Distribution of GABAergic interneurons immunoreactive for calretinin, calbindin D28K, and parvalbumin in the cerebral cortex of the lizard Podarcis hispanica

GABA-immunoreactive neurons in the Central Nervous System of the viviparous teleost Poecilia sphenops

Gamma-aminobutyric acid (GABA) functions as the primary inhibitory neurotransmitter within the central nervous system (CNS) of vertebrates. In this study, we examined the distribution pattern of GABA-immunoreactive (GABA-ir) cells and fibres in the CNS of the viviparous teleost Poecilia sphenops using immunofluorescence method. GABA immunoreactivity was seen in the glomerular, mitral, and granular layers of the olfactory bulbs, as well as in most parts of the dorsal and ventral telencephalon. The preoptic area consisted of a small cluster of GABA-ir cells, whereas extensively labelled GABA-ir neurons were observed in the hypothalamic areas, including the paraventricular organ, tuberal hypothalamus, nucleus recessus lateralis, nucleus recessus posterioris, and inferior lobes. In the thalamus, GABA-positive neurons were only found in the ventral thalamic and central posterior thalamic nuclei, whereas the dorsal part of the nucleus pretectalis periventricularis consisted of a few GABA-ir cells. GABA-immunoreactivity was extensively seen in the alar and basal subdivisions of the midbrain, whereas in the rhombencephalon, GABA-ir cells and fibres were found in the cerebellum, motor nucleus of glossopharyngeal and vagal nerves, nucleus commissuralis of Cajal, and reticular formation. In the spinal cord, GABA-ir cells and fibres were observed in the dorsal horn, ventral horn, and around the central canal. Overall, the extensive distribution of GABA-ir cells and fibres throughout the CNS suggests several roles for GABA, including the neuroendocrine, viscerosensory, and somatosensory functions, for the first time in a viviparous teleost.

Analysis of pallial/cortical interneurons in key vertebrate models of Testudines, Anurans and Polypteriform fishes

The organization of the pallial derivatives across vertebrates follows a comparable elementary arrangement, although not all of them possess a layered cortical structure as sophisticated as the cerebral cortex of mammals. However, its expansion along evolution has only been possible by the development and coevolution of the cellular networks formed by excitatory neurons and inhibitory interneurons. Thus, the comparative analysis of interneuron types in vertebrate models of key evolutionary significance will provide important information, due to the extraordinary anatomical sophistication of their interneuron systems with simpler behavioral implications. Particularly in mammals, the main consensus for classifying interneuron types is based on non-overlapping markers, which do not form a single population, but consist of several distinct classes of inhibitory cells showing co-expression of other markers. In our study, we analyzed immunohistochemically the expression of the main markers like somatostatin (SOM), parvalbumin (PV), calretinin (CR), calbindin (CB), neuropeptide Y (NPY) and/or nitric oxide synthase (NOS) at the pallial regions of three different models of Osteichthyes. First, we selected two tetrapods, one amniote from the genus Pseudemys belonging to the order Testudine, at the base of the amniote diversification and with a three-layered simple cortex, and the Anuran Xenopus laevis, an anamniote tetrapod with a non-layered evaginated pallium, and finally the order Polypteriform, a small fish group at the base of the actinopterygian diversification with an everted telencephalon. SOM was the most conserved interneuron type in terms of its distribution and co-expression with other markers such as CR, in contrast to PV, which showed a different pattern between the models analyzed. In addition, the SOM expression supports a homological relationship between the medial pallial derivatives in all the models. CR and CB expressions in the tetrapods were observed, particularly, CR expressing cells were detected in the medial and the dorsal pallial derivatives, in contrast to CB, which appeared only in discrete scattered populations. However, the pallium of Polypteriforms fishes was almost devoid of CR cells, in contrast to the important number of CB cells observed in all the pallial regions. The NPY immunoreactivity was detected in all the pallial domains of all the models, as well as cells coexpressing CR. Finally, the pallial nitrergic expression was also conserved, which allows to postulate the homological relationships between the ventropallial and the amygdaloid derivatives. In summary, even in basal pallial models the neurochemically characterized interneurons indicate that their first appearance took place before the common ancestor of amniotes. Thus, our results suggest a shared pattern of interneuron types in the pallium of all Osteichthyes.

Region specific knockdown of Parvalbumin or Somatostatin produces neuronal and behavioral deficits consistent with those observed in schizophrenia

The anterior hippocampus and prefrontal cortex are regions linked to symptoms of schizophrenia. The anterior hippocampus is believed to be a key regulator of the mesolimbic dopamine system and is thought to be the driving force contributing to positive symptoms, while the prefrontal cortex is involved in cognitive flexibility and negative symptoms. Aberrant activity in these regions is associated with decreases in GABAergic markers, indicative of an interneuron dysfunction. Specifically, selective decreases are observed in interneurons that contain parvalbumin (PV) or somatostatin (SST). Here, we used viral knockdown in rodents to recapitulate this finding and examine the region-specific roles of PV and SST on neuronal activity and behaviors associated with positive, negative and cognitive symptoms. We found that PV and SST had differential effects on neuronal activity and behavior when knocked down in the ventral hippocampus (vHipp) or medial prefrontal cortex (mPFC). Specifically, SST or PV knockdown in the vHipp increased pyramidal cell activity of the region and produced downstream effects on dopamine neuron activity in the ventral tegmental area (VTA). In contrast, mPFC knockdown did not affect the activity of VTA dopamine neuron activity; however, it did produce deficits in negative (social interaction) and cognitive (reversal learning) domains. Taken together, decreases in PV and/or SST were sufficient to produce schizophrenia-like deficits that were dependent on the region targeted.

