Search PubMed⌕ Search

Biomedical subjects

R Wessel

Publications and source records attributed to R Wessel.

At least 19 recordsLinked to original sources
PubMed2001

Chattering and differential signal processing in identified motion-sensitive neurons of parallel visual pathways in the chick tectum.

At least three identified cell types in the stratum griseum centrale (SGC) of the chick optic tectum mediate separate pathways from the retina to different subdivisions of the thalamic nucleus rotundus. Two of these, SGC type I and type II, constitute the major direct inputs to rotundal subdivisions that process various aspects of visual information, e.g., motion and luminance changes. Here, we examined the responses of these cell types to somatic current injection and synaptic input. We used a brain slice preparation of the chick tectum and applied whole-cell patch recordings, restricted electrical stimulation of dendritic endings, and subsequent labeling with biocytin. Type I neurons responded with regular sequences of bursts ("chattering") to depolarizing current injection. Electrical stimulation of retinal afferents evoked a sharp-onset EPSP/burst response that was blocked with CNQX. The sharp-onset EPSP/burst response to synaptic stimulation persisted when the soma was hyperpolarized, thus suggesting the presence of dendritic spike generation. In contrast, the type II neurons responded to depolarizing current injection solely with an irregular sequence of individual spikes. Electrical stimulation of retinal afferents led to slow and long-lasting EPSPs that gave rise to one or several action potentials. In conclusion, the morphological distinct SGC type I and II neurons also have different response properties to retinal inputs. This difference is likely to have functional significance for the differential processing of visual information in the separate pathways from the retina to different subdivisions of the thalamic nucleus rotundus.

Action Potentials↗
PubMed2001

Elaboration and characterization of spatially controlled assemblies of complementary polyphenol oxidase-alkaline phosphatase activities on electrodes.

The electrooxidation of a biotin pyrrole has allowed the formation of biotinylated polypyrrole films. Gravimetric measurements based on a quartz crystal microbalance demonstrate the efficient coupling of avidin, biotinylated polyphenol oxidase (PPO-B) and avidin-labeled alkaline phosphatase (AP-A) with the underlying biotinylated polymer film. The estimated mass increase corresponds to the anchoring of 1.6-1.8 equivalent layer of proteins. A step-by-step construction of bienzyme multilayers composed of PPO-B and AP-A was carried out on the electrode surface modified by the biotinylated polypyrrole film through avidin-biotin bridges. A spatially controlled distribution of the two enzymes was performed by the formation of one AP-A layer on 1, 5, and 10 PPO-B layers. The resulting bienzyme electrodes were applied to the determination of phenyl phosphate on the basis of amperometric detection of enzymically generated o-quinone at -0.2 V. Their analytical performances were analyzed in relation to the design of the enzyme architectures and in comparison with the amperometric behavior of the monoenzymatic electrodes (PPO-B electrode and AP-A electrode). It appears that at the 10-layer-PPO-B polypyrrole electrode only 4% of phenol is transformed, whereas 42-69% of phenyl phosphate is enzymatically consumed and detected at the AP-A polypyrrole electrode, depending on the enzyme activity. For the bienzymatic AP-A/PPO-B polypyrrole electrodes, the activity of each immobilized enzyme clearly affects the biosensor performance, the main limiting factor being the very low efficiency of PPO-B at pH 8.8.

Alkaline Phosphatase↗
PubMed1999

Dendritic Ca(2+)-activated K(+) conductances regulate electrical signal propagation in an invertebrate neuron.

Activity-dependent changes in the short-term electrical properties of neurites were investigated in the anterior pagoda (AP) cell of leech. Imaging studies revealed that backpropagating Na(+) spikes and synaptically evoked EPSPs caused Ca(2+) entry through low-voltage-activated Ca(2+) channels that are distributed throughout the neurites. Voltage-clamp recordings from the soma revealed a TEA-sensitive outward current that was reduced when Ca(2+) entry was blocked with Co(2+) or when the intracellular concentration of free Ca(2+) was reduced by a high-affinity Ca(2+) buffer. Ca(2+) released in the neurite from a caged Ca(2+) compound caused a hyperpolarization of the membrane potential. These data imply that the AP cell expresses Ca(2+)-activated K(+) conductances, and that these conductances are present in the neurites. When the Ca(2+)-activated K(+) current was reduced through the block of Ca(2+) entry, backpropagating Na(+) spikes and synaptically evoked EPSPs increased in amplitude. Hence, the activity-dependent changes in the intracellular [Ca(2+)] together with the Ca(2+)-activated K(+) conductances participate in the regulation of dendritic signal propagation.

