Enzymatic kinetic analyses that employ high-performance liquid chromatography. Competition between orotate- and hypoxanthine/guanine-phosphoribosyltransferases for a common substrate

Enzymatic assay procedures that employ high-performance liquid chromatography (HPLC) have been proven to be sensitive and versatile methods for accomplishing kinetic analyses of enzyme-catalyzed reactions, with nucleotides as substrates or products. Both orotate phosphoribosyltransferase (OPRTase) and hypoxanthine/guanine phosphoribosyltransferase (HGPRTase) have been purified from Baker's yeast and analyzed kinetically using a modification of published HPLC procedures. Because these two enzymes exist in the cytosol of yeast and might compete for the limiting (approximately equal to 15 microM) concentration of phosphoribosyl alpha-1-pyrophosphate (PRibPP), we elected to examine both equilibrium and steady-state effects of one enzymatic reaction on the other with HPLC. First, under the condition of equivalent mass concentrations of OPRTase and HGPRTase, the initial rate of orotidine monophosphate synthesis and the equilibrium state were greatly affected by the presence of HGPRTase activity. In contrast, the presence of the OPRTase activity had no effect on the HGPRTase-catalyzed reaction under these conditions. Second, to examine a competition by these enzymes for PRibPP in vivo, we have established that the total activities (units/ml) of OPRTase and HGPRTase in yeast cell extracts were 740 units/ml and 450 units/ml, respectively (a 1.7:1 ratio). These relative activities were then employed in an in vitro reaction competition analysis. The results were similar to the those obtained from experiments where equivalent OPRTase and HGPRTase activities were employed and reveal profound initial velocity and equilibrium effects of one reaction on the other. Thus a real competition between these enzymes for PRibPP may occur in the yeast cell cytosol, as determined by this unique HPLC competition assay procedure.

Effects of divalent metal ions on the uptake of glutamate and GABA from synaptosomal fractions

The effects of divalent metal ions on high affinity uptake glutamate and GABA were examined, using crude and purified synaptosomal fractions prepared from brains of DBA/2CBI. The uptake velocities of both amino acids are severely reduced in the presence of Cu2+, Fe2+ and Zn2+ but remain unaffected by Co2+.

Duck lymphocytes. I. Purification and preliminary observations on surface markers

High yields of lymphocytes were obtained by centrifuging duck blood, collected into an equal volume of heparinized phosphate-buffered saline, over Ficoll-diatrizoate, sg. 1.077, for 25 min at 200 X g. These lymphocytes did not form E rosettes under a variety of conditions, or EA rosettes with sheep erythrocytes sensitized with rabbit antibodies. High proportions (50-80%) of blood and organ (thymus, bursa of Fabricius, spleen, cervical lymph node) lymphocytes had surface immunoglobulin (SmIg). The proportion of blood lymphocytes with SmIg was reduced (to 16-73%) by previous incubation in serum-free medium; less reduction occurred after similar incubation of organ lymphocytes. With or without previous treatment with neuraminidase duck lymphocytes did not express receptors for Helix pomatia lectin. However, some untreated lymphocytes did have receptors for peanut agglutinin (PNA) and the number was increased after desialation. The proportions and organ distribution of lymphocytes with SmIg or receptors for PNA did not follow a pattern consistent with any expected distribution of T and B cells. Duck blood lymphocytes were separated into 2 bands (mean densities 1.026 and 1.068 g/cm3) in Percoll gradients. These did not differ in expression of SmIg and receptors for PNA, but only the upper band (sg. 1.026) responded to phytohaemagglutinin, concanavalin A, pokeweed mitogen and rabbit anti-duck immunoglobulin serum in lymphocyte transformation tests. Blood lymphocytes were also separated according to their adherence to nylon wool. There was no difference in surface markers between the adherent and non-adherent cells, but only the adherent population responded to mitogens. It is suggested that since ducks are phylogenetically close to the amphibians and reptiles their lymphocyte populations may not express surface markers similar to those of chickens and mammals.

