Anterior thalamic nucleus projections to the dorsal pallium in ranid frogs

Amphibian thalamic nuclear organization during larval development and in the adult frog Xenopus laevis: Genoarchitecture and hodological analysis

The early patterning of the thalamus during embryonic development defines rostral and caudal progenitor domains, which are conserved from fishes to mammals. However, the subsequent developmental mechanisms that lead to the adult thalamic configuration have only been investigated for mammals and other amniotes. In this study, we have analyzed in the anuran amphibian Xenopus laevis (an anamniote vertebrate), through larval and postmetamorphic development, the progressive regional expression of specific markers for the rostral (GABA, GAD67, Lhx1, and Nkx2.2) and caudal (Gbx2, VGlut2, Lhx2, Lhx9, and Sox2) domains. In addition, the regional distributions at different developmental stages of other markers such as calcium binding proteins and neuropeptides, helped the identification of thalamic nuclei. It was observed that the two embryonic domains were progressively specified and compartmentalized during premetamorphosis, and cell subpopulations characterized by particular gene expression combinations were located in periventricular, intermediate and superficial strata. During prometamorphosis, three dorsoventral tiers formed from the caudal domain and most pronuclei were defined, which were modified into the definitive nuclear configuration through the metamorphic climax. Mixed cell populations originated from the rostral and caudal domains constitute most of the final nuclei and allowed us to propose additional subdivisions in the adult thalamus, whose main afferent and efferent connections were assessed by tracing techniques under in vitro conditions. This study corroborates shared features of early gene expression patterns in the thalamus between Xenopus and mouse, however, the dynamic changes in gene expression observed at later stages in the amphibian support mechanisms different from those of mammals.

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.

Thalamo-telencephalic pathways in the fire-bellied toad Bombina orientalis

It was suggested that among extant vertebrates, anuran amphibians display a brain organization closest to the ancestral tetrapod condition, and recent research suggests that anuran brains share important similarities with the brains of amniotes. The thalamus is the major source of sensory input to the telencephalon in both amphibians and amniote vertebrates, and this sensory input is critical for higher brain functions. The present study investigated the thalamo-telencephalic pathways in the fire-bellied toad Bombina orientalis, a basal anuran, by using a combination of retrograde tract tracing and intracellular injections with the tracer biocytin. Intracellular labeling revealed that the majority of neurons in the anterior and central thalamic nuclei project to multiple brain targets involved in behavioral modulation either through axon collaterals or en passant varicosities. Single anterior thalamic neurons target multiple regions in the forebrain and midbrain. Of note, these neurons display abundant projections to the medial amygdala and a variety of pallial areas, predominantly the anterior medial pallium. In Bombina, telencephalic projections of central thalamic neurons are restricted to the dorsal striato-pallidum. The bed nucleus of the pallial commissure/thalamic eminence similarly targets multiple brain regions including the ventral medial pallium, but this is accomplished through a higher variety of distinct neuron types. We propose that the amphibian diencephalon exerts widespread influence in brain regions involved in behavioral modulation and that a single dorsal thalamic neuron is in a position to integrate different sensory channels and distribute the resulting information to multiple brain regions.

Organization of the sensory input to the telencephalon in the fire-bellied toad, Bombina orientalis

The functional organization of sensory activity in the amphibian telencephalon is poorly understood. We used an in vitro brain preparation to compare the anatomy of afferent pathways with the localization of electrically evoked sensory potentials and single neuron intracellular responses in the telencephalon of the toad Bombina orientalis. Anatomical tracing showed that the anterior thalamic nucleus innervates the anterior parts of the medial, dorsal, and lateral pallia and the rostralmost part of the pallium in addition to the subpallial amygdala/ventral pallidum region. Additional afferents to the medial telencephalon originate from the thalamic eminence. Electrical stimulation of diverse sensory nerves and brain regions generated evoked potentials with distinct characteristics in the pallium, subpallial amygdala/ventral pallidum, and dorsal striatopallidum. In the pallium, this sensory activity is generated in the anterior medial region. In the case of olfaction, evoked potentials were recorded at all sites, but displayed different characteristics across telencephalic regions. Stimulation of the anterior dorsal thalamus generated a pattern of activity comparable to olfactory evoked potentials, but it became similar to stimulation of the optic nerve or brainstem after bilateral lesion of the lateral olfactory tract, which interrupted the antidromic activation of the olfactohabenular tract. Intracellular bimodal sensory responses were obtained in the anterior pallium, medial amygdala, ventral pallidum, and dorsal striatopallidum. Our results demonstrate that the amphibian anterior pallium, medial amygdala/ventral pallidum, and dorsal striatopallidum are multimodal sensory centers. The organization of the amphibian telencephalon displays striking similarities with the brain pathways recently implicated in mammalian goal-directed behavior.

