Biomedical subjects

R Malach

Publications and source records attributed to R Malach.

12 recordsLinked to original sources

PubMed1992

Organization of layers II-III connections in human visual cortex revealed by in vitro injections of biocytin.

In the search for cortical mechanisms subserving psychological phenomena, a better understanding of human cortical circuitry is crucial. In this report we describe aspects of intrinsic connectivity of supragranular layers in human visual cortex, revealed by extracellular injections of the anterograde tracer biocytin in vitro. Human cortical slices were obtained from visual association cortex in the posterior-medial portion of the dorsal bank of the occipital lobe, removed during neurosurgical tumor ablations. Small iontophoretic injections of biocytin into layers II-III revealed intense Golgi-like staining of axonal projections emanating from the injection sites. Vertically descending axons are grouped in bundles 20 microns in diameter which are spaced 15 microns apart. Some of these axons enter the white matter and send long oblique and horizontal collaterals. The main horizontal spread of the axons could be observed in layers II-III and V. The bulk of projections extends to a distance of 1.5 mm in layers II-III and 1.1 mm in layer V. Few individual axons could be observed at greater distances. In contrast, layer IV is almost devoid of horizontal connections, forming a clear gap between supra- and infragranular layers. Axon collaterals in the infragranular layers project mostly in a descending oblique direction with long horizontal collaterals in lower layer VI.

Axons↗

PubMed1992

Excitatory inputs to layer V pyramidal cells of rat primary visual cortex revealed by acetylcholine activation.

Cells in layers II-III or VI were activated by microdrop application of acetylcholine (ACh), while monitoring the intracellular response of layer V pyramidal cells. This enabled the tracing of functional connections between the cells of layers II-III or VI with those of layer V. ACh activation of layer II-III or VI cells resulted in a small depolarization of these cells, accompanied by a burst of excitatory postsynaptic potentials (EPSPs) from layer V pyramidal cells. These effects of ACh were blocked by tetrodotoxin (TTX), suggesting the involvement of action potentials in their production. The input resistance of layer V pyramidal cells during and after the EPSP burst was not significantly different from control values, further suggesting an indirect effect of ACh on layer V pyramidal cells. Isolation of the supragranular layer, by horizontal cutting, did not prevent the EPSP burst evoked by ACh application to the lower layer VI, suggesting a direct input from layer VI to layer V pyramidal cells. ACh applied near pyramidal cells in layers II-III, V or VI caused transient hyperpolarization associated with a decrease in input resistance followed by a large depolarization, an increase in input resistance, and action potential discharges. The ACh-mediated hyperpolarization and the train of action potentials of layer II-III pyramidal cells were blocked by TTX. Thus the ACh-activated cells in layers II-III and VI make an excitatory synaptic contact with layer V pyramidal cells, producing the EPSP burst observed in layer V.

Acetylcholine↗

PubMed1992

Golgi-like staining of visual cortex cells obtained by extracellular biocytin application in vitro.

We report here the application of biocytin (a biotin-lysine complex) as an extracellular tracer in vitro. Biocytin was applied extracellularly, revealing Golgi-like staining of cells in the adult in vitro rat visual cortex. Micropipettes were filled with a solution of 2.3-2.6% biocytin dissolved in 0.05 M Tris buffer, pH 7.4. Biocytin was applied by one of 3 methods: diffusion, pressure injection or drop application. Cell bodies and dendrites around the application site and their efferent axonal processes were stained; dendritic spines were often visible. The injection sites varied in size from a single cell to a diameter of 400 microns. When applied in layer I-III, few filled cells were also seen in layers IV and V, outside the application site. The drop application (5-10 microliters) of biocytin resulted in filling of cells throughout the cortex. The combination of biocytin and the slice preparation was found to be very useful in revealing cell morphology and tracing interlaminar connections in the visual cortex. The advantages of this technique are its ease of application, the precise and restricted injection sites, and Golgi-like morphological detail.

Animals↗

PubMed1992

Dendritic sampling across processing streams in monkey striate cortex.

