striatal-like extended amygdala macrostructure. These heuristic frameworks provide a neuroanatomical basis for adaptively translating motivation into behavior. The ancient amygdala-to-striatum pathways remain a current functional thread not only for stimulus-response valuation, but also for the psychopathological plasticity that underlies addiction-related memory, craving and relapse.

Characterization of the hypothalamus of Xenopus laevis during development. I. The alar regions

The patterns of expression of a set of conserved developmental regulatory transcription factors and neuronal markers were analyzed in the alar hypothalamus of Xenopus laevis throughout development. Combined immunohistochemical and in situ hybridization techniques were used for the identification of subdivisions and their boundaries. The alar hypothalamus was located rostral to the diencephalon in the secondary prosencephalon and represents the rostral continuation of the alar territories of the diencephalon and brainstem, according to the prosomeric model. It is composed of the supraoptoparaventricular (dorsal) and the suprachiasmatic (ventral) regions, and limits dorsally with the preoptic region, caudally with the prethalamic eminence and the prethalamus, and ventrally with the basal hypothalamus. The supraoptoparaventricular area is defined by the orthopedia (Otp) expression and is subdivided into rostral and caudal portions, on the basis of the Nkx2.2 expression only in the rostral portion. This region is the source of many neuroendocrine cells, primarily located in the rostral subdivision. The suprachiasmatic region is characterized by Dll4/Isl1 expression, and was also subdivided into rostral and caudal portions, based on the expression of Nkx2.1/Nkx2.2 and Lhx1/7 exclusively in the rostral portion. Both alar regions are mainly connected with subpallial areas strongly implicated in the limbic system and show robust intrahypothalamic connections. Caudally, both regions project to brainstem centers and spinal cord. All these data support that in terms of topology, molecular specification, and connectivity the subdivisions of the anuran alar hypothalamus possess many features shared with their counterparts in amniotes, likely controlling similar reflexes, responses, and behaviors.

Characterization of the bed nucleus of the stria terminalis in the forebrain of anuran amphibians

Major common features have been reported for the organization of the basal telencephalon in amniotes, and most characteristics were thought to be acquired in the transition from anamniotes to amniotes. However, gene expression, neurochemical, and hodological data obtained for the basal ganglia and septal and amygdaloid complexes in amphibians (anamniotic tetrapods) have strengthened the idea of a conserved organization in tetrapods. A poorly characterized region in the forebrain of amniotes has been the bed nucleus of the stria terminalis (BST), but numerous recent investigations have characterized it as a member of the extended amygdala. Our study analyzes the main features of the BST in anuran amphibians to establish putative homologies with amniotes. Gene expression patterns during development identified the anuran BST as a subpallial, nonstriatal territory. The BST shows Nkx2.1 and Lhx7 expression and contains an Islet1-positive cell subpopulation derived from the lateral ganglionic eminence. Immunohistochemistry for diverse peptides and neurotransmitters revealed that the distinct chemoarchitecture of the BST is strongly conserved among tetrapods. In vitro tracing techniques with dextran amines revealed important connections between the BST and the central and medial amygdala, septal territories, medial pallium, preoptic area, lateral hypothalamus, thalamus, and prethalamus. The BST receives dopaminergic projections from the ventral tegmental area and is connected with the laterodorsal tegmental nucleus and the rostral raphe in the brainstem. All these data suggest that the anuran BST shares many features with its counterpart in amniotes and belongs to a basal continuum, likely controlling similar reflexes, reponses, and behaviors in tetrapods.

The vertebrate mesolimbic reward system and social behavior network: a comparative synthesis

All animals evaluate the salience of external stimuli and integrate them with internal physiological information into adaptive behavior. Natural and sexual selection impinge on these processes, yet our understanding of behavioral decision-making mechanisms and their evolution is still very limited. Insights from mammals indicate that two neural circuits are of crucial importance in this context: the social behavior network and the mesolimbic reward system. Here we review evidence from neurochemical, tract-tracing, developmental, and functional lesion/stimulation studies that delineates homology relationships for most of the nodes of these two circuits across the five major vertebrate lineages: mammals, birds, reptiles, amphibians, and teleost fish. We provide for the first time a comprehensive comparative analysis of the two neural circuits and conclude that they were already present in early vertebrates. We also propose that these circuits form a larger social decision-making (SDM) network that regulates adaptive behavior. Our synthesis thus provides an important foundation for understanding the evolution of the neural mechanisms underlying reward processing and behavioral regulation.