Action Potentials↗
PubMed1999

Supralinear summation of synaptic inputs by an invertebrate neuron: dendritic gain is mediated by an "inward rectifier" K(+) current.

Dendritic processing of glutamatergic synaptic inputs was investigated in the anterior pagoda cell of leech. We observed that below spike threshold, the amplitude of individual EPSPs decreased with hyperpolarization and that simultaneous stimulation of pairs of synaptic inputs leads to the supralinear summation of EPSPs. Voltage-clamp measurements revealed a hyperpolarization-activated, Ba(2+)-sensitive, fast, noninactivating K(+) conductance that depends on the external [K(+)]. These features are those of an "inward rectifier," Kir. Microsurgery experiments, in combination with electrophysiological measurements, revealed an inhomogeneous spatial distribution of the Kir conductance. Furthermore, on surgical removal of the neurites that contain the Kir conductance, the amplitude of EPSPs from the remaining synaptic inputs increased with hyperpolarization. A model cell, with the Kir conductance as the sole voltage-dependent conductance, reproduced qualitatively the observed voltage dependence of individual EPSPs as well as the supralinear summation of EPSP pairs.

2-Amino-5-phosphonovalerate↗
PubMed1999

Modulation of EphA receptor function by coexpressed ephrinA ligands on retinal ganglion cell axons.

The Eph family is thought to exert its function through the complementary expression of receptors and ligands. Here, we show that EphA receptors colocalize on retinal ganglion cell (RGC) axons with EphA ligands, which are expressed in a high-nasal-to-low-temporal pattern. In the stripe assay, only temporal axons are normally sensitive for repellent axon guidance cues of the caudal tectum. However, overexpression of ephrinA ligands on temporal axons abolishes this sensitivity, whereas treatment with PI-PLC both removes ephrinA ligands from retinal axons and induces a striped outgrowth of formerly insensitive nasal axons. In vivo, retinal overexpression of ephrinA2 leads to topographic targeting errors of temporal axons. These data suggest that differential ligand expression on retinal axons is a major determinant of topographic targeting in the retinotectal projection.

Animals↗
PubMed1998

Feature extraction by burst-like spike patterns in multiple sensory maps.

In most sensory systems, higher order central neurons extract those stimulus features from the sensory periphery that are behaviorally relevant (e.g.,Marr, 1982; Heiligenberg, 1991). Recent studies have quantified the time-varying information carried by spike trains of sensory neurons in various systems using stimulus estimation methods (Bialek et al., 1991; Wessel et al., 1996). Here, we address the question of how this information is transferred from the sensory neuron level to higher order neurons across multiple sensory maps by using the electrosensory system in weakly electric fish as a model. To determine how electric field amplitude modulations are temporally encoded and processed at two subsequent stages of the amplitude coding pathway, we recorded the responses of P-type afferents and E- and I-type pyramidal cells in the electrosensory lateral line lobe (ELL) to random distortions of a mimic of the fish's own electric field. Cells in two of the three somatotopically organized ELL maps were studied (centromedial and lateral) (Maler, 1979; Carr and Maler, 1986). Linear and second order nonlinear stimulus estimation methods indicated that in contrast to P-receptor afferents, pyramidal cells did not reliably encode time-varying information about any function of the stimulus obtained by linear filtering and half-wave rectification. Two pattern classifiers were applied to discriminate stimulus waveforms preceding the occurrence or nonoccurrence of pyramidal cell spikes in response to the stimulus. These signal-detection methods revealed that pyramidal cells reliably encoded the presence of upstrokes and downstrokes in random amplitude modulations by short bursts of spikes. Furthermore, among the different cell types in the ELL, I-type pyramidal cells in the centromedial map performed a better pattern-recognition task than those in the lateral map and than E-type pyramidal cells in either map.

Action Potentials↗
PubMed1998

Branched phosphatidylcholines stimulate activity of cytochrome P450SCC (CYP11A1) in phospholipid vesicles by enhancing cholesterol binding, membrane incorporation, and protein exchange.

