Role of fracture toughness in impact-abrasion wear.

Two new low alloyed steels were developed with different fracture toughness values but at similar level of hardness with same composition and microstructural phase. The steels were subjected to impact-abrasion wear test. This work examines specifically the additional role of toughness during impact-abrasion wear, using a newly developed high toughness steel. Microstructural characterisation of the damaged samples revealed that better toughness helps resist both impact and abrasion damage.

CryoEM at 100†keV: a demonstration and prospects.

100 kV is investigated as the operating voltage for single-particle electron cryomicroscopy (cryoEM). Reducing the electron energy from the current standard of 300 or 200 keV offers both cost savings and potentially improved imaging. The latter follows from recent measurements of radiation damage to biological specimens by high-energy electrons, which show that at lower energies there is an increased amount of information available per unit damage. For frozen hydrated specimens around 300 Šin thickness, the predicted optimal electron energy for imaging is 100 keV. Currently available electron cryomicroscopes in the 100-120 keV range are not optimized for cryoEM as they lack both the spatially coherent illumination needed for the high defocus used in cryoEM and imaging detectors optimized for 100 keV electrons. To demonstrate the potential of imaging at 100 kV, the voltage of a standard, commercial 200 kV field-emission gun (FEG) microscope was reduced to 100 kV and a side-entry cryoholder was used. As high-efficiency, large-area cameras are not currently available for 100 keV electrons, a commercial hybrid pixel camera designed for X-ray detection was attached to the camera chamber and was used for low-dose data collection. Using this configuration, five single-particle specimens were imaged: hepatitis B virus capsid, bacterial 70S ribosome, catalase, DNA protection during starvation protein and haemoglobin, ranging in size from 4.5 MDa to 64 kDa with corresponding diameters from 320 to 72 Å. These five data sets were used to reconstruct 3D structures with resolutions between 8.4 and 3.4 Å. Based on this work, the practical advantages and current technological limitations to single-particle cryoEM at 100 keV are considered. These results are also discussed in the context of future microscope development towards the goal of rapid, simple and widely available structure determination of any purified biological specimen.

Author Correction: Magnetism and high magnetic-field-induced stability of alloy carbides in Fe-based materials.

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Magnetism and high magnetic-field-induced stability of alloy carbides in Fe-based materials.

Understanding the nature of the magnetic-field-induced precipitation behaviors represents a major step forward towards unravelling the real nature of interesting phenomena in Fe-based alloys and especially towards solving the key materials problem for the development of fusion energy. Experimental results indicate that the applied high magnetic field effectively promotes the precipitation of M23C6 carbides. We build an integrated method, which breaks through the limitations of zero temperature and zero external field, to concentrate on the dependence of the stability induced by the magnetic effect, excluding the thermal effect. We investigate the intimate relationship between the external field and the origins of various magnetics structural characteristics, which are derived from the interactions among the various Wyckoff sites of iron atoms, antiparallel spin of chromium and Fe-C bond distances. The high-magnetic-field-induced exchange coupling increases with the strength of the external field, which then causes an increase in the parallel magnetic moment. The stability of the alloy carbide M23C6 is more dependent on external field effects than thermal effects, whereas that of M2C, M3C and M7C3 is mainly determined by thermal effects.

Synthesis, resolution, and absolute configuration of the isomers of the neuronal excitant 1-amino-1,3-cyclopentanedicarboxylic acid.

The endogenous amino acids glutamate and aspartate depolarize mammalian neurons to produce excitation, and the rigid glutamate analogue 1-amino-1,3-cyclopentanedicarboxylic acid also has this effect. This compound exists as two pairs of geometric isomers, and in the present study the absolute configuration of the four isomers is assigned. The known (+)-S and (-)-R isomers of 3-oxocyclopentanecarboxylic acid were used as the basis for the synthesis. The cis and trans amino acids were obtained by fractional crystallization. Spectral data, including optical rotation, circular dichroism, and 13C nuclear magnetic resonance, are presented. The compounds were evaluated as excitants by microiontophoretic ejection into the dendritic region of impaled CA1 pyramidal neurons of rat hippocampal slices. One isomer, cis-1R,3R, mimicked completely the actions elicited by N-methyl-D-aspartic acid; the other three isomers were alpha-kainic acid like.

4,5,6,7-Tetrahydroisothiazolo[5,4-c]pyridin-3-ol and related analogues of THIP. Synthesis and biological activity.