Cytochrome oxidase (CO) dense blobs in primate striate cortex provide a striking example of parallel processing of visual information. The level of isolation of the blobs from the surrounding interblob tissue was investigated in the present study by combining CO staining with Golgi impregnation of dendritic arbors in the same tissue sections. The data are based on material from two marmoset and three squirrel monkeys. The analysis was conducted on two types of Golgi preparations. In the first preparation, dense networks of overlapping dendrites were impregnated over blob margins. The results of analyzing these networks with transmission and confocal microscopy revealed that dendritic arbors penetrate freely through blob margins. Statistical analysis revealed that the density of dendritic crossings at blob margins was similar to that found at blob and interblob centers. In the second type of Golgi preparation, single, isolated neurons were impregnated. Studies of such neurons revealed occasional examples of dendritic arbors that appeared to reflect back from blob margins, but counter examples were equally abundant. Bias index analysis indicated that dendritic arbors were generally unaffected by the presence of a nearby blob margin. Scanning a large number of impregnated arbors indicated that at least half of the population of blob-related neurons had dendrites in both blob and interblob territory. Under the conditions of free dendritic penetration of blob margins, the sole factor that determines the level of blob/interblob mixing appears to be the relationship between blob size and the dendritic spread of blob neurons. Interestingly, in both the marmoset and squirrel monkeys this size ratio is similar despite a large difference in their cortical surface area. Thus, it is hypothesized that blob size is optimally matched to the dendritic span so as to create a smooth transition of dendritic sampling from blob to interblob-related processing streams.

Animals↗

PubMed1991

Patterns of sensory intermodality relationships in the cerebral cortex of the rat.

Patterns of connections underlying cross-modality integration were studied by injecting distinguishable, retrograde tracers (Fluoro-Gold and diamidino yellow) in pairwise manner into different sensory representations (visual, somatosensory, and auditory) in the cerebral cortex of the rat. In agreement with previous single tracer studies, our results indicate that the central core of sensory areas receives projections mainly from a set of association areas located in a ringlike fashion along the margin of the cortical mantle. The visual cortex received projections from areas 48/49, area 29d, posterior agranular medial cortex (AGm), area 11, area 13, and area 35. All these areas were also connected to the auditory cortex with the exception of areas 29d and AGm. However, lateral to area 29d and posterior AGm, a band of neurons projecting to the auditory cortex was present. Somatosensory cortex was connected mainly with the more anterior aspect of the hemisphere, which included primary motor area, area 11, and area 13. The patterns of intermodality relationships revealed in the present study were of two main categories. In the anterior and lateral areas, an intermingling of cells projecting to different sensory modalities was observed. In contrast, in areas located along the medial aspect of the hemisphere, cells connected to different sensory modality representations tended to be segregated from each other. Postsubicular cortex (areas 48/49) contained both intermingled and segregated groups of cells. The incidence of clearly identified double-labeled cells concurrently projecting to two different sensory representations was extremely rare. These patterns may form a substrate for different levels of cross-modal sensory integration in the rat cortex.

Animals↗

PubMed1991

Hypertension induced by hypothalamic transplantation from genetically hypertensive to normotensive rats.

The role of the hypothalamus (HTH) in the pathogenesis of genetic hypertension was studied in spontaneously hypertensive rats (SHR). It is currently believed that, in this strain, the genetic defect manifests itself mainly in the HTH. We examined this hypothesis by grafting HTH neurons from embryos of SHR or control Wistar Kyoto (WKY) rats into the HTH of adult normotensive WKY rats. Changes in host systolic blood pressure (SBP) were monitored, and alterations in vasoactive intestinal polypeptide (VIP) gene expression of the host brain were studied. In rats grafted with HTH tissue from SHR embryos (G-SHR), the blood pressure rose by 31% as compared with that in the grafted control group. The blood pressure climbed gradually over a period of 6 weeks to its highest level, which was maintained for at least 3 months following grafting. Along with the elevated blood pressure, the heart weight increased by 80% compared to controls. Behavioral changes were also evident in the G-SHR rats, and these were similar to those of the native SHR strain. In situ hybridization histochemistry showed a 40% elevation in VIP transcripts in the suprachiasmatic nucleus of the host G-SHR brain compared to controls. These studies demonstrate that transplantation of embryonic SHR HTH tissue into brains of adult normotensive rats results in the development of hypertensive characteristics in the host. It thus appears that the HTH is a prime candidate for the source of changes leading to spontaneous hypertension in mammals.

