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Members of the semaphorin family have been implicated in mediating axonal guidance in the nervous system by their ability to collapse growth cones and to function as chemorepellents. The present findings show that recombinant Semaphorin D has similar effects on cortical axons and, in addition, inhibits axonal branching. In contrast, semaphorin E acts as an attractive guidance signal for cortical axons. Attractive effects were only observed when growth cones encountered increasing concentrations or a patterned distribution of Semaphorin E, but not when they are exposed to uniform concentrations of this molecule. Specific binding sites for Semaphorin D and Semaphorin E were present on cortical fibers both in vitro and in vivo at the time when corticofugal projections are established. In situ hybridization analysis revealed that the population of cortical neurons used in our experiments express neuropilin-1 and neuropilin-2, which are essential components of receptors for the class III semaphorins. Moreover, semD mRNA was detected in the ventricular zone of the neocortex whereas semE mRNA was restricted to the subventricular zone. Taken together, these results indicate that semaphorins are bifunctional molecules whose effects depend on their spatial distribution. The coordinated expression of different semaphorins, together with their specific activities on cortical axons, suggests that multiple guidance signals contribute to the formation of precise corticofugal pathways.

Neurotrophin-3 and its receptor TrkC are expressed during the development of the mammalian cerebral cortex. To examine whether neurotrophin-3 might play a role in the elaboration of layer-specific cortical circuits, slices of layer 6 and layers 2/3 neurons were cultured in the presence of exogenously applied neurotrophin-3. Results indicate that neurotrophin-3 promotes axonal branching of layer 6 axons, which target neurotrophin-3-expressing layers in vivo, and that it inhibits branching of layers 2/3 axons, which avoid neurotrophin-3-expressing layers. Such opposing effects of neurotrophin-3 on axonal branching were also observed with embryonic cortical neurons, indicating that the response to neurotrophin-3 is specified at early developmental stages, prior to cell migration. In addition to its effects on fiber branching, axonal guidance assays also indicate that neurotrophin-3 is an attractive signal for layer 6 axons and a repellent guidance cue for layers 2/3 axons. Experiments with specific antibodies to neutralize neurotrophin-3 in cortical membranes revealed that endogenous levels of neurotrophin-3 are sufficient to regulate branching and targeting of cortical axons. These opposing effects of neurotrophin-3 on specific populations of axons demonstrate that it could serve as one of the signals for the elaboration of local cortical circuits.

Slice cultures from the visual cortex of young rats were prepared using the roller culture technique (Gähwiler 1984). After 10 days in vitro the cortical cultures flattened to 1–3 cell layers, surviving for up to 12 weeks. The cultures were organotypically organized, the typical layered structure of the cortex was preserved. The neuronal composition of slice cultures was studied using intracellular staining, Golgi impregnation and GABA immunohistochemistry. Both pyramidal cells and several types of nonpyramidal cells were identified in the slice cultures. Electrophysiological recordings showed that the electrical properties of cells in culture were similar to those measured in acute slice preparations; for some cells, however, the spontaneous activity was higher. The maintained activity was strongly increased by application of the GABA antagonist bicuculline and decreased by GABA, suggesting that GABAergic inhibition is present in these preparations. We could observe the postnatal maturation of some characteristic morphological features in culture. For example, pyramidal cells in 6 day-old rats in situ have very short basal dendrites with growth-cones, and the dendrites are free of spines. After 2–3 weeks in culture growth-cones were no longer observed. Instead, the cells had developed a large basal dendritic field and the dendrites were covered with spines. Slice cultures therefore may provide a useful tool for physiological, anatomical, pharmacological and developmental studies of cortical neurons in an organotypical environment.

The behavior of the proton magnetic form factor is studied within the modified hard scattering picture, which takes into account gluonic radiative corrections in terms of transverse separations. We parallel the analysis given previously by Li and make apparent a serious objection. The appropriate cut-off needed to render the form-factor calculation finite is both detailed and analyzed by considering different cut-off prescriptions. The use of the maximum interquark separation as a common infrared cut-off in the Sudakov suppression factor is proposed, since it avoids difficulties with the as-singularrities and yields a proton form factor insensitive to the inclusion of the soft region which therefore can be confidently attributed to perturbative QCD. Results are presented for a variety of proton wave functions including also their intrinsic transverse momentum. It turns out that the perturbative contribution, although theoretically self-consistent forQ2 larger than about 6 GeV2 to 10 GeV2, is too small compared to the data.

