Ascending connections to the forebrain in the Tegu lizard

Structural organization of the cerebral cortex of the neotropical lizard Tropidurus hispidus

Lizards belonging to the Tropiduridae family are "sit-and-wait" foragers, relying mainly on visual identification to catch prey that cross their visual fields. Little is known about the neurobiology of Tropiduridae lizards. We have used neurohistological techniques to study the structural organization of the telencephalon of the neotropical lizard Tropidurus hispidus, paying special attention to the cerebral cortex. As revealed by the Nissl technique and Golgi staining, the telencephalon of T. hispidus follows the squamate pattern, with some differences: the lateral cortex appears relatively atrophic, and most of the neuronal somata of the dorsal cortex are dispersed without forming a conspicuous cell layer. Golgi staining has revealed ten different neuronal types in the three cortical layers, based on somata shape and dendritic morphology: the granular (unipolar, bipolar, and multipolar), pyramidal (normal, inverted, open, bipyramidal, and horizontal), spherical horizontal, and fusiform neuronal types. The axon direction could be traced in five of the subtypes. We have also studied the distribution of zinc-enriched terminals in the telencephalon of T. hispidus by the Neo-Timm method. Some portions of the cortex, septum, striatum, and amygdaloid complex stain heavily, with patterns resembling those described for other lizard families. Thus, T. hispidus appears to be an interesting representative of the Tropiduridae family for further neurobiological comparative studies.

Mammalian and avian neuroanatomy and the question of consciousness in birds

Some birds display behavior reminiscent of the sophisticated cognition and higher levels of consciousness usually associated with mammals, including the ability to fashion tools and to learn vocal sequences. It is thus important to ask what neuroanatomical attributes these taxonomic classes have in common and whether there are nevertheless significant differences. While the underlying brain structures of birds and mammals are remarkably similar in many respects, including high brain-body ratios and many aspects of brain circuitry, the architectural arrangements of neurons, particularly in the pallium, show marked dissimilarity. The neural substrate for complex cognitive functions that are associated with higher-level consciousness in mammals and birds alike may thus be based on patterns of circuitry rather than on local architectural constraints. In contrast, the corresponding circuits in reptiles are substantially less elaborated, with some components actually lacking, and in amphibian brains, the major thalamopallial circuits involving sensory relay nuclei are conspicuously absent. On the basis of these criteria, the potential for higher-level consciousness in these taxa appears to be lower than in birds and mammals.

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.

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

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

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.

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

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

Conservation of K1 immunoreactivity against early cortical neurones in the vertebrate telencephalon

A monoclonal antibody, K1, immunostains neurones generated earliest in the rat neocortex. The K1 immunoreactivity was found in both mouse and human embryos. In the human marginal zone, the subpial granular layer and the inner sublayer were stained at the 19th and 20th week of gestation, respectively. Western blot analysis revealed that the K1 immunoreactivity was conserved in a variety of vertebrates. While a protein of low molecular weight (200 kDa) reacted dominantly in an amphibian (Xenopus laevis) and a reptile (Agkistrodon blomhoffii), proteins of higher molecular weights (280 and 290 kDa) reacted dominantly in mammals (mouse, rat and macaque). In the brain of the reptile (Lacerta triliniata) embryo, K1 stained a marginal part of the superficial molecular layer in the dorsal cortex that is probably homologous to the mammalian marginal zone in the neocortex. In the chick embryo at the 8th day of incubation, the immunoreactivity was observed on neurones generated earliest in the dorsal cortex but not in the superficial molecular layer. The dorsal ventricular ridge and pallial thickening in either the reptile or chick were not stained. The K1 antigen could be a good marker for evolutional study of the mammalian neocortex.

Calcium-binding proteins in the dorsal ventricular ridge of the lizard Psammodromus algirus

The aim of the present work was to study further the intrinsic organization of the dorsal ventricular ridge of lizards. For that purpose, the morphology and distribution of cells and fibers containing the calcium-binding proteins calbindin-D28k, parvalbumin, and calretinin were investigated by using immunohistochemical methods. Colocalization of calcium-binding proteins with the neurotransmitter gamma-aminobutyric acid (GABA) was also studied because they are shown to coexist in many areas of the telencephalon where they define distinct subpopulations of GABAergic local circuit neurons. Neurons containing calcium-binding proteins are limited to the anterior part of the dorsal ventricular ridge (ADVR), whereas the posterior or caudal portion of the ridge is devoid of immunoreactive cells. This result gives further evidence for defining both regions of the dorsal ventricular ridge. Calcium-binding proteins mark three distinct populations of neurons within the ADVR. Two of them, parvalbumin- and calretinin-expressing cells, are GABAergic. On the other hand, calbindin-containing neurons do not express GABA, and the possibility is discussed that these cells are projection neurons. The distribution and overall density of fibers immunoreactive to calcium-binding proteins suggests that most fibers are of extrinsic origin, the thalamic nuclei projecting to the ADVR and the lateral amygdala being good candidates for their origin. The comparison of data on the populations of calcium-binding protein-containing neurons in the reptilian ADVR with those of mammals illustrate the difficulty in finding a mammalian homologue for this controversial region of the reptilian telencephalon.

Identification of the reptilian basolateral amygdala: an anatomical investigation of the afferents to the posterior dorsal ventricular ridge of the lizard Podarcis hispanica

The presence of multimodal association in the telencephalon of reptiles has been investigated by tracing the afferent connections to the posterior dorsal ventricular ridge (PDVR) of the lizard Podarcis hispanica. The PDVR receives telencephalic afferents from the lateral (olfactory) and dorsal cortices, and from the three unimodal areas of the anterior dorsal ventricular ridge, in a convergent manner. From the diencephalon, it receives afferents from the dorsomedial anterior and medial posterior thalamic nuclei, and from several hypothalamic nuclei. Brainstem afferents to the PDVR originate in the dorsal interpeduncular nucleus, the nucleus of the lateral lemniscus and parabrachial nucleus. The afferents to the thalamic nuclei that project to the PDVR have also been studied. The dorsomedial anterior thalamic nucleus receives projections mainly from limbic structures, whereas the medial posterior thalamic nucleus is the target of projections from structures with a clear sensory significance (optic tectum, torus semicircularis, nuclei of the lateral and spinal lemniscus, superior olive and trigeminal complex). As a result, the PDVR appears as an associative centre that receives visual, auditory, somatosensory and olfactory information from several telencephalic and non-telencephalic centres, and a multimodal projection from the medial posterior thalamic nucleus. This pattern of afferents of the PDVR is similar to that of the caudal neostriatum in birds and the basolateral division of the mammalian amygdala. These results indicate that a multimodal amygdala is already present in reptiles, and has probably played a key role in the evolution of the vertebrate brain.

