Structure of anterior dorsal ventricular ridge in a turtle (Pseudemys scripta elegans)

Dynamics and sources of response variability and its coordination in visual cortex

The trial-to-trial response variability in sensory cortices and the extent to which this variability can be coordinated among cortical units have strong implications for cortical signal processing. Yet, little is known about the relative contributions and dynamics of defined sources to the cortical response variability and their correlations across cortical units. To fill this knowledge gap, here we obtained and analyzed multisite local field potential (LFP) recordings from visual cortex of turtles in response to repeated naturalistic movie clips and decomposed cortical across-trial LFP response variability into three defined sources, namely, input, network, and local fluctuations. We found that input fluctuations dominate cortical response variability immediately following stimulus onset, whereas network fluctuations dominate the response variability in the steady state during continued visual stimulation. Concurrently, we found that the network fluctuations dominate the correlations of the variability during the ongoing and steady-state epochs, but not immediately following stimulus onset. Furthermore, simulations of various model networks indicated that (i) synaptic time constants, leading to oscillatory activity, and (ii) synaptic clustering and synaptic depression, leading to spatially constrained pockets of coherent activity, are both essential features of cortical circuits to mediate the observed relative contributions and dynamics of input, network, and local fluctuations to the cortical LFP response variability and their correlations across recording sites. In conclusion, these results show how a mélange of multiscale thalamocortical circuit features mediate a complex stimulus-modulated cortical activity that, when naively related to the visual stimulus alone, appears disguised as high and coordinated across-trial response variability.

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.

Multivariate statistical analysis of Golgi stained neurons

The multivariate statistical analysis provides a useful method to study neuronal populations. It allows both the objective classification of neuronal types and the study of the morphological variation within a neuronal population. We report a particular example of the use of these techniques on a Golgi study of a complex telencephalic structure, the reptilian anterior dorsal ventricular ridge (aDVR). We present a R-mode factor analysis and a cluster analysis on the results of the factor analysis. Sixteen original variables were chosen for the study in order to obtain the greatest information about the cell body, the dendritic field and the location of 96 Golgi-stained cells. Six factors were obtained after the R-mode factor analysis, the interpretation of which was clear in five of them. This contributed to explain the morphological variation of the neuronal types within the aDVR. The cluster analysis classified the 96 cells into eight groups. Some groups could be ascribed to specific regions of the aDVR.

Organization of the anterior dorsal ventricular ridge of the lizard Podarcis hispanica: cytoarchitecture and GABA-immunohistochemistry

The anterior dorsal ventricular ridge (ADVR) of Podarcis hispanica comprises 3 cytoarchitectonic subareas: the rostromedial, caudomedial and lateral ADVR. All 3 subareas showed gamma-aminobutyric (GABA)-like immunoreactivity. GABA-ergic neurons were classified as multipolar (large), stellate (small), bipolar and unipolar cells. Multipolar and stellate GABA-ergic neurons often formed clusters together with GABA-negative cells. Reactive puncta were seen around unreactive somata of the 3 subareas. Like the telencephalic sensory areas of mammals and birds, the ADVR of lizards shows a heterogeneous population of widely distributed GABA-ergic cells that may be the basis for lateral and vertical local inhibition.

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.

Organization of corticogeniculate projections in the turtle, Pseudemys scripta

The efferent pathways from the visual cortex to the dorsal lateral geniculate complex of turtles have been studied by using the orthograde and retrograde transport of horseradish peroxidase (HRP). Injections of HRP in the lateral thalamus retrogradely label neurons throughout the visual cortex. The majority of labeled neurons have somata in layer 2 of the lateral part of dorsal cortex (D2); a minority have somata in layer 3. Labeled neurons in layer 2 tend to have vertically oriented, fusiform somata and dendrites that ascend into layer 1. Labeled neurons in layer 3 have fusiform somata and dendrites, both oriented horizontally. Injections of HRP in visual cortex orthogradely label corticofugal axons. Those projecting to the lateral geniculate complex course laterally from the visual cortex, pass through the striatum (occasionally bearing varicosities), and enter the diencephalon in the ventral peduncle of the lateral forebrain bundle. Individual axons leave the ventral peduncle and run dorsally in the transverse plane, entering the dorsal lateral geniculate complex from its ventral edge. They continue dorsally, principally in the cell plate of the geniculate complex, where they bear varicosities.