Organization of the pallium in the fire-bellied toad Bombina orientalis. I: Morphology and axonal projection pattern of neurons revealed by intracellular biocytin labeling

The cytoarchitecture and axonal projection pattern of pallial areas was studied in the fire-bellied toad Bombina orientalis by intracellular injection of biocytin into a total of 326 neurons forming 204 clusters. Five pallial regions were identified, differing in morphology and projection pattern of neurons. The rostral pallium receiving the bulk of dorsal thalamic afferents has reciprocal connections with all other pallial areas and projects to the septum, nucleus accumbens, and anterior dorsal striatum. The medial pallium projects bilaterally to the medial pallium, septum, nucleus accumbens, mediocentral amygdala, and hypothalamus and ipsilaterally to the rostral, dorsal, and lateral pallium. The ventral part of the medial pallium is distinguished by efferents to the eminentia thalami and the absence of contralateral projections. The dorsal pallium has only ipsilateral projections running to the rostral, medial, and lateral pallium; septum; nucleus accumbens; and eminentia thalami. The lateral pallium has ipsilateral projections to the olfactory bulbs and to the rostral, medial, dorsal, and ventral pallium. The ventral pallium including the striatopallial transition area (SPTA) has ipsilateral projections to the olfactory bulbs, rostral and lateral pallium, dorsal striatopallidum, vomeronasal amygdala, and hypothalamus. The medial pallium can be tentatively homologized with the mammalian hippocampal formation, the dorsal pallium with allocortical areas, the lateral pallium rostrally with the piriform and caudally with the entorhinal cortex, the ventral pallium with the accessory olfactory amygdala. The rostral pallium, with its projections to the dorsal and ventral striatopallidum, resembles the mammalian frontal cortex.

Integration of sensory and motor processing underlying social behaviour in túngara frogs

Social decision making involves the perception and processing of social stimuli, the subsequent evaluation of that information in the context of the individual's internal and external milieus to produce a decision, and then culminates in behavioural output informed by that decision. We examined brain networks in an anuran communication system that relies on acoustic signals to guide simple, stereotyped motor output. We used egr-1 mRNA expression to measure neural activation in male túngara frogs, Physalaemus pustulosus, following exposure to conspecific and heterospecific calls that evoke competitive or aggressive behaviour. We found that acoustically driven activation in auditory brainstem nuclei is transformed into activation related to sensory-motor interactions in the diencephalon, followed by motor-related activation in the telencephalon. Furthermore, under baseline conditions, brain nuclei typically have correlated egr-1 mRNA levels within brain divisions. Hearing conspecific advertisement calls increases correlations between anatomically distant brain divisions; no such effect was observed in response to calls that elicit aggressive behaviour. Neural correlates of social decision making thus take multiple forms: (i) a progressive shift from sensory to motor encoding from lower to higher stages of neural processing and (ii) the emergence of correlated activation patterns among sensory and motor regions in response to behaviourally relevant social cues.

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.

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.