Predictive reward signal of dopamine neurons

The effects of lesions, receptor blocking, electrical self-stimulation, and drugs of abuse suggest that midbrain dopamine systems are involved in processing reward information and learning approach behavior. Most dopamine neurons show phasic activations after primary liquid and food rewards and conditioned, reward-predicting visual and auditory stimuli. They show biphasic, activation-depression responses after stimuli that resemble reward-predicting stimuli or are novel or particularly salient. However, only few phasic activations follow aversive stimuli. Thus dopamine neurons label environmental stimuli with appetitive value, predict and detect rewards and signal alerting and motivating events. By failing to discriminate between different rewards, dopamine neurons appear to emit an alerting message about the surprising presence or absence of rewards. All responses to rewards and reward-predicting stimuli depend on event predictability. Dopamine neurons are activated by rewarding events that are better than predicted, remain uninfluenced by events that are as good as predicted, and are depressed by events that are worse than predicted. By signaling rewards according to a prediction error, dopamine responses have the formal characteristics of a teaching signal postulated by reinforcement learning theories. Dopamine responses transfer during learning from primary rewards to reward-predicting stimuli. This may contribute to neuronal mechanisms underlying the retrograde action of rewards, one of the main puzzles in reinforcement learning. The impulse response releases a short pulse of dopamine onto many dendrites, thus broadcasting a rather global reinforcement signal to postsynaptic neurons. This signal may improve approach behavior by providing advance reward information before the behavior occurs, and may contribute to learning by modifying synaptic transmission. The dopamine reward signal is supplemented by activity in neurons in striatum, frontal cortex, and amygdala, which process specific reward information but do not emit a global reward prediction error signal. A cooperation between the different reward signals may assure the use of specific rewards for selectively reinforcing behaviors. Among the other projection systems, noradrenaline neurons predominantly serve attentional mechanisms and nucleus basalis neurons code rewards heterogeneously. Cerebellar climbing fibers signal errors in motor performance or errors in the prediction of aversive events to cerebellar Purkinje cells. Most deficits following dopamine-depleting lesions are not easily explained by a defective reward signal but may reflect the absence of a general enabling function of tonic levels of extracellular dopamine. Thus dopamine systems may have two functions, the phasic transmission of reward information and the tonic enabling of postsynaptic neurons.

A Golgi study of the principal projection neurons of the medial cortex of the lizard Podarcis hispanica

The medial cortex of lizards is a simple three-layered brain region displaying many characteristics that parallel the hippocampal fascia dentata of mammals. Its principal neurons form a morphologically diverse population, partly as a result of the prominent continuous growth of this nervous center. By using the classic Golgi impregnation method, we describe here the morphology of the principal neurons populating the medial cortex of Podarcis hispanica. These were projection neurons giving off descending axons. These axons displayed deep collateral branches provided with prominent axonal boutons, while the main axonal branch reached adjacent cortical areas and the bilateral septum. According to three main classification criteria, dendritic tree pattern, dendritic spine covering, and soma size, we have distinguished eight different types of projection neurons. Five of them, "heavily spiny granular" (monotufted, medium-sized), "heavily spiny bitufted" (large), "spiny bitufted" (medium-sized), "sparsely spiny bitufted" (small), and "superficial multipolar" (small), were found in the cell layer, whereas the three others lay outside this layer and were regarded as ectopic types ("outer plexiform ectopic bitufted," "inner plexiform ectopic bitufted", and "inner plexiform monotufted"). Additional secondary criteria, soma position and shape, allowed us to further classify bitufted neurons into three distinct subtypes each: "superficial-round," "intermediate-fusiform," and "deep-pyramidal." Moreover, a variety of small impregnated cells were observed; they probably represented newly generated immature neurons that had not yet completed their development. These cell types were compared with those reported previously in Golgi, immunocytochemical, and electron-microscopy studies, both in the reptilian medial cortex and in the mammalian dentate area. Presumably age-related changes and synaptic relationships of these projection cells in the medial cortex circuitry were analyzed.

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.

The septal complex of the telencephalon of the lizard Podarcis hispanica. I. Chemoarchitectonical organization

In this paper we study the septal complex architecture in the lizard Podarcis hispanica (Lacertidae). Histochemical and immunohistochemical techniques were used to define the distribution of zinc (Timm stain), acetyl cholinesterase (AChase), gamma-aminobutyric acid (GABA), tyrosine hydroxylase (TH), dopamine (DA), serotonin (5-HT), and two neuropeptides: leu-enkephalin (L-ENK) and substance P (SP). These reactions delineate a coherent map of nine septal nuclei that are named with a topographical nomenclature: anterior, lateral, ventromedial, medial, dorsolateral, ventrolateral, and dorsal septal nuclei, nucleus septalis impar, and nucleus of the posterior pallial commissure. The anterior septal nucleus is characterized by intense reaction for zinc and the presence of fibers immunoreactive for GABA, 5-HT, and L-ENK, which form pericellular nests. The lateral septal nucelus shows intense reaction for zinc, a high density of GABA-immunoreactive cells, and L-ENK-immunoreactive fibers forming basketlike figures around unstained somata. The ventromedial septal nucleus shows intense AChase reactivity, a dense network of 5-HT-immunoreactive fibers, and virtually no labeling for the other histochemical stains. The medial septal nucleus is defined by heavy reactivity for zinc, dense DA/TH and L-ENK innervations, and the presence of L-ENK-immunoreactive cells. The dorsolateral septal nucleus shows intense AChase staining in the neuropile and a dense network of fibers immunoreactive for 5-HT and DA/TH, but it shows low staining for zinc. The ventrolateral septal nucleus shows L-ENK-immunoreactive cells and a dense L-ENK innervation, but low reactivity for zinc. The dorsal septal nucleus, intermingled with the fimbrial fibers, shows a dense population of GABA-immunoreactive cells and terminals, but it is unreactive for zinc. Two subdivisions can be established in this dorsal septal nucleus: the dorsal part, intensely reactive for AChase and innervated by 5-HT fibers, and the central part, which shows L-ENK-immunoreactive neurons and fibers without reactivity for either AChase or 5-HT. The nucleus septalis impar, traversed by the fibers of the anterior pallial commissure (mildly reactive for zinc), shows reaction for AChase but low (if present) reactivity for the remaining markers. The nucleus of the posterior pallial commissure shows a generally low reactivity for the histochemical reactions employed. The distribution of these markers is similar to that found in other squamate reptiles and allows for a direct comparison with the septal formation of mammals. Such a comparison reinforces the view that the limbic system has undergone a conservative evolution within vertebrates.