The effects on retinal ganglion cells of iontophoretically applied glycine, taurine and strychnine were studied in the optically intact eye of the cat. Glycine and taurine suppressed the light‐evoked discharge of all on‐centre and off‐centre brisk ganglion cells, regardless of the visual stimulus used. Strychnine blocked the action of externally applied glycine and taurine. The light‐evoked response of all ganglion cells was raised by strychnine. The tonic discharge of the light response was suppressed or raised by the drugs more than the phasic response. A population of amacrine cells, which was heavily labelled by [3H]glycine, did not take up [3H]taurine. [3H]taurine was only weakly accumulated by inner nuclear layer neurones and was predominantly located in the outer retina.

The effects of iontophoretically applied GABA (gamma‐aminobutyric acid) and bicuculline on retinal ganglion cells were studied in the optically intact eye of the anaesthetized cat. GABA suppressed both the spontaneous activity and light‐evoked discharge of all retinal ganglion cells, regardless of their type and regardless of the visual stimulus used. Bicuculline antagonized the action of iontophoretically applied GABA. Bicuculline enhanced the spontaneous activity of on‐centre cells, but suppressed the spontaneous activity of most off‐centre cells. The light‐evoked response of on‐centre cells was increased by bicuculline. A more complicated picture emerged for off‐centre cells. Weak light responses were suppressed by bicuculline, but during strong light responses the initial transient phase of the response was dramatically enhanced. Amacrine cells of the inner nuclear layer and displaced amacrine cells of the ganglion cell layer were labelled, using glutamic acid decarboxylase (GAD) immunohistochemistry and [3H]muscimol uptake. GAD‐positive dendrites were found throughout the inner plexiform layer and no sign of dendritic stratification was detected.

1. Several methods for evaluating light‐evoked response latency and its variability in brisk‐sustained (X) and brisk‐transient (Y) retinal ganglion cells were tested. The most accurate procedure proved to be that described by Levick (1973), in which the time of the occurrence of the fourth impulse after stimulus onset is taken as an estimate of the latency.

A method is proposed to extend the hard scattering picture of Brodsky and Lepage to transitions between hadrons with orbital angular momentuml=0 andl=1. The use of covariant spin wave functions turns out to be very helpful in formulating that method. As a first application we construct a light-cone wave function of the nucleon resonance N*(1535) in the quark-diquark picture. Using this wave function and the extended hard scattering picture, theN-N* transition form factors are calculated at large momentum transfer and the results compared to experimental data. As a further application of our method we briefly discuss thep- a1 form factors in an appendix.

Summary Auditory brain stem evoked responses (BSERs) were recorded from schizophrenic and depressed patients and healthy subjects. Between these groups no differences in latencies, amplitudes and variability of the BSERs were found.

Tethered walking imagines of the mealworm beetle Tenebrio molitor wave their heads in random fashion. If a periodic pattern of vertical black and white stripes is rotated around the animal a regular nystagmic head movement is superimposed upon the random waving, the frequency of the latter equals the contrast frequency within large ranges of the angular velocity of the pattern. The nystagmus is inverted: After a short period of tracking, during which the angular velocity of the head is the same as that of the panorama, the head returns slowly toward its normal position according to an exponential-like function. Resting animals do not wave their heads. However, if the above panorama is rotated, the beetle turns its head in the direction of the movement of the panorama and holds it in a side-way position, as long as the rotation is maintained. The angular position reached depends in the same manner on the angular velocity of the panorama as the turning tendency of walking animals established in open loop experiments using the spherical Y-maze method.

We have implanted Sn, Ge, and Si into Nb films. The resulting Nb-Sn compounds and their annealing behavior have been analyzed by the Mössbauer effect and compared to samples obtained by diffusion of Sn into Nb foils. Mössbauer spectra show that Nb3Sn is obtained just by implantation, but with a Tcof only 5 K. The 925°C annealing temperature necessary to form the A15 structure with long-range order of Nb chains and Tcvalues up to 17.8 K is at least 100 °C higher in implanted samples than in samples prepared by diffusion of Sn into Nb. This is explained in terms of implantation-induced lattice defects. The metastable A15 phases of Nb3Ge and Nb3Si could not be formed by Ge or Si implantation, regardless of target or annealing temperature. It is suggested that the high-energy ions only form phases stable at high temperatures and with low Tcvalues.