Hodological characterization of the septum in anuran amphibians: I. Afferent connections

On the basis of Nissl-stained sections, we subdivided the septum of the gray treefrog Hyla versicolor in the lateral, central, and medial septal complex. The afferent projections of the different septal nuclei were studied by combined retrograde and anterograde tracing with biotin ethylendiamine (Neurobiotin). The central and medial septal complex receives direct input from regions of the olfactory bulb and from all other limbic structures of the telencephalon (e.g., amygdalar regions, nucleus accumbens), whereas projections to the lateral septal complex are absent or less extensive. The medial pallium projects to all septal nuclei. In the diencephalon, the anterior thalamic nucleus provides the main ascending input to all subnuclei of the anuran septum, which can be interpreted as a limbic/associative pathway. The ventromedial thalamic nucleus projects to the medial and lateral septal complex and may thereby transmit multisensory information to the limbic system. Anterior preoptic nucleus, suprachiasmatic nucleus, and hypothalamic nuclei innervate the central and lateral septal complex. Only the nuclei of the central septal complex receive input from the brainstem. Noteworthy is the relatively strong projection from the nucleus raphe to the central septal complex, but not to the other septal nuclei.

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.

A Pallial Visual Area in the Telencephalon of the Bony Fish Polypterus

The dorsal (P2) and lateral (P3) pallial zones of bichirs receive a substantial projection from nucleus medianus of the posterior tuberculum. Although nucleus medianus does not receive a direct retinal input, its close proximity to ascending tectal efferents suggests that it might receive a tectal input and form a segment of a retinotectal-tubercular pathway to the pallium. In order to test this possibility, evoked responses to light flashes were electrophysiologically recorded and tectal efferents were experimentally determined in bichirs. These experiments suggest that a single visual field exists across the dorsal and lateral pallial zones and that this field is mediated by nucleus medianus, which does receive a direct tectal projection. This visual pathway appears to be uniquely derived and not homologous to any other known pathway in tetrapods. Furthermore, these results support the contention that the P2 and P3 pallial zones in bichirs are subdivisions of a single pallial zone.

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.

Morphology, axonal projection pattern, and responses to optic nerve stimulation of thalamic neurons in the fire-bellied toad Bombina orientalis

Intracellular recording and biocytin labeling were carried out in the fire-bellied toad Bombina orientalis to study the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation. Labeled neurons (n = 142) were divided into the following groups: TH1 neurons projecting to the dorsal striatum; TH2 neurons projecting to the amygdala, nucleus accumbens, and septal nuclei; TH3 neurons projecting to the medial or dorsal pallium; TH4 neurons with projections ascending to the dorsal striatum or ventral striatum/amygdala and descending to the optic tectum, tegmentum, and rostral medulla oblongata; TH5 neurons with projections to the tegmentum, rostral medulla oblongata, prectectum, or tectum; and TH6 neurons projecting to the hypothalamus. TH1 neurons are found in the central, TH2 neurons in the anterior and central, TH3 neurons in the anterior dorsal nucleus, and TH4 and TH5 neurons in the posterior dorsal or ventral nucleus. Neurons with descending projections arborize in restricted parts of retinal afferents; neurons with ascending projections do not substantially arborize within retinal afferents. At electrical optic nerve stimulation, neurons in the ventral thalamus respond with excitation at latencies of 10.8 msec; one-third of them follow repetitive stimulation and possibly are monosynaptically driven. Neurons in the dorsal thalamus respond mostly with inhibition at latencies of 42.3 msec and are polysynaptically driven. This corroborates the view that neurons in the dorsal thalamus projecting to the telencephalon receive no substantial direct retinal input and that the thalamopallial pathway of amphibians is not homologous to the mammalian retinogeniculocortical pathway.

Connections of the pallial telencephalon in the Senegal bichir, Polypterus

The connections of the dorsal telencephalon (pallium) of the Senegal bichir (Polypterus senegalus) were investigated in order to test the hypothesis that the simple everted pallium of polypterid fishes is directly comparable to the evaginated pallium of most vertebrates. Neuroanatomical tracers (DiI or biotinylinated dextran amines) were injected into the three traditionally recognized divisions of the bichir pallium to determine their afferent and efferent connections. The connections were corroborated with injections into regions identified as projecting to, or receiving input from, one or more pallial divisions. The results suggest that the bichir pallium consists not of three, but only two primary divisions: a dorsomedial one and a dorsolateral one. This reinterpretation refutes the hypothesis of a direct one-to-one comparison with the tripartite pallium of most vertebrates. Homologues of the bichir dorsomedial and dorsolateral pallium are recognized in the lateral and medial pallium of anuran amphibians, respectively. The pallium of bichirs is compared to that of derived ray-finned fishes as a link between derived ray-finned fishes and other vertebrates. The available information on the connections of the pallium of teleosts suggests that only the olfactory recipient (pars posterior) of the pallium can be directly compared to bichirs and amphibians and that the remaining divisions of the pallium in teleosts are uniquely derived features of the teleost telencephalon.