Organization of the avian "corticostriatal" projection system: a retrograde and anterograde pathway tracing study in pigeons

Birds have well-developed basal ganglia within the telencephalon, including a striatum consisting of the medially located lobus parolfactorius (LPO) and the laterally located paleostriatum augmentatum (PA). Relatively little is known, however, about the extent and organization of the telencephalic "cortical" input to the avian basal ganglia (i.e., the avian "corticostriatal" projection system). Using retrograde and anterograde neuroanatomical pathway tracers to address this issue, we found that a large continuous expanse of the outer pallium projects to the striatum of the basal ganglia in pigeons. This expanse includes the Wulst and archistriatum as well as the entire outer rind of the pallium intervening between Wulst and archistriatum, termed by us the pallium externum (PE). In addition, the caudolateral neostriatum (NCL), pyriform cortex, and hippocampal complex also give rise to striatal projections in pigeon. A restricted number of these pallial regions (such as the "limbic" NCL, pyriform cortex, and ventral/caudal parts of the archistriatum) project to such ventral striatal structures as the olfactory tubercle (TO), nucleus accumbens (Ac), and bed nucleus of the stria terminalis (BNST). Such "limbic" pallial areas also project to medialmost LPO and lateralmost PA, while the hyperstriatum accessorium portion of the Wulst, the PE, and the dorsal parts of the archistriatum were found to project primarily to the remainder of LPO (the lateral two-thirds) and PA (the medial four-fifths). The available evidence indicates that the diverse pallial regions projecting to the striatum in birds, as in mammals, are parts of higher order sensory or motor systems. The extensive corticostriatal system in both birds and mammals appears to include two types of pallial neurons: 1) those that project to both striatum and brainstem (i.e., those in the Wulst and the archistriatum) and 2) those that project to striatum but not to brainstem (i.e., those in the PE). The lack of extensive corticostriatal projections from either type of neuron in anamniotes suggests that the anamniote-amniote evolutionary transition was marked by the emergence of the corticostriatal projection system as a prominent source of sensory and motor information for the striatum, possibly facilitating the role of the basal ganglia in movement control.

NADPH diaphorase-positive neurons in the lizard hippocampus: a distinct subpopulation of GABAergic interneurons

We analyzed the distribution and light-microscopic features of the NADPH diaphorase-containing structures in the lizard hippocampus, likely to correspond to nitric oxide synthase-containing cells and fibers, and thus likely to release nitric oxide. We also studied co-localization of NADPH diaphorase with the neurotransmitter GABA, the calcium-binding protein parvalbumin, and the neuropeptide somatostatin, in order to examine whether putative nitric oxide-synthesizing neurons represent a different subpopulation of GABA cells, on which the authors recently reported in lizards. We also studied co-localization of NADPH diaphorase with parvalbumin or somatostatin in mice to ascertain whether the characteristics of this population in reptiles parallel the situation in mammals. Most of the positive NADPH diaphorase neurons were stained in a Golgi-like manner and were in the plexiform layers of the lizard hippocampus with morphologies ranging from bipolar to multipolar. Co-localization with GABA was 100%, and NADPH diaphorase-positive neurons in the lizard hippocampus did not contain parvalbumin or somatostatin. The results indicate that putative nitric oxide-synthesizing neurons represent a distinct subpopulation of GABA interneurons in the lizard hippocampus. Two different types of fibers were described in the plexiform layers: one type bearing thick varicosities, and the other thinner ones. We discuss the possibility that at least part of the positive fibers arise from a hypothalamic aminergic nucleus contacting the third ventricle, the periventricular hypothalamic organ. Most radial glia were stained almost completely and formed typical end-feet both at the pia and around capillaries. The results of this study confirm that the capacity for synthesizing nitric oxide is linked to a determined set of neuronal markers depending on the specific brain region, and they provide new resemblances between hippocampal regions in different classes of vertebrates.

The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

The evolution of the dorsal pallium in the telencephalon of amniotes: cladistic analysis and a new hypothesis

The large body of evidence that supports the hypothesis that the dorsal cortex and dorsal ventricular ridge of non-mammalian (non-synapsid) amniotes form the dorsal pallium and are homologous as a set of specified populations of cells to respective sets of cells in mammalian isocortex is reviewed. Several recently taken positions that oppose this hypothesis are examined and found to lack a solid foundation. A cladistic analysis of multiple features of the dorsal pallium in amniotes was carried out in order to obtain a morphotype for the common ancestral stock of all living amniotes, i.e., a captorhinomorph amniote. A previous cladistic analysis of the dorsal thalamus (Butler, A.B., The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis, Brain Res. Rev., 19 (1994) 29-65; this issue, previous article) found that two fundamental divisions of the dorsal thalamus can be recognized--termed the lemnothalamus in reference to predominant lemniscal sensory input and the collothalamus in reference to predominant input from the midbrain roof. These two divisions are both elaborated in amniotes in that their volume is increased and their nuclei are laterally migrated in comparison with anamniotes. The present cladistic analysis found that two corresponding, fundamental divisions of the dorsal pallium were present in captorhinomorph amniotes and were expanded relative to their condition in anamniotes. Both the lemnothalamic medial pallial division and the collothalamic lateral pallial division were subsequently further markedly expanded in the synapsid line leading to mammals, along with correlated expansions of the lemnothalamus and collothalamus. Only the collothalamic lateral pallial division--along with the collothalamus--was subsequently further markedly expanded in the non-synapsid amniote line that gave rise to diapsid reptiles, birds and turtles. In the synapsid line leading to mammals, an increase in the degree of radial organization of both divisions of the dorsal pallium also occurred, resulting in an 'outside-in' migration pattern during development. The lemnothalamic medial division of the dorsal pallium has two parts. The medial part forms the subicular, cingulate, prefrontal, sensorimotor, and related cortices in mammals and the medial part of the dorsal cortex in non-synapsid amniotes. The lateral part forms striate cortex in mammals and the lateral part of dorsal cortex (or pallial thickening or visual Wulst) in non-synapsid amniotes. Specific fields within the collothalamic lateral division of the dorsal pallium form the extrastriate, auditory, secondary somatosensory, and related cortices in mammals and the visual, auditory, somatosensory, and related areas of the dorsal ventricular ridge in non-synapsid amniotes.

