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At least 1,387 records · Page 77Linked to original sources
PubMed2001

Phase shifting prior to spatial filtering enhances optical recordings of cardiac action potential propagation.

Optical imaging of cardiac electrical activity using a voltage-sensitive dye provides high spatial resolution maps of action potential propagation and repolarization. Charge-coupled-device (CCD) camera-based imaging systems, however, are limited by their low signal-to-noise ratio. We have developed an image processing method to enhance the quality of optical signals recorded using a CCD camera. The method is based on the observation that within a small neighborhood of adjacent pixels, the morphology of the optical action potential varies little except for a phase shift in time resulting from the propagation of the wavefront. The method uses a phase-correlation technique to first correct for this time shift before spatially filtering with a 5 x 5 Gaussian convolution kernel (sigma = 1.179). A length 5 median filter is then applied to further reduce noise by filtering in the temporal domain. The image-processing scheme allows for more accurate extraction of maps of electrical activation, repolarization, and action potential duration.

Action Potentials↗
PubMed2001

Characterization of a new brain-derived proteoglycan inhibiting retinal ganglion cell axon outgrowth.

A proteoglycan was identified and isolated from physiological saline extracts of chick embryo brains by using a new monoclonal antibody (hybridoma clone mab Te38). The purified proteoglycan displayed an apparent molecular mass of 2500-3500 kDa, which became reduced to 370 and 600 kDa after digestion with chondroitinase ABC or chondroitinase AC. After additional treatment with keratanase the 600-kDa band was no longer detectable in Western blots. The specific epitope recognized by mab Te38 is an O-linked carbohydrate associated with the core protein. Tenascin-C, an extracellular matrix protein known to associate with several proteoglycans, copurified with the mab Te38 proteoglycan on the immunoaffinity column. Mab Te38 binds to the surface of nonneuronal cells; in sections from the primary visual system, expression is restricted to cells in the optic fissure, the dorsal optic nerve, and the chiasm. No retinal cells were found to express the mab Te38 epitope. The isolated molecule inhibited axon outgrowth from retinal explants when offered bound to a substrate consisting of either matrigel or collagen, chondroitinase treatment did not alter the inhibitory properties. The distribution and in vitro function of the Te38 proteoglycan indicate that it may serve a role in guidance of retinal ganglion cell axons.

Animals↗
PubMedDate not supplied

The re-establishment of the representation of the dorso-ventral retinal axis in the chiasmatic region of the ferret.

This study has examined the representation of the dorso-ventral retinal axis in the optic nerve and tract of the ferret, as well as the associated fiber transformations which take place within the chiasmatic region. In one series of experiments, dorsal or ventral retinal lesions were made to induce fiber degeneration along the pathway, from which semi-thin sections were then stained for degenerating myelin. In a second series, implants of the carbocyanine dye, DiI, were made into the caudo-medial or rostro-lateral optic tract in order to label retrogradely the axons as they course through the chiasmatic region. Additional observations were made from the optic pathways of ferrets that had been similarly lesioned or implanted, but employing either a reduced-silver technique to reveal the degenerating axons or horseradish peroxidase as the retrograde label. The axons arising from the dorsal and ventral retina course in the dorsal and ventral parts of the optic nerve posterior to the eye, but as they continue along the nerve they disperse producing a highly impoverished retinotopy in the prechiasmatic portion of the nerve. As they course through the chiasmatic region, however, they become segregated again: dorsal fibers cross the midline relatively caudally while ventral fibers cross further rostrally, although there is overlap between them. Nearer the threshold of the optic tract, the fibers from dorsal and ventral retina undergo a further and more striking segregation, placing the dorsal fibers caudo-medially and the ventral fibers rostro-laterally within the tract. This re-emergence of retinotopic order implicates a fiber-substrate interaction as being responsible for the axonal reordering, and suggests that fiber pre-ordering in the tract contributes to the formation of the orderly projection of the dorso-ventral retinal axis upon central visual targets.