Rapid neuroendocrine responses evoked at the onset of social challenge

At the onset of agonistic social challenge, individuals must assess the degree of threat the opponent represents in order to react appropriately. We aimed to characterize the neuroendocrine changes accompanying this period of initial social assessment using the lizard Anolis carolinensis. Conveyance of aggressive intent by male A. carolinensis is facilitated by rapid postorbital skin darkening (eyespot), whereas eyespot presence inhibits opponent aggression. By manipulating this visual signal, we also investigated whether differing neuroendocrine changes were evoked by initial presentation of varying levels of social threat. Subjects were painted postorbitally either with black paint (high threat level), green paint (low threat level) or water (controls). Painted animals were presented with a mirror and sampled immediately upon exhibiting aggressive intent towards the reflected simulated opponent, but before producing behaviors such as motor pattern-based displays. Control animals (blank surface presented) were sampled at times derived from averaging response times of painted subjects. Brains and plasma were analyzed for monoamine activity and catecholamine levels using electrochemical HPLC. Social threat evoked increases in plasma catecholamine levels indistinguishable from those caused by brief environmental disturbance. However, brief social challenge caused distinct rapid increases in amygdala and nucleus accumbens (NAc) dopamine and serotonin levels. Amygdalar changes were associated with general social threat presence, but NAc monoamines were affected by both threat level and subject motivation to engage in confrontation. This suggests that specific rapid activity changes in key forebrain limbic nuclei differ according to the degree of social threat perceived at the start of the interaction.

The turtle thalamic anterior entopeduncular nucleus shares connectional and neurochemical characteristics with the mammalian thalamic reticular nucleus

Neurochemical and key connectional characteristics of the anterior entopeduncular nucleus (Enta) of the turtle (Testudo horsfieldi) were studied by axonal tracing techniques and immunohistochemistry of parvalbumin, gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD). We showed that the Enta, which is located within the dorsal peduncle of the lateral forebrain bundle (Pedd), has roughly topographically organized reciprocal connections with the dorsal thalamic visual nuclei, the nucleus rotundus (Rot) and dorsal lateral geniculate nucleus (GLd). The Enta receives projections from visual telencephalic areas, the anterior dorsal ventricular ridge and dorsolateral cortex/pallial thickening. Most Enta neurons contained GABA and parvalbumin, and some of them were retrogradely labeled when the tracer was injected into the visual dorsal thalamic nuclei. Further experiments using double immunofluorescence revealed colocalization of GAD and parvalbumin in the vast majority of Enta neurons, and many of these cells showed retrograde labeling with Fluoro-gold injected into the Rot and/or GLd. According to these data, the Enta may be considered as a structural substrate for recurrent inhibition of the visual thalamic nuclei. Based on morphological and neurochemical similarity of the turtle Enta, caiman Pedd nucleus, the superior reticular nucleus in birds, and the thalamic reticular nucleus in mammals, we suggest that these structures represent a characteristic component which is common to the thalamic organization in amniotes.