Phosphatidylcholines (PCs) with branched fatty acyl chains substituted in the two positions of the main chains (branched PCs) have been shown to be potent activators of the side chain cleavage activity of cytochrome P450SCC (CYP11A1) (Schwarz, D., Kisselev, P., Wessel, R., Jueptner, O., and Schmid, R. D. (1996) J. Biol. Chem. 271, 12840-12846). The present study reports on the effect of a series of branched PC on cholesterol binding, membrane integration, and protein exchange in large unilamellar vesicles prepared by an extrusion technique. Enzyme kinetics using vesicles as well as optical titration using a micelle system with the detergent Tween 20 demonstrate that activation is correlated with the fraction of P450SCC in the high spin form. The potency of branched PCs both to activate the enzyme and to induce spin state changes increases with increasing lengths of both the branched and main fatty acyl chains. We found that the extent as well as the rate of integration of P450SCC into vesicle membranes studied by gel chromatography and stopped flow kinetics were increased by branched PC. Finally, it is demonstrated by measurement of the enzymatic activity in primary and secondary vesicles that branched PCs are potent in retaining a very rapid exchange of P450SCC between vesicles, in contrast to cardiolipin, that partially inhibits this exchange process. The data suggest that different properties of P450SCC in membrane systems including cholesterol binding, membrane integration, and protein exchange are affected by branched PCs and probably by other phospholipids, too, and therefore must be considered in an explanation of the observed high stimulation of activity.

Animals↗
PubMed1997

Antioxidative activity of ginkgolides against superoxide in an aprotic environment.

The terpene lactones ginkgolide A, ginkgolide B, ginkgolide C, ginkgolide J and bilobalide, which are components of a standardized extract (EGb 761) from leaves of Ginkgo biloba, as well as ginkgolide M from roots of G. biloba were studied regarding their reaction against superoxide (O2-) and hydroperoxyl radicals (HO2) in dimethyl sulfoxide as an aprotic solvent. It was found that the ginkgolides B, C, J, M as well as bilobalide react with superoxide and its protonated form as demonstrated by EPR and UV/VIS spectroscopy. The initial reaction rate with these oxygen-derived radicals is in the order of 100 M-1/s and below. Ginkgolide A does not react with superoxide under these conditions. From these findings it can be suggested that the superoxide scavenging effect of the ginkgolides B, C, J, M and bilobalide contributes to the antioxidant properties of G. biloba.

Antioxidants↗
PubMed1997

Possible involvement of nonbilayer lipids in the stimulation of the activity of cytochrome P450SCC (CYP11A1) and its propensity to induce vesicle aggregation.

Phosphatidylcholines with saturated branched fatty acyl chains substituted in the 2-positions of the main chains have a significant stimulating effect on both the side chain cleavage activity of cytochrome P450SCC and the ability of P450SCC to induce a specific vesicle aggregation in lipid-vesicle reconstituted systems. Stimulation increases with increasing content of the branched phosphatidylcholines and is sensitive to its acyl chain structure and composition. Because branched phosphatidylcholines containing 2-alkyl substituted fatty acid chains belong to a new class of phosphatidylcholines forming inverted nonbilayer phases the results indicate that nonbilayer lipids might play an important role in the function of mitochondrial P450SCC.

Cholesterol Side-Chain Cleavage Enzyme↗
PubMed1996

From stimulus encoding to feature extraction in weakly electric fish.

Animals acquire information about sensory stimuli around them and encode it using an analogue or a pulse-based code. Behaviourally relevant features need to be extracted from this representation for further processing. In the electrosensory system of weakly electric fish, single P-type electroreceptor afferents accurately encode the time course of random modulations in electric-field amplitude. We applied a stimulus estimation method and a signal-detection method to both P-receptor afferents and their targets, the pyramidal cells in the electrosensory lateral-line lobe. We found that although pyramidal cells do not accurately convey detailed information about the time course of the stimulus, they reliably encode up- and downstrokes of random modulations in electric-field amplitude. The presence of such temporal features is best signalled by short bursts of spikes, probably caused by dendritic processing, rather than by isolated spikes. Furthermore, pyramidal cells outperform P-receptor afferents in signalling the presence of temporal features in the stimulus waveform. We conclude that the sensory neurons are specialized to acquire information accurately with little processing, whereas the following stage extracts behaviourally relevant features, thus performing a nonlinear pattern-recognition task.

Action Potentials↗
PubMed1996

Alpha-branched 1,2-diacyl phosphatidylcholines as effectors of activity of cytochrome P450SCC (CYP11A1). Modeling the structure of the fatty acyl chain region of cardiolipin.