The cerebellar and vestibular nuclear complexes in the turtle. II. Projections to the prosencephalon

Prosencephalic projections from the cerebellar and vestibular nuclear complexes in the turtle Pseudemys scripta elegans were investigated with anterograde tracing. Following injections of 35S-methionine at various locations within the cerebellar and vestibular nuclear complexes, labeled ascending fibers were found to arise from the lateral cerebellar and the rostral (superior and/or dorsolateral) vestibular nuclei. The great majority of these fibers coursed within the ipsilateral ascending periventricular tract. There were possible terminations in the hypothalamosuprapeduncular region, the ovalis-complex, and the nucleus commissuralis anterior, but scarcely any indication of terminal labeling within the dorsal thalamus. The labeled fibers, however, continued rostralward, entered the lateral forebrain bundle, and terminated in the anterior dorsal ventricular ridge--in all but one case, exclusively ipsilaterally. The terminal area within the lateral division (referred to as area L) of the anterior dorsal ventricular ridge was sharply delimited, being situated ventrolateral to the visually oriented area D of the anterior dorsal ventricular ridge (Balaban and Ulinski, '81), medial to the lateral cortex, and ventral to the pallial thickening (motor pallium of Johnston, '16). The findings are compared with related ones in mammals, particularly those pertaining to telencephalic somatosensorimotor regions and their interactions with the vestibular nuclear complex and the cerebellum.

The distribution of cholecystokinin-8 in the central nervous system of turtles: an immunohistochemical and biochemical study

Immunohistochemical techniques, radioimmunoassay (RIA) and high performance liquid chromatography (HPLC) were used to: (1) determine the regional distribution and amounts of cholecystokinin-8 (CCK8)-like immunoreactivity in the turtle central nervous system, and (2) chemically characterize the CCK8-like material present in the turtle central nervous system. High levels of CCK8-like immunoreactivity were found in the turtle central nervous system, with the highest levels being present in the hypothalamus and neurohypophysis. Moderate levels of the CCK8-like material were found in all other regions of the turtle nervous system except the cerebellum, the olfactory bulbs and the dorsal ventricular ridge of the telencephalon, which contained low levels. The bulk (87%) of the CCK8-like material in turtle central nervous system co-eluted with CCK8-sulfate in gradient elution HPLC. The distribution of CCK8-like immunoreactivity (CCK8LI) observed using immunohistochemistry was consistent with the results of the RIA studies. Numerous CCK8LI-containing neurons and fibers were observed in the hypothalamus and neurohypophysis. Neurons and fibers containing CCK8 were, however, more sparsely distributed outside the hypothalamus. The immunohistochemical data provided evidence for the existence of two major CCK8-containing pathways in turtles that have been previously described in mammals: a pathway from the supraoptic and paraventricular magnocellular nuclei to the external zone of the median eminence and neurohypophysis and a pathway from dorsal root ganglia to the dorsal horn of the spinal cord. Overall, the present results, in conjunction with several previous studies, indicate that CCK8 has had a relatively stable evolutionary history as a CNS neuropeptide among land vertebrates. The molecular structure of CCK8 appears to have been largely (if not entirely) conserved, as has its concentration in many brain regions. A noteworthy exception to such conservatism in the localization of CCK8 is that the concentration of CCK8 in the telencephalon, particularly in the telencephalic cortex, is much lower in turtles than in mammals. The present results therefore suggest that CCK8 may not have become a prominent peptide in the telencephalic cortex (or its anatomical equivalents) until the evolution of neocortex in the mammalian lineage.

Synaptic organization of dorsal area of the turtle, Pseudemys scripta elegans

The dorsal ventricular ridge is a subcortical structure receiving sensory information from the thalamus in reptiles. In the red-eared turtle, Pseudemys scripta elegans, it contains four cytoarchitectonic areas each characterized by distinct thalamic projections. This is an electron microscopic study of one of these, the dorsal area, which receives its thalamic input from the tectorecipient nucleus rotundus. It contains four concentric zones, internal to the ependymal zone, each of which is distinguished by the distribution of spiny and aspiny neurons. The ependymal zone of dorsal area contains tanycytes whose tails extend into zones 2 and 4. Synapses, usually with asymmetric junctional complexes and round synaptic vesicles, occur on these processes. Zone 1 neurons have fusiform somata and dendrites that parallel the ventricular surface. Their cytoplasm contains rough endoplasmic reticulum located primarily in Nissl bodies, lipofuchsin granules, multivesicular bodies, extensive arrays of Golgi apparatus, and large numbers of mitochondria. Synapses occur mainly on dendritic spines and shafts of zone 1 neurons and less frequently on somata. The majority have round vesicles and asymmetric junctional complexes. In contrast to those in zone 1, neurons in zones 2 and 4 have large amounts of rough endoplasmic reticulum, giving their cytoplasm an electron-dense quality. Synapses occur mainly on spines and shafts of zone 2 and 4 neurons. As in zone 1, the majority have round synaptic vesicles and contain asymmetric junctional complexes. Zones 2 and 4 contain clusters of neurons distributed among isolated neurons. The clusters are larger and less frequent in zone 2. Protoplasmic and fibrous glial processes, axon boutons, dendrites, and axon fascicles surround the neuron clusters. Though less numerous, the same structures also occur inside the clusters. Most synapses inside the clusters have round synaptic vesicles, asymmetric junctional complexes, and occur mainly on spines. Some neurons in clusters have somata whose plasma membranes are in direct apposition. In contrast to dorsal ventricular ridge in snakes, no specialized intercellular contacts were seen between somata in clusters.