Morphology, axonal projection pattern, and responses to optic nerve stimulation of thalamic neurons in the salamander Plethodon jordani

In the salamander Plethodon jordani, the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation were determined by intracellular recording and biocytin labeling under in vitro, whole-brain conditions. Based on their axonal projections, labeled neurons (n = 76) were divided into the following groups: TH1 neurons, with mostly ipsilateral projections to the striatum; TH2 neurons, with ipsilateral or bilateral projections to the medial amygdala and nucleus accumbens; TH3 neurons, with bilateral projections to the medial and dorsal pallium; TH4 neurons, with mostly ipsilateral projections to the striatum and ipsilateral projections to the tectum opticum, tegmentum, and rostral medulla oblongata; and TH5 neurons, with ipsilateral projections to the tegmentum, medulla oblongata, and rostral spinal cord without (TH5.1) or with (TH5.2) additional projections to the optic tectum. TH1-TH4 neurons are found in the dorsal thalamus and around the sulcus medialis, and TH5 neurons are found in the ventral thalamus. Labeled neurons with ascending projections, i.e., the more dorsally situated TH1-TH4 neurons, are mostly inhibited by electrical stimulation of the optic nerve and have significantly longer latencies (mean +/- S.D., 42.1 +/- 11.6 msec) than neurons with exclusively descending projections, i.e., the ventrally located TH5 neurons (8.5 +/- 6.1 msec), which receive the bulk of retinal afferents and show excitation at electrical optic nerve stimulation. Neurons recorded without labeling in the dorsal thalamus likewise exhibit mostly inhibition and have significantly longer latencies (35.7 +/- 18.9 msec) than those recorded in the ventral thalamus (10.9 +/- 7.7 msec), which mostly show excitation. None of the neurons recorded in the dorsal thalamus followed repetitive stimulation of the optic nerve. Thus, neurons situated in the dorsal thalamus and projecting to pallial or subpallial telencephalic targets are unlikely to receive monosynaptic or oligosynaptic, excitatory retinal input. Accordingly, no retino-thalamo-telencephalic pathway homologous to that found in amniotes appears to exist in salamanders.

The effects of telencephalic lesions on visually mediated prey orienting behavior in the leopard frog (Rana pipiens). I. The effects of complete removal of one telencephalic lobe, with a comparison to the effects of unilateral tectal lobe lesions

In this paper, we report studies aimed at characterizing the relationship between forebrain and midbrain systems involved in the control of prey orienting behavior in the leopard frog. In frogs, unilateral forebrain lesions, like unilateral tectal lobe lesions, have their most prominent effects in the contralateral monocular visual field. Such lesions produce partial reductions in response frequency in the binocular visual field as well. Similar sequelae follow unilateral tectal lobe removal. These findings suggest that the effects of unilateral forebrain removal can be largely attributed to removal of a facilitating influence on the tectal lobe on the same side of the brain. In the case of both forebrain and midbrain lesions, behavior was assayed not only in terms of the frequency with which animals responded to stimuli at various locations in the visual field (as is usually done) but also in terms of the latency of whatever responses were observed. A striking inverse relationship between response frequency and response latency was found, both in lesioned and in normal frogs. This relationship has not previously been noticed, doesn't appear to be an obvious consequence of any existing models of the neuronal circuitry underlying anuran orienting behavior, and is difficult to account for in terms of the time scales associated with axonal conduction times and synaptic delays. It may be easier to account for in terms of the responses to perturbation of large interacting systems of neurons, and this possibility seems worthy of further exploration.