Connections of the auditory forebrain in the pigeon (Columba livia)

Ascending auditory efferents in birds terminate mainly within Field L2, a cytoarchitectonically distinct region of the caudomedial telencephalon. The organization of Field L2, and that of its flanking regions, L1 and L3, was investigated with 14C-2-deoxyglucose (14C-2-DG), cytochrome oxidase, and both retrograde and anterograde tracing techniques. Field L2 was found to contain a high concentration of cytochrome oxidase. Following auditory stimulation, 14C-2-DG autoradiography revealed that Field L2 consists of two adjacent but seemingly discontinuous zones, designated Field L2a, which lies ventromedially, and Field L2b, which lies dorsolaterally. Termination of thalamic efferents: The thalamic auditory nuclei ovoidalis (Ov) and semilunaris parovoidalis (SPO) project predominantly upon Field L2, and possibly sparsely upon L1, L3 and the overlying hyperstriatum ventrale (HV). Ov subnuclei project upon L2a and SPO projects predominantly upon L2b. The topography of the projections is inverted along the ventromedial-to-dorsolateral axis of L2, and is in accord with an inverted tonotopic representation of frequencies; high frequencies (< 3.5 kHz) being found in the more ventromedial parts of L2a, and low frequencies and broad band responses in L2b. Intra- and extratelencephalic connections: Field L2a also receives a substantial projection from HV, but the efferent projections of L2a appear confined to adjacent "neostriatal" regions. The subsequent projections of L2b were not identified in this study. L1 and L3 project predominantly to the dorsal neostriatum (Nd) caudolateral to Field L, and have fewer projections to the caudomedial paleostriatum and anterior hyperstriatum accessorium. Nd projects massively upon the ventromedial nucleus of the intermediate archistriatum (Aivm), which has bilateral projections upon the caudomedial telencephalon and is the origin of a major descending pathway having dense terminations surrounding the ovoidalis complex (Ov and SPO), MLd, the lateral lemniscal nuclei, and sparse terminations within SPO itself. It is suggested that within the telencephalon the major components of the auditory pathway consist of cell groups which collectively correspond to the populations of neurons found within the auditory cortex of mammals.

Subpopulations of GABA neurons containing somatostatin, neuropeptide Y, and parvalbumin in the dorsomedial cortex of the lizard Psammodromus algirus

Different subpopulations of GABA neurons containing the neuropeptides somatostatin and neuropeptide Y, and the calcium binding protein parvalbumin were studied by immunocytochemistry using light and electron microscopy in the dorsomedial cortex of the lizard Psammodromus algirus to investigate the connectivity of different subsets of GABA neurons in the lizard dorsomedial cortical circuitry and to compare cortical regions of reptiles and mammals. GABA neurons were classified into different subsets by using the peroxidase anti-peroxidase immunohistochemical method on adjacent Araldite-embedded semithin sections. GABA neurons in the dorsomedial cortex fall into three major subsets: 1) neurons with somatostatin (and neuropeptide Y), which accounted for about 44% of the GABA population; 2) neurons with parvalbumin, which accounted for about 13% of the GABA neurons; and 3) neurons without parvalbumin or neuropeptides, which represented 40% of all GABA cells. This division of GABA neurons in non-overlapping subpopulations of neuropeptide- and parvalbumin-containing cells is similar to that found in the mammalian hippocampal formation. On the basis of the nerve terminal fields, somatostatin- and parvalbumin-immunoreactive neuronal populations appear to be functionally different, acting on different portions of the projection neurons. Parvalbumin-immunoreactive neurons inhibit the pyramidal neurons at the cell body level, whereas somatostatin-immunoreactive neurons inhibit them on distal dendrites. The results of the present study add more similarities between the lizard dorsomedial cortex and parts of the mammalian hippocampus.

Neurotransmitter organization and connections of turtle cortex: implications for the evolution of mammalian isocortex

Telencephalic cortex in turtles is a simple three layered-structure. The dorsal most part of this structure is thought to resemble the reptilian forerunner of at least parts of mammalian isocortex. This dorsal part of turtle cortex contains several functionally distinct regions that show similarity in their connections and function to specific areas in mammalian isocortex. The types of neurons found in turtle dorsal cortex (as defined by their morphology and neurotransmitter content) also show great similarity to those observed in mammals, with the major exception that turtle cortex appears to lack the types of neurons found in granular and supragranular layers of mammalian isocortex. Similar results have also been observed in other living reptiles. Thus, one major step in the evolution of reptilian cortex into mammalian cortex must have been the addition of the types of neurons found in the granular and supragranular layers of mammalian isocortex. These observations for turtles also suggest that turtle cortex in particular and reptilian telencephalic cortex in general must differ functionally from mammalian isocortex with respect to those features associated with the laminar and columnar organization of isocortex. These issues are discussed in more detail below and in Reiner (1991).

GABA-immunoreactive basal forebrain afferents innervate GABA-immunoreactive non-pyramidal cells in the cerebral cortex of the lizard Podarcis hispanica

The basal forebrain projection to the cerebral cortex was studied in the lizard Podarcis hispanica by anterograde transport of Phaseolus vulgaris leucoagglutinin. After injections of the lectin into the septal-basal forebrain area, Phaseolus vulgaris leucoagglutinin-labelled fibres were mainly detected in the outer plexiform layer of the medial cortex and in the inner plexiform layer of the dorsal and dorsomedial cortices. Ultrathin sections from these areas were obtained and processed for postembedding immunogold staining for GABA. Most of the Phaseolus vulgaris leucoagglutinin-labelled boutons in the dorsal and dorsomedial cortical areas were GABA immunoreactive and all the double-labelled boutons established symmetric synaptic contacts on cell bodies and dendrites that were also found to be GABA immunoreactive in all cases. In contrast, Phaseolus vulgaris leucoagglutinin-labelled varicosities in the outer plexiform layer of the medial cortex made asymmetric synaptic contacts on GABA-immunonegative profiles and they were themselves negative for GABA. In double-labelled sections, GABA-, calbindin D28k- and neuropeptide Y-immunoreactive neurons were found to be innervated by multiple Phaseolus vulgaris leucoagglutinin-labelled varicosities in the dorsal and dorsomedial cortical areas, whereas in the medial cortex Phaseolus vulgaris leucoagglutinin-labelled fibres were not observed in contact with any subpopulation of GABAergic cells. The results demonstrate that in lizards the septal-basal forebrain projection to the cortex has a GABAergic component, which selectively terminates on GABAergic non-pyramidal cells including the neuropeptide Y- and the calbindin D28k-containing subpopulations. This synaptic organization is remarkably similar to that in mammals, and suggests that the mechanisms of control of the cortical activity by the basal forebrain have been highly preserved during phylogeny.