In an investigation of the possible coexistence of superconductivity and magnetism, we have measured the Mössbauer effect of151Eu and119Sn in EuzSn1-zMo6Sy(0=z=1,y˜7) between 0.05 and 4.2 K. The Eu spectra show a magnetic hyperfine splitting with a field of 27.3 T below 0.1 K; no fields in excess of 0.1 T have been observed at the Sn nuclei. For 0.1

Mössbauer spectra have been taken for Nb3Sn films on Hastelloy substrate bombarded with 25-MeV O4+ions at fluences of 9.6 × 1015and 4.0 × 1016cm−2, and annealed at temperatures up to 1050 ‡C. The bombarded samples are found to contain highly damaged Nb3Sn and Nb6Sn5, and show superconducting transitions at 7.3 and 3.3 K, respectively. Annealing at higher temperatures does not restore the properties of the samples to those known for undamaged Nb3Sn films. The results rather indicate the presence of Nb3Sn and also Nb6Sn5even after annealing at 850 ‡C. Annealing at 1050 ‡C seems to produce only regions of heavily damaged Nb3Sn.

Proposed future concepts for space transportation systems offer new services and a more complex field of applications regarding payload transfer, launch frequency and time in orbit. Planned transportation systems like the – formerly – European shuttle Hermes, as well as the hypersonic winged launcher concepts of Sanger, HOTOL and NASP, demand advanced developments in the field of high–temperarure stable materials in order to meet the requirements dictared by the aerothermal and aeromechanical loads arising during ascent, re–entry and cruising at high speed in the upper atmosphere. Considerable improvements in metallic materials properties have been achieved due to the development of advanced processing and manufacturing technologies and by combining lightweight metal matrix composites with suitable protective coatings in order to enhance the thermomechanical capabilities of the material. Modifications of fibre–reinforced ceramic materials and the development of carbon–carbon composites present a very promising approach to ultra–high temperature resistant materials for special applications. This paper intends to address space specific material requirements, and briefly discuss improvements to and critical aspects of material applications.

Retinal ganglion cells are the projection neurons that link the retina to the brain. Peptide immunoreactive cells in the ganglion cell layer (GCL) of the mammalian retina have been noted but their identity has not been determined1–3. We now report that, in the rabbit, 25–35% of all retinal ganglion cells contain substance P-like (SP) immunoreactivity. They were identified by either retrograde transport of fluorescent tracers injected into the superior colliculus, or by retrograde degeneration after optic nerve section. SP immunoreactive cells are present in all parts of the retina and have medium to large cell bodies with dendrites that ramify extensively in the proximal inner plexiform layer. Their axons terminate in the dorsal lateral geniculate nucleus, superior colliculus and accessory optic nuclei, and these terminals disappear completely after contralateral optic nerve section and/or eye enucleation. In the dorsal lateral geniculate nucleus large, beaded, immunoreactive axons and varicosities make up a narrow plexus just below the optic tract, where they define a new geniculate lamina. The varicosities make multiple synaptic contacts with dendrites of dorsal lateral geniculate nucleus projection neurons and presumptive interneurons in complex glomerular neuropil. This is direct evidence that some mammalian retinal ganglion cells contain substance P-like peptides and strongly suggests that, in the rabbit, substance P (or related tachykinins) may be a transmitter or modulator in a specific population or populations of retinal ganglion cells.

Mössbauer spectra of119Sn in Sn1-zEuzMo6S8 (z=0, 1/3, 2/3) have been measured in the temperature range 4.9 K?T?293 K. The spectra consist of a quadrupole doublett with unequal intensities at all temperatures. The Debye-Waller factor, the isomeric shift and the asymmetry of the intensity of the quadrupole components show irregularities between 50 K and 110 K which are interpreted as arising from a structural phase transition. The temperature dependence of the Debye-Waller factor can be approximated by a simple phonon spectrum. No quantitative explanation can be given for the large values of the asymmetry. Evidence is presented that this behavior is connected with the properties of the librational modes of the Mo6S8-units in the crystal.

Mössbauer spectra for119Sn in crystalline and disordered Sn, as well as in crystalline and liquid-like amorphous Sn1-xCux (X=0.10-0.18), have been measured at 2.6 K?T?108 K. The Debye-Waller-Factor (DWF) obtained from the spectra is identical for the crystalline and for the disordered phase. The DWF of the amorphous phase is smaller than the DWF of the crystalline phase athigh temperatures, but it shows a stronger temperature dependence than the DWF of the crystalline phase and reaches the latter one at about 4 K. From this low-temperature result we conclude that the differences of the Eliashberg functiona2(?)F(?) and of the superconducting transition temperatureTc in these two phases cannot be related to changes in the phonon spectrumF(?), but must result from changes of the interaction parametera2 (?). A comparison between DWF,a2F, and specific heat data is performed. From the values for the isomeric shift of the Mössbauer line we can show that the hybridisation and covalency of the electronic bonds present in the crystalline and in the disordered phases are destroyed in the amorphous phase. Both, the DWF and the isomer shift demonstrate that the electronic properties of crystalline and amorphous Sn(Cu) differ appreciably. The electronic and superconducting properties of amorphous Sn(Cu) are similar to the properties of the high pressure phase of tin.