Enhancement of brain pyridoxal 5'-phosphate level following chronic ingestion of ethanol

The concentrations of pyridoxal 5'-phosphate in the brains of DBA/2J mice during the withdrawal period following chronic ethanol ingestion were measured fluorimetrically. The brain of the ethanol-withdrawn mouse contains 22.4 +/- 3.1% more pyridoxal 5'-phosphate than the brain of a control mouse. The enhanced susceptibility to epileptic seizures during the withdrawal phase may in part be due to the increased cerebral content of this coenzyme.

Alteration of amino acid metabolism in epileptogenic mice by elevation of brain pyridoxal phosphate

A single intraperitoneal injection of pyridoxal-5'-phosphate (PLP) in a species of mouse, DBA/2J, that is normally susceptible to sound-induced convulsion exacerbated its epileptic condition. The effect of injection was most pronounced about 30 min after the administration and subsided gradually within the following 4 h. Correlated with this increased seizure susceptibility were enhanced levels of synaptosomal aspartate and glutamate, and a diminished gamma-aminobutyric acid (GABA) level. The concentrations of nonneuroactive amino acids remained unchanged. When stimulated with veratrine, synaptosomes prepared from PLP-injected mice showed an increased release of aspartate and glutamate and a decreased release of GABA compared to those prepared from control mice. The activity of glutamate decarboxylase in the brains of PLP-treated mice was lowered, whereas the activity of GABA-transaminase was enhanced. Finally, the epileptic condition of DBA mice could be ameliorated by maintenance on a diet composed of vitamin B6-deficient feed and cellulose.

L-cycloserine: a potent anticonvulsant

The L-isomer of 4-amino-3- isoxazolidinone (L-cycloserine), a potent transaminase inhibitor, was tested for its anticonvulsant action. Intraperitoneal injection of 25 mg kg-1 protects the inbred epileptic mouse (DBA/2J) from convulsions. The drug exerts its protective influence 3 h after administration, and its effect subsides gradually thereafter. A normal mouse (CBA/Ca) can be made prone to sound-induced epilepsy by enhancing its cerebral concentrations of Zn2+ and pyridoxal-5'-phosphate (PLP). Prior administration of 25-50 mg kg-1 L-cycloserine counteracts the convulsive effects of these substances. The pretreated animal is resistant to seizures. The concentrations of glutamate and aspartate in the inferior colliculus of the treated animal are diminished, whereas the concentration of gamma-aminobutyrate is enhanced. The time course of the changes in the amino acid concentrations broadly mirrors the changes in seizure susceptibility following the treatment. Proton nuclear magnetic resonance spectra of a mixture containing equimolar concentrations of L-cycloserine, PLP, and ZnSO4 were obtained. From the resonance peaks of such an adduct, we have ascertained that a molecule of L-cycloserine forms an irreversible Schiff base with the 4-aldehyde group of PLP, and this adduct is stabilized by a zinc ion. The significance of this finding for the anticonvulsant action of the drug is discussed.

Studies on sound-induced epilepsy in mice

The cerebral concentrations of pyridoxal-5'-phosphate and divalent transition metal ions (Cu2+ and Zn2+) are appreciably higher in the seizure-susceptible strain of mouse (DBA/2J) than those in normal strains (CBA/Ca and Parkes ). By injecting metal ions intracranially and pyridoxal-5'-phosphate intraperitoneally, we could render the normal mouse prone to sound-induced epilepsy. The behaviour of the treated seizure-susceptible strain of mouse. The levels of glutamate and aspartate in its inferior colliculus were elevated and the concentration of gamma-aminobutyrate was lowered. Glutaminase inhibitors, 6-diazo-5-oxo-L-norleucine (DON) and 0-diazo-acetyl-L-serine (azaserine), and a transaminase inhibitor, 4-amino-3- isoxazolidone (L-cycloserine), when injected intraperitoneally, protected the seizure-susceptible mouse from undergoing convulsions, whereas pyridoxal-5'-phosphate and methionine sulphoximine, a glutamine synthetase inhibitor, exacerbated its epileptic condition. We propose a possible sequence of biochemical events associated with susceptibility to audiogenic seizures.