Evidence of GABA-immunopositive neurons in the dorsal part of the lateral geniculate nucleus of reptiles: morphological correlates with interneurons

The distribution and staining pattern of gamma-aminobutyric acid immunoreactivity have been examined by both light and electron microscopy in the dorsal part of the lateral geniculate nucleus of three reptilian species: the turtle Chinemys reevesi, the lizard Ophisaurus apodus and the snake Vipera aspis. After perfusion of the animals with 1% paraformaldehyde and 1% glutaraldehyde and polyethyleneglycol embedding of the brains, the analysis of sections processed immunocytochemically with an anti-GABA antiserum has revealed a moderate-to-dense labeling of the neurons of the dorsal part of the lateral geniculate complex in these species. Labeled cell bodies are small-sized, either rounded or fusiform and the GABA-positive dendrites emerging from them are not preferentially oriented in any particular direction. Quantitative studies in Vipera indicate that GABA-positive neurons make up about 14% of the population of neurons of the dorsal part of the lateral geniculate nucleus. Electron microscopy of specimens treated by either pre- or post-embedding techniques has confirmed that these cells corresponded to neurons. No glial cells were ever observed to be immunopositive. These GABA-positive neurons, characterized by the presence of pleiomorphic synaptic vesicles localized either in their perikaryon or more often in presynaptic dendrites, established symmetrical synaptic contacts. In this case, the latter were involved both pre- and postsynaptically in serial and, more rarely, in triadic arrangements, a synaptic organization specific to interneurons. The involvement of such GABA-positive neurons in local circuits is discussed.

A second auditory area in the non-cortical telencephalon of a reptile

Injections of horseradish peroxidase (HRP) into a caudocentral portion of the non-cortical telencephalon of Caiman known as the dorsolateral area (dorsal ventricular ridge) resulted in retrogradely labeled neurons throughout the entire extent of the ipsilateral nucleus reuniens. HRP-positive cells were most numerous in nucleus reuniens pars diffusa with only sparse labeling of neurons in nucleus reuniens pars centralis. The results of the present experiment, when compared with those of a prior study that determined the telencephalic connections of nucleus reuniens pars centralis, suggested that these two forebrain areas are separate. Staining with succinate dehydrogenase and acetylcholinesterase revealed that nucleus reuniens pars centralis and pars diffusa and their respective telencephalic projection areas can be differentiated on the basis of histochemical features. These findings in Caiman suggest that certain thalamic and telencephalic auditory areas in birds and crocodilians are most likely the result of common ancestry rather than examples of parallel evolution.

An immunocytochemical analysis of the lateral geniculate complex in the pigeon (Columba livia)

The lateral geniculate complex (GL) of pigeons was investigated with respect to its immunohistochemical characteristics, retinal afferents, and the putative transmitters/modulators of its neurons. The distributions of serotonin-, choline acetyltransferase-, glutamic acid decarboxylase-, tyrosine hydroxylase-, neuropeptide Y- (NPY), substance P- (SP), neurotensin- (NT), cholecystokinin- (CCK), and leucine-enkephalin- (L-ENK) like immunoreactive perikarya and fibers were mapped. Retinal projections were studied following injections of Rhodamine-B-isothiocyanate into the vitreous. Transmitter-specific projections onto the visual Wulst and the optic tectum were studied by simultaneous double-labelling of retrograde tracer molecules and immunocytochemical labelling. The GL can be divided into three major subdivisions, the n. geniculatus lateralis, pars dorsalis (GLd; previously designated as the n. opticus principalis thalami, OPT), the n. marginalis tractus optici (nMOT), and the n. geniculatus lateralis, pars ventralis (GLv). All three subdivisions are retinorecipient. The GLd can be further subdivided into at least five components differing in their immunohistochemical characteristics: n. lateralis anterior (LA); n. dorsolateralis anterior thalami, pars lateralis (DLL), n. dorsolateralis anterior thalami, pars magnocellularis (DLAmc); n. lateralis dorsalis nuclei optici principalis thalami (LdOPT); and n. suprarotundus (SpRt). The LdOPT consists of an area of dense CCK-like and NT-like terminals of probable retinal origin. Three subnuclei (DLL, DLAmc, SpRt) were shown to project to the visual Wulst. Cholinergic and cholecystokinergic relay neurons participated in this projection. The nMOT occupies a position between the GLd and GLv and encircles the rostral pole of n. rotundus and the LA. It is characterized mainly by medium sized NPY-like perikarya which were shown to project onto the ipsilateral optic tectum. Bands of NPY-like fibers in the tectal layers 2, 4, and 7 could at least in part be due to this projection of the nMOT. Most of the antisera used revealed transmitter/modulator-specific fiber systems in the GLv which often showed a layer-specific distribution. Perikaryal labelling was only obtained with glutamic acid decarboxylase. On the basis of its chemoarchitectonics, topography, and connectional pattern, the GLd complex of pigeons is most directly equivalent to the mammalian GLd. However, although the different subdivisions of the avian GLd may represent functionally different channels within the thalamofugal pathway similar to the lamina-specific differentiation within the mammalian geniculostriate projection, direct comparison of subnuclei of birds and mammals is not justified at this time. The nMOT appears similar to the intergeniculate leaflet (IGL) and the avian GLv clearly corresponds in many features to the mammalian GLv.