Chemoarchitecture and afferent connections of the "olfactostriatum": a specialized vomeronasal structure within the basal ganglia of snakes

The olfactostriatum, a portion of the striatal complex of snakes, is the major tertiary vomeronasal structure in the ophidian brain, receiving substantial afferents from the nucleus sphericus, the primary target of accessory olfactory bulb efferents. In the present study, we have characterized the olfactostriatum of garter snakes (Thamnophis sirtalis) on the basis of chemoarchitecture (distribution of serotonin, neuropeptide Y and tyrosine hydroxylase) and hodology (afferent connections). The olfactostriatum is densely immunoreactive for serotonin and neuropeptide Y and shows moderate-to-weak immunoreactivity for tyrosine hydroxylase. In addition to afferents from the nucleus sphericus, the olfactostriatum receives inputs from the dorsal and lateral cortices, nucleus of the accessory olfactory tract, external and dorsolateral amygdalae, dorsomedial thalamic nucleus, ventral tegmental area and raphe nuclei. Double labeling experiments demonstrated that the distribution of serotonin and neuropeptide Y in this area almost completely overlaps the terminal field of projections from the nucleus sphericus. Also, serotonergic and dopaminergic innervation of the olfactostriatum likely arise, respectively, from the raphe nuclei and the ventral tegmental area, whereas local circuit neurons originate the neuropeptide Y immunoreactivity. These results indicate that the olfactostriatum of snakes could be a portion of the nucleus accumbens, with features characteristic of the accumbens shell, devoted to processing vomeronasal information. Comparative data suggest that a similar structure is present in the ventral striatum of amphibians and mammals.

The "olfactostriatum" of snakes: a basal ganglia vomeronasal structure in tetrapods

The olfactostriatum is a portion of the basal ganglia of snakes situated ventromedially to the nucleus accumbens proper. It receives a major vomeronasal input from the nucleus sphericus, the primary target of accessory olfactory bulb efferents. Recently, the ophidian olfactostriatum has been characterized on the basis of chemoarchitecture (distribution of serotonin, neuropeptide Y and tyrosine hydroxylase) and hodology (afferent and efferent connections). In contrast to the nucleus accumbens proper, the olfactostriatum is densely immunoreactive for serotonin and neuropeptide Y and sparsely immunoreactive for tyrosine hydroxylase. The nucleus accumbens proper and the olfactostriatum share most afferent connections except those originating in the nucleus sphericus, which are exclusively directed to the olfactostriatum. Similarly, the nucleus accumbens proper and the olfactostriatum show a similar pattern of efferent connections including those going to the ventral pallidum, although the olfactostriatum alone projects to the main and accessory olfactory bulbs as well as some amygdaloid nuclei. On the basis of its chemoarchitecture, the olfactostriatum resembles the mammalian ventral pallidum (but also the shell of the nucleus accumbens). Its connections, however, suggests that the olfactostriatum could be a specialized portion of the shell of nucleus accumbens extended more ventromedially than previously believed and devoted to processing vomeronasal information. Comparative data suggest that a similar structure is present in the basal ganglia of amphibians and mammals.

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.

Tyrosine hydroxylase expression is affected by sexual vigor and social environment in male Cnemidophorus inornatus

Although the distribution of catecholamine-synthesizing cells has been described for a variety of taxa, less is known about the functional significance of particular populations in nonmammalian species, especially reptiles. To understand the role of these populations in the display of social behaviors in lizards, we studied the interactive effects of sexual vigor (sexually vigorous vs. sluggish) and social condition (housing in isolation vs. with females) on the number and somal areas of cells expressing tyrosine hydroxylase (TH), a rate-limiting enzyme in catecholamine synthesis, in male whiptail lizards, Cnemidophorus inornatus. We found that, regardless of social condition, sexually vigorous males had more TH-immunoreactive (TH-ir) cells in the dorsal hypothalamus (DH) relative to sluggish males. Sexually vigorous males also had more TH-ir cells in the substantia nigra pars compacta (SNpc), but this difference was significant only among males housed with females. Sexually vigorous males that had been housed with females had smaller TH-ir cells in the preoptic area (POA) than vigorous males housed in isolation. On the other hand, no significant differences were found in the anterior hypothalamus. These results highlight the regional heterogeneity in the plasticity of TH expression and suggest that, just as in other species, the DH, SNpc, and POA might be involved in the expression of social behaviors and in behavioral plasticity following social experiences in lizards.

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.

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.