Cardiolipin has been shown to be the most effective activator of cholesterol side chain cleavage activity of cytochrome P450SCC, and evidence has been provided for a lipid effector site on the enzyme. Results suggested the headgroup of cardiolipin as major determinant of lipid interaction with P450SCC (Lambeth, J. D. (1981) J. Biol. Chem. 256, 4757-4762). The role of unsaturation is contradictory and open to question (Igarashi, Y. and Kimura, T. (1986) Biochemistry 25, 6461-6466). We synthesized phosphatidylcholines with fully saturated branched fatty acyl chains substituted in the 2-positions of the main chains and studied the influence of these lipids on the activity and other properties of P450SCC in vesicle-reconstituted systems. These saturated branched lipids, with regard to the fatty acyl moiety in molecular shape similar to cardiolipin but with the headgroup of phosphatidylcholines retained, showed a stimulatory efficiency higher than any other phospholipid and at least comparable to cardiolipin. Activation is sensitive to the acyl chain structure and composition. Results suggest that the shape of the molecule at least partially plays an important role in the process of stimulation of the activity of P450SCC. Because binding of cholesterol was increased by the branched lipids monitored optically by the fraction of P450SCC in the high spin form, it was concluded that these lipids, like cardiolipin and other lipids, exert their effects by regulating the binding of cholesterol to P450SCC. These data suggest that polymorphic lipids such as branched phosphatidylcholines and cardiolipin might influence P450SCC function by maintenance of the membrane curvature at a value optimal for activity.

Acylation↗
PubMed1996

Coding of time-varying electric field amplitude modulations in a wave-type electric fish.

1. The coding of time-varying electric fields in the weakly electric fish, Eigenmannia, was investigated in a quantitative manner. The activity of single P-type electroreceptor afferents was recorded while the amplitude of an externally applied sinusoidal electric field was stochastically modulated. The amplitude modulation waveform (i.e., the stimulus) was reconstructed from the spike trains by mean square estimation. 2. From the stimulus and the reconstructions we calculated the following: 1) the signal-to-noise ratio and thus an effective temporal bandwidth of the units; 2) the coding fraction, i.e., a measure of the fraction of the time-varying stimulus encoded in single spike trains; and 3) the mutual information provided by the reconstructions about the stimulus. 3. Signal-to-noise ratios as high as 7:1 were observed and the bandwidth ranged from 0 up to 200 Hz, consistent with the limit imposed by the sampling theorem. Reducing the cutoff frequency of the stimulus increased the signal-to-noise ratio at low frequencies, indicating a nonlinearity in the receptors' response. 4. The coding fraction and the rate of mutual information transmission increased in parallel with the standard deviation (i.e., the contrast) of the stimulus as well as the mean firing rate of the units. Significant encoding occurred 20-40 Hz above the spontaneous discharge of a unit. 5. When the temporal cutoff frequency of the stimulus was increased between 80 and 400 Hz, 1) the coding fraction decreased, 2) the rate of mutual information transmission remained constant over the same frequency range, and 3) the reconstructed filter changed. This is in agreement with predictions obtained in a simplified neuronal model. 6. Our results suggest that 1) the information transmitted by single spike trains of primary electrosensory afferents to higherorder neurons in the fish brain depends on the contrast and the cutoff frequency of the stimulus as well as on the mean firing rate of the units; and 2) under optimal conditions, more than half of the information about a Gaussian stimulus that can in principle be encoded is carried in single spike trains of P-type afferents at rates up to 200 bits per second.

Action Potentials↗
PubMed1996

Mitral valve repair for mitral regurgitation utilizing intraoperative transesophageal echocardiography--late results.

