Innervation from the claustrum of the frontal association and motor areas: axonal transport studies in the cat

Regional and cell-type-specific afferent and efferent projections of the mouse claustrum

The claustrum (CLA) is a conspicuous subcortical structure interconnected with cortical and subcortical regions. Its regional anatomy and cell-type-specific connections in the mouse remain not fully determined. Using multimodal reference datasets, we confirmed the delineation of the mouse CLA as a single group of neurons embedded in the agranular insular cortex. We quantitatively investigated brain-wide inputs and outputs of CLA using bulk anterograde and retrograde viral tracing data and single neuron tracing data. We found that the prefrontal module has more cell types projecting to the CLA than other cortical modules, with layer 5 IT neurons predominating. We found nine morphological types of CLA principal neurons that topographically innervate functionally linked cortical targets, preferentially the midline cortical areas, secondary motor area, and entorhinal area. Together, this study provides a detailed wiring diagram of the cell-type-specific connections of the mouse CLA, laying a foundation for studying its functions at the cellular level.

Neural activity in the mouse claustrum in a cross-modal sensory selection task

The claustrum, a subcortical nucleus forming extensive connections with the neocortex, has been implicated in sensory selection. Sensory-evoked claustrum activity is thought to modulate the neocortex's context-dependent response to sensory input. Recording from claustrum neurons while mice performed a tactile-visual sensory-selection task, we found that neurons in the anterior claustrum, including putative optotagged claustrocortical neurons projecting to the primary somatosensory cortex (S1), were rarely modulated by sensory input. Rather, they exhibited different types of direction-tuned motor responses. Furthermore, we found that claustrum neurons encoded upcoming movement during intertrial intervals and that pairs of claustrum neurons exhibiting synchronous firing were enriched for pairs preferring contralateral lick directions, suggesting that the activity of specific ensembles of similarly tuned claustrum neurons may modulate cortical activity. Chemogenetic inhibition of claustrocortical neurons decreased lick responses to inappropriate sensory stimuli. Altogether, our data indicate that the claustrum is integrated into higher-order premotor circuits recently implicated in decision-making.

Quantitative 3D Ultrastructure of Thalamocortical Synapses from the "Lemniscal" Ventral Posteromedial Nucleus in Mouse Barrel Cortex

Thalamocortical synapses from “lemniscal” neurons of the dorsomedial portion of the rodent ventral posteromedial nucleus (VPMdm) are able to induce with remarkable efficacy, despite their relative low numbers, the firing of primary somatosensory cortex (S1) layer 4 (L4) neurons. To which extent this high efficacy depends on structural synaptic features remains unclear. Using both serial transmission (TEM) and focused ion beam milling scanning electron microscopy (FIB/SEM), we 3D-reconstructed and quantitatively analyzed anterogradely labeled VPMdm axons in L4 of adult mouse S1. All VPMdm synapses are asymmetric. Virtually all are established by axonal boutons, 53% of which contact multiple (2–4) elements (overall synapse/bouton ratio = 1.6). Most boutons are large (mean 0.47 μm³), and contain 1–3 mitochondria. Vesicle pools and postsynaptic density (PSD) surface areas are large compared to others in rodent cortex. Most PSDs are complex. Most synapses (83%) are established on dendritic spine heads. Furthermore, 15% of the postsynaptic spines receive a second, symmetric synapse. In addition, 13% of the spine heads have a large protrusion inserted into a membrane pouch of the VPMdm bouton. The unusual combination of structural features in VPMdm synapses is likely to contribute significantly to the high efficacy, strength, and plasticity of these thalamocortical synapses.

Organization of the connections between claustrum and cortex in the mouse

The connections between the claustrum and the cortex in mouse are systematically investigated with adeno-associated virus (AAV), an anterograde viral tracer. We first define the boundary and the three-dimensional structure of the claustrum based on a variety of molecular and anatomical data. From AAV injections into 42 neocortical and allocortical areas, we conclude that most cortical areas send bilateral projections to the claustrum, the majority being denser on the ipsilateral side. This includes prelimbic, infralimbic, medial, ventrolateral and lateral orbital, ventral retrosplenial, dorsal and posterior agranular insular, visceral, temporal association, dorsal and ventral auditory, ectorhinal, perirhinal, lateral entorhinal, and anteromedial, posteromedial, lateroposterior, laterointermediate, and postrhinal visual areas. In contrast, the cingulate and the secondary motor areas send denser projections to the contralateral claustrum than to the ipsilateral one. The gustatory, primary auditory, primary visual, rostrolateral visual, and medial entorhinal cortices send projections only to the ipsilateral claustrum. Primary motor, primary somatosensory and subicular areas barely send projections to either ipsi- or contralateral claustrum. Corticoclaustral projections are organized in a rough topographic manner, with variable projection strengths. We find that the claustrum, in turn, sends widespread projections preferentially to ipsilateral cortical areas with different projection strengths and laminar distribution patterns and to certain contralateral cortical areas. Our quantitative results show that the claustrum has strong reciprocal and bilateral connections with prefrontal and cingulate areas as well as strong reciprocal connections with the ipsilateral temporal and retrohippocampal areas, suggesting that it may play a crucial role in a variety of cognitive processes. J. Comp. Neurol. 525:1317-1346, 2017. © 2016 Wiley Periodicals, Inc.