Efferent connections of the dorsomedial thalamic nuclei of the domestic chick (Gallus domesticus)

Small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were placed in the thalamic anterior dorsomedial nucleus (DMA) of domestic chicks. The projections of the DMA covered the rostrobasal forebrain, ventral paleostriatum, nucleus accumbens, septal nuclei, Wulst, hyperstriatum ventrale, neostriatal areas, archistriatal subdivisions, dorsolateral corticoid area, numerous hypothalamic nuclei, and dorsal thalamic nuclei. The rostral DMA projects preferentially on the hypothalamus, whereas the caudal part is connected mainly to the dorsal thalamus. The DMA is also connected to the periaqueductal gray, deep tectum opticum, intercollicular nucleus, ventral tegmental area, substantia nigra, locus coeruleus, dorsal lateral mesencephalic nucleus, lateral reticular formation, nucleus papillioformis, and vestibular and cranial nerve nuclei. This pattern of connectivity is likely to reflect an important role of the avian DMA in the regulation of attention and arousal, memory formation, fear responses, affective components of pain, and hormonally mediated behaviors.

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.

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

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

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

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

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.

Understanding the basic circuitry of the cerebral hemispheres: the case of lizards and its implications in the evolution of the telencephalon

The organization of the cerebral hemispheres of mammals is characterized by corticostriatal glutamatergic projections and striatopallidal GABAergic ones, plus the descending projections of the pallium and subpallium to extratelencephalic targets. The present review of the available neuroanatomical data on the forebrain of lizards suggests that the telencephalon of reptiles also follows this basic pattern of connectivity. In addition, we show that this basic circuitry includes a pallido-cortical projection, therefore forming a cortico-striato-pallido-cortical circuit. The analysis of this circuitry for the medial, dorsal, lateral, and ventral pallial divisions in reptiles and mammals leads to the following conclusions: (1) The medial and dorsal cortices of lizards together appear to be equivalent to the medial pallium of mammals. (2) The projection from the lacertilian dorsal cortex to the striatum proper resembles the subiculo-striatal projection of mammals, rather than the isocortical projection to the caudatus-putamen. (3) Most of the dorsal striatum of reptiles is engaged in the corticostriatal circuit corresponding to the ventral pallium (the anterior dorsal ventricular ridge), and therefore, it is not equivalent to the mammalian caudatus-putamen, which is involved in the circuit of the dorsal pallium. (4) The main and accessory olfactory bulbs also follow this pattern of connections.

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.

Immunohistochemical localization of DARPP-32 in the brain of the lizard, Gekko gecko: co-occurrence with tyrosine hydroxylase

To assess the relationship between dopaminergic neuronal structures and dopaminoceptive structures in a reptile, single and double immunohistochemical procedures with antibodies directed against DARPP-32 (dopamine- and cAMP-regulated phosphoprotein with an apparent molecular mass of 32,000 daltons),a phosphoprotein related to the dopamine D(1)-receptor, and tyrosine hydroxylase (TH) were applied to the brain of the lizard, Gekko gecko. The DARPP-32 antibody yielded a well-differentiated pattern of staining in the brain of Gekko. In general, areas that are densely innervated by TH-immunoreactive, putative dopaminergic fibers, such as the nucleus accumbens, striatum, dorsal ventricular ridge, and amygdaloid complex, display strong immunoreactivity for DARPP-32 in somata and neuropil. Distinct cellular DARPP-32 immunoreactivity was also found in the lateral cortex, ventral hypothalamus, habenula, central nucleus of the torus semicircularis, midbrain tectum, parvicellular isthmic nucleus, raphe nuclei, caudal rhombencephalic tegmentum, and spinal cord. Striatal projections to the midbrain and their target, i.e., the substantia nigra pars reticulata, were found to be strongly immunoreactive. Double immunofluorescence staining revealed that dopaminergic cells generally do not stain for DARPP-32, except for cells in the ventral hypothalamus and at caudal rhombencephalic levels. In conclusion, the distribution of DARPP-32 in the brain of the lizard Gekko gecko largely resembles the pattern observed in birds and mammals, at least as far as basal ganglia structures are concerned. On the other hand, there are several specific features of DARPP-32 distribution in the gekkonid brain that deserve further attention, such as cellular colocalization of DARPP-32 and TH immunoreactivity in hypothalamic and caudal rhombencephalic areas, and cellular DARPP-32 immunoreactivity in the tectum and central nucleus of the torus semicircularis of the midbrain, the superior and inferior raphe nuclei, and the spinal cord.