Calcitonin-gene related peptide is an evolutionarily conserved marker within the amniote thalamo-telencephalic auditory pathway

The distribution of neurons and fibers containing calcitonin-gene-related peptide (CGRP) was mapped in the thalamo-telencephalic auditory pathways of four amniote species, rats, pigeons (Columba livia), caiman (Caiman crocodilus), and turtles (Pseudemys scripta). In colchicine-treated turtles and pigeons, numerous CGRP+ perikarya were observed in the auditory relay nucleus of the thalamus (n. reuniens of reptiles, and n. ovoidalis of birds). In pigeons, these neurons were most abundant in the outer circumference of the nucleus and were not observed without colchicine pretreatment. In the telencephalon of turtles, caiman, and pigeons, CGRP+ fibers were observed within portions of the dorsal ventricular ridge previously shown to receive projections from the auditory thalamus, thus implying that the thalamic CGRP+ neurons observed here in fact project to these telencephalic areas. In colchicine treated rats, numerous CGRP+ perikarya were observed along the ventral margin of the medial geniculate nucleus extending into the posterior intralaminar and peripeduncular nuclei, as well as occasionally within the ventral subdivision of the medial geniculate nucleus. Injections of fluorogold into the auditory cortex combined with immunofluorescence labeling for CGRP revealed that CGRP+ cells in these areas do, in fact, project to the auditory cortices. The present results are interpreted as providing strong support for the theory, advanced previously, that the medial geniculate nucleus of mammals, nucleus ovoidalis of birds, and nucleus reuniens of reptiles contain at least some homologous cell populations. Although the data are consistent with the theory that the telencephalic projection fields are homologous, other interpretations are also consistent with the data presented here. These include the possibility that auditory thalamic projections to the telencephalon arose independently in the lines of evolution leading to mammals and sauropsids.

Afferent and efferent connections of the olfactory bulbs in the lizard Podarcis hispanica

The connections of the olfactory bulbs of Podarcis hispanica were studied by tract-tracing of injected horseradish peroxidase. Restricted injections into the main olfactory bulb (MOB) resulted in bilateral terminallike labeling in the medial part of the anterior olfactory nucleus (AON) and in the rostral septum, lateral cortex, nucleus of the lateral olfactory tract, and ventrolateral amygdaloid nucleus. Bilateral retrograde labeling was found in the rostral lateral cortex and in the medial and dorsolateral AON. Ipsilaterally the dorsal cortex, nucleus of the diagonal band, lateral preoptic area, and dorsolateral amygdala showed labeled cell bodies. Retrogradely labeled cells were also found in the midbrain raphe nucleus. Results from injections into the rostral lateral cortex and lateral olfactory tract indicate that the mitral cells are the origin of the centripetal projections of the MOB. Injections in the accessory olfactory bulb (AOB) produced ipsilateral terminallike labeling of the ventral AON, bed nucleus of the accessory olfactory tract, central and ventromedial amygdaloid nuclei, medial part of the bed nucleus of the stria terminalis, and nucleus sphericus. Retrograde labeling of neurons was observed ipsilaterally in the bed nucleus of the accessory olfactory tract and stria terminalis, in the central amygdaloid nucleus, dorsal cortex, and nucleus of the diagonal band. Bilateral labeling of somata was found in the ventral AON, the nucleus sphericus (hilus), and in the mesencephalic raphe nucleus and locus coeruleus. Injections into the dorsal amygdala showed that the mitral neurons are the cells of origin of the AOB centripetal projections. Reciprocal connections are present between AOB and MOB. To our knowledge, this is the first study to address the afferent connections of the olfactory bulbs in a reptile. On the basis of the available data, a discussion is provided of the similarities and differences between the reptilian and mammalian olfactory systems, as well as of the possible functional role of the main olfactory connections in reptiles.

Neuronal typology of the thalamic area triangularis of Gallotia galloti (reptilia, sauria)

In a Golgi study of the area triangularis (AT), a rostral nucleus of the ventral thalamus of Gallotia galloti, we have identified four major neuronal types on the basis of their morphological characteristics: medium-sized fusiforms with two processes, medium-sized fusiforms with three or four processes, small bipolars, and small and medium-sized multipolars. These neurons are characterized by a simple morphology and radial arrangement. Cell size varies from small to medium, and all axons project laterally. These characteristics distinguish AT neurons from those of neighboring nuclei. In addition, we found some evidence of differential topographic distribution of each neuronal type within the nucleus. Medium-sized fusiform neurons with two processes are located in the most ventral part, where they constitute the ventral nuclear limit. Small multipolar neurons prevail in the dorsal and ventromedial parts, and in the rest of the nucleus medium-sized neurons, including both fusiform with three or four processes and multipolar types, are normally found. Finally, we discuss a putative homology of the reptilian AT with a part of the mammalian zona incerta.

Serotonin innervation of the cerebral cortex in lizards

The serotonin (5-HT) innervation of the cerebral cortex in two species of lizards has been studied. Results show no differences between both species. Most of the cerebral cortex of these lizards is innervated by serotoninergic fibers, which are fine and varicose. Their density varies greatly from one cortical region to another: the areas with higher density of serotoninergic fibers and terminals are parts of the medial and dorsal cortices. There is a laminar pattern of distribution of serotoninergic fibers. In the medial cortex, 5-HT fibers are found preferentially in both plexiform layers just above and below the cellular layer. In the dorsomedial cortex, there is an immunoreactive plexus in the outermost third of the superficial plexiform layer and another in the depth of the layer, whereas 5-HT fibers are distributed evenly in the deep plexiform layer of this cortex. In the pars medialis of the dorsal cortex, serotoninergic fibers are abundant in all layers, whereas in the pars lateralis, fibers are found predominantly in the external third of the superficial plexiform layer. The lateral cortex is almost devoid of immunoreactive fibers. These results show a different organization of the cortical serotonin innervation between lizards and turtles.