Release of endogenous Zn2+ from brain tissue during activity

The role of divalent transition metal ions in neural function is poorly understood. In excess, these ions are associated with neurological disorders such as Wilson's disease, Pick's disease and epileptic seizures. We suggest that zinc ions, which are contained in nerve terminals, are extruded into the extracellular space during neuronal activity. Excessive levels of zinc may be released during intense neuronal activation, and contribute to the paroxysm and toxic damage observed. Zinc ions are contained in high concentrations in mossy fibres of the hippocampal formation, and it is the postsynaptic neurones of these fibres which are most susceptible to the toxic effects of kainic acid, a potent convulsant, or to chronic exposure to organometallic compounds. Here we demonstrate for the first time that Zn2+ is released into the extracellular space during excitation of hippocampal slices.

Divalent transition-metal ions (Cu2+ and Zn2+) in the brains of epileptogenic and normal mice

The concentrations of Cu2+ and Zn2+ in 3 strains of mice were determined spectrophotometrically. The brain of the inborn audiogenic mouse (DBA/2J) contains higher levels of Zn2+ and Cu2+ than those found in the normal mouse (CBA/Ca or Parkes). Small differences in the metallic content in the whole brains of audiogenic and normal mice are accentuated in the hippocampus and the colliculus.

Determination of pyridoxal phosphate levels in the brains of audiogenic and normal mice

The concentrations of pyridoxal-5'-phosphate in the brains of audiogenic and normal mice were measured fluorimetrically. The brain of the audiogenic mouse (DBA/2J) contains 25% more pyridoxal-5'-phosphate than the brain of a control mouse. Intraperitoneal injection of this substance causes a transient increase of its concentration in the brain, lasting a few hours. The substance thereafter is degraded to pyridoxal and pyridoxic acid.

Experimentally induced susceptibility to audiogenic seizure

By injecting metal ions and pyridoxal-5'-phosphate, we made the normal mouse prone to sound-induced epilepsy. We showed that the levels of glutamate and aspartate in its inferior colliculus were elevated and the concentration of gamma-aminobutyrate was lowered. The time course of audiogenic susceptibility after the treatment was closely mirrored by changes in the concentrations of glutamate, aspartate, and gamma-aminobutyrate in the inferior colliculus.

The relationship between retinal and tectal growth in larval Xenopus: implications for the development of the retino-tectal projection

The modes of retinal and tectal histogenesis, as well as the patterns of terminal distribution of optic axons in larval Xenopus were studied, using anatomical techniques. We show that the retina grows by adding strips of cells at its ciliary margin. The pattern of retinal growth is asymmetrical along the dorso-ventral axis of the retina. On the other hand, the tectum grows by adding newly formed cells caudo-medially. The most rapid tectal growth takes place between stages 50 and 53, and thereafter only a small proportion of cells are added near the midline of the caudal tectum. Despite such incongruent modes of retinal and tectal growth, retinotopic order is maintained throughout larval life. We present here further evidence supporting the idea that connexions between the arrays of retinal and tectal cells shift progressively caudo-medially on the tectum during the period of growth. When the temporal pole of the retina is destroyed at various developmental stages, the density of degenerating synapses is always highest in the rostral pole of the tectum. Moreover, optic terminals stemming from the central retina spread caudally, invading newly maturing regions of the tectum. Quantitative analysis of the terminal distributions of optic axons suggest that orderly shifts in synaptic contacts between optic axons and tectal dendrites take place in the course of development. Ultrastructural observations on the maturing tectal neuropil are consistent with this view.