Regional chemoarchitecture of the brain of lungfishes based on calbindin D-28K and calretinin immunohistochemistry

Lungfishes are the closest living relatives of land vertebrates, and their neuroanatomical organization is particularly relevant for deducing the neural traits that have been conserved, modified, or lost with the transition from fishes to land vertebrates. The immunohistochemical localization of calbindin (CB) and calretinin (CR) provides a powerful method for discerning segregated neuronal populations, fiber tracts, and neuropils and is here applied to the brains of Neoceratodus and Protopterus, representing the two extant orders of lungfishes. The results showed abundant cells containing these proteins in pallial and subpallial telencephalic regions, with particular distinct distribution in the basal ganglia, amygdaloid complex, and septum. Similarly, the distribution of CB and CR containing cells supports the division of the hypothalamus of lungfishes into neuromeric regions, as in tetrapods. The dense concentrations of CB and CR positive cells and fibers highlight the extent of the thalamus. As in other vertebrates, the optic tectum is characterized by numerous CB positive cells and fibers and smaller numbers of CR cells. The so-called cerebellar nucleus contains abundant CB and CR cells with long ascending axons, which raises the possibility that it could be homologized to the secondary gustatory nucleus of other vertebrates. The corpus of the cerebellum is devoid of CB and CR and cells positive for both proteins are found in the cerebellar auricles and the octavolateralis nuclei. Comparison with other vertebrates reveals that lungfishes share most of their features of calcium binding protein distribution with amphibians, particularly with salamanders.

The cerebral cortex of the pygmy hippopotamus, Hexaprotodon liberiensis (Cetartiodactyla, Hippopotamidae): MRI, cytoarchitecture, and neuronal morphology

The structure of the hippopotamus brain is virtually unknown because few studies have examined more than its external morphology. In view of their semiaquatic lifestyle and phylogenetic relatedness to cetaceans, the brain of hippopotamuses represents a unique opportunity for better understanding the selective pressures that have shaped the organization of the brain during the evolutionary process of adaptation to an aquatic environment. Here we examined the histology of the cerebral cortex of the pygmy hippopotamus (Hexaprotodon liberiensis) by means of Nissl, Golgi, and calretinin (CR) immunostaining, and provide a magnetic resonance imaging (MRI) structural and volumetric dataset of the anatomy of its brain. We calculated the corpus callosum area/brain mass ratio (CCA/BM), the gyrencephalic index (GI), the cerebellar quotient (CQ), and the cerebellar index (CI). Results indicate that the cortex of H. liberiensis shares one feature exclusively with cetaceans (the lack of layer IV across the entire cerebral cortex), other features exclusively with artiodactyls (e.g., the morphologiy of CR-immunoreactive multipolar neurons in deep cortical layers, gyrencephalic index values, hippocampus and cerebellum volumetrics), and others with at least some species of cetartiodactyls (e.g., the presence of a thick layer I, the pattern of distribution of CR-immunoreactive neurons, the presence of von Economo neurons, clustering of layer II in the occipital cortex). The present study thus provides a comprehensive dataset of the neuroanatomy of H. liberiensis that sets the ground for future comparative studies including the larger Hippopotamus amphibius.

Characterization of GABAergic neurons in the mouse lateral septum: a double fluorescence in situ hybridization and immunohistochemical study using tyramide signal amplification

Gamma-aminobutyric acid (GABA) neurotransmission in the lateral septum (LS) is implicated in modulating various behavioral processes, including emotional reactivity and maternal behavior. However, identifying the phenotype of GABAergic neurons in the CNS has been hampered by the longstanding inability to reliably detect somal immunoreactivity for GABA or glutamic acid decarboxylase (GAD), the enzyme that produces GABA. In this study, we designed unique probes for both GAD65 (GAD2) and GAD67 (GAD1), and used fluorescence in Situ hybridization (FISH) with tyramide signal amplification (TSA) to achieve unequivocal detection of cell bodies of GABAergic neurons by GAD mRNAs. We quantitatively characterized the expression and chemical phenotype of GABAergic neurons across each subdivision of LS and in cingulate cortex (Cg) and medial preoptic area (MPOA) in female mice. Across LS, almost all GAD65 mRNA-expressing neurons were found to contain GAD67 mRNA (approximately 95-98%), while a small proportion of GAD67 mRNA-containing neurons did not express GAD65 mRNA (5-14%). Using the neuronal marker NeuN, almost every neuron in LS (> 90%) was also found to be GABA-positive. Interneuron markers using calcium-binding proteins showed that LS GABAergic neurons displayed immunoreactivity for calbindin (CB) or calretinin (CR), but not parvalbumin (PV); almost all CB- or CR-immunoreactive neurons (98-100%) were GABAergic. The proportion of GABAergic neurons immunoreactive for CB or CR varied depending on the subdivisions examined, with the highest percentage of colocalization in the caudal intermediate LS (LSI) (approximately 58% for CB and 35% for CR). These findings suggest that the vast majority of GABAergic neurons within the LS have the potential for synthesizing GABA via the dual enzyme systems GAD65 and GAD67, and each subtype of GABAergic neurons identified by distinct calcium-binding proteins may exert unique roles in the physiological function and neuronal circuitry of the LS.