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.

Evolution of the basal ganglia: new perspectives through a comparative approach

The basal ganglia (BG) have received much attention during the last 3 decades mainly because of their clinical relevance. Our understanding of their structure, organisation and function in terms of chemoarchitecture, compartmentalisation, connections and receptor localisation has increased equally. Most of the research has been focused on the mammalian BG, but a considerable number of studies have been carried out in nonmammalian vertebrates, in particular reptiles and birds. The BG of the latter 2 classes of vertebrates, which together with mammals constitute the amniotic vertebrates, have been thoroughly studied by means of tract-tracing and immunohistochemical techniques. The terminology used for amniotic BG structures has frequently been adopted to indicate putative corresponding structures in the brain of anamniotes, i.e. amphibians and fishes, but data for such a comparison were, until recently, almost totally lacking. It has been proposed several times that the occurrence of well developed BG structures probably constitutes a landmark in the anamniote-amniote transition. However, our recent studies of connections, chemoarchitecture and development of the basal forebrain of amphibians have revealed that tetrapod vertebrates share a common pattern of BG organisation. This pattern includes the existence of dorsal and ventral striatopallidal systems, reciprocal connections between the striatopallidal complex and the diencephalic and mesencephalic basal plate (striatonigral and nigrostriatal projections), and descending pathways from the striatopallidal system to the midbrain tectum and reticular formation. The connectional similarities are paralleled by similarities in the distribution of chemical markers of striatal and pallidal structures such as dopamine, substance P and enkephalin, as well as by similarities in development and expression of homeobox genes. On the other hand, a major evolutionary trend is the progressive involvement of the cortex in the processing of the thalamic sensory information relayed to the BG of tetrapods. By using the comparative approach, new insights have been gained with respect to certain features of the BG of vertebrates in general, such as the segmental organisation of the midbrain dopaminergic cell groups, the occurrence of large numbers of dopaminergic cell bodies within the telencephalon itself and the variability in, among others, connectivity and chemoarchitecture. However, the intriguing question whether the basal forebrain organisation of nontetrapods differs essentially from that observed in tetrapods still needs to be answered.

Nucleus accumbens in the lizard Psammodromus algirus: chemoarchitecture and cortical afferent connections

To better understand the organization and evolution of the basal ganglia of vertebrates, in the present study we have analyzed the chemoarchitecture and the cortical input to the nucleus accumbens in the lacertid lizard Psammodromus algirus. The nucleus accumbens contains many gamma-aminobutyric acid (GABA)-positive neurons and calbindin-positive neurons, the majority of which may be spiny projection neurons, and a few dispersed neuropeptide Y-positive neurons that likely represent aspiny interneurons. The nucleus accumbens contains two chemoarchitectonically different fields: a rostromedial field that stains heavily for substance P, dopamine, GABA(A) receptor, and a caudolateral field that stains only lightly to moderately for them, appearing more similar to the adjacent striatum. Injections of biotinylated dextran amine were placed in either the medial, dorsomedial, or dorsal cortices of Psammodromus. The medial and the dorsal cortices project heavily to the rostromedial field of the accumbens, whereas they project lightly to moderately to the caudolateral field. Cortical terminals make asymmetric, presumably excitatory, synaptic contacts with distal dendrites and the head of spines. Our results indicate that the hippocampal-like projection to the nucleus accumbens is similar between mammals and reptiles in that cortical terminals make mainly excitatory synapses on spiny, putatively projection neurons. However, our results and results from previous investigations indicate that important differences exist between the nucleus accumbens of mammals and reptiles regarding local modulatory interactions between cortical, dopaminergic, and cholinergic elements, which suggest that the reptilian nucleus accumbens may be as a whole comparable to the shell of the mammalian nucleus accumbens.