Psychedelics

Psychedelics (serotonergic hallucinogens) are powerful psychoactive substances that alter perception and mood and affect numerous cognitive processes. They are generally considered physiologically safe and do not lead to dependence or addiction. Their origin predates written history, and they were employed by early cultures in many sociocultural and ritual contexts. After the virtually contemporaneous discovery of (5R,8R)-(+)-lysergic acid-N,N-diethylamide (LSD)-25 and the identification of serotonin in the brain, early research focused intensively on the possibility that LSD and other psychedelics had a serotonergic basis for their action. Today there is a consensus that psychedelics are agonists or partial agonists at brain serotonin 5-hydroxytryptamine 2A receptors, with particular importance on those expressed on apical dendrites of neocortical pyramidal cells in layer V. Several useful rodent models have been developed over the years to help unravel the neurochemical correlates of serotonin 5-hydroxytryptamine 2A receptor activation in the brain, and a variety of imaging techniques have been employed to identify key brain areas that are directly affected by psychedelics. Recent and exciting developments in the field have occurred in clinical research, where several double-blind placebo-controlled phase 2 studies of psilocybin-assisted psychotherapy in patients with cancer-related psychosocial distress have demonstrated unprecedented positive relief of anxiety and depression. Two small pilot studies of psilocybin-assisted psychotherapy also have shown positive benefit in treating both alcohol and nicotine addiction. Recently, blood oxygen level-dependent functional magnetic resonance imaging and magnetoencephalography have been employed for in vivo brain imaging in humans after administration of a psychedelic, and results indicate that intravenously administered psilocybin and LSD produce decreases in oscillatory power in areas of the brain's default mode network.

Comparative organization of the claustrum: what does structure tell us about function?

The claustrum is a subcortical nucleus present in all placental mammals. Many anatomical studies have shown that its inputs are predominantly from the cerebral cortex and its outputs are back to the cortex. This connectivity thus suggests that the claustrum serves to amplify or facilitate information processing in the cerebral cortex. The size and the complexity of the cerebral cortex varies dramatically across species. Some species have lissencephalic brains, with few cortical areas, while others have a greatly expanded cortex and many cortical areas. This evolutionary diversity in the cerebral cortex raises several questions about the claustrum. Does its volume expand in coordination with the expansion of cortex and does it acquire new functions related to the new cortical functions? Here we survey the organization of the claustrum in animals with large brains, including great apes and cetaceans. Our data suggest that the claustrum is not always a continuous structure. In monkeys and gorillas there are a few isolated islands of cells near the main body of the nucleus. In cetaceans, however, there are many isolated cell islands. These data suggest constraints on the possible function of the claustrum. Some authors propose that the claustrum has a more global role in perception or consciousness that requires intraclaustral integration of information. These theories postulate mechanisms like gap junctions between claustral cells or a “syncytium” to mediate intraclaustral processing. The presence of discontinuities in the structure of the claustrum, present but minimal in some primates, but dramatically clear in cetaceans, argues against the proposed mechanisms of intraclaustral processing of information. The best interpretation of function, then, is that each functional subdivision of the claustrum simply contributes to the function of its cortical partner.

Topographical distribution and morphology of NADPH-diaphorase-stained neurons in the human claustrum

We studied the topographical distribution and morphological characteristics of NADPH-diaphorase-positive neurons and fibers in the human claustrum. These neurons were seen to be heterogeneously distributed throughout the claustrum. Taking into account the size and shape of stained perikarya as well as dendritic and axonal characteristics, Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPHd)-positive neurons were categorized by diameter into three types: large, medium and small. Large neurons ranged from 25 to 35 μm in diameter and typically displayed elliptical or multipolar cell bodies. Medium neurons ranged from 20 to 25 μm in diameter and displayed multipolar, bipolar and irregular cell bodies. Small neurons ranged from 14 to 20 μm in diameter and most often displayed oval or elliptical cell bodies. Based on dendritic characteristics, these neurons were divided into spiny and aspiny subtypes. Our findings reveal two populations of NADPHd-positive neurons in the human claustrum-one comprised of large and medium cells consistent with a projection neuron phenotype, the other represented by small cells resembling the interneuron phenotype as defined by previous Golgi impregnation studies.