Reptiles: a new model for brain evo-devo research

Vertebrate brains exhibit vast amounts of anatomical diversity. In particular, the elaborate and complex nervous system of amniotes is correlated with the size of their behavioral repertoire. However, the evolutionary mechanisms underlying species-specific brain morphogenesis remain elusive. In this review we introduce reptiles as a new model organism for understanding brain evolution. These animal groups inherited ancestral traits of brain architectures. We will describe several unique aspects of the reptilian nervous system with a special focus on the telencephalon, and discuss the genetic mechanisms underlying reptile-specific brain morphology. The establishment of experimental evo-devo approaches to studying reptiles will help to shed light on the origin of the amniote brains.

Sites of origin and developmental dynamics of the neurons in the core and shell regions of torus semicircularis in the Chinese softshell turtle (Pelodiscus sinensis)

To know the embryogenesis of the core and shell regions of the midbrain auditory nucleus, a single dose of [(3)H]-thymidine was injected into the turtle embryos at peak stages of neurogenesis in the shell and core of the torus semicircularis. Following sequential survival times, labeled neurons and the dynamics of cell proliferation were examined. The expression of vimentin (VM), reelin, calbindin, parvalbumin, and substance P were also studied. The results showed that: 1) progenitor cells for the core and shell regions were generated in different sites of the ventricular zone; 2) the length of the cell cycle or S-phase for the shell region were both longer than those for the core region (4.7 and 3.2 hours longer, respectively), suggesting that mitotic activity in the core region is higher than it is in the shell region; 3) the elongated cell bodies of the labeled core and shell cells had close apposition to VM fibers, suggesting that the migration of these cells is guided by VM fibers; 4) the germinal sites of the core and shell constructed by projecting the orientation of radial VM fibers back to the ventricular zone was consistent with those obtained by short and sequential survival [(3)H]-thymidine radiography; and 5) the beginning of positive staining for parvalbumin in the core region was interposed between those for calbindin and substance P in the shell regions. This study contributes to the understanding of how auditory nuclei are organized and how their components developed and evolved.

Immunohistochemical localization of calbindin-D28k and calretinin in the spinal cord of lungfishes

A common pattern of distribution of neurons and fibers containing the calcium-binding proteins calbindin-D28k (CB) and calretinin (CR) in the spinal cord of terrestrial vertebrates has been recently demonstrated. Lungfishes are considered the closest living relatives of tetrapods, but practically no experimental data exist on the organization of their spinal cord. By means of immunohistochemical techniques, the localization of CB and CR was investigated in the spinal cord of the African (Protopterus dolloi) and Australian (Neoceratodus forsteri) lungfishes. Abundant cell bodies and fibers immunoreactive for either CB or CR were widely distributed throughout the spinal cord. A large population of immunoreactive cells was found in the dorsal column of the gray matter in both species, and abundant cells were distributed in the lateral and ventral columns. Ventrolateral motoneurons and multipolar cells were only intensely CB and CR immunoreactive in Neoceratodus. For the most part, separate cell populations contained either CB or CR, but a small subset of dorsally located neurons contained both in the two lungfishes. Colocalization was found in motoneurons and in ventrolaterally located cells only in Neoceratodus. Fiber labeling showed a predominance of CR-containing axons in the lateral and ventral funiculi of presumed supraspinal origin. These results show that lung-fishes and tetrapods have many features in common, suggesting that primitive anatomical, and likely functional, organization of the spinal cord of tetrapods is present in lungfishes.

Immunohistochemical and hodological characterization of calbindin-D28k-containing neurons in the spinal cord of the turtle,Pseudemys scripta elegans

Neurons and fibers containing the calcium-binding protein calbindin-D28k (CB) were studied by immunohistochemical techniques in the spinal cord of adult and juvenile turtles, Pseudemys scripta elegans. Abundant cell bodies and fibers immunoreactive for CB were widely and distinctly distributed throughout the spinal cord. Most neurons and fibers were labeled in the superficial dorsal horn, but numerous cells were also located in the intermediate gray and ventral horn. In the dorsal horn, most CB-containing cells were located in close relation to the synaptic fields formed by primary afferents, which were not labeled for CB. Double immunohistofluorescence demonstrated distinct cell populations in the dorsal horn labeled only for CB or nitric oxide synthase, whereas in the dorsal part of the ventral horn colocalization of nitric oxide synthase was found in about 6% of the CB-immunoreactive cells in this region. Choline acetyltransferase immunohistochemistry revealed that only about 2% of the neurons in the dorsal part of the ventral horn colocalized CB, whereas motoneurons were not CB-immunoreactive. The involvement of CB-containing neurons in ascending spinal projections to the thalamus, tegmentum, and reticular formation was demonstrated combining the retrograde transport of dextran amines and immunohistochemistry. Similar experiments demonstrated supraspinal projections from CB-containing cells mainly located in the reticular formation but also in the thalamus and the vestibular nucleus. The revealed organization of the neurons and fibers containing CB in the spinal cord of the turtle shares distribution and developmental features, colocalization with other neuronal markers, and connectivity with other tetrapods and, in particular with mammals.