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.

Evolution of the basal ganglia in tetrapods: a new perspective based on recent studies in amphibians

It has been postulated frequently that the fundamental organization of the basal ganglia (BG) in vertebrates arose with the appearance of amniotes during evolution. An alternative hypothesis, however, is that such a condition was already present in early anamniotic tetrapods and, therefore, characterizes the acquisition of the tetrapod phenotype rather than the anamniotic-amniotic transition. Re-examination of the BG organization in tetrapods in the light of recent findings in amphibians strongly supports the notion that elementary BG structures were present in the brain of ancestral tetrapods and that they were organized according to a general plan shared today by all extant tetrapods.

Afferent and efferent connections of the nucleus sphericus in the snake Thamnophis sirtalis: convergence of olfactory and vomeronasal information in the lateral cortex and the amygdala

This paper is an account of the afferent and efferent projections of the nucleus sphericus (NS), which is the major secondary vomeronasal structure in the brain of the snake Thamnophis sirtalis. There are four major efferent pathways from the NS: 1) a bilateral projection that courses, surrounding the accessory olfactory tract, and innervates several amygdaloid nuclei (nucleus of the accessory olfactory tract, dorsolateral amygdala, external amygdala, and ventral anterior amygdala), the rostral parts of the dorsal and lateral cortices, and the accessory olfactory bulb; 2) a bilateral projection that courses through the medial forebrain bundle and innervates the olfactostriatum (rostral and ventral striatum); 3) a commissural projection that courses through the anterior commissure and innervates mainly the contralateral NS; and 4) a meager bilateral projection to the lateral hypothalamus. On the other hand, important afferent projections to the NS arise solely in the accessory olfactory bulb, the nucleus of the accessory olfactory tract, and the contralateral NS. This pattern of connections has three important implications: first, the lateral cortex probably integrates olfactory and vomeronasal information. Second, because the NS projection to the hypothalamus is meager and does not reach the ventromedial hypothalamic nucleus, vomeronasal information from the NS is not relayed directly to that nucleus, as previously reported. Finally, a structure located in the rostral and ventral telencephalon, the olfactostriatum, stands as the major tertiary vomeronasal center in the snake brain. These three conclusions change to an important extent our previous picture of how vomeronasal information is processed in the brain of reptiles.

Afferent connections of the nucleus accumbens of the snake, Elaphe guttata, studied by means of in vitro and in vivo tracing techniques in combination with TH immunohistochemistry

The aim of the present study was to determine the afferent connections of the nucleus accumbens in snakes, in particular its catecholaminergic input. For that purpose, in vitro and in vivo applications of retrograde tracers in the nucleus accumbens of Elaphe guttata were combined with tyrosine hydroxylase (TH) immunohistochemistry. Both techniques revealed telencephalic inputs to the nucleus accumbens originating from the diagonal band of Broca, ventral pallidum, amygdaloid complex, and dorsal cortex. Major diencephalic inputs arise from the dorsomedial thalamic nucleus and the hypothalamus. In the brainstem, a few retrogradely labeled cells were observed in the raphe nucleus and the locus coeruleus. Considerably more cells were found in the midbrain tegmentum. Within the confines of the locus coeruleus and, in particular, the midbrain tegmentum, retrogradely labeled cells stained also for TH suggesting that those areas constitute the major catecholaminergic input to the nucleus accumbens of snakes. The experimental approach used in the present study, in particular the in vitro technique, seems to be very suited for studying the development of basal ganglia organization of reptiles in the near future.