The thio analogues of the GABA (gamma-aminobutyric acid) agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol), the GABA uptake inhibitor THPO (4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol), and the glycine antagonist THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-3-ol) have been synthesized and tested biologically on single neurons in the cat spinal cord and in vitro by using synaptic membrane preparations obtained from rat brains. In contrast to THIP, thio-THIP (4,5,6,7-tetrahydroisothiazolo[5,4-c]pyridin-3-ol, 5) was only a weak GABA agonist. Thio-THPO (4,5,6,7-tetrahydroisothiazolo[4,5-c]pyridin-3-ol, 10) was slightly weaker than THPO as an inhibitor of GABA uptake in vitro, and these two compounds were approximately equipotent in enhancing the inhibition of the firing of cat spinal neurons by GABA. Like THAZ and structurally related bicyclic isoxazole zwitterions, thio-THAZ (5,6,7,8-tetrahydro-4H-isothiazolo[4,5-d]azepin-3-ol, 15) was an antagonist at glycine receptors on cat spinal neurons. The I/U ratios, which reflect the ability of neutral amino acids to penetrate the blood-brain barrier (BBB), were calculated for 5 (I/U = 16), 10 (63), and 15 (200). These low I/U ratios, compared with the findings that THIP (I/U = 500 or 1500) and THPO (I/U = 2500) enter the brain after systemic administration, suggest that the thio analogues may penetrate the BBB very easily.

2-Amino-5-phosphonovalerate and Co2+ selectively block depolarization and burst firing of rat hippocampal CA1 pyramidal neurones by N-methyl-D-aspartate.

Intracellular recordings from pyramidal neurones during microiontophoretic ejection of N-methyl-D-aspartate and quisqualate into the pyramidal cell layer of the CA1 region of the rat hippocampal slice showed that both amino acids caused depolarization and evoked spike activity. Whereas quisqualate evoked tetrodotoxin-sensitive spikes, those produced by N-methyl-D-aspartate consisted of bursts of tetrodotoxin-sensitive action potentials superimposed on an underlying depolarizing shift of membrane potential. Both membrane depolarization and the superimposed depolarizing shifts associated with N-methyl-D-aspartate excitation were selectively and reversibly antagonized by the D(-) isomer of 2-amino-5-phosphonovalerate and Co2+. Both amino acids caused an increase in membrane conductance when small ejection currents were used, and the depolarizing response to these compounds was prevented by current injection. However, only the increase by N-methyl-D-aspartate was blocked by 2-amino-5-phosphonovalerate and Co2+. These results provide evidence to support the suggestion that different mechanisms underlie the excitatory response to N-methyl-D-aspartate and quisqualate in CA1 pyramidal neurones.

The N-methyl-D-aspartate receptor and burst firing of CA1 hippocampal pyramidal neurons.

Previous intracellular investigations in the rat hippocampus have demonstrated that N-methyl-D-aspartate, ibotenate and 2,3-pyridine dicarboxylate (quinolinate) all evoke burst firing of CA1 pyramidal neurons, whereas kainate and quisqualate, which are thought to react with different receptors, do not. The purpose of the present study has been to investigate the ability of a series of compounds either to trigger burst firing or to antagonize this pattern of excitation. We report here that N-methyl-L-aspartate, 1,2-benzene dicarboxylate (phthalate) and methylene succinate (itaconate) are also capable of evoking burst firing. The results of this investigation suggest that since both quinolinate and phthalate are rigid planar molecules and only the 2 and 3 positioning of the carboxylates of pyridine was active, a cis configuration of the carboxyls with respect to the 2,3 carbon bond appears to be necessary for excitation. While a nitrogen atom is not necessary for activity (this is absent in phthalate and itaconate) a third functional group, bearing at least a partial positive charge, and in a position alpha to one of the carboxyl groups is required. The requirements for pyridine derivatives to trigger burst firing is similar to that reported as necessary for evoking convulsions and neurotoxicity after intrahippocampal infusion and a correlation between N-methyl-D-aspartate-like burst firing and depolarization and this neuropathology is considered. An important observation has been that the addition of a benzene ring to either quinolinate or phthalate to yield 2,3-quinoline dicarboxylate and 2,3-napthalene dicarboxylate, respectively, converted these excitants into antagonists of burst firing.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal interneurone depression by DS103-282.

The depressant effect of 5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzothiodiazole (DS103-282) on the polysynaptic excitation of interneurones in the cat spinal cord appears to be related to a postsynaptic reduction in the effectiveness of excitatory transmitters than to interference with their presynaptic release.

Photoperiodic control of gonadotrophin secretion in the ram: a detailed study of the temporal changes in plasma levels of follicle-stimulating hormone, luteinizing hormone and testosterone following an abrupt switch from long to short days.