Calcium-binding proteins, neuronal nitric oxide synthase, and GABA help to distinguish different pallial areas in the developing and adult chicken. I. Hippocampal formation and hyperpallium

To better understand the formation and adult organization of the avian pallium, we studied the expression patterns of gamma-aminobutyric acid (GABA), calbindin (CB), calretinin (CR), and neuronal nitric oxide synthase (nNOS) in the hippocampal formation and hyperpallium of developing and adult chicks. Each marker showed a specific spatiotemporal expression pattern and was expressed in a region (area)-specific but dynamic manner during development. The combinatorial expression of these markers was very useful for identifying and following the development of subdivisions of the chicken hippocampal formation and hyperpallium. In the hyperpallium, three separate radially arranged subdivisions were present since early development showing distinct expression patterns: the apical hyperpallium (CB-rich); the intercalated hyperpallium (nNOS-rich, CB-poor); the dorsal hyperpallium (nNOS-poor, CB-moderate). Furthermore, a novel division was identified (CB-rich, CR-rich), interposed between hyper- and mesopallium and related to the lamina separating both, termed laminar pallial nucleus. This gave rise at its surface to part of the lateral hyperpallium. Later in development, the interstitial nucleus of the apical hyperpallium became visible as a partition of the apical hyperpallium. In the hippocampal formation, at least five radial divisions were observed, and these were compared with the divisions proposed recently in adult pigeons. Of note, the corticoid dorsolateral area (sometimes referred as caudolateral part of the parahippocampal area) contained CB immunoreactivity patches coinciding with Nissl-stained cell aggregates, partially resembling the patches described in the mammalian entorhinal cortex. Each neurochemical marker was present in specific neuronal subpopulations and axonal networks, providing insights into the functional maturation of the chicken pallium.

Calbindin-D28k and calretinin immunoreactivity in the spinal cord of the lizard Gekko gecko: Colocalization with choline acetyltransferase and nitric oxide synthase

The distribution of the calcium-binding proteins calbindin-D28k (CB) and calretinin (CR) was investigated in the spinal cord of the lizard Gekko gecko, by means of immunohistochemical techniques. Abundant cell bodies and fibers immunoreactive for either CB or CR were widely distributed throughout the spinal cord. Most neurons and fibers were labeled in the superficial dorsal horn, but numerous cells were also located in the intermediate gray and ventral horn. Distinct CB- and CR-containing cell populations were observed, although double immunohistochemistry revealed that 17-20% of the single-labeled cells for CB or CR in the dorsal horn contained both proteins. In addition, nitric oxide synthase was immunodetected in about 6% of the CB-positive neurons in the dorsal horn and in 10% in the ventral horn, whereas nitric oxide synthase was present in 9-13% of CR-positive cells in the dorsal horn and in 14% in the ventral horn. These doubly immunoreactive cells were restricted to areas IV, VII and VIII. Similar colocalization experiments revealed that 18-24% of the cholinergic cells in the ventral horn contained CB and 21-30% CR, with some variations throughout the length of the spinal cord. The pattern of distribution for CB and CR immunoreactivity in the spinal cord of the lizard, reported in the present study, is largely comparable to those reported for mammals, birds and anuran amphibians suggesting a high degree of conservation of the spinal systems modulated by these calcium-binding proteins.

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

We investigated whether gamma-amino butyric acidergic (GABAergic) cell populations correlate positionally with specific Dlx-expressing histogenetic territories in an anamniote tetrapod, the frog Xenopus laevis. To that end, we cloned a fragment of Xenopus GAD67 gene (xGAD67, expressed in GABAergic neurons) and compared its expression with that of Distal-less-4 gene (xDll-4, ortholog of mouse Dlx2) in the forebrain at late larval and adult stages. In Xenopus, GABAergic neurons were densely concentrated in xDll-4-positive territories, such as the telencephalic subpallium, part of the hypothalamus, and ventral thalamus, where nearly all neurons expressed both genes. In contrast, the pallium of Xenopus generally contained dispersed neurons expressing xGAD67 or xDll-4, which may represent local circuit neurons. As in amniotes, these pallial interneurons may have been produced in the subpallium and migrated tangentially into the pallium during development. In Xenopus, the ventral division of the classic lateral pallium contained extremely few GABAergic cells and showed only low signal of the pallial gene Emx1, suggesting that it may represent the amphibian ventral pallium, homologous to that of amniotes. At caudal forebrain levels, a number of GABAergic neurons was observed in several areas (dorsal thalamus, pretectum), but no correlation to xDll-4 was observed there. The location of GABAergic neurons in the forebrain and their relation to the developmental regulatory genes Dll and Dlx were very similar in Xenopus and in amniotes. The close correlation in the expression of both genes in rostral forebrain regions supported the notion that Dll/Dlx are among the genes involved in the acquisition of the GABAergic phenotype.

Calbindin and calretinin immunoreactivities identify different types of neurons in the adult lamprey spinal cord

The central pattern generator for locomotion in vertebrates is composed of different spinal neuronal populations that generate locomotor movement. In the lamprey spinal cord, several classes of interneurons have been identified based on morphologic and physiological criteria and integrated in the spinal cord circuits implicated in the generation of locomotion. However, the lack of histochemical markers for most of the interneurons makes it difficult to study whole populations along the spinal cord. We have investigated the immunoreactivity with antibodies raised against calbindin and calretinin. Several types of neurons could be classified: (1). strongly immunoreactive neurons located dorsomedially, (2). moderately immunoreactive neurons located laterally, (3). small weakly immunoreactive neurons, d). ventromedial neurons, (4). liquor contacting cells, and (5). motoneurons. The ventromedial group of calbindin-immunoreactive neurons also is immunoreactive for serotonin and, therefore, represents the ventromedial group of dopamine/serotonin spinal neurons. Some of the lateral calbindin-immunoreactive neurons may be CC-type cells (cells with caudal-crossed axons), because they are retrogradely labeled by tracer injections into the contralateral spinal cord. Other well-characterized cell types, such as sensory dorsal cells, lateral interneurons, descending propriospinal edge cells, and spinobulbar giant interneurons are negative for both calbindin and calretinin. Therefore, calbindin and calretinin are useful markers for the study of cell populations that may be integrated in locomotor circuits.