Animals↗
PubMed2006

A practical three-dimensional dosimetry system for radiation therapy.

There is a pressing need for a practical three-dimensional (3D) dosimetry system, convenient for clinical use, and with the accuracy and resolution to enable comprehensive verification of the complex dose distributions typical of modern radiation therapy. Here we introduce a dosimetry system that can achieve this challenge, consisting of a radiochromic dosimeter (PRESAGE) and a commercial optical computed tomography (CT) scanning system (OCTOPUS). PRESAGE is a transparent material with compelling properties for dosimetry, including insensitivity of the dose response to atmospheric exposure, a solid texture negating the need for an external container (reducing edge effects), and amenability to accurate optical CT scanning due to radiochromic optical contrast as opposed to light-scattering contrast. An evaluation of the performance and viability of the PRESAGE/OCTOPUS, combination for routine clinical 3D dosimetry is presented. The performance of the two components (scanner and dosimeter) was investigated separately prior to full system test. The optical CT scanner has a spatial resolution of < or = 1 mm, geometric accuracy within 1 mm, and high reconstruction linearity (with a R2 value of 0.9979 and a standard error of estimation of approximately 1%) relative to independent measurement. The overall performance of the PRESAGE/OCTOPUS system was evaluated with respect to a simple known 3D dose distribution, by comparison with GAFCHROMIC EBT film and the calculated dose from a commissioned planning system. The "measured" dose distribution in a cylindrical PRESAGE dosimeter (16 cm diameter and 11 cm height) was determined by optical-CT, using a filtered backprojection reconstruction algorithm. A three-way Gamma map comparison (4% dose difference and 4 mm distance to agreement), between the PRESAGE, EBT and calculated dose distributions, showed full agreement in measurable region of PRESAGE dosimeter (approximately 90% of radius). The EBT and PRESAGE distributions agreed more closely with each other than with the calculated plan, consistent with penumbral blurring in the planning data which was acquired with an ion chamber. In summary, our results support the conclusion that the PRESAGE optical-CT combination represents a significant step forward in 3D dosimetry, and provides a robust, clinically effective and viable high-resolution relative 3D dosimetry system for radiation therapy.

Algorithms↗
PubMed1989

A-to-D conversion from paper records with a desktop scanner and a microcomputer.

A novel method for digitizing signals contained in paper records is presented. This method is based on the use of an inexpensive optical scanner to translate the image on paper into a binary, bit map data structure. Several algorithms which recognize the signal line in the bit map and translate it into a series of numbers which are equivalent to the output of electronic analog-to-digital converters are described. The method was validated by comparison both with idealized test patterns of varying frequency content and with electronically digitized pressure and pressure time derivative tracings from chronically instrumented dogs. The root mean square error for the physiological signals was 3.5-3.9% of peak-to-peak full scale, corresponding to roughly 50% more than the thickness of the signal line on the paper.

Algorithms↗
PubMed2005

Orientation selectivity without orientation maps in visual cortex of a highly visual mammal.

In mammalian neocortex, the orderly arrangement of columns of neurons is thought to be a fundamental organizing principle. In primary visual cortex (V1), neurons respond preferentially to bars of a particular orientation, and, in many mammals, these orientation-selective cells are arranged in a semiregular, smoothly varying map across the cortical surface. Curiously, orientation maps have not been found in rodents or lagomorphs. To explore whether this lack of organization in previously studied rodents could be attributable to low visual acuity, poorly differentiated visual brain areas, or small absolute V1 size, we examined V1 organization of a larger, highly visual rodent, the gray squirrel. Using intrinsic signal optical imaging and single-cell recordings, we found no evidence of an orientation map, suggesting that formation of orientation maps depends on mechanisms not found in rodents. We did find robust orientation tuning of single cells, and this tuning was invariant to stimulus contrast. Therefore, it seems unlikely that orientation maps are important for orientation tuning or its contrast invariance in V1. In vertical electrode penetrations, we found little evidence for columnar organization of orientation-selective neurons and little evidence for local anisotropy of orientation preferences. We conclude that an orderly and columnar arrangement of functional response properties is not a universal characteristic of cortical architecture.