A role of the claustrum in auditory scene analysis by reflecting sensory change

The biological function of the claustrum remains speculative, despite many years of research. On the basis of its widespread connections it is often hypothesized that the claustrum may have an integrative function mainly reflecting objects rather than the details of sensory stimuli. Given the absence of a clear demonstration of any sensory integration in claustral neurons, however, we propose an alternative, data-driven, hypothesis: namely that the claustrum detects the occurrence of novel or salient sensory events. The detection of new events is critical for behavior and survival, as suddenly appearing objects may require rapid and coordinated reactions. Sounds are of particular relevance in this regard, and our conclusions are based on the analysis of neurons in the auditory zone of the primate claustrum. Specifically, we studied the responses to natural sounds, their preference to various sound categories, and to changes in the auditory scene. In a test for sound-category preference claustral neurons responded to but displayed a clear lack of selectivity between monkey vocalizations, other animal vocalizations or environmental sounds (Esnd). Claustral neurons were however able to detect target sounds embedded in a noisy background and their responses scaled with target signal to noise ratio (SNR). The single trial responses of individual neurons suggest that these neurons detected and reflected the occurrence of a change in the auditory scene. Given its widespread connectivity with sensory, motor and limbic structures the claustrum could play the essential role of identifying the occurrence of important sensory changes and notifying other brain areas-hence contributing to sensory awareness.

Quantitative mapping of the local and extrinsic sources of GABA and Reelin to the layer Ia neuropil in the adult rat neocortex

Inputs to apical dendritic tufts have been considered to be crucial for associative learning, attention and similar ''feedback'' interactions and are located in neocortical layer Ia. Excitatory thalamic projections to apical tufts in layer Ia have been well characterized and their role in the cortical circuit has been emphasized. In addition, the neuropil and the extracellular matrix surrounding apical tufts are highly reactive to GABA and to the glycoprotein Reelin, respectively. Recently it has been shown that the GABA inhibition on apical dendrites can reduce the output of pyramidal cells in layer V, however, the origin of 89% of the symmetric synapses in layer I still remains unknown. In the present study we have systematically analyzed the origin of the GABAergic neuropil in neocortical layer Ia in a qualitative and quantitative manner, and investigated the possible extrinsic origin of the rich extracellular Reelin content of the same layer. We show that the inhibitory inputs in a given spot in layer I come from cortical projections and arise mainly from Martinotti cells located directly under that same spot. Double bouquet and bipolar cells may also project to layer Ia although to a lesser extent and the external globus pallidus and zona incerta provide the remaining inhibitory inputs. Finally, our results suggest that Martinotti cells are also the main source of Reelin in layer Ia. The present data will help in the understanding of the cortical circuit and why it changes in pathological conditions.

Neurochemically defined cell types in the claustrum of the cat

The claustrum is a subcortical structure reciprocally and topographically connected with all sensory and motor domains of the cerebral cortex. Previous anatomical and electrophysiological data suggested that most cells in the claustrum are large neurons that both receive cortical input and project back to cortex, forming excitatory connections with their cortical targets. These data have been interpreted to imply a relay function for the claustrum, with information from different functional cortical domains remaining segregated. The possibility that the claustrum might mediate a more "global" function has been recently been developed by Crick and Koch [Crick, F. C., Koch, C., 2005. What is the function of the claustrum? Philos. Trans. R. Soc. Lond., B Biol. Sci. 360, 1271-1279]. We have reexamined the anatomical substrate for information processing in the claustrum of the cat by analyzing the patterns of immunoreactivity to calcium-binding proteins, GAD, serotonin, nNOS and the glutamate transporter EAAC1. We found multiple neurochemically defined cell types, suggesting multiple classes of projection neurons and interneurons. Each class was found throughout the entire claustrum, in all functionally defined subdivisions. Many neurons in the claustrum were surrounded by parvalbumin, calretinin, GAD or nNOS immunoreactive terminals, suggesting that many neurons of the claustrum make extensive intraclaustral connections. The entire claustrum also receives a serotonergic input. The identification of multiple neurochemical cell classes, their distribution and the extent of their dendritic arborizations relative to functional compartments suggest a substrate for information processing in the claustrum that may allow integration of information across functional subdivisions.

Immunohistochemical localization of the vesicular glutamate transporter VGLUT2 in the developing and adult mouse claustrum

We studied the immunoreactive expression pattern for the vesicular glutamate transporter VGLUT2 in the embryonic, postnatal and adult mouse dorsal claustrum, at the light and electron microscopic levels. VGLUT2 immunoreactivity in the dorsal claustrum starts to be observed at E16.5, with a dramatic increase towards P0. At this age, abundant VGLUT2-immunoreactive axons and puncta are observed in all pallial regions, including the claustral complex. From the first postnatal week, VGLUT2 immunoreactivity declines in several telencephalic areas, including the pallium, but abundant VGLUT2-immunoreactive fine axons and puncta remain in the claustrum. Beginning at E18.5, VGLUT2 immunoreactivity within the claustrum shows a characteristic arrangement: a central part of the region is practically devoid of VGLUT2 immunoreactivity, and it is surrounded by plenty of immunoreactive axon terminals forming a shell around it. This core/shell arrangement of the VGLUT2 immunoreactivity resembles the complementary expression of parvalbumin and calretinin described in the mouse claustrum [Real, M.A., Dávila, J.C., Guirado, S., 2003. Expression of calcium-binding proteins in the mouse claustrum. J. Chem. Neuroanat. 25, 151-160]. We observed immunoreactive neuronal cell bodies as well in the dorsal claustrum, but only at P0. Electron microscopic analysis reveals that VGLUT2 immunoreactivity in the developing and adult dorsal claustrum consists predominantly of presynaptic boutons making asymmetric synaptic contacts. These VGLUT2-immunoreactive boutons are observed as early as E16.5 and may be related to thalamo-claustral incoming fibers.