The lizard cerebral cortex as a model to study neuronal regeneration

The medial cerebral cortex of lizards, an area homologous to the hippocampal fascia dentata, shows delayed postnatal neurogenesis, i.e., cells in the medial cortex ependyma proliferate and give rise to immature neurons, which migrate to the cell layer. There, recruited neurons differentiate and give rise to zinc containing axons directed to the rest of cortical areas, thus resulting in a continuous growth of the medial cortex and its zinc-enriched axonal projection. This happens along the lizard life span, even in adult lizards, thus allowing one of their most important characteristics: neuronal regeneration. Experiments in our laboratory have shown that chemical lesion of the medial cortex (affecting up to 95% of its neurons) results in a cascade of events: first, massive neuronal death and axonal-dendritic retraction and, secondly, triggered ependymal-neuroblast proliferation and subsequent neo-histogenesis and regeneration of an almost new medial cortex, indistinguishable from a normal undamaged one. This is the only case to our knowledge of the regeneration of an amniote central nervous centre by new neuron production and neo-histogenesis. Thus the lizard cerebral cortex is a good model to study neuronal regeneration and the complex factors that regulate its neurogenetic, migratory and neo-synaptogenetic events.

Neurogenesis of heterotopic gray matter in the brain of the microcephalic mouse

Neurogenesis of heterotopic gray matter in the brain of the microcephalic mouse prenatally exposed to X-rays at embryonic day 13 (E13) was studied immunohistochemically. Bromodeoxyuridine (BrdU) as a marker to label the migrating position of neuroblasts generated at various embryonic stages showed that no "inside-out" pattern of neuronal migration occurred in the heterotopic cell mass similar to that seen in the laminated cortex. Further results in which midkind (MK) immunoreactive radial glial fibers did not appear in the heterotopic cell mass demonstrated that heterotopia formed in the absence of radial glia system. Different types of cells (pyramidal and non-pyramidal neurons) in the heterotopic cell mass were identified with immunoreactivity for anti-parvalbumin and anti-calbindin D-28K antibodies in addition to current histological methods. Two major types of neurons were mixed together with random distribution in the heterotopic cell mass. This finding indicates that irradiation might have no selective effects on the precursors of pyramidal and non-pyramidal neurons. Moreover, anti-glial fibrillary acidic protein (GFAP) immunostaining showed that numerous astrocytes were present in the heterotopic cell mass. The fact that astrocytes appeared in the heterotopia without the transition from classic radial glial cells to astrocytes suggests that astrocytes might be generated directly from a separate astroglial precursor.

Changes in the distribution and connectivity of interneurons in the epileptic human dentate gyrus

The distribution, size, dendritic morphology and synaptic connections of calbindin-, calretinin- and substance P receptor-positive interneurons and pathways have been examined in control and epileptic human dentate gyrus. In the epileptic dentate gyrus, calbindin-containing interneurons are preserved, but their dendrites become elongated and spiny, and several cell bodies appear hypertrophic. The relative laminar distribution of calretinin-containing cells did not change, but their number was considerably reduced. The calretinin-positive axonal bundle at the top of the granule cell layer originating from the supramammillary nucleus expanded, forming a dense network in the entire width of the stratum moleculare. Substance P receptor-immunopositive cells were partially lost in epileptic samples, and in addition, the laminar distribution and dendritic morphology of the surviving cells differed considerably from the controls. In the control human dentate gyrus, the majority of substance P receptor-positive cells can be seen in the hilus, while most are present in the stratum moleculare in the epileptic tissue. Their synaptic input is also changed. The extent of individual pathological abnormalities correlates with each other in most cases. Our data suggest, that although a large proportion of inhibitory interneurons are preserved in the epileptic human dentate gyrus, their distribution, morphology and synaptic connections differ from controls. These functional alterations of inhibitory circuits in the dentate gyrus are likely to be compensatory changes with a role to balance the enhanced excitatory input in the region.

Distribution of parvalbumin immunoreactivity in the brain of the tench (Tinca tinca L., 1758)

The distribution of parvalbumin (PV) immunoreactivity in the tench brain was examined by using the avidin-biotin-peroxidase immunocytochemical method. This protein was detected in neuronal populations throughout all main divisions of the tench brain. In the telencephalic hemispheres, PV-immunopositive neurons were distributed in both the dorsal and ventral areas, being more abundant in the area ventralis telencephali, nucleus ventralis. In the diencephalon, the scarce distribution of PV-containing cells followed a rostrocaudal gradient, and the most evident staining was observed in the nucleus periventricularis tuberculi posterioris and in a few nuclei of the area praetectalis. In the mesencephalon, abundant PV-immunoreactive elements were found in the tectum opticum, torus semicircularis, and tegmentum. In the tectum opticum, PV-immunoreactivity presented a laminar distribution. Three PV-containing neuronal populations were described in the torus semicircularis, whereas in the tegmentum, the PV staining was mainly located in the nucleus tegmentalis rostralis and in the nucleus nervi oculomotorii. In the metencephalon, Purkinje cells were PV-immunopositive in the valvula cerebelli, lobus caudalis cerebelli, and in the corpus cerebelli. In the myelencephalon, PV immunoreactivity was abundant in the nucleus lateralis valvulae, in the nucleus nervi trochlearis, nucleus nervi trigemini, nucleus nervi abducentis, nucleus nervi glossopharyngei, and in the formatio reticularis. Mauthner cells were also PV immunostained. By contrast to other vertebrate groups, only a restricted population of PV-containing neurons was GABA-immunoreactive in the tench, demonstrating that this calcium-binding protein cannot be considered a marker for GABAergic elements in the teleost brain. This study demonstrates a low phylogenetic conservation of the distribution of PV comparing teleosts and tetrapods.