Laminar distribution and morphology of gamma-aminobutyric acid (GABA)-immunoreactive neurons in the medial and dorsomedial areas of the cerebral cortex of the lizard Podarcis hispanica

The morphology and laminar distribution of immunolabeled neurons in the medial and dorsomedial telencephalic cortices of the lizard Podarcis hispanica were examined in vibratome sections after preembedding gamma-aminobutyric acid (GABA)-immunocytochemistry. In both cortical areas and at all rostrocaudal levels, GABA-immunoreactive neurons were found in all cortical layers, with the largest number (74%) of GABA-positive cells in layer 3. GABA-positive neurons were classified into pyramidlike, vertical-fusiform, multipolar, and horizontal neurons. Cells that could be so classified were counted in each cortical lamina. In the medial cortex, multipolar and horizontal-bipolar cells dominated layer 1. Layer 2 displayed mainly horizontal and pyramidlike cells at its outer margin and pyramidlike cells at its inner margin. In layer 3, horizontal cells were the prevalent group. In the dorsomedial cortex, layer 1 mainly contained small multipolar neurons (35% of layer-1 cells) in its outer third and vertical-fusiform neurons (37% of layer-1 cells) in its inner two thirds. In layer 2, 47% of the few GABA-positive perikarya were pyramidlike. The largest population of neurons in layer 3 was that formed by multipolar cells (45% of layer-3 cells). Ultrastructural examination revealed that GABA-immunoreactive neurons possessed indented euchromatic nuclei with a central nucleolus. Their cytoplasm contained numerous mitochondria and a very well-developed granular endoplasmic reticulum. Their somata were contacted by numerous unstained boutons making asymmetric contacts and by a few symmetric synapses of GABA-positive terminals. Dendrites of GABA-immunoreactive cells were thin, with irregular outlines, and generally aspinous. Like the somata, dendrites were contacted by many unstained asymmetric synapses. Some dendritic profiles also received symmetric contacts from GABA-positive boutons. GABA-positive terminal-like puncta were found throughout the layers, with a maximal concentration in layer 2. Electron microscopy confirmed that nearly all of the puncta represent GABA-positive terminal boutons. Comparison of GABA-immunoreactive cells in Podarcis with those found in the mammalian hippocampus suggests that these cells may be inhibitory neurons, as in the hippocampus of mammals.

Projections from the medial cortex in the brain of lizards: correlation of anterograde and retrograde transport of horseradish peroxidase with Timm staining

Efferent projections of the medial cortex of the lizards Podarcis hispanica and Gallotia stehlinii were studied by examining the transport of horseradish peroxidase; results were correlated with those from Timm-stained sections. Two efferent systems were found. The first reaches the distal part of the outer plexiform layer in the medial, dorsomedial, and dorsal cortices, i.e., zones that are negative to Timm staining, and possibly originates from horizontal fusiform neurons. The second reaches the Timm-positive zones in the cortex and septum and is topographically arranged: the vertical portion of the intermediate and caudal medial cortex and the entire rostral medial cortex project to the inner two-thirds of the outer plexiform layer of the dorsomedial cortex and of the medial subfield of the dorsal cortex; to the paraventricular zone of the inner plexiform layer of the medial cortex; and bilaterally to the dorsal part of the dorsal precommissural septum. The dorsal part of the intermediate and caudal medial cortex and the ventralmost folded part of its caudal edge project rostrally to the juxtasomatic zone of the outer plexiform layer and the entire inner plexiform layer of the intermediate and lateral subfields of the dorsal cortex and to the ventral part of the dorsal septum. In its intense Timm reaction and its ultrastructural properties, as reported in earlier studies, the Timm-positive fiber system of the lizard brain shows a close resemblance to the mossy fiber system of the mammalian hippocampus.

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

The efferent connections of the striatum and the nucleus accumbens of the lizard Gekko gecko were studied with the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). These structures were found to have segregated output systems. The striatum shows a major projection to the globus pallidus. Striatal fibers which are more caudally directed run through the lateral forebrain bundle and can be traced as far caudally as the pars reticularis of the substantia nigra where they exhibit many varicosities. Along its course, the lateral forebrain bundle issues fibers with varicosities to the anterior and posterior entopeduncular nuclei. The major recipient structure of the nucleus accumbens is the ventral pallidum. The nucleus accumbens, in addition, projects to the portion of the lateral hypothalamus in the path of the medial forebrain bundle and to the ventral tegmental area, which is its most caudal target. Subsequently, the same technique was used in an attempt to study the efferents of the globus pallidus and the ventral pallidum, the major recipient structures of the striatum and the nucleus accumbens. The globus pallidus was found to project to the rostral part of the suprapeduncular nucleus in the ventral thalamus and, in addition, may distribute fibers to the same structures as does the striatum. The ventral pallidum distributes fibers to the ventromedial thalamic nucleus. It probably also projects diffusely to the hypothalamus, the habenula, and the mesencephalic tegmentum.

Delayed postnatal neurogenesis in the cerebral cortex of lizards

Labelled cells were consistently observed in the medial cortex of the lizard brain after i.p. injections of tritiated thymidine (5 microCi/g b. wt.), 1, 7, 18 or 28 days of survival and posterior autoradiographic evaluation. In 3 groups of specimens (postnatal, young and adult) of the species Podarcis hispanica, after one day of survival, labelled cells were located in the ependymal cell layer underlying the medial cortex. After intermediate survival times (7, 18 days), labelled cells were found in 3 zones: the ependymal layer, the inner plexiform layer and the granular layer. After one month of survival, most labelled cells were observed in the granular layer. In the granular layer, these cells were distributed at random. These results show that postnatal neurogenesis in the medial cortex of the lizard occurs following a spatio-temporal pattern reminiscent of that found in the fascia dentata of the mammalian hippocampus.

Long-spined polymorphic neurons of the medial cortex of lizards: a Golgi, Timm, and electron-microscopic study

The morphology, ultrastructure, and principal synaptic input of long-spined neurons located in the inner plexiform layer of the medial cortex in three related species of lizards is described. Golgi impregnations have been used to define the external morphology of these neurons and their axonal trajectories. Their most striking characteristic is the presence of very long spines or "microdendrites" especially abundant on the distal dendritic segments. Axons have ascendent trajectories, pass through the cell layer, and ramify in the outer plexiform layer. Combined Golgi-electron microscopy as well as standard electron microscopy permitted the definition of the ultrastructure of these neurons. Timm and sulfide-osmium methods permitted the detection and characterization of their principal synaptic input (i.e., zinc-containing boutons). Gamma aminobutyric acid (GABA)-immunostained sections in one of the species studied allowed the identification of GABA-immunoreactive somata which had the same morphology and ultrastructure as long-spined neurons; these GABA-immunoreactive somata and their processes were found in the same location as long-spined neurons. This suggests that at least some long-spined polymorphic neurons are GABA-ergic and presumably inhibitory. Finally, the neurobiological significance of these long-spined neurons is discussed and briefly compared with that of similar neurons of the hilus of the fascia dentata of the rat.