Action Potentials↗
PubMed1995

Three distinct axonal transport rates for tau, tubulin, and other microtubule-associated proteins: evidence for dynamic interactions of tau with microtubules in vivo.

Microtubule-associated proteins (MAPs), such as tau, modulate neuronal shape and process outgrowth by influencing the stability and organization of microtubules. The dynamic nature of MAP-microtubule interactions in vivo, however, is poorly understood. Here, we have assessed the stability of these interactions by investigating the synthesis and axoplasmic transport of tau in relation to that of tubulin and other MAPs within retinal ganglion cells of normal adult mice in vivo. Using immunoprecipitation and Western blot analysis with anti-tau monoclonal and polyclonal antibodies, we unequivocally identified in optic axons a family of 50-60 kDa tau isoforms and a second 90-95 KDa tau family, the members of which were shown to contain the domain of tau encoded by exon 4A. To measure the rates of translocation of tau proteins in vivo, we injected mice with 35S-methionine intravitreously and, after 6-30 d, quantitated the radiolabeled tau isoforms immunoprecipitated from eight consecutive 1.1 mm segments of the nerve and optic tract and separated by electrophoresis. Linear regression analysis of protein transport along optic axons showed that the tau isoforms advanced at a rate of 0.2-0.4 mm/d, and other radiolabeled MAPs, identified by their association with taxol-stabilized microtubules, moved three- to fivefold more rapidly. By contrast, tubulins advanced at 0.1-0.2 mm/d, significantly more slowly than tau or other MAPs. These studies establish that tau is not cotransported with tubulin or microtubules, indicating that associations of tau with microtubules within axons are not as stable as previously believed. Our findings also reveal differences among various MAPs in their interactions with microtubules and provide evidence that assembly and reorganization of the microtubule network is an active process even after axons establish connections and fully mature.

Animals↗
PubMed1991

Fibre divergence in the distal optic radiation: possible basis of functional plasticity in adult primate visual cortex.

The precision of retinotopy in primate visual cortex is commonly thought to result from highly ordered arrangement of fibres in the visual pathways. However, rigid point-to-point representation is hardly compatible with findings of a substantial reorganization of visual cortical maps after peripheral and central lesions. Such observations could be accounted for by divergence in the optic radiation. To explore the hypothesis of fibre divergence, we made small knife cuts in the distal optic radiation of macaca fascicularis. After subsequent axonal tracing by injecting WGA-HRP into lateral geniculate nucleus, we studied the course of distal fibres in white matter. The amount of divergence was assessed by measuring, relative to the prevailing fibre course, length and orientation of labelled fibres between lesion and entry into cortex. Lesion sizes between 1 mm to 3 mm did not result in any detectable diminution of terminal labelling in layer IVC of striate cortex. Individual labelled fibres were found to diverge symmetrically from both sides into the gap distal to the lesion. Divergence starts at a distance of about 3 mm before cortex. At the white matter boundary, less than 10% of all fibres still retain the original direction, with the remaining fibres taking any other orientation without preference. We estimate that this corresponds to a divergence of visual afferents encompassing about 6-10 mm of cortical distance, if intracortical arborization of terminal fibres is taken into account. Possible consequences for functional plasticity in the adult primate visual cortex are discussed.

Animals↗
PubMed1989

In vivo phosphorylation of distinct domains of the 70-kilodalton neurofilament subunit involves different protein kinases.