Six adult Soay rams were housed under artificial lighting conditions of long days (16 h light:8 h darkness) for 4 months and this caused the animals to lapse into a state of reproductive quiescence with low levels of gonadotrophins in the circulation and regressed testes secreting very low amounts of testosterone. The photoperiod was changed abruptly to short days (8 h light:16 h darkness) to induce a resurgence of sexual activity, and a detailed study was made of the pituitary and testicular responses over the first 100 days. Plasma levels of LH and FSH first began to increase between days 6 and 12 of short days, and rose progressively until days 33-54 before declining again. Testicular growth of the rams began on days 19-26 and continued for most of the remaining period of study. Plasma testosterone levels rose in parallel with the growth of the testes, and were greatly increased by day 100 when gonadotrophin levels were reduced. At most stages there were short-term fluctuations in the plasma levels of FSH, LH and testosterone indicative of episodic secretion. Peaks in plasma levels of LH were especially conspicuous and from the changes in frequency and amplitude of these peaks it was possible to predict the way in which photoperiod influenced gonadotrophin secretion by its effect on hypothalamic LH-RH secretion. A slight 24 h rhythm in the plasma levels of all three hormones was observed, and the significance of this in relation to the photoperiodic response is discussed.

Seasonal and circadian changes in the episodic release of follicle-stimulating hormone, luteinizing hormone and testosterone in rams exposed to artificial photoperiods.

Six rams of an ancient breed of domesticated sheep (SOAY) were subjected to an artificial light régime of alternating periods of long days (16 h light: 8 h darkness) and short days (8 h light: 16 h darkness) which induced seasonal development and regression of the testes during a period of 36 weeks. Over 2000 blood samples were taken, and the changes in plasma levels of FSH, LH and testosterone were related to the cycle of testicular activity. During long days plasma levels of gonadotrophins became very low and the testes regressed to about 20% of their maximum size; there was a corresponding reduction in plasma testosterone levels. When the rams were returned to short days reproductive development was again stimulated after 2-3 weeks with a progressive increase in plasma FSH and LH levels and consequent hypertrophy of the testes. It took about 16 weeks of short days for testicular activity to become maximal. Blood samples collected at hourly intervals for 24 h on ten occasions during the study revealed transitory peaks in plasma FSH and LH levels indicative of episodic release. Changes in gonadotrophin secretion were modulated primarily by alterations in the frequency of episodic releas; less than 1 spike per 24 h during long days increased to a maximum of 10 spikes/24 h under short daylengths. The peaks of FSH release were of smaller amplitude than those of LH, although during periods of frequent episodic release basal levels of fsh were increased to a greater extent than those of LH. A circadian rhythm was observed in the plasma levels of FSH, LH and testosterone, which was related to increased gonadotrophin release during the dark phase of the 24 h cycle; changes in blood haematocrit were also observed. The circadian changes appeared to be correlated with the activity cycle of the animals which in turn was dictated by daylight. A possible interrelationship between the circadian cycle and the seasonal cycle is discussed.

A novel spinal cord slice preparation from the rat.

A spinal cord slice preparation is described. The lumbar enlargement of weanling rats is exposed by laminectomy, the dorsal and ventral roots cut and a portion of cord consisting of about 4 segments removed and chilled to 3 degrees C. A modified tissue chopper is used to produce 400 micron dorsoventral longitudinal slices. The slices are maintained in a low-volume, continuously perfused tissue chamber at the interface between warm moist 95% O2/5% CO2 and oxygenated artificial cerebrospinal fluid, and remain viable for over 8 h. An extracellular recording of excitatory amino acid-induced activity of a dorsal horn lamina IV cell is shown. Excitatory amino acid antagonists applied iontophoretically and in the perfusate have actions similar to those in the adult spinal cord in vivo. This in vitro preparation of the cord has permitted stable extracellular recordings from single cells of 2 h or more, and has the potential for intracellular investigation of spinal cord neurones.

Post-excitatory depression of neuronal firing by acidic amino acids and acetylcholine in the cat spinal cord.

In the spinal cord of cats anaesthetized with pentobarbitone, the excitation of interneurones and Renshaw cells by acidic amino acids or acetylcholine (Renshaw cells) is followed by a period of depressed excitability. This depression appears to be the consequence of prolonged repetitive firing rather than of the enzymic or chemical conversion of the excitants to neuronal depressants.

Antagonists of synaptic and amino acid excitation of neurones in the cat spinal cord.