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.

Cellular distribution of the calcium-binding proteins parvalbumin, calbindin, and calretinin in the neocortex of mammals: phylogenetic and developmental patterns

The three calcium-binding proteins parvalbumin, calbindin, and calretinin are found in morphologically distinct classes of inhibitory interneurons as well as in some pyramidal neurons in the mammalian neocortex. Although there is a wide variability in the qualitative and quantitative characteristics of the neocortical subpopulations of calcium-binding protein-immunoreactive neurons in mammals, most of the available data show that there is a fundamental similarity among the mammalian species investigated so far, in terms of the distribution of parvalbumin, calbindin, and calretinin across the depth of the neocortex. Thus, calbindin- and calretinin-immunoreactive neurons are predominant in layers II and III, but are present across all cortical layers, whereas parvalbumin-immunoreactive neurons are more prevalent in the middle and lower cortical layers. These different neuronal populations have well defined regional and laminar distribution, neurochemical characteristics and synaptic connections, and each of these cell types displays a particular developmental sequence. Most of the available data on the development, distribution and morphological characteristics of these calcium-binding proteins are from studies in common laboratory animals such as the rat, mouse, cat, macaque monkey, as well as from postmortem analyses in humans, but there are virtually no data on other species aside of a few incidental reports. In the context of the evolution of mammalian neocortex, the distribution and morphological characteristics of calcium-binding protein-immunoreactive neurons may help defining taxon-specific patterns that may be used as reliable phylogenetic traits. It would be interesting to extend such neurochemical analyses of neuronal subpopulations to other species to assess the degree to which neurochemical specialization of particular neuronal subtypes, as well as their regional and laminar distribution in the cerebral cortex, may represent sets of derived features in any given mammalian order. This could be particularly interesting in view of the consistent differences in neurochemical typology observed in considerably divergent orders such as cetaceans and certain families of insectivores and metatherians, as well as in monotremes. The present article provides an overview of calcium-binding protein distribution across a large number of representative mammalian species and a review of their developmental patterns in the species where data are available. This analysis demonstrates that while it is likely that the developmental patterns are quite consistent across species, at least based on the limited number of species for which ontogenetic data exist, the distribution and morphology of calcium-binding protein-containingneurons varies substantially among mammalian orders and that certain species show highly divergent patterns compared to closely related taxa. Interestingly, primates, carnivores, rodents and tree shrews appear closely related on the basis of the observed patterns, marsupials show some affinities with that group, whereas prototherians have unique patterns. Our findings also support the relationships of cetaceans and ungulates, and demonstrates possible affinities between carnivores and ungulates, as well as the existence of common, probably primitive, traits in cetaceans and insectivores.

The functions of the preplate in development and evolution of the neocortex and hippocampus

Recently, it has been shown that the early developmental organization of the archicortical hippocampus resembles that of the neocortex. In both cortices at embryonic stages, a preplate is present, which is split by the formation of the cortical plate into a marginal zone and a subplate layer. The pioneer neurons of the preplate are believed to form a phylogenetically ancient cortical structure. Neurons in these preplate layers are the first postmitotic neurons and have important roles in the development of the cerebral cortex. Cajal-Retzius cells in the marginal zone regulate the phenotype of radial glial cells and may direct neuronal migration establishing the inside-out gradient of corticogenesis. Furthermore, pioneer neurons form the initial axonal connections with other (sub)cortical structures. A significant difference between the hippocampus and neocortex, however, is that in the hippocampus, most afferents are guided by the pioneer neurons in the prominent marginal zone, while in the neocortex most ingrowing afferent axons enter via the subplate. At later developmental periods, most pioneer neurons disappear by cell death or transform into other neuronal shapes. Here, we review the early developmental organization of the mammalian cerebral cortex (both neocortex and hippocampus) and discuss the functions and fate of pioneer neurons in cortical development, in particular that of Cajal-Retzius cells. Evaluating the developmental properties of the hippocampus and neocortex, we present the hypothesis that the distribution of the main ingrowing afferent systems in the developing neocortex, which differs from the one in the hippocampal region, may have enabled the specific evolution of the neocortex.

A Golgi study of the short-axon interneurons of the cell layer and inner plexiform layer of the medial cortex of the lizard Podarcis hispanica

The medial cortex of lizards is a three-layered brain region displaying cyto- and chemoarchitectonical, connectional, and ontogenetic characteristics that relate it to the hippocampal fascia dentata of mammals. Three interneuron types located in the cell layer and ten others in the inner plexiform layer (six in the juxtasomatic zone and four in the deep zone) are described in this study. The granuloid neurons, web-axon neurons, and deep-fusiform neurons lay within the cell layer. These neurons were scarce; they were probably gamma-aminobutyric acid (GABA)-, and parvalbumin-immunoreactive and presumably participated in feed forward as well as in feed back inhibition of the principal projection cells of the lizard medial cortex. In the juxtasomatic inner plexiform layer, the smooth vertical neurons, smooth horizontal neurons, small radial neurons, large radial neurons, pyramidal-like radial neurons, and spheroidal neurons were found. They were all probably GABA-, and parvalbumin-immunoreactive and were involved in feed forward inhibition of principal medial cortex cells. In the deep inner plexiform layer lay the giant-multipolar neurons, long-spined polymorphic neurons, periventricular neurons, and alveus-horizontal neurons. These neurons were probably GABA-immunoreactive and either neuropeptide- (somatostatin-neuropeptide Y) or parvalbumin-immunoreactive. They seemed to be involved in feed back or even occasionally in feed forward inhibition phenomena.