Embryonic and postnatal development of GABA, calbindin, calretinin, and parvalbumin in the mouse claustral complex

We analyzed the development of immunoreactive expression patterns for the neurotransmitter gamma-aminobutyric acid (GABA) and the calcium-binding proteins calbindin, calretinin, and parvalbumin in the embryonic and postnatal mouse claustral complex. Each calcium-binding protein shows a different temporal and spatial pattern of development. Calbindin-positive cells start to be seen very early during embryogenesis and increase dramatically until birth, thus becoming the most abundant cell type during embryonic development, especially in the ventral pallial part of the claustrum. The distribution of calbindin neurons throughout the claustrum during embryonic development partly parallels that of GABA neurons, suggesting that at least part of the calbindin neurons of the claustral complex are GABAergic and originate in the subpallium. Parvalbumin cells, on the other hand, start to be seen only postnatally, and their number then increases while the density of calbindin neurons decreases. Based on calretinin expression in axons, the core/shell compartments of the dorsal claustrum start to be clearly seen at embryonic day 18.5 and may be related to the development of the thalamoclaustral input. Comparison with the expression of Cadherin 8, a marker of the developing dorsolateral claustrum, indicates that the core includes a central part of the dorsolateral claustrum, whereas the shell includes a peripheral area of the dorsolateral claustrum, plus the adjacent ventromedial claustrum. The present data on the spatiotemporal developmental patterns of several subtypes of GABAergic neurons in the claustral complex may help for future studies on temporal lobe epilepsies, which have been related to an alteration of the GABAergic activity.

Expression of Dbx1, Neurogenin 2, Semaphorin 5A, Cadherin 8, and Emx1 distinguish ventral and lateral pallial histogenetic divisions in the developing mouse claustroamygdaloid complex

We studied the lateral and ventral pallial divisions of the claustroamygdaloid complex by means of analysis of expression patterns of the developmental regulatory genes Tbr1, Dbx1, Neurogenin 2, Emx1, Cadherin 8, and Semaphorin 5A in mouse developing telencephalon, from embryonic day 12.5 until birth. Our results indicate that these genes help to distinguish distinct lateral and ventral pallial histogenetic divisions in the embryonic telencephalon. Tbr1 is broadly expressed in both lateral and ventral pallial histogenetic divisions (the lateroventral migratory stream plus the mantle) during early and intermediate embryonic development; its signal becomes weak in parts of the mantle during late embryonic development. Dbx1 is strongly and specifically expressed in progenitor cells (ventricular zone) of the ventral pallium during early embryonic development, but there is no signal of this gene in the rest of the pallium nor the subpallium. Neurogenin 2 and Semaphorin 5A are both expressed in a ventral subdivision of the lateroventral migratory stream (called by us the ventral migratory stream). Further, specific nuclei of the claustral complex and pallial amygdala show strong expression of Neurogenin 2 and/or Semaphorin 5A, including the ventromedial claustrum and endopiriform nuclei, the lateral and basomedial amygdalar nuclei, the anterior and posteromedial cortical amygdalar areas, plus the amygdalo-hippocampal area. We interpret these nuclei or areas of the claustroamygdaloid complex as possible derivatives of the ventral pallium. In contrast, during embryonic development the dorsolateral claustrum, the basolateral amygdalar nucleus, and the posterolateral cortical amygdalar area do not express or show weak expression of Neurogenin 2 or Semaphorin 5A, but express selectively and strongly Cadherin 8 plus Emx1, and may be derivatives of the lateral pallium. The lateral pallial and ventral pallial divisions of the claustroamygdaloid complex appear to have some different sets of connections, although this requires further investigation.

Further evidence for a role of the anterior claustrum in epileptogenesis

The anatomy of the claustrum (CLA) has been well characterized, but its functional role remains uncertain. The results of recent research suggest that the CLA may be part of a network of structures involved in seizure generalization, and we set out to test this idea. To test persistence, seizures were kindled in the anterior CLA. Following a 14-day suspension of kindling, all rats required only one stimulation to evoke a stage 5 seizure. In another experiment, groups of rats received bilateral lesions of the anterior CLA before and after amygdaloid kindling. We found that small lesions of the anterior CLA retard amygdaloid kindling, but do not block the expression of generalized seizures. Lesions produced after amygdaloid kindling resulted in a shorter seizure duration, but had no marked effect on seizure expression. Another group of rats was tested for transfer of kindling between the anterior CLA and contralateral amygdala. We found an asymmetrical transfer of kindling to the CLA from the amygdala wherein amygdaloid kindling facilitated subsequent kindling of the CLA but kindling of the anterior CLA failed to facilitate kindling of the amygdala. The results add support to the notion that the CLA contributes to the development of generalized limbic seizures.