Animals↗

PubMed1990

Factors regulating the expression of acetylcholinesterase-containing neurons in striatal cultures: effects of chemical depolarization.

The influence of chemical depolarization on the survival and differentiation of acetylcholinesterase (AChE)-containing neurons was examined in primary rat striatal cultures, maintained in different types of media (serum-free and serum-supplemented) and substrate (poly-ornithine and astrocyte monolayer). Chronic application of 5 microM veratridine resulted in a significant loss of neurites by AChE-positive cells, while a higher concentration (20 microM) reduced the number of stained cell bodies. These effects appeared to be selective with regard to AChE-positive cells, as indicated by morphological observations of the cells in the treated cultures and receptor binding measurements. Similarly, elevation of extracellular KCl levels (20-60 mM) produced a dose-dependent neurite loss by AChE-containing cells. Blockers of voltage-sensitive Ca2+ channels--verapamil (1 microM) and nifedipine (1 microM)--did not affect the veratridine-induced neurite loss, while tetrodotoxin (0.1 microM) had a partial effect. When cultures treated with 5 microM veratridine were allowed to recuperate for several days, the number of AChE-positive cells possessing neurites returned close to control values, thus indicating the reversibility of the effect of chemical depolarization. The possibility that chronic neuronal depolarization in the striatum might play a role in regulation of the neuronal processes outgrowth by AChE-containing cells is discussed.

Acetylcholinesterase↗

PubMed1989

VIP-mRNA is increased in hypertensive rats.

Vasoactive intestinal peptide (VIP) is a potent vasodilator. We therefore set out to investigate VIP-gene expression in spontaneous hypertensive rats. By quantitative in situ hybridization histochemistry as well as by RNA blot hybridization experiments we discovered a significant increase in VIP transcripts in the brains of those hypertensive rats. We suggest that the increase in VIP-gene expression may play a compensatory role in these rats where otherwise the rise in blood pressure may have had a much more adverse effect.

Animals↗

PubMed1989

Strabismus does not prevent recovery from monocular deprivation: a challenge for simple Hebbian models of synaptic modification.

It has been suggested that development of central connections in the mammalian visual system is governed by a simple Hebbian rule of synaptic modifiability. Under such a rule, simultaneity of presynaptic and postsynaptic action potentials is a prerequisite for enhanced synaptic efficacy. The present paper reports the results of a study designed to test whether this hypothesis is applicable to the development of the thalamo-cortical visual pathway. In four-week-old kittens, exposure to a 2-d period of monocular deprivation was used to render the vast majority of cortical cells capable of being activated only by the nondeprived eye. During a subsequent 3-5 month recovery period, both eyes were open but surgically misaligned. This combination of conditions was chosen so that during the recovery period presynaptic activity originating from the initially deprived eye would be decorrelated from postsynaptic action potentials in cortical neurons. If synaptic modification is regulated by a simple Hebbian mechanism, then in this situation the deprived eye should be unable to recover control of cortical cells. In fact, the present results indicate that during the recovery period the proportion of cortical neurons dominated by the deprived eye rose to a level equal to that of the nondeprived eye--a result contrary to that predicted by a simple Hebbian rule of development. Histological analysis indicated that a similar level of recovery was present both within and outside of cortical layer IV, the main thalamo-recipient layer. As expected, the induced strabismus resulted in a failure of cortical binocularity to recover in these kittens. Although these results argue against a simple Hebbian mechanism of development, they are compatible with the hypothesis that synaptic modifiability is dependent upon correlations between presynaptic activity and local, subthreshold, postsynaptic changes. This alternative hypothesis has the advantage of allowing modification of local synaptic circuits within the dendritic arbors of a single neuron.

Action Potentials↗

PubMed1989

Patterns of connections in rat visual cortex.