Basal ganglia organization in amphibians: catecholaminergic innervation of the striatum and the nucleus accumbens

The aim of the present study was to determine the origin of the catecholaminergic inputs to the telencephalic basal ganglia of amphibians. For that purpose, retrograde tracing techniques were combined with tyrosine hydroxylase immunohistochemistry in the anurans Xenopus laevis and Rana perezi and the urodele Pleurodeles waltl. In all three species studied, a topographically organized dopaminergic projection was identified arising from the posterior tubercle/mesencephalic tegmentum and terminating in the striatum and the nucleus accumbens. Although essentially similar, the organization of the mesolimbic and mesostriatal connections in anurans seems to be more elaborate than in urodeles. The present study has also revealed the existence of a noradrenergic projection to the basal forebrain, which has its origin in the locus coeruleus. Additional catecholaminergic afferents to the striatum and the nucleus accumbens arise from the nucleus of the solitary tract, where catecholaminergic neurons appear to give rise to the bulk of the projections to the basal forebrain. In other regions, such as the olfactory bulb, the anterior preoptic area, the suprachiasmatic nucleus, and the thalamus, retrogradely labeled neurons (after basal forebrain tracer-applications) and catecholaminergic cells were intermingled, but none of these centers contained double-labeled cell bodies. It is concluded that the origin of the catecholaminergic innervation of the striatum and the nucleus accumbens in amphibians is largely comparable to that in amniotes. The present study, therefore, strongly supports the existence of a common pattern in the organization of the catecholaminergic inputs to the basal forebrain among tetrapod vertebrates.

Basal ganglia organization in amphibians: afferent connections to the striatum and the nucleus accumbens

As part of a research program to determine if the organization of basal ganglia (BG) of amphibians is homologous to that of amniotes, the afferent connections of the BG in the anurans Xenopus laevis and Rana perezi and the urodele Pleurodeles waltl were investigated with sensitive tract-tracing techniques. Hodological evidence is presented that supports a division of the amphibian BG into a nucleus accumbens and a striatum. Both structures have inputs in common from the olfactory bulb, medial pallium, striatopallial transition area, preoptic area, ventral thalamus, ventral hypothalamic nucleus, posterior tubercle, several mesencephalic and rhombencephalic reticular nuclei, locus coeruleus, raphe, and the nucleus of the solitary tract. Several nuclei that project to both subdivisions of the BG, however, show a clear preference for either the striatum (lateral amygdala, parabrachial nucleus) or the nucleus accumbens (medial amygdala, ventral midbrain tegmentum). In addition, the anterior entopeduncular nucleus, central thalamic nucleus, anterior and posteroventral divisions of the lateral thalamic nucleus, and torus semicircularis project exclusively to the striatum, whereas the anterior thalamic nucleus, anteroventral, and anterodorsal tegmental nuclei provide inputs solely to the nucleus accumbens. Apart from this subdivision of the basal forebrain, the results of the present study have revealed more elaborate patterns of afferent projections to the BG of amphibians than previously thought. Moreover, regional differences within the striatum and the nucleus accumbens were demonstrated, suggesting the existence of functional subdivisions. The present study has revealed that the organization of the afferent connections to the BG in amphibians is basically similar to that of amniotes. According to their afferent connections, the striatum and the nucleus accumbens of amphibians may play a key role in processing olfactory, visual, auditory, lateral line, and visceral information. However, contrary to the situation in amniotes, only a minor involvement of pallial structures on the BG functions is present in amphibians.

Ultrastructural morphology of synapses formed by corticostriatal terminals in the avian striatum

We studied the ultrastructural morphology of corticostriatal projections from two different avian 'neocortical' regions, namely, the hyperstriatum accessorium (HA) and the pallium externum (PE). Biotinylated dextran amine (BDA) was used to label the corticostriatal projection from either HA or PE to the striatum. The corticostriatal axons from both the PE and HA possessed numerous beaded varicosities with the striatum. These varicosities were filled with numerous round vesicles characterizing them as terminals. These terminals formed asymmetric synapses with spine heads and with dendrites of striatal neurons. The axospinous synapses outnumbered the axodendritic synapses by more than two to one. The diameters of labeled axons were typically 250-500 nm. The labeled terminals were typically 400-750 nm in diameter. No obvious differences between the ultrastructural morphology of the HA and the PE corticostriatal projections were observed. These data show that corticostriatal terminals and their synaptic contacts in birds are similar to those described in mammals.