The corticostriatal junction: a crucial region for forebrain development and evolution

Most parts of the brain are conserved across reptiles and birds (sauropsids) and mammals. Two major qualitative differences occur in the upper part, or pallium, of the telencephalon, the most rostral part of the brain. Mammals have a six-layered neocortex and also exhibit a different morphological organization in the lateral half, or sector, of their pallium than do sauropsids. These differences of lateral pallial construction may derive from small but crucial differences in migration patterns of neuronal precursors generated at or above the corner of the lateral ventricle, the corticostriatal junction (CS). Sauropsids have a large structure, the dorsal ventricular ridge, that is proliferated from this region, and its anterior part (ADVR) receives ascending projections from the dorsal thalamus. Mammals have multiple structures in this same region-the lateral part of neocortex, amygdala, and claustrum-endopiriform formation. We propose here that, as the degree of development of structures that form the deeper tier of the pallium varies across the stages of embryology and across phylogeny, mutations may have occurred during evolution at the origin of mammals that had profound consequences for the fate of neural populations generated in the region of the CS and its neighboring pallial germinal zone.

Differentiation in the immunocytochemical features of intrinsic and cortically projecting neurons in the rat claustrum -- combined immunocytochemical and axonal transport study

Retrograde axonal transport method of the fluorescent tracer FluoroGold (FG) was combined with immunocytochemistry to investigate the occurrence of nitric oxide synthase (NOS), somatostatin (SOM), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) in both intrinsic and cortically projecting neurons of the rat claustrum. Only NOS was detected in both the scattered projecting neurons and internal neurons of the claustrum. Approximately 20% of NOS-immunoreactive neurons in the claustrum were also retrogradely labeled with FG after tracer injections into the frontal cortex. The other substances were exclusively confined to the population of interneurons, which mainly displayed an oval, round or fusiform shape and a medium size. Apart from the neuronal somata, the proximal parts of the dendritic arborization were clearly visible. The immunoreactive neurons were randomly distributed in the claustrum and their neuronal size and shape did not differ in the various parts of the studied structure. Co-localization of NOS and SOM or NOS and NPY was reported. In conclusion, SOM, VIP and NPY do not appear to play a significant role in the claustro-cortical projection but are most probably involved in modulation and information transfer in the claustrum. The appearance of NOS in both cortically projecting and intrinsic neurons of the claustrum may be indicative of a fundamentally different role in the functioning of the claustro-cortical loop.

Susceptibility to kindling and neuronal connections of the anterior claustrum

The claustrum has been implicated in the kindling of generalized seizures from limbic sites. We examined the susceptibility of the anterior claustrum itself to kindling and correlated this with an anatomical investigation of its afferent and efferent connections. Electrical stimulation of the anterior claustrum resulted in a pattern of rapid kindling with two distinct phases. Early kindling involved extremely rapid progression to bilaterally generalized seizures of short duration. With repeated daily kindling stimulations, early-phase generalized seizures abruptly became more elaborate and prolonged, resembling limbic-type seizures as triggered from the amygdala. We suggest that the rapid rate of kindling from the anterior claustrum is an indication that the claustrum is functionally close to the mechanisms of seizure generalization. In support of our hypothesis, we found significant afferent, efferent, and often reciprocal connections between the anterior claustrum and areas that have been implicated in the generation of generalized seizures, including frontal and motor cortex, limbic cortex, amygdala, and endopiriform nucleus. Additional connections were found with various other structures, including olfactory areas, nucleus accumbens, midline thalamus, and brainstem nuclei including the substantia nigra and the dorsal raphe nucleus. The anatomical connections of the anterior claustrum are consistent with its very high susceptibility to kindling and support the view that the claustrum is part of a forebrain network of structures participating in the generalization of seizures.

Kindling of claustrum and insular cortex: comparison to perirhinal cortex in the rat

The perirhinal cortex has recently been implicated in the kindling of limbic generalized seizures. The following experiments in rats tested the selectivity of the perirhinal cortex's epileptogenic properties by comparing its kindling profile with those of the adjacent insular cortex, posterior (dorsolateral) claustrum and amygdala. The first experiment examined the kindling and EEG profiles, and found that both the claustrum and insular cortex demonstrated rapid epileptogenic properties similar to the perirhinal cortex, including very rapid kindling rates and short latencies to convulsion. Furthermore, electrical stimulation of all three structures led to a two-phase progression through stage-5 seizures which had characteristics of both neocortical and amygdaloid kindling. In a second experiment rats were suspended in a harness to allow for more detailed documentation of both forelimb and hindlimb convulsions. With this procedure we were able to detect subtle yet unique differences in convulsion characteristics from each of the kindled sites and stage-5 seizure phases. Some of these convulsive parameters were correlated with changes in FosB/DeltaFosB protein and BDNF mRNA expression measured two hours after the last convulsion. Overall, it appears that the perirhinal cortex is not unique in its property of rapid epileptogenesis. Moreover, the posterior claustrum exhibited the fastest kindling and most vigorous patterns of clonus, suggesting that it may be even more intimately associated with the motor substrates responsible for limbic seizure generalization than is the perirhinal cortex.