The effects of telencephalic lesions on visually mediated prey orienting behavior in the leopard frog (Rana pipiens). II. The effects of limited lesions to the telencephalon

Unilateral removal of the telencephalon in the leopard frog, Rana pipiens, produces a contralateral deficit in visual prey orienting behavior [Patton and Grobstein, 1997]. In mammals, such deficits are most commonly associated with damage to the isocortex, a pallial derived structure. In contrast, we here report that in leopard frogs, lesions that remove substantial areas of one telencephalic lobe, including virtually the entire pallium, have no discernible effect on visual orienting behavior. Restricted lesions to the ventrocaudal telencephalon, however, produce an effect that closely resembles that produced by the complete removal of one telencephalic lobe. The 'critical area' that is both included in all lesions that are effective in producing a severe deficit and excluded from all ineffective lesions includes a portion of the caudal striatum. The striatum is known to play a significant role in anuran vision. It thus seems likely that the deficit produced by unilateral removal of the telencephalon in the leopard frog is due specifically to the removal of the caudal striatum. Unilateral lesions to the striatum have previously been shown to produce a contralateral deficit in visual orienting behavior in cats, and a role for the striatonigral pathway in the production of the visual orienting deficit that follows visual cortex lesions has been proposed. The current findings call attention to the possible general importance of the striatum in the control of vertebrate visual orienting behaviors.

Intrinsic connections in the anterior dorsal ventricular ridge of the lizard Psammodromus algirus

We have studied the intrinsic connections of the anterior dorsal ventricular ridge (ADVR) in the lacertid lizard Psammodromus algirus by means of retrograde transport of horseradish peroxidase (HRP) and fluorescent labeling with the lipophilic carbocyanine dye DiI. We injected HRP into different regions in the ADVR arrayed in a medial-to-lateral sequence, with each consisting of three distinct superficial-to-deep zones. When HRP was injected into a given region, many labeled neurons (always located ipsilateral to the injection site) were found at all mediolateral regions of ADVR in locations rostrally distant from the injection site. DiI crystals were applied on different superficial-to-deep zones within each region. Two patterns could be recognized: DiI crystals applied on the periventricular (most superficial) zone resulted in a labeling of cells widely distributed throughout the ADVR independently of the mediolateral region of the application site, whereas DiI crystals applied on deeper zones resulted in a staining of cells mostly restricted to a narrow radial area. Results from both types of labeling confirm that the ADVR has a prominent radial component in its intrinsic organization, but they also demonstrate that some areas of the ADVR receive projections from distant, rostrally located neurons in every ipsilateral region of the ridge itself, which establishes a clear non-radial component. This organization may have important functional properties with regard to a putative integration of different sensory modalities conveyed by thalamic afferent fibers to the ADVR. Last, we analyzed some evolutionary implications of our results.

Anuran dorsal column nucleus: organization, immunohistochemical characterization, and fiber connections in Rana perezi and Xenopus laevis

As part of a research program on the evolution of somatosensory systems in vertebrates, the dorsal column nucleus (DCN) was studied with (immuno)histochemical and tract-tracing techniques in anurans (the large green frog, Rana perezi, and the clawed toad, Xenopus laevis). The anuran DCN contains some nicotinamide adenine dinucleotide phosphate diaphorase-positive neurons, very little calbindin D-28k, and a distinct parvalbumin-positive cell population. The anuran DCN is innervated by primary and non-primary spinal afferents, by primary afferents from cranial nerves V, VII, IX, and X, by serotonin-immunoreactive fibers, and by peptidergic fibers. Non-primary DCN afferents from the spinal cord appear to arise throughout the spinal cord, but particularly from the ipsilateral dorsal gray. The present study focused on the efferent connections of the DCN, in particular the targets of the medial lemniscus. The medial lemniscus could be traced throughout the brainstem and into the diencephalon. Along its course, the medial lemniscus gives off collaterals to various parts of the reticular formation, to the octavolateral area, and to the granular layer of the cerebellum. At mesencephalic levels, the medial lemniscus innervates the lateral part of the torus semicircularis as well as various tegmental nuclei. A striking difference between the two species studied is that while in R. perezi medial lemniscal fibers do not reach the tectum mesencephali, in X. laevis, intermediate and deep tectal layers are innervated. Beyond the midbrain, both dorsal and ventral thalamic areas are innervated by the medial lemniscus. The present study shows that the anuran "lemniscal pathway" is basically similar to that of amniotes.