Telencephalic connections in lizards. II. Projections to anterior dorsal ventricular ridge

Three distinct cytoarchitectonic regions were identified within the anterior dorsal ventricular ridge (ADVR) of two species of lizards, Gekko gecko and Iguana iguana. These regions have been named according to their general topographical positions: medial area, caudolateral area, and rostrolateral area. Injections of horseradish peroxidase throughout the ADVR demonstrated that each of the three areas of the ADVR receives projections from specific thalamic nuclei which are associated with specific sensory modalities. The medial area receives an auditory thalamic projection from nucleus medialis. The caudolateral area receives thalamic projections from nucleus medialis posterior and nucleus posterocentralis. The latter two nuclei were shown to receive projections from the spinal cord and, therefore, are presumed to be associated with body somatosensory information. The rostrolateral area receives a thalamic projection from nucleus rotundus, which receives visual information. In addition, the mesencephalic tegmentum and the thalamic nucleus dorsomedialis project to the entire ADVR. The latter projection is similar to the diffuse cortical projections of the intralaminar thalamic nuclei in mammals. These findings support previous suggestions that the ADVR is comparable to sensory regions of the mammalian neocortex.

The effects of lesions of telencephalic visual structures on visual discriminative performance in turtles (Chrysemys picta picta)

Ascending thalamotelencephalic visual pathways that terminate in specific telencephalic regions have been described in all reptiles studied. Although the anatomical data suggests that such telencephalic regions may play a role in visual processing in reptiles, few behavioral data are available. In the present study, the effects of destruction of either the core nucleus (CN) of the dorsal ventricular ridge (telencephalic terminus of the tectothalamofugal pathway) or the dorsal cortex (telencephalic terminus of the retinothalamofugal pathway) on visual discriminative performance in the turtle were examined. Following extensive bilateral destruction of the CN, turtles were severely impaired in their performance of both a simultaneous pattern discrimination and a simultaneous visual intensity discrimination. The extent of the discriminative impairment was found to be specifically correlated with the amount of CN damage. In contrast to the effects of CN lesions, lesions of the dorsal cortex had no evident effect on the performance of either a simultaneous pattern discrimination or a simultaneous visual intensity discrimination. The present results suggest that, as in birds and mammals, telencephalic visual areas play an important role in visual functions in reptiles. As in at least some birds (such as pigeons), the telencephalic terminus of the tectothalamofugal visual pathway appears to play a larger, or at least more readily measurable, role in visual discrimination than does the telencephalic terminus of the retinothalamofugal pathway.

A forebrain atlas and stereotaxic technique for the lizard, Anolis carolinensis

A forebrain atlas and stereotaxic neurosurgical techniques were developed for use in anatomical and behavioral experiments on the green anolis lizard (Anolis carolinensis). Green anoles are convenient and robust experimental subjects with a rich behavioral repertoire, the social components of which are partly under hormonal control. The technique and atlas were devised to conduct neuroethological investigations of the effect of lesions on species-typical display behavior. The atlas consists of 12 transverse sections from an average size adult male. The figures (4-15) are based on Nissl material and supplemented with fiber-stained material from adjacent sections. They appear at the end of the article. Limitations on the accuracy of stereotaxic coordinates are discussed and tables of correlative nomenclature for principal telencephalic and diencephalic nuclei are provided.

Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. III. The tectorotundal projection

Nucleus rotundus is the primary thalamic recipient of projections from the optic tectum in pond turtles. Although the projection of the retina to the optic tectum is known to be topographically organized, earlier studies suggest that the tectorotundal projection is not topographically organized. Three types of analyses are used in this paper to characterize the organization of the projection of the optic tectum to nucleus rotundus. First, large iontophoretic injections of horseradish peroxidase into the optic tectum anterogradely fill axons with reaction product after the use of a cobalt-enhanced diaminobenzidine procedure. These preparations show that shafts of axons in the tectothalamic tract give rise to thinner, primary collaterals that enter nucleus rotundus from its caudolateral aspect and form sparsely branching arbors within the nucleus. Very thin secondary collaterals branch from these collateral bear terminal collaterals with frequent varicosities. Although the total size of such arbors is unknown, the evidence suggests that each arbor is large in relation to the size of nucleus rotundus. Thus, injection sites restricted to central tectum label axons throughout nucleus rotundus. Second, subtotal lesions of the tectum produce degeneration throughout nucleus rotundus in silver degeneration preparations. Finally, analysis of electron microscopic degeneration material indicates that tectal boutons are distributed along the full lengths of the dendrites of rotundal neurons, but not on their somata. These boutons form asymmetric synaptic junctions and contain round synaptic vesicles. In view of the relatively large size of the dendritic fields of rotundal neurons, these data suggest that the tectorotundal projection is both strongly convergent on individual neurons and strongly divergent from single tectorotundal axons. This type of organization is consistent with physiological evidence that rotundal neurons have receptive fields that cover at least one-half of the contralateral visual field and often include the entire hemifield. It seems unlikely that nucleus rotundus can be involved in neuronal transactions that preserve detailed spatial information, but it may be involved in processing information on other visual parameters such as stimulus velocity or color.

Visual properties of cells in anterior dorsal ventricular ridge of turtle

Single units in the anterior dorsal ventricular ridge (ADVR), a structure in the major afferent visual pathway of turtle, were investigated electrophysiologically for response properties to varied light patterns. The majority of cells responded to a broad range of spatially, temporally, and chromatically varied stimuli over most of the monocular visual field. One category of cells, seemingly specialized for 'novelty' detection, indicates one possible role for ADVR in visual sensory processing.

Organization of thalamic afferents to anterior dorsal ventricular ridge in turtles. II. Properties of the rotundo-dorsal map

This study describes some properties of the map of nucleus rotundus onto dorsal area of anterior dorsal ventricular ridge (ADVR) in emydid turtles by correlating results of anterograde and retrograde tracing experiments with observations from Golgi- and myelin-stained brains. An earlier paper (Balaban and Ulinski, '81) demonstrated that this projections is restricted to zone 4 of dorsal area of ADVR. This paper indicates that the rotundal pathway is organized such that longitudinally aligned groups of neurons in nucleus rotundus project to longitudinal regions in zone 4 of dorsal area. The projections field spans the dorso-ventral (or concentric) dimensions of zone 4 at each transverse level. Comparisons of experimental and Golgi preparations suggest that each rotundal neuron projects, via collaterals, to the entire rostrocaudal extent of rotundorecipient zone 4. Individual terminal branches span the dorsoventral dimension of zone 4 and are confined with both sagittal and transverse planes. Lesion experiments suggest that collaterals of a single rotundal axon are also distributed over at least one-third to one-half of the superficial-deep dimension of zone 4. This is also reflected in the observation that neurons from disjoint dorsal, dorsolateral and medial rotundal loci project to overlapping, concentric regions of dorsal area. Both this prominent concentric component of terminal branches and the extensive overlap is projections of neurons at distinct rotundal loci preclude the possibility of a topographic representation of either dorsoventral or mediolateral rotundal axes in zone 4 of dorsal area.

Organization of thalamic afferents to anterior dorsal ventricular ridge in turtles. I. Projections of thalamic nuclei

Dorsal ventricular ridge (DVR) is a thalamorecipient, subcortical telencephalic structure in reptiles and birds. Although there is a fair amount of information about sources of afferents to DVR, little is known about the relationship of projections from individual thalamic nuclei to the organization of the structure. This study examines the relationship between thalamic projections and both areal and zonal divisions of anterior DVR (ADVR; Balaban, '78a) of emydid turtles with orthograde degeneration, autoradiographic and horseradish peroxidase techniques. Individual thalamic nuclei contribute either a diffuse or a restricted projection to ADVR. Diffuse projections arise primarily from the dorsomedial anterior nucleus. These fine-caliber axons distribute bilaterally over a wide region of the telencephalon via both medial and lateral thalamotelencephalic pathways. The terminal regions include septum, striatum and the medial bank of cortex caudal to the lamina terminalis. In ADVR, the fibers are distributed sparsely in zones 2-4 of dorsal, medial and ventral areas. Restricted projections to ADVR originate in nucleus rotundus, nucleus reuniens and nucleus caudalis. They ascend ipsilaterally in the lateral thalamotelencephalic pathway (lateral forebrain bundle), and enter ADVR rostral to the anterior commissure. Nucleus rotundus projects to zone 4 of dorsal area, nucleus caudalis projects to zones 2-4 of dorsal division of medial area, and nucleus reuniens projects to zones 2-4 of both the ventral division of medial area and the ventral area. Comparison of these results with thalamotelencephalic projections in mammals suggests that diffuse and restricted thalamic projection systems are a common feature of both groups. Restricted thalamic projections in reptiles, birds and mammals terminating in anatomically distinct regions, also appear to be associated with different sensory modalities. The significance of diffuse systems is not clear.