Connections of the auditory cortex with the claustrum and the endopiriform nucleus in the cat

We studied the connections of eleven auditory cortical areas with the claustrum and the endopiriform nucleus in the cat, by means of cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase, or biotinylated dextran amines. Unlike previously accepted reports, all auditory areas have reciprocal connections with the ipsi- and contralateral claustrum, though they differ in strength and/or topography. The areas that send the strongest projections are the intermediate region of the posterior ectosylvian gyrus and the insular cortex, followed by the primary auditory cortex and the dorsal portion of the posterior ectosylvian gyrus. The high degree of convergence of cortical axons in the intermediate region of the claustrum, arising from tonotopic and nontonotopic areas, suggests that claustral neurons are unlikely to be well tuned to the frequency of the acoustic stimulus. Corticoclaustral axons from any given area cover territories largely overlapping with those occupied by the claustrocortical neurons projecting back to the same area. The location of cortically projecting neurons in the claustrum matches the position of the target cortical area in the cerebral hemisphere, both rostrocaudally and dorsoventrally. These findings suggest that the intermediate region of the claustrum integrates inputs from all auditory cortical areas, and then sends the result of such processing back to every auditory cortical field. On the other hand, the endopiriform nucleus, a limbic-related structure thought to play a role in the acquisition of conditioned fear, would process mostly polymodal information, since it only receives projections from the insular and temporal cortices.

Specific amygdaloid nuclei are involved in suppression or propagation of epileptiform activity during transition stage between oral automatisms and generalized clonic seizures

Kindling is a model of the neural plasticity that occurs following stimulation to the brain, which can result in epileptogenesis. The amygdala (Am), one of the most sensitive structures from which to induce electrical kindling, is comprised of distinct nuclei that possess differences in threshold for seizure initiation, unique cellular and molecular morphology, and specific neuroanatomical connections within the amygdala and, to other cortical and subcortical brain structures. The aim of this study was to map the spread of epileptiform activity throughout the ipsilateral and contralateral hemispheres during the transition stage between oral automatisms and generalized clonic seizures, by measuring changes in mRNA expression for c-fos, NGFI-A, and BDNF. The stimulating electrode was implanted in either the basolateral (BL) or the lateral (CeL) or medial (CeM) subdivisions of the central nucleus of the amygdala. The rats were kindled once daily using afterdischarge-threshold electrical stimulation until the first forelimb clonic seizure was induced. They were sacrificed 30 min later, and their brains were prepared for in situ hybridization to measure mRNA expression of c-fos, NGFI-A and BDNF. The results demonstrate that: (1) the threshold to elicit an afterdischarge from the BL was lower than that of either the medial (CeM) or lateral (CeL) subdivisions of the Ce, which did not differ from each other; (2) the patterns of mRNA expression for c-fos, NGFI-A and BDNF were highly similar to each other when the stimulation site was the BL or the CeL, and included mainly limbic cortical and subcortical areas ipsilateral to the electrode; (3) c-fos was the only probe to be expressed in the contralateral hemisphere following the first motor seizure, and the pattern of its expression reflected a subset of structures recruited in the ipsilateral hemisphere including the claustrum, insular and perirhinal cortices; (4) unexpectedly, stimulation of the CeM elicited seizures and afterdischarges of shorter duration than those evoked by stimulation of the BL or CeL, and failed to increase mRNA expression for any of the probes in the hippocampus or in the contralateral hemisphere. A neuroanatomical model of Am-induced seizure propagation is proposed suggesting that the Claust-Ins-PRh play a pivotal role during the transition between oral automatisms and generalized clonic convulsions.

Percentage incidence of gamma-aminobutyric acid neurons in the claustrum of the rabbit and comparison with the cortex and putamen

We describe the incidence of gamma-aminobutyric acid (GABA)ergic neurons after post-embedding immunocytochemistry on semithin sections of the claustrum, putamen and lateral, dorsal and medial cortical areas. Twelve percent of the neurons counted in the claustrum of 11 rabbits were GABAergic. This incidence was significantly higher in the dorsal halves of both the insular and endopiriform claustra than in the ventral (13 vs. 10%). The incidence of GABAergic cells was 4% in the putamen, 14% in the insular cortex, 15% in areas 17 and 18 and 13% in area 29d. Thus, our results indicate that in contrast to the putamen the incidence of GABAergic cells was similar in the claustrum and cortical areas. We interpret this in the light of the pallial origin of the claustrum, which has recently been substantiated.