A combination of in vivo and in vitro approaches were used to characterize phosphorylation sites on the 70,000-kilodalton (kDa) subunit of neurofilaments (NF-L) and to identify the protein kinases that are likely to mediate these modifications in vivo. Neurofilament proteins in a single class of neurons, the retinal ganglion cells, were pulse-labeled in vivo by injecting mice intravitreously with [32P]orthophosphate. Radiolabeled neurofilaments were isolated after they had advanced along optic axons, and the individual subunits were separated on sodium dodecyl sulfate-polyacrylamide gels. Two-dimensional alpha-chymotryptic phosphopeptide map analysis of NF-L revealed three phosphorylation sites: an intensely labeled peptide (L-1) and two less intensely labeled peptides (L-2 and L-3). The alpha-chymotryptic peptide L-1 was identified as the 11-kDa segment containing the C terminus of NF-L. The ability of these peptides to serve as substrates for specific protein kinases were examined by incubating neurofilament preparations with [gamma-32P]ATP in the presence of purified cAMP-dependent protein kinase or appropriate activators and/or inhibitors of endogenous cytoskeleton-associated protein kinases. The heparin-sensitive, calcium- and cyclic nucleotide-independent kinase associated with the cytoskeleton selectively phosphorylated L-1 and L-3 but had little, if any, activity toward L-2. When this kinase was inhibited with heparin, cAMP addition to the neurofilament preparation stimulated the phosphorylation of L-2, and addition of the purified catalytic subunit of cAMP-dependent protein kinase induced intense labeling of L-2. At higher labeling efficiencies, the exogenous kinase also phosphorylated L-3 and several sites at which labeling was not detected in vivo; however, L-1 was not a substrate. Calcium and calmodulin added to neurofilament preparations in the presence of heparin modestly stimulated the phosphorylation of L-1 and L-3, but not L-2, and the stimulation was reversed by trifluoperazine. The selective phosphorylation of different polypeptide domains on NF-L by second messenger-dependent and -independent kinases suggests multiple functions for phosphate groups on this protein.

Axons↗
PubMed2000

Neurotrophin-4/5 alters responses and blocks the effect of monocular deprivation in cat visual cortex during the critical period.

The mechanisms underlying changes in neural responses and connections in the visual cortex may be studied by occluding one eye during a critical period in early postnatal life. Under these conditions, neurons in the visual cortex rapidly lose their responses to the deprived eye and ultimately lose many of their inputs from that eye. Cats at the peak of the critical period received infusions of exogenous neurotrophin NT-4/5 into primary visual cortex beginning before a short period of monocular deprivation. Within areas affected by NT-4/5, cortical cells remained responsive to the deprived eye, and maps of ocular dominance were no longer evident using intrinsic-signal optical imaging. Cortical cells also became broadly tuned for stimulus orientation and less responsive to visual stimulation through either eye. These effects required at least 48 hr exposure to the neurotrophin and were specific for trkB, because they were not seen with the trkA or trkC ligands NGF or NT-3. Even after neurons had already lost their responses to the deprived eye, subsequent NT-4/5 infusion could restore them. The NT-4/5 effects were not seen after the critical period. Together, these results suggest that trkB activation during the critical period may promote promiscuous connections independent of correlated activity.

Aging↗
PubMed1999

Local and global patterns during morphogenesis of the retinotectal topographical mapping in the vertebrate brain.

The highly ordered neuronal projections from the retina to the tectum mesencephali (optic tectum) in several vertebrate groups have been intensively studied. Several hypotheses so far have been proposed, suggesting mechanisms to explain the topographical and biochemical specificity of the retinotectal projections during ontogeny. In the present paper we compare the main hypotheses of retinotectal development with respect to the nature of specificity envisaged, the activity-dependence versus inheritance criterium and the strategy of argument, in casu the descriptive versus interferential type of argument. Matching of the current developmental hypotheses and mechanisms for elimination or persistence of contralateral, respectively ipsilateral connections, is attempted with respect to the known neuroanatomical connectivity data in the amphibian optic tectum and with respect to the symmetry relations between ipsilateral and contralateral connection patterns described in amphibians and other vertebrate groups. Local mechanisms influencing the survival potential of synaptic contacts between retinal afferents and tectal neurons are probably essential for generating global symmetry patterns. Finally, a global topological approach is discussed with respect to its applicability in the amphibian retinotectal projection system. Basic assumptions of topological modeling appear to rely on anatomical and functional properties of the visual system like left-right symmetry, dichotomy, (absence of) convergence and segregation of fibers and neurons into columnar information units. It is concluded however, that more neuroanatomical, physiological and ultrastructural data are needed to establish a formalized operation model of the amphibian retinotectal system.