Interactions between tectal radial cells in the red-eared turtle, Pseudemys scripta elegans: an analysis of tectal modules

The optic tectum is a major subdivision of the visual system in reptiles. Previous studies have characterized the laminar pattern, the neuronal populations, and the afferent and efferent connections of the optic tectum in a variety of reptiles. However, little is known about the interactions that occur between neurons within the tectum. This study describes two kinds of interactions that occur between one major class of neurons, the radial cells, in the optic tectum of Pseudemys using Nissl, Golgi and electron microscopic preparations. Radial cells have somata which bear long, radially oriented apical dendrites from their upper poles and short, basal dendrites from their lower poles. They are divided into two populations on the basis of the distribution of their somata in the tectum. Deep radial cells have somata densely packed in the stratum griseum periventriculare. Their plasma membranes form casual appositions. Middle radial cells have somata scattered throughout the stratum griseum centrale and stratum fibrosum et griseum superficiale and do not contact each other. The apical dendrites of both populations of radial cells participate in vertically oriented, dendritic bundles. The plasma membranes of the dendrites in these bundles form casual appositions in the deeper tectal layers and chemical, dendrodenritic synapses within the stratum fibrosum et griseum superficiale. The synapses have clear, round synaptic vesicles and slightly asymmetric membrane densities. Thus, radial cells interact via both casual appositions and chemical synapses. These interactions suggest that radial cells may form a basic framework in the tectum. Because both populations of radial cells extend into the stratum fibrosum et griseum superficiale and stratum opticum, they may receive input from some of the same tectal afferent systems. Because the deep radial cells alone have somata and dendrites in the deep tectal layers, they may receive additional inputs that the middle radial cells do not. Neurons in the two populations interact via chemical dendrodentritic synapses, thereby forming vertically oriented modules in the tectum.

Quantitative studies of retinal ganglion cells in a turtle, Pseudemys scripta elegans. I. Number and distribution of ganglion cells

Multiple pathways for the transmission of visual information from retina to brain have been described in reptiles, but little is known about their functional organization. These parallel channels begin at the retina, and we have therefore begun to study the functional organization of retinal ganglion cells in the turtle, Pseudemys scripta elegans. This paper describes the numbers and distribution of cells in the ganglion cell layer. To develop criteria for the identification of ganglion cells, we labelled them retrogradely by applying horseradish peroxidase (HRP) to the optic nerve. Ganglion cells were found to vary substantially in size and cytology. In low density areas of the retina, ganglion cells typically have cytoplasm with well developed Nissl substance, a distinct, pale nucleus, and a large nucleolus. In high density areas of retina, ganglion cells are small, densely staining, and gliaform. The average minimum proportion of ganglion cells in the ganglion cell layer is 75--80% of total profiles. No more than five or six percent of profiles in the ganglion cell layer are neurons which do not send an axon into the optic nerve (displaced amacrine cells or intraretinal association cells). The ganglion cell layer of P. s. elegans can be divided into a number of regions on the basis of cell density. Isodensity maps constructed from Nissl-stained, wholemounted retinas indicate that there is an elongated region of high ganglion cell density, the visual streak, which extends from nasal to temporal retina and is oriented such that its long axis follows the horizontal axis of the eye. The streak is aligned with the externally visible iris line. Seen in cross-section, the ganglion cell layer in the streak is three to four cells thick; in nonstreak retina, ganglion cells form only a monolayer of somas. Ganglion cell density drops off more rapidly above the streak than below it. The temporal arm of the streak is both shorter and broader than the nasal arm. There is a peak in ganglion cell density at the midpoint of the streak, in the approximate center of the retina. Here, ganglion cell densities exceed 20,000 cells mm-2. The total number of ganglion cells in the retina is 350,000--390,000.