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.

Ontogeny of somatostatin immunoreactive neurons in the medial cerebral cortex and other cortical areas of the lizard Podarcis hispanica

The ontogeny of somatostatin immunoreactive interneurons in the cerebral cortex of the lizard Podarcis hispanica has been studied in histological series of embryos, perinatal specimens, and adults. Somatostatin immunoreactive interneurons appear in the early stages of lizard cerebral cortex ontogeny, their number increases during embryonary development, reaches a peak in early postnatal life, and decreases in adult lizards. The first somatostatin immunoreactive somata in the lizard forebrain appeared on E36, and they were located in non cortical areas. Then, on E39 and later, somatostatin immunoreactive neurons were seen in the lizard cortex in a rostral-to-caudal spatial gradient, which parallels that of the normal histogenesis of the lizard cerebral cortex. On E39, labelled somata were seen in the medial and dorsal cortex inner plexiform layers; immunoreactive puncta and dendritic processes were detectable in the inner plexiform layer of the medial cortex. On E40, labelled neurons were observed in the inner plexiform layer of the lateral cortex; labelled processes were found in the inner plexiform layers (dorsomedial, dorsal, and lateral cortices) and the outer plexiform layers (medial and dorsomedial cortices). At hatching (P0), some somatostatin immunoreactive neurons populated the external plexiform layer of the dorsomedial cortex. On P28, groups of labelled neurons appeared in the cell layer of dorsal and lateral cortices, reaching the adult-mature pattern of somatostatin immunoreactivity in the lizard cerebral cortex, i.e., labelled somata and dendritic processes populating the inner plexiform layers in addition to an axonic labelled plexus in the outermost part of the outer plexiform layers. Immunoreactive somata and processes occupied all the cortical areas, but they were especially abundant in the dorsomedial cortex. Proliferating Cell Nuclear Antigen (PCNA) immunostaining in the same histological series revealed that the number of PCNA immunoreactive nuclei in the subjacent proliferative neuroepithelium followed an inverse-complementary evolution to somatostatin, suggesting some temporal relationship between somatostatin immunoreactive cells and neurogenesis in the lizard cerebral cortex.

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.

Changes in GABA and parvalbumin immunoreactivities in the cerebral cortex of lizards after narine occlusion

Olfactory deprivation produced by narine occlusion has been suggested to reduce the activity in the cerebral cortex of lizards. Here we analyzed the short-term changes in GABA and parvalbumin (PV) immunoreactivities in the cerebral cortex of lizards after narine occlusion. The number and distribution of GABA- and parvalbumin-immunoreactive (IR) cells have been studied by immunocytochemistry in the cerebral cortex of control and olfactory-deprived lizards. The distribution of GABA-IR cells as well as that of PV-IR cells was similar in control and deprived animals, and PV-IR cells were GABA-IR in all cases. However, significant changes were observed in the absolute number of GABA- and PV-IR cells. GABA-IR cells were more abundant in deprived animals than in control ones. In contrast, the number of PV-IR cells decreased significantly and PV immunoreactivity in dendrites and boutons was lower in deprived animals. These results suggest that the reduction in the number of PV-IR cells in olfactory-deprived lizards occurs without loss of GABA cells, and that PV expression is under the control of olfactory activity and remains plastic in the cerebral cortex of adult lizards.

Serotoninergic innervation of nonprincipal cells in the cerebral cortex of the lizard Podarcis hispanica

The mechanism of serotoninergic transmission in the neo- and archicortex of mammals is complex, including both synaptic and nonsynaptic components, direct actions on principal cells, and indirect effects mediated by GABAergic interneurons. Here we studied the termination pattern and synaptic organization of the serotoninergic afferents in the cerebral cortex of the lizard, Podarcis hispanica, which is considered to correspond in part to the mammalian hippocampal formation, with the aim of unraveling basic, phylogenetically preserved rules in the connectivity of this pathway. We demonstrate that serotoninergic afferents, visualized by immunostaining for serotonin itself, establish multiple synaptic contacts with different subpopulations of nonprincipal cells containing parvalbumin, neuropeptide Y, and opioid peptides. The former two subpopulations contain GABA, whereas the opioid-immunoreactive neurons are most likely GABA-negative cells. Evidence is provided at the electron microscopic level that serotonin-immunoreactive varicosities establish conventional asymmetric synaptic contacts with their nonprincipal targets, but nonsynaptic varicosities also exist. We conclude that, similarly to mammals, a selective synaptic innervation of nonprincipal, possibly inhibitory, neurons is among the mechanisms of serotoninergic modulation of cerebral cortical activity in the lizard.

Neurons of the medial cortex outer plexiform layer of the lizard Podarcis hispanica: Golgi and immunocytochemical studies