The definition of visual areas is one of the central problems in visual cortex research. Rodent extrastriate cortex offers a striking example of the complexity of this issue, in that different parcelation schemes identify within it from 2 to as many as 13 separate visual areas. In the experiments reported here, patterns of connections within rat visual cortex were studied in an effort to better define its organizational layout. The experimental paradigm used consisted of the following steps: first, the pattern of callosal connections was revealed in vivo with the fluorescent tracer bisbenzimide. Then, using the callosal pattern as a landmark, single injections of WGA-HRP were placed at various sites in striate and extrastriate cortex. Subsequently, the relation between the tangential distribution of ipsilateral corticocortical connections, the callosal connections, and the borders of striate cortex were examined in the flattened cortex preparation. The experiments revealed widespread, patchy connections within rat visual cortex. These connections appeared to reflect 3 organizational trends. First, neighboring sites were more extensively connected than distant ones. Second, extrastriate sites receiving common striate cortex inputs tended to be interconnected. Finally, projections from opposite poles in striate cortex tended to form interdigitating patterns of connections in regions of overlap. Altogether these trends suggest that the extrastriate band adjoining striate cortex has a single, global map organization. However, within the global map, a clear modular organization was evident, which appeared to correspond to the multiple visuotopic representations reported for this region. Based on its location, and some organizational similarities. it is suggested that the global map may constitute the rat homolog of area V2 in cat and monkey.

Animals↗

PubMed1989

Disruption of the optic pathway during development affects vasoactive intestinal peptide mRNA expression.

Vasoactive intestinal polypeptide (VIP) is a regulatory peptide widely distributed in the central and peripheral nervous systems. To understand the activities of VIP it is necessary to study the mechanisms governing its production. The highest concentration of VIP-producing cells occurs in the suprachiasmatic nucleus (SCN) of the hypothalamus. Because the SCN is directly innervated by the optic nerve, we decided to investigate the effect of visual input on VIP gene expression. By means of Northern blot hybridization, we measured VIP mRNA levels in the hypothalami of 36-day-old normal rats and rats that had been enucleated at birth. The concentration of VIP mRNA in the hypothalami of enucleated rats was approximately double that in the hypothalami of normal rats. In contrast, the concentration of VIP mRNA in the cerebral cortex significantly decreased after enucleation. The concentrations of VIP mRNA were also measured by in situ hybridization to brain sections. The hypothalamic VIP mRNA was located mainly in the SCN. Enucleation resulted in an increase of VIP transcripts in the SCN. These results indicate that visual input may participate in the regulation of VIP production.

Animals↗

PubMed1988

In vivo visualization of callosal pathways: a novel approach to the study of cortical organization.

I describe here the successful visualization of interhemispheric callosal connections in the live mammalian cortex. The development of this method was prompted by the finding that fluorescent tracer labeling of groups of cortical neurons, when done under optimal conditions, is sufficiently intense to be visible even in the whole brain preparation. The new approach could provide a useful tool for enhanced precision in localizing cortical modules in vivo. In a typical experiment, rats had their left cortical hemisphere extensively injected with the fluorescent tract-tracer bisBenzimide (BB). After appropriate survival, the right cortical hemisphere was illuminated with UV light and the fluorescing callosal pattern could be discerned under the network of blood vessels even with the unaided eye. The pattern, although diffuse, was grossly similar to the pattern of callosal connections as seen in flattened, sectioned cortex. Features that could be discerned were: the main callosal band straddling the lateral border of area 17, several rings and bands in extrastriate areas 18a and 18b, and a major band straddling the lateral border of area 3. The vitally visualized callosal pattern was used to guide injections of either wheat germ agglutinin conjugated to HRP (WGA-HRP) or rhodamine-labeled microspheres (RLM) into precisely localized sites in occipital cortex. There were numerous instances of doubly labeled neurons stained both with BB and WGA-HRP or RLM, suggesting that uptake of BB combined with UV exposure did not hinder the ability of stained neurons to take up and transport a second tracer. It is suggested that vital tract tracing be used as a tool for enhanced precision in studies of cortical connectivity.

Animals↗