In the spinal cord of the anaesthetized cat microelectrophoretically administered (+/-)-cis-2,3-piperidine dicarboxylate (2,3-PDA), (+/-)-cis-2,5-piperidine dicarboxylate (2,5-PDA), gamma-D-glutamylglycine (gamma DGG), beta-D-aspartyl-beta-alanine (beta DAA), (+/-)-2-amino-4-phosphonobutyrate (2-APB), (+/-)-2-amino-5-phosphonovalerate (2-APV) and (+/-)-2-amino-7-phosphonoheptanoate (2-APH) were assessed as antagonists of chemical excitation of dorsal horn interneurones and Renshaw cells by N-methyl-D-aspartate (NMDA), L-aspartate, quisqualate (QUIS), kainate and L-glutamate, and of monosynaptic and polysynaptic excitation by impulses in primary afferent fibres of muscle and cutaneous origin. Whereas polysynaptic excitation of interneurones was readily and reversibly depressed by 2-APV, 2-APH, beta DAA, gamma DGG and 2,3-PDA, all of which also reduced excitation by NMDA (and L-aspartate) more than that by QUIS (and L-glutamate), no selective antagonism of monosynaptic excitation could be demonstrated. In particular, 2,3-PDA, which depressed excitation by kainate to a greater extent than that by either QUIS or NMDA, appeared to have no effect on monosynaptic excitation. The results support the involvement of L-aspartate as the transmitter of some spinal excitatory interneurones, but none of the antagonists tested were considered suitable for assessing the role of L-glutamate as the transmitter of some spinal primary afferent fibres.

Effects of noradrenaline and 5-hydroxytryptamine on spinal Ia afferent terminations.

When administered microelectrophoretically, noradrenaline (NA) and 5-hydroxytryptamine (5-HT) increased the thresholds of the terminal portions of extensor muscle Ia afferents stimulated extracellularly near lumbar motoneurons of anesthetized cats. This effect, and the concomitant increase in the electrical resistance of the extracellular medium near the orifices of multibarrel micropipettes, could be reversibly altered by ouabain. The results suggest further evidence is required of a direct effect of these amines at transmitter-related receptors on Ia terminations since the observed increase in threshold may be indirect, resulting from the sodium-dependent uptake of the administered amines by neurons and glia.

Acridinic acid: a new antagonist of amino acid-induced excitations of central neurones.

The actions of acridinic acid (2,3-quinoline dicarboxylic acid), a new derivative of quinolinic acid, as an antagonist of amino acid-induced excitations are described. Acridinate, like kynurenate, in the cerebral cortex reduced the effects of all amino acids equally, but in the spinal cord was significantly less active against quisqualate.

Glutamic acid agonists. Stereochemical and conformational studies of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and related compounds.

Microelectrophoretic techniques were used to study the effects of the optical isomers of the L-glutamic acid (GLUT) agonist AMPA on cat spinal neurones. Both enantiomers excited spinal interneurones, L-AMPA being more potent than D-AMPA, and, like GLUT, this excitation was blocked by L-glutamic acid diethyl ester but not by 2-amino-5-phosphonovaleric acid. ATPA and ABPA, in which the methyl group of AMPA was replaced by more bulky substituents, were also GLUT agonists, although weaker than AMPA. O-methyl-AMPA was inactive, suggesting that a necessary condition for GLUT agonist or antagonist actions of this class of compound is the presence of an acidic group in the position equivalent with the omega-position of GLUT.

Pre-and postsynaptic actions of baclofen: blockade of the late synaptically-evoked hyperpolarization of CA1 hippocampal neurones.

Using intracellular recording techniques, the effects of beta-p-chlorophenyl-GABA (baclofen) on passive membrane properties and postsynaptic potentials of CA1 pyramidal neurones were investigated. In experiments where only the hyperpolarizing action of baclofen was precluded by conventional current clamp techniques, 20 microM ( +/- ) baclofen blocked the early GABA-mediated IPSP and also a late hyperpolarization which, since it could be evoked by orthodromic stimulation subthreshold for spike firing, would not be expected to be produced by a Ca2+-activated increase in potassium conductance (AHP), but to be a transmitter-mediated event. In addition the conductance increase associated with this late IPSP evoked by subthreshold stimulation and also that associated with the AHP produced by spike activation were abolished. Baclofen also appeared to increase the duration of EPSPs, an event possibly related to loss of IPSPs. The hyperpolarization produced by baclofen was associated with an increased conductance of the resting membrane, an event possibly associated with an elevated potassium flux. To preclude this postsynaptic effect as a cause of reduced synaptic responses, tetraethylammonium chloride (TEA), a compound which decreases conductance and depolarizes the membrane of CA1 pyramidal neurones by a reduction of a 'leak' or resting potassium conductance (gK), was added to the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)