Ninety consecutive patients underwent mitral valve repair for mitral regurgitation (MR) utilizing intraoperative transesophageal echocardiography (TEE). Fifty-nine males and 31 females between the ages of 31 and 88 with a mean age of 67.9 years were evaluated. Preoperative TEE demonstrated pathology involving the posterior leaflet in 28 patients, anterior leaflet in 21 patients, both leaflets in 19 patients, annular dilatation in 19 patients, and restricted leaflet in three patients. Surgical procedures attempted included quadrangle resection of posterior leaflet pathology (40), Duran "flip over" operation (13), or Goretex suture for anterior leaflet pathology (20), and ring alone for central/ischemic mitral regurgitation (20). TEE immediately following repair showed either no regurgitation or a trace in 78 patients (86%). Time elapsed since repair ranged from one month to 55 months, with a mean of 29 months. Long-range evaluation of mitral valve competence was done by clinical examination and transthoracic or transesophageal echocardiography. Three patients died postoperatively. Fifty-six of 87 patients (64%) had either no or trivial MR within the first year of follow-up. Ring alone for annular dilatation and Goretex suture for anterior leaflet prolapse had the highest incidence of progression of MR. Among the 65 patients followed over one year, 42 (64%) continue to have either none or trivial MR. Three patients had worsening MR requiring mitral valve replacement. Quadrangle resection for posterior leaflet repair and Duran "flip over" operation for anterior leaflet pathology had the highest success rate in long-term follow-up.

Adult↗
PubMed1995

In vitro study of phase resetting and phase locking in a time-comparison circuit in the electric fish, Eigenmannia.

The electric fish Eigenmannia generates on oscillating weak electric field. The amplitude and timing information of this electric field is perceived by electroreceptors distributed on its skin. The pathway of timing information, consisting of spherical cells and giant cells, was studied in an in vitro preparation. The giant cells were identified to be endogenous oscillators and thus have the functional advantage of phase locking more easily to a periodic stimulus with a frequency in the range of the intrinsic frequency. Their spontaneous rhythmic activity was perturbed by delivering excitatory single pulses or periodic pulses via their synaptic inputs. The regular and irregular dynamics produced by periodic stimulation were discussed in the context of a mathematical analysis of the response to single pulses. Ambiguous representations of the timing of the stimulus pulse were observed and could be related to this analysis. Some spontaneously firing cells could be silenced with periodic excitatory stimulation in a narrow frequency and amplitude range. Some irregularly firing cells continued to fire periodically for several seconds after phase locking to a periodic stimulus. This study is the first description of an endogenous oscillator in a system devoted to the precise timing of sensory events.

Animals↗
PubMed1992

The interaction of SV40 large T antigen with unspecific double-stranded DNA: an electron microscopic study.

T antigen, an early protein encoded by simian virus 40 (SV40), is a specific DNA-binding protein with high affinity for elements in the viral origin of replication where it forms a double-hexameric complex as a prerequisite for DNA untwisting and, in the presence of ATP hydrolysis, for DNA unwinding. Like other specific DNA-binding proteins, T antigen also associates with DNA strands of random sequence albeit at reduced affinity. In addition, T antigen is able to unwind unspecific DNA sequences starting from internal binding sites. This property could be a step in the pathway leading to the chromosomal rearrangements that are frequently observed in SV40-transformed cells. This possibility prompted us to investigate the binding of T antigen to unspecific DNA using electron microscopy. We observed that the protein binds randomly to many unspecific DNA sites excluding a preference for particular DNA sequences or structural features. Addition of ATP to the binding buffer induces the formation of oligomeric, possibly hexameric, T antigen complexes that frequently align to form long arrays of DNA-bound protein. Magnesium salts induce the formation of tightly packed T antigen aggregates which bind to DNA to form many DNA branches and loops that emanate from the aggregated protein core. Upon ATP hydrolysis, aggregated T antigen catalyzes the unwinding of DNA duplices.

Adenosine Triphosphate↗
PubMed1992

Simian virus 40 T-antigen DNA helicase is a hexamer which forms a binary complex during bidirectional unwinding from the viral origin of DNA replication.

The role of simian virus 40 (SV40) large tumor antigen (T antigen) as a DNA helicase at the replication fork was studied. We found that a T-antigen hexamer complex acts during the unidirectional unwinding of appropriate DNA substrates and is localized directly in the center of the fork, contacting the adjacent double strand as well as the emerging single strands. When bidirectional DNA unwinding, initiated at the viral origin of DNA replication, was analyzed, a larger T-antigen complex that is simultaneously active at both branch points of an unwinding bubble was observed. The size and shape of this helicase complex imply that the T-antigen dodecamer complex, assembled at the origin and active in the localized melting of duplex DNA, is subsequently also used to continue DNA unwinding bidirectionally. Then, however, the dodecamer complex does not split into two hexamer subunits that track along the DNA; rather, the DNA is threaded through the intact complex, with the concomitant extrusion of single-stranded loops.

Antigens, Polyomavirus Transforming↗