The combined retrograde transport and unbiased stereological study of the claustrocortical connections in the rabbit

The quantitative analysis of the claustrocortical connections in the rabbit, labeled with the fluorescent retrograde tracer Fluoro-Gold (FG), was conducted by means of unbiased stereology. The FG was injected into selected regions of the motor, somatosensory, auditory and visual cortices and then a comparison of the various claustrocortical projections was carried out. This was achieved by comparing (1) the numerical densities of projecting neurones for each claustral projection zone and (2) the distribution of the labeled neurones throughout the rostro-caudal extent of the claustrum. No significant differences between the numerical densities of labeled neurones in the various projection zones are reported. The motor and primary somatosensory projections dominated in the anterior and central parts of the claustrum, whereas the secondary somatosensory, auditory and visual projections--in the posterior part. The difference in the distributions was significant (p < 0.001). Summarizing, the cortical projections in the claustrum, although varying topographically, do not reveal a quantitative differentiation. This may speak in favour of the integrative and modulating function of this structure in relationship to the neocortex.

The cortico-related zones of the rabbit claustrum-study of the claustrocortical connections based on the retrograde axonal transport of fluorescent tracers

The claustrocortical connections in the rabbit were assessed for the first time by the method of axonal retrograde transport of two fluorescent tracers (Fast Blue and Diamidino Yellow). The material consisted of 23 adult New Zealand rabbits. Projection zones of spindle-like form, connected with the precentral, postcentral, temporal and occipital cortices have been delineated. They are organized topographically both in the anteroposterior and ventrodorsal direction. The precentral (motor) projection zone is localized in the anterodorsal part of the claustrum. It may be divided into two separate parts that project to the medial and lateral part of the precentral cortex. The large postcentral (somatosensory) zone occupies mainly the central part, whereas the temporal (auditory) and occipital (visual) zones are situated in the posteroventral part of the claustrum. The overlap of various claustral projection zones is differentiated, the largest being that of the somatosensory zones. In comparison to the results of study of claustral projection zones performed on other species, presumably on the rat and cat, its seems plausible to conclude that the extension of claustral projection zones and degree of their overlap in the rabbit represent an intermediate character.

Cross-modal transfer of information between the tactile and the visual representations in the human brain: A positron emission tomographic study

Positron emission tomography in three-dimensional acquisition mode was used to identify the neural populations involved in tactile-visual cross-modal transfer of shape. Eight young male volunteers went through three runs of three different matching conditions: tactile-tactile (TT), tactile-visual (TV), and visual-visual (VV), and a motor control condition. Fifteen spherical ellipsoids were used as stimuli. By subtracting the different matching conditions and calculating the intersections of statistically significant activations, we could identify cortical functional fields involved in the formation of visual and tactile representation of the objects alone and those involved in cross-modal transfer of the shapes of the objects. Fields engaged in representation of visual shape, revealed in VV-control, TV-control and TV-TT, were found bilaterally in the lingual, fusiform, and middle occipital gyri and the cuneus. Fields engaged in the formation of the tactile representation of shape, appearing in TT-control, TV-control and TV-VV, were found in the left postcentral gyrus, left superior parietal lobule, and right cerebellum. Finally, fields active in both TV-VV and TV-TT were considered as those involved in cross-modal transfer of information. One field was found, situated in the right insula-claustrum. This region has been shown to be activated in other studies involving cross-modal transfer of information. The claustrum may play an important role in cross-modal matching, because it receives and gives rise to multimodal cortical projections. We propose here that modality-specific areas can communicate, exchange information, and interact via the claustrum.

Insular cortex and neighboring fields in the cat: a redefinition based on cortical microarchitecture and connections with the thalamus

The insular areas of the cerebral cortex in carnivores remain vaguely defined and fragmentarily characterized. We have examined the cortical microarchitecture and thalamic connections of the insular region in cats, as a part of a broader study aimed to clarify their subdivisions, functional affiliations, and eventual similarities with other mammals. We report that cortical areas, which resemble the insular fields of other mammals, are located in the cat's orbital gyrus and anterior rhinal sulcus. Our data suggest four such areas: (a) a "ventral agranular insular area" in the lower bank of the anterior rhinal sulcus, architectonically transitional between iso- and allocortex and sparsely connected to the thalamus, mainly with midline nuclei; (b) a "dorsal agranular insular area" in the upper bank of the anterior rhinal sulcus, linked to the mediodorsal, ventromedial, parafascicular and midline nuclei; (c) a "dysgranular insular area" in the anteroventral half of the orbital gyrus, characterized by its connections with gustatory and viscerosensory portions of the ventroposterior complex and with the ventrolateral nucleus; and (d) a "granular insular area", dorsocaudal in the orbital gyrus, which is chiefly bound to spinothalamic-recipient thalamic nuclei such as the posterior medial and the ventroposterior inferior. Three further fields are situated caudally to the insular areas. The anterior sylvian gyrus and dorsal lip of the pseudosylvian sulcus, which we designate "anterior sylvian area", is connected to the ventromedial, suprageniculate, and lateralis medialis nuclei. The fundus and ventral bank of the pseudosylvian sulcus, or "parainsular area", is associated with caudal portions of the medial geniculate complex. The rostral part of the ventral bank of the anterior ectosylvian sulcus, referred to as "ventral anterior ectosylvian area", is heavily interconnected with the lateral posterior-pulvinar complex and the ventromedial nucleus. Present results reveal that these areas interact with a wide array of sensory, motor, and limbic thalamic nuclei. In addition, these data provide a consistent basis for comparisons with cortical fields in other mammals.