Summary We have been studying the fine structural organization of slice cultures prepared from the visual cortex of 6-day-old rats and cultured for 2 weeks using a roller culture technique. Neurons in culture exhibited the characteristic cytological differences between perikarya, axons and dendrites. Neuronal and glial processes formed a dense neuropil with minimal extracellular spaces, and within the neuropil there were numerous synaptic contacts. Both morphological types of cortical synapses, type I (asymmetrical) and type II (symmetrical) could be readily identified in slice cultures. The pattern of synaptic connections in culture was remarkably similar to that observed in normal cerebral cortex: asymmetrical synapses were usually found in contact with dendritic spines, less frequently with dendritic shafts, and never on perikarya, whereas symmetrical synapses were found mostly on perikarya, occasionally on dendritic shafts but never on dendritic spines. Synaptic morphology appeared mature after 2 weeksin vitro and did not show the immature features observed at the time of culture preparation. Taken together with our previous light microscopic studies, these results indicate that cortical slice cultures are organotypically organized and serve as a useful model to study mechanisms of cortical development and plasticity.

Current light-cone wave functions for the nucleon are unsatisfactory since they are in conflict with the data of the nucleon’s Dirac form factor at large momentum transfer. Therefore, we attempt a determination of a new wave function respecting theoretical ideas on its parameterization and satisfying the following constraints: It should provide a soft overlap contribution to the proton’s form factor in agreement with data; it should be consistent with current parameterizations of the valence quark distribution functions and lastly it should provide an acceptable value for theJ/ψ → N Ndecay width. The latter process is calculated within the modified perturbative approach to hard exclusive reactions. A simultaneous fit to the three sets of data leads to a wave function whosex-dependent part, the distribution amplitude, shows the same type of asymmetry as those distribution amplitudes constrained by QCD sum rules. The assymetry is however much more moderate as in those amplitudes. Our distribution amplitude resembles the asymptotic one in shape but the position of the maximum is somewhat shifted.

Current light-cone wave functions for the nucleon are unsatisfactory since they are in conflict with the data of the nucleon’s Dirac form factor at large momentum transfer. Therefore, we attempt a determination of a new wave function respecting theoretical ideas on its parameterization and satisfying the following constraints: It should provide a soft overlap contribution to the proton’s form factor in agreement with data; it should be consistent with current parameterizations of the valence quark distribution functions and lastly it should provide an acceptable value for the J/ψ → NNdecay width. The latter process is calculated within the modified perturbative approach to hard exclusive reactions. A simultaneous fit to the three sets of data leads to a wave function whose x-dependent part, the distribution amplitude, shows the same type of asymmetry as those distribution amplitudes constrained by QCD sum rules. The asymmetry is however much more moderate as in those amplitudes. Our distribution amplitude resembles the asymptotic one in shape but the position of the maximum is somewhat shifted.

In the imagines of the mealworm Tenebrio molitor vigorous head movements during walk have been observed. This head waving is absent in resting animals. The time course of the head movement in the horizontal plane of the tethered animals was registered by means of a television camera and its power spectrum, autocorrelation function as well as the distribution of the head's angular position have been calculated. In non-homogeneous stationary panoramas, which are symmetrical with respect to the body axis the process governing the head waving is quasi-Gaussian, its power spectrum is a slowly declining exponential function of the frequency. In asymmetrical panoramas the distribution of the angular position of the head becomes also asymmetrical, and its average value is shifted with respect to the body axis. In homogeneously bright surroundings the process is the superposition of a quasi Gaussian and of a symmetrical periodic function, thus indicating a regular searching movement in addition to the random head waving. The possible role of the head waving for the fixation response is discussed.

Head movements of different species of walking beetles elicited by rotating stripe patterns have been investigated. They are of the usual type in contrast to an “inverted nystagmus” reported forTenebrio molitor in similar experimental situations. Reexamination of theTenebrio records revealed that the sign of the stimulus was interchanged by a mistake while plotting the results. Thus, the head nystagmus inTenebrio is also of the usual type, consisting of a smooth pursuit head movement followed by a faster returning phase.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.