Amphibians↗
PubMed1985

Synthesis of thymosin alpha 1 by fragment condensation using tert.-butyl side chain protection.

A novel synthesis of thymosin alpha 1 by classical methods using seven tert.-butyl side chain protected fragments is described. Optimum conditions were found for the final DCC/HOBt coupling of the two key intermediates; decapeptide and octadecapeptide. Thymosin alpha 1 was purified by two stages of preparative HPLC (partial purification with C8 and final purification with C18 reverse phase silica gel) to give a 30% overall yield for the final four stages of synthesis (including catalytic hydrogenation of octadecapeptide, coupling, deprotection and purification). The product was shown to be homogeneous by thin-layer and paper high voltage electrophoresis, isoelectric focusing analysis, thin-layer chromatography and high performance liquid chromatography. Amino acid analysis, optical rotation, 1H-n.m.r. spectroscopy, FAB mass spectroscopy and peptide mapping after tryptic digestion confirmed the structure of thymosin alpha 1. Three minor stereoisomer contaminants were isolated by HPLC and characterized as [D-Lys14]-thymosin alpha 1, [D-Lys17]-thymosin alpha 1 and [D-Ala3]-thymosin alpha 1 resulting from racemization at Lys14, Lys17 and Ala3 during the coupling of the fragments. A final contaminant, isolated by HPLC, was characterized as N alpha-isobutyloxycarbonyl-thymosin alpha 1 (15-28), which results from "wrong way opening" of an activated mixed anhydride.

Amino Acid Sequence↗
PubMed1984

An anatomical and electrophysiological study of the centrifugal visual system in the lamprey (Lampetra fluviatilis).

The centrifugal visual system of the lamprey Lampetra fluviatilis was investigated using various neurohistophysical methods: intraocular injections of [3H]adenosine, fluorescent tracer (Evans Blue) and the iontophoretic deposit of HRP on the optic nerve. Retrogradely labeled neurons were identified bilaterally within the nucleus M5 of Schober and contralaterally in the reticular mesencephalic area (RMA). Comparison of the various orthograde and retrograde labeling results indicated that the neurons of M5 and RMA were labeled via retrograde axonal transport of the different tracers in the retinopetal system and not by orthograde transneuronal processes or from extraretinal pathways. Part of the anatomical data regarding RMA as a site of origin of the centrifugal visual system was confirmed using electrophysiological techniques involving evoked potential and unit cell recordings in RMA following electrical stimulation of the optic nerve. The experiments were performed in the curarized animal under conditions of either normal blood circulation, perfusions of adapted physiological saline, or with a solution known to block chemical synaptic transmission. Various electrophysiological criteria, including the results obtained during the conditions of reversible chemical synaptic blockade, indicated that the responses in RMA reflect an antidromic process. The anatomical organization of the centrifugal visual system in the lamprey is compared to that found in different gnathostome vertebrate species. Several hypotheses concerning the marked interspecies differences related either to the number and the topographical location of the centrifugal neurons as well as the evolution of this system are advanced.

Animals↗
PubMed1979

Thermal dosimetry and temperature measurements.

A crucial ingredient of any hyperthermia procedure is the accurate measurement of achieved temperature. In this paper, we present some accuracy and resolution suggestions and address the problem of temperature measurements when electromagnetic energy is used as the mode of heating. In such cases, conventional metallic thermometers can cause severe errors due to reradiated fields, high internal healing, and electronic interference. To avoid these problems, several groups have developed either high-resistance lead thermometers or optical-fiber probes using a variety of sensors, including birefringent and semiconductor crystals. A new noninvasive approach uses ultrasound-computed tomography and relies upon the change in the speed of sound versus temperature to achieve maps of internal tissue temperature.