The study of Golgi-impregnated lizard brains has revealed a scarce but heterogeneous neuronal population in the outer plexiform layer of the medial cortex. Some of the neuronal types detected here resemble the neurons of the dentate molecular layer of the mammalian hippocampus. According to their morphology, five intrinsic neuronal types have been clearly identified: short axon aspinous bipolar neuron (type 1, or sarmentous neuron), short axon aspinous juxtasomatic neuron (type 2, or coral neuron), short axon sparsely spinous multipolar neuron (type 3, or stellate neuron), short axon sparsely spinous juxtasomatic multipolar neuron (type 4, or deep stellate neuron), and sparsely spinous juxtasomatic horizontal neuron (type 5, or couchant neuron). Most neuronal types were identified as gamma-aminobutyric acid (GABA) and parvalbumin immunoreactive, and are thus probably involved in medial cortex inhibition. Moreover, a small fraction of them displayed beta-endorphin immunoreactivity. The distribution of these neuronal types is not uniform in the laminae of the outer plexiform layer. Type 1 (sarmentous) and type 3 (stellate) neurons overlap the axonal field projection coming from the dorsal cortex and the thalamus, whereas types 4 (deep stellate) and 5 (couchant) neurons overlap ipsi- and contralateral dorsomedial projection fields as well as raphe serotoninergic and opioid immunoreactive axonal plexi. Thus, these neuronal types may be involved in the control of specific inputs to the medial cortex by presumably feed-forward inhibition; nevertheless, feed-back inhibition may also occur regarding type 4 (deep stellate) neurons that extend deep dendrites to the zinc-rich bouton field.

Postnatal increase of GABA- and PV-IR cells in the cerebral cortex of the lizard Podarcis hispanica

The number and distribution of GABA- and parvalbumin (PV)-immunoreactive (IR) cells have been studied by immunocytochemistry in the cerebral cortex of newborn and adult lizards. The distribution of GABA-IR cells as well as that of PV-IR cells were similar in newborn and adult lizards, and PV-IR cells were GABA-IR in all cases. However, the absolute number of GABA- and PV-IR cells increased significantly during development. In addition, the rate of of GABA-IR cells also displaying PV immunoreactivity also increased after birth. Moreover, dendrites were rarely found to be PV-IR in newborn lizards, whereas they appeared stained in a Golgi-like manner in adult animals. These results suggest that the GABAergic neuronal population of the cerebral cortex of lizards experiments a significant increment in number and neurochemical maturation after birth.

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.

Parvalbumin-containing neurons in the cerebral cortex of the lizard Podarcis hispanica: morphology, ultrastructure, and coexistence with GABA, somatostatin, and neuropeptide Y

The morphology, fine structure, and degree of colocalization with GABA, somatostatin, and neuropeptide Y of parvalbumin-containing cells were studied with immunocytochemistry in the cerebral cortex of the lizard Podarcis hispanica. Parvalbumin-containing cells make up a morphologically heterogeneous population of spine-free neurons, displaying the morphological features of nonprincipal cells previously described in Golgi studies. Electron microscopically, parvalbumin-immunoreactive cell bodies are similar in all cortical areas and layers. The perisomatic input is moderate in number, and boutons with either round clear vesicles or flattened vesicles were observed making asymmetric or symmetric synaptic contacts, respectively. Parvalbumin-immunoreactive dendrites are smooth and almost completely covered with synaptic boutons of different types, most of which establish asymmetric contacts. Parvalbumin-immunoreactive boutons are concentrated around cell bodies of principal cells. They are large, containing abundant mitochondria and small pleomorphic vesicles, and establishing symmetric synaptic contacts with somata, proximal dendritic shafts, and axon initial segments of principal cells. Colocalization studies revealed that all the parvalbumin-containing cells are GABA-immunoreactive, representing only a fraction of the GABA-immunopositive cell population, and that parvalbumin- and peptide- (somatostatin and neuropeptide Y) containing cells show a negligible overlap. These results demonstrate that in the cerebral cortex of the lizard Podarcis hispanica, parvalbumin-containing cells represent a subset of nonprincipal GABAergic neurons largely involved in perisomatic inhibition, which are different from the peptide-containing cells, and suggest that they may include both axosomatic and axoaxonic cells.

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.

Calretinin is present in non-pyramidal cells of the rat hippocampus--II. Co-existence with other calcium binding proteins and GABA

The possible co-existence of calretinin with other calcium binding proteins, parvalbumin and calbindin D28k, and with GABA, was studied in non-pyramidal cells of the rat dorsal hippocampal formation, using the mirror technique. The majority of the calretinin-containing neurons (83%) were found to be immunoreactive for GABA (79% in the dentate gyrus, 84% in the CA2-3, and 88% in the CA1 subfield). Most of the GABA-negative calretinin-immunoreactive neurons were located in the hilus of the dentate gyrus and in stratum lucidum of the CA3 subfield. Detailed analysis of the calretinin-immunoreactive cells of these subfields revealed that the two morphologically distinct types of calretinin neurons, i.e. the spiny and the spine-free cells, differ in their immunoreactivity for GABA. The overwhelming majority (92%) of the spine-free neurons were GABA-positive, whereas the immunoreactivity of spiny cells was ambiguous. At the sensitivity threshold of the immunocytochemical techniques used in the present study, most of the spiny cells (89%) had to be considered as GABA-negative, although the staining intensity in their cell bodies was somewhat above background level. Colchicine treatment resulted in a degeneration of calretinin-immunoreactive neurons; therefore, its effect on the GABA content of spiny neurons could not be evaluated. Nevertheless, the observations suggest that calretinin-containing neurons are heterogeneous both morphologically and neurochemically. Examination of the co-existence of calcium binding proteins revealed that none of the hippocampal cells contained both calretinin and parvalbumin in any regions of the hippocampal formation. Some overlap was detected between the calretinin- and the calbindin D28k-containing cell populations, 5.1% of the former and 6.2% of the latter were immunoreactive for both calcium binding proteins. This may be due to a small degree of cross-reactivity of the calbindin D28k antiserum with calretinin. Thus, our results demonstrate that the majority of calretinin-immunoreactive neurons are GABAergic and represent a subpopulation of non-pyramidal cells with no or only a negligible overlap with the subpopulations containing the other calcium binding proteins, parvalbumin and calbindin.