Spinothalamic projections in amphibians as revealed with anterograde tracing techniques

Direct spinothalamic pathways were demonstrated in anurans (Rana ridibunda, Xenopus laevis) and in the ribbed newt, Pleurodeles waltl. With the powerful anterograde tracers Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine, rather extensive spinothalamic projections were found, including the ventromedial thalamic nucleus, the dorsal thalamus and several posterior diencephalic nuclei (anurans), and the neuropil lateral to the pars ventralis thalami as well as to the anteroventral and posterodorsal zones (P. waltl), respectively.

Secondary olfactory projections and pallial topography in the Pacific hagfish, Eptatretus stouti

The extent of the secondary olfactory projections shows great variation among different groups of craniates. Gnathostomes typically display restricted secondary olfactory projections, whereas lampreys have more extensive projections. Any attempt to determine the phylogenetic polarity of these characters, that is, to decide which is primitive and which is derived, requires an investigation of the secondary olfactory system in the sister group of lampreys and gnathostomes, the hagfishes. Therefore the secondary olfactory projections of the Pacific hagfish, Eptatretus stouti, were traced with the use of horseradish peroxidase and the lipophilic fluorescent tracing compound DiI. The projections are bilateral and massive to all pallial areas and the septum, moderate to the striatum, and relatively weak to the preoptic and infundibular regions of the hypothalamus, reaching caudally to the diencephalic-mesencephalic boundary. Afferents to the olfactory bulb arise from the pallium, the preoptic area, and the ventral thalamus. We compare the secondary olfactory projections in hagfishes with those in lampreys and in gnathostomes, and we conclude that the presence of extensive secondary olfactory projections is a primitive character of craniate brains.

Anatomical evidence for an intergeniculate leaflet in Rana pipiens

The projections of the nucleus of Bellonci and the anterior thalamic nucleus in Rana pipiens appear to be remarkably similar to those that have been described for the mammalian intergeniculate leaflet. The connections of these nuclei were examined using both the anterograde and retrograde transport of horseradish peroxidase. Afferents to the neuropil of Bellonci and its nucleus include bilateral projections from the retina, the contralateral nucleus of Bellonci, and anterior thalamic nucleus as well as bilateral projections from the pretectum and the ipsilateral suprachiasmatic nucleus. Efferent projections observed following HRP injections in the anterior thalamus consist of three components: (1) a ventral hypothalamic-suprachiasmatic and commissural projection, (2) a dorsal descending tract to the pretectum and tectum, and (3) a ventral descending tract to the somatomotor brainstem.

Topologic and connectional analysis of the dorsal thalamus of Triturus alpestris (amphibia, urodela, salamandridae)

A morphological and connectional analysis was performed on the dorsal thalamus of the alpine newt, Triturus alpestris. We have used a graphic reconstruction technique for the evaluation of the connectional (HRP) data. On the basis of these reconstructions, we propose a subdivision of the salamandrid dorsal thalamus into subhabenular, anteroventral, and posterodorsal zones. Each of these zones is defined by its telencephalic projections ("ascending thalamofugal systems"). The posterodorsal zone projects to the striatum, the anteroventral zone to the pallium. The subhabenular zone projects to the subpallial telencephalon and to the tegmentum. This zonal subdivision allows a more detailed comparison of the salamandrid dorsal thalamic features with ranid dorsal thalamic structures. We compare our dorsal thalamic zones to the ones proposed by Herrick (J. Comp. Neurol. 62:239-261, '35, The Brain of the Tiger Salamander. Chicago: The University of Chicago Press, '48). Furthermore, using the same reconstructive technique, we undertook an analysis of the spatial relations of various inputs to the salamandrid dorsal thalamus ("thalamopetal systems"). Besides the well-known retinal inputs, we identified the tectum and the tegmentum as sources of inputs to the thalamus. We provide evidence that there is no extensive multi- or unimodal overlap of these thalamopetal systems.