Electromagnetic Fields↗
PubMed2001

Spontaneous and stimulus-evoked intrinsic optical signals in primary auditory cortex of the cat.

Spontaneous and tone-evoked changes in light reflectance were recorded from primary auditory cortex (A1) of anesthetized cats (barbiturate induction, ketamine maintenance). Spontaneous 0.1-Hz oscillations of reflectance of 540- and 690-nm light were recorded in quiet. Stimulation with tone pips evoked localized reflectance decreases at 540 nm in 3/10 cats. The distribution of patches "activated" by tones of different frequencies reflected the known tonotopic organization of auditory cortex. Stimulus-evoked reflectance changes at 690 nm were observed in 9/10 cats but lacked stimulus-dependent topography. In two experiments, stimulus-evoked optical signals at 540 nm were compared with multiunit responses to the same stimuli recorded at multiple sites. A significant correlation (P < 0.05) between magnitude of reflectance decrease and multiunit response strength was evident in only one of five stimulus conditions in each experiment. There was no significant correlation when data were pooled across all stimulus conditions in either experiment. In one experiment, the spatial distribution of activated patches, evident in records of spontaneous activity at 540 nm, was similar to that of patches activated by tonal stimuli. These results suggest that local cerebral blood volume changes reflect the gross tonotopic organization of A1 but are not restricted to the sites of spiking neurons.

Acoustic Stimulation↗
PubMed1996

Retinal-specific guanylate cyclase gene mutations in Leber's congenital amaurosis.

Leber's congenital amaurosis (LCA, MIM 204,000), the earliest and most severe form of inherited retinopathy, accounts for at least 5% of all inherited retinal dystrophies. This autosomal recessive condition is usually recognized at birth or during the first months of life in an infant with total blindness or greatly impaired vision, normal fundus and extinguished electroretinogram (ERG). Nystagmus (pendular type) and characteristic eye poking are frequently observed in the first months of life (digito-ocular sign of Franceschetti). Hypermetropia and keratoconus frequently develop in the course of the disease. The observation by Waardenburg of normal children born to affected parents supports the genetic heterogeneity of LCA. Until now, however, little was known about the pathophysiology of the disease, but LCA is usually regarded as the consequence of either impaired development of photoreceptors or extremely early degeneration of cells that have developed normally. We have recently mapped a gene for LCA to chromosome 17p13.1 (LCA1) by homozygosity mapping in consanguineous families of North African origin and provided evidence of genetic heterogeneity in our sample, as LCA1 accounted for 8/15 LCA families in our series. Here, we report two missense mutations (F589S) and two frameshift mutations (nt 460 del C, nt 693 del C) of the retinal guanylate cyclase (RETGC, GDB symbol GUC2D) gene in four unrelated LCA1 probands of North African ancestry and ascribe LCA1 to an impaired production of cGMP in the retina, with permanent closure of cGMP-gated cation channels.

Blindness↗
PubMed1990

Retinofugal and retinopetal projections in the Pacific hagfish, Eptatretus stouti (Myxinoidea).

The retinofugal and retinopetal connections of the Pacific hagfish were determined using the fluorescent compound DiI as an in vitro tracer. The retinofugal connections were found to be bilateral and to reach preoptic, thalamic and pretectal, as well as tectal, targets. No distinct basal optic root was found. A retinopetal projection in hagfishes is described here for the first time. It originates from two cells groups in the rostral mesencephalic tegmentum. These retinal connections in hagfishes are compared to the organization of the visual system in the second Recent group of jawless fishes, the lampreys. Both visual systems are similar in the number and location of the targets of retinal fibers. Furthermore, there is a striking similarity in the retinopetal systems. Major differences are found in the organization of the optic tract. These observations allow the identification of some cell groups not previously recognized in myxinoid brains or interpreted differently. Finally, the findings are discussed in relation to visually guided behavior in hagfishes.

Animals↗