Role of the claustrum in convulsive evolution of visual afferent and partial nonconvulsive seizure in primates

PURPOSE

We tested cross-species validity of the role of the claustrum in the convulsive evolution of the visual afferent and amygdaloid seizure and the specificity of the claustral lesioning effect.

METHODS

In 7 Senegalese baboons, we examined the effect of unilateral claustral lesioning on generalized convulsive seizures either kindled from the amygdaloid nucleus (AM) and cingulate cortex (CG) or induced by intermittent photic stimulation (IPS) after systemic administration of D,L-allylglycine (AG).

RESULTS

A lesioned area common to all animals was the anterior half of the left claustrum. Postoperative restimulation of the kindled left AM or CG evoked only nonconvulsive seizures. When few convulsive seizures emerged in 1 CG-kindled animal, they were mirror image of the kindled seizure and arose from the nonlesioned right hemisphere. Restimulation of the kindled right AM or CG reactivated kindled seizures. An IPS-induced generalized convulsive seizure was transformed into a secondarily generalized seizure arising from the nonlesioned right hemisphere.

CONCLUSIONS

The primate claustrum regulates the convulsive evolution of partial seizures originating from nonmotor structures such as the AM and CG and also regulates the convulsive development that follows IPS. Our findings suggest that predisposed susceptibility expressed at the claustrum may be involved in the clinical variation with respect to convulsive evolution of nonmotor partial seizures and convulsive susceptibility to IPS in human primates.

Rat's claustrum shows two main cortico-related zones

Methods of retrograde axonal transport were employed to evaluate the topography and overlap of claustroneocortical connections in the rat. Fluorescent tracers Fast Blue (FB) and Diamidino Yellow (DY) were injected simultaneously in various combinations into the motor, somatosensory, auditory and visual cortical areas. Experiments showed that claustroneocortical projections are organized in two main cortico-related zones: sensorimotor and visuoauditory. The sensorimotor zone occupies the anterodorsal part whereas the visuoauditory occupies the posteroventral part of the claustrum. Between these two main zones only a scanty overlap was observed. In the sensorimotor zone a large overlap between neurons projecting to the motor and somatosensory cortical areas exists. The visuoauditory zone is characterized by a full overlap of neuronal populations projecting to the visual and auditory areas.

Mapping subcortical extrarelay afferents onto primary somatosensory and visual areas in cats

Projections from the claustrum (Cl) and the thalamic anterior intralaminar nuclei (AIN) to different representations within the primary somatosensory (S1) and visual (V1) areas were studied using the multiple retrograde fluorescent tracing technique. The injected cortical regions were identified electrophysiologically. Retrograde labeling in Cl reveals two different projection patterns. The first pattern is characterized by a clear topographic organization and is composed of two parts. The somatosensory Cl shows a dorsoventral progression of cells projecting to the hindpaw, forepaw, and face representations of S1. The visual Cl has cells projecting to the vertical meridian representation of V1 surrounded dorsally by neurons projecting to the representation of retinal periphery. A second pattern of Cl projections is composed of neurons that are distributed diffusely through the nucleus. In both somatosensory and visual sectors, these intermingle with the topographically projecting cells. Neurons retrogradely labeled from cortical injections are always present in the AIN. In the central medial nucleus, the segregation of modality is evident: The visual-projecting sector is dorsal, and the somatosensory is ventral. Projections from the central lateral nucleus display detectable somatotopic and retinotopic organization: Individual regions are preferentially connected with specific representations of S1 or V1. In the paracentral nucleus, no clear regional preferences are detectable. Also performed were comparisons of the proportions of neurons projecting to different sensory representations. Projections to V1 from both AIN and Cl are biased towards the retinal periphery representation. S1 projection preference is for the forepaw representation in Cl and for the hindpaw in the AIN. The quantitative analysis of multiply labeled cells reveals that, compared to Cl, the AIN contains a higher proportion of neurons branching between different representations of S1 or V1. The concept of topographic vs. diffuse projecting systems is reviewed and discussed, and functional implications of quantitative analysis are considered.

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.