At least one thalamotelencephalic pathway in cartilaginous fishes projects to the medial pallium

Injections of horseradish peroxidase into the medial pallium of the spiny dogfish, Squalus acanthias, retrogradely labeled cells in the superficial portion of the dorsal pallium, the medial pallium and the septal area of the telencephalon, and in the posterior tubercle and posterior lateral thalamic nucleus of the diencephalon. Except for the septal projection, all projections to the medial pallium are bilateral. Labeled fibers could be traced from the injection site to caudal telencephalic levels but not to the hypothalamus. The projection from the posterior lateral thalamic nucleus, which is known to be an electrosensory relay center in ray and skate, provides evidence that at least one ascending thalamotelencephalic pathway in Squalus and perhaps other elasmobranchs terminates in the medial pallium rather than in the dorsal pallium. In this respect Squalus shows striking similarities to amphibians.

Evidence for parallel processing in the frog's auditory thalamus

We have conducted anatomical and physiological experiments to investigate the functional organization of the dorsal thalamus in the northern leopard frog (Rana pipiens pipiens). Our studies provide evidence for parallel auditory processing at this level of the frog's brain. Acoustically evoked potentials were recorded from the posterior and central thalamic nuclei and several differences in sound-evoked activity were noted between them: the amplitude of acoustically evoked potentials (AEPs), in response to a standard search stimulus, was always greater in the central, as opposed to the posterior, nucleus; the posterior, but not central, nucleus exhibited the phenomenon of nonlinear summation when 350-Hz and 1,700-Hz tones were presented simultaneously rather than individually; and the central, but not posterior, nucleus showed selectivity for the repetition rate of pulsed sound signals. The posterior and central thalamic nuclei also possessed distinct innervation patterns as revealed by the HRP transport patterns arising from these structures. The central nucleus was reciprocally connected with the major auditory relay stations along the frog's central auditory pathway including the superior olive, nucleus of the lateral lemniscus, and the torus semicircularis. Major projections to the lateral thalamic nucleus, ventral hypothalamus, and the telencephalic striatal complex were also observed. The posterior nucleus, on the other hand, established reciprocal connections primarily with the medial reticular nucleus, ventral midbrain tegmentum, and structures constituting of the ventral thalamic nuclei, particularly the nucleus of Bellonci. Thus, time and frequency cues contained within the species mating call, and conveying information concerning species identity, appear to be processed independently within the frog's thalamus with separate neural channels for each.

Auditory pathways to the hypothalamus in ranid frogs

Three multisynaptic pathways from the midbrain auditory center (torus semicircularis) of the bullfrog, Rana catesbeiana, to the infundibular hypothalamus were found using the axonal tracer wheat germ agglutinin-horseradish peroxidase. Toral neurons project to the secondary visceral nucleus of the isthmus and to the central and anterior thalamic nuclei of the dorsal thalamus. All 3 of these nuclei project to the infundibular hypothalamus. These findings indicate multiple connections between two centers presumed important for reproductive behavior in frogs, a midbrain sensory region processing acoustic communication signals and a hypothalamic endocrine control area regulating gonadotropin and gonadal steroid secretion.

Two thalamo-telencephalic pathways in a urodele, Triturus alpestris

Ascending thalamo-telencephalic projection systems have been investigated in an urodele, Triturus alpestris, using the horseradish peroxidase technique. Two separate dorsal thalamic projections onto the telencephalon have been identified; one arises from the posterior dorsal thalamus and terminates in the ipsilateral striatum, the other originates from anterior dorsal thalamic cells and reaches the medial pallium and a part of the dorsal pallium bilaterally. Both systems, which are spatially well segregated, might carry visual information to the telencephalon, as the posterior dorsal thalamus receives tectal, and the anterior dorsal thalamus direct retinal input. The urodele projection scheme as described here shows great similarities to the one described in anurans, although there are remarkable cytoarchitecture differences between the anuran and the urodele thalamus.