Organization of zinc-containing terminal fields in the brain of the lizard Podarcis hispanica: a histochemical study

The Timm method for the histochemical detection of metals defines accurately many terminal fields in the brain of mammals. This pattern is based on the presence of zinc within the synaptic vesicles of some boutons. The aim of this study was to use the Timm method for the anatomical description of the brain in a reptile. In the telencephalon, zinc staining was observed in the inner layer of the medial cortex, the inner and outer layers of both dorsomedial and dorsal cortices, the inner layer of the lateral cortex pars anterior ventralis, the lateral cortex pars profunda, the intermediate and caudal aspects of the anterior dorsal ventricular ridge, the marginal layer and hilus of the nucleus sphericus, the perifascicular nucleus of the accessory olfactory tract, the striatum pars medialis, the olfactory tubercle, the septum pars anterior, and embedded in the fibres of both pallial and anterior commissures. In the diencephalon, staining was observed in the ventromedial hypothalamic nucleus and among the fibers of the stria terminalis. Stained somata and dendrites were observed in the infundibulum. In the mesencephalon and rhombencephalon, sparse staining was observed in the central gray, torus semicircularis, nucleus interpeduncularis, raphe, reticular formation, Purkinje and granular cell layers in the cerebellum, and nucleus cerebellaris medialis. The present results suggest that the histochemical detection of zinc may be a useful method for the accurate definition of terminal fields in the brain of reptiles also. The presence of zinc-containing terminal fields is discussed in relation to the connections and histochemistry in the reptilian brain. Similarities in the pattern of staining for zinc between mammals and reptiles are mentioned.

Evolution of neurotransmitter-related markers in the vertebrate telencephalon. Comparative microchemical study in discrete brain regions of a frog and a turtle

1. Neurochemical markers related to cholinergic, GABAergic and glutamatergic/aspartatergic neurotransmission have been measured in telencephalic areas obtained by microdissection from frog (Rana esculenta) and turtle (Pseudemys scripta elegans) brain. 2. In both species, pallial areas showed remarkably higher levels of synaptosomal D-3H-aspartate high affinity uptake than basal regions. Conversely, striatal and septal areas possessed higher levels of the GABAergic marker glutamate decarboxylase (GAD) than the pallium. 3. A differential distribution of GAD was noticed in striatal regions, highest levels of the enzyme being present in the ventral striatum, followed by the nucleus accumbens and the dorsal striatum. 4. Cholinergic markers choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) were rather uniformly distributed in the frog telencephalon, while, in the turtle, cholinergic markers were several-fold higher in the basal telencephalon, particularly in the striatum, than in the pallium. 5. The turtle dorsal ventricular ridge possessed ChAT levels more similar to the striatal than to the cortical ones. On the contrary, D-3H-aspartate uptake in the dorsal ventricular ridge was close to the highest levels found in cortical areas. 6. The quantitative neurochemical approach adopted for the present study appears to be a useful tool to investigate the problem of homologies and to gain new information on the evolution of neuron populations and neuronal connections in the vertebrate telencephalon.

Brain stem afferents to the anterior dorsal ventricular ridge in a lizard (Varanus exanthematicus)

The anterior dorsal ventricular ridge (ADVR), a large intraventricular protrusion in the reptilian forebrain, receives information from many different sensory modalities and in turn, projects massively onto the striatum. The ADVR possesses functional similarities to the mammalian isocortex and may perform complex sensory integrations. The ADVR in lizards is composed of three longitudinal zones which receive visual, somatosensory and acustic information, respectively. These projections are relayed via thalamic nuclei. Previous retrograde tracer studies also suggested brain stem projections to the ADVR arising in the midbrain reticular formation and in certain monoaminergic brain stem nuclei (substantia nigra, locus coeruleus and nucleus raphes superior). In the present study the powerful retrograde fluorescent tracer 'Fast Blue' was applied as a slow-release gel to the ADVR of the savanna monitor lizard, Varanus exanthematicus. Thalamic projections were confirmed and various direct brain stem projections to the ADVR were demonstrated. Brain stem afferents to the ADVR were found from the laminar nucleus of the torus semicircularis (possibly comparable to the mammalian periaqueductal gray), from the midbrain reticular formation, from the substantia nigra (pars compacta and reticulata) and the adjacent ventral tegmental area, from the nucleus raphes superior, from the locus coeruleus, from the parabrachial region, from the nucleus of the lateral lemniscus and even from the most caudal part of the brain stem (a few neurons in the nucleus of the solitary tract and lateral reticular formation, possibly comparable to the mammalian A2 and A1 groups, respectively). These data strongly suggest direct ADVR projections from the parabrachial region (related to visceral and taste information) as well as distinct catecholaminergic (presumably dopaminergic: substantia nigra, ventral tegmental area and, noradrenergic: locus coeruleus, respectively) and serotonergic projections (nucleus raphes superior).

Percentage of intrinsic and relay cells in a thalamic nucleus projecting to general cortex in reptiles, Caiman crocodilus

Small injections of horseradish peroxidase (HRP) restricted solely to the general cortex in reptiles, Caiman crocodilus, resulted in retrogradely labeled cells located bilaterally in the nucleus dorsolateralis anterior of the thalamus. Larger cortical injections of HRP labeled greater numbers of nucleus dorsolateralis anterior cells. HRP injections designed to label as much cortex as possible resulted in very few unlabeled ipsilateral neurons in the nucleus dorsolateralis anterior. The percentage of these unlabeled cells was less than 1%. These data indicate that the nucleus dorsolateralis anterior contains few, if any, intrinsic or local circuit neurons.

Histochemical identification of pallidal and striatal structures in the lizard Gekko gecko: evidence for compartmentalization

The present study provides a description of the distribution patterns of enkephalin, substance P and dopamine immunoreactivity, and acetylcholinesterase activity in the striatum, nucleus accumbens, globus pallidus, and ventral pallidum of the Gekko gecko. The pallidal structures were identified on the basis of characteristic enkephalin and substance P immunoreactive plexus. The globus pallidus contains a very dense enkephalin immunoreactive woolly fiber plexus and a light and less extensive substance P immunoreactive plexus. The ventral pallidum is characterized by substance P and enkephalin immunoreactive woolly fiber plexus, which show a dense and moderate staining, respectively. The striatum and the nucleus accumbens were found to be inhomogeneous with respect to staining intensities for all four markers. Except for the enkephalin immunoreactivity distribution pattern in the striatum, the compartments that can be distinguished in material stained for these markers appear to coincide quite well.