The orientation of the visuotectal map in Xenopus: developmental aspects

Rotations and translocations of the eye anlage were performed in Xenopus embryos of stages ranging from 21/22 to 30. Some of the operations involved grafting wild-type eye anlagen into albino host orbits. Operations were performed under a variety of operating media and conditions. In later larval life, or after metamorphosis, the visuotectal maps from the operated eyes were recorded electrophysiologically. Results fell into two classes. In the majority, the orientation of the visuotopic map corresponded to the orientation of the eye at the time of recording. In the minority the visuotopic maps were 'compound', consisting of two parts each with its own independent orientation. The organization of the compound maps was such that one component was oriented in correspondence with the orientation of the eye, while the other component was normally oriented. Histological analysis and observations on genetically marked grafts indicated that the component parts of the compound eye were of dual cellular origin. The component giving the rotated (or translocated) map belonged to the originally operated eye tissue; whereas the component giving the normally oriented map was derived from newly grown eye tissue coming from the optic stalk. In no case was a normally oriented map obtained from a rotated or translocated eye. The results are discussed in relation to mechanisms proposed to account for the determination of map-related retinal specificity.

A simple interspike interval analyzer for study of neuronal spike trains

A simple, low cost interspike interval analyzer for the analysis of trains of nerve impulses is described. The analyzer is built with readily available integrated circuits and has been used to analyze spike trains in the lateral vestibular nucleus of cats.

Changes in diffraction patterns with length in single muscle fibres at rest

The addition of a simple X--Y sampling circuit to a closed circuit television system (CCTV) permits measurements of a narrow profile of the laser diffraction patterns from single frog muscle fibres at rest. Results confirm that maximum intensity occurs at 2.95-3.00 micron, but a positive linear relationship between the dispersions of sarcomeres and sarcomere lengths from 2.7--3.8 micron is obtained in four isolated single fibres.

Observations on the formation of the brain and of nerve connections following embryonic manipulation of the amphibian neural tube

A small piece of tissue in the neural tube of Xenopus laevis was cut out and either replaced in the same embryo after rotating it by 180°, or trans­planted into another embryo. We show that, within 24 h after fertilization, the neural tube consists of a mosaic of regions, each with a fixed tendency to develop into a specific part of the nervous system. If a piece of tissue is grafted into another embryo, a pre-determined structure is formed without reference to its surroundings. If a piece of tissue is rotated by 180°, the neural structures resulting from it are correspondingly reversed. Despite such determination of the anatomical polarity, the polarity for selective nerve connections remains labile. When a part or the whole of the tectum is reversed, the trajectory of the optic nerve is sometimes altered, but the array of optic axons invading the tectum ignores the anatomical reversal of the target structure and forms a normal retino-tectal projection with respect to the whole animal. Whenever the position of the diencephalon relative to the tectum is altered, however, the axial polarity of the retino-tectal projection is altered correspondingly.

Dynamics of the amphibian middle ear

Using laser speckle interferometry, we show that the directional information of acoustic signals is encoded in the motion of eardrum. Moreover, the frequency sensitivity of the auditory system is determined by the mechanical properties of the middle ear, which acts like a damped resonator.

Neurophysiological basis of directional hearing in amphibia

Discrete regions of neural tissue along the anterior-posterior axis of the torus semicircularis act as resonators, each one broadly tuned to a different range of frequencies, and each one responding optimally to the incident sound from a different direction. In this way, a map of auditory space is represented in the midbrain.

Correlative study of pain perception and masticatory muscle reflexes in man

An attempt was made to explore the possibility of using the masticatory muscle reflexes as pain indices in human subjects. It was found that the digastric reflex, so readily evoked by electrodental stimulation in experimental animals, could not be consistently elicited in man. The reflex silent period of the masseteric muscles, on the other hand, may be used as a pain index for evaluating dental analgesia, provided that analgesic interference is anticipated at points along the reflex arc.

Polarity of structure and of ordered nerve connections in the developing amphibian brain

Tectal polarity for retinal connections remains reversible long after the anatomical pattern of neural structures has been determined. Cells in the diencephalon seem to control this polarity. Following certain embryonic operations, the diencephalon developed behind the tectum. In such cases, the polarity of the retino-tectal projection was reversed.

The structural and functional development of the retina in larval Xenopus

The structural transformations of the larval Xenopus retina at successive stages of development, and concomitant changes in response characteristics of retinal ganglion cells, were studied using histological and electrophysiological techniques. The first sign of visually evoked electrical responses appears at about the time when the ganglion cells spread out into a single layer and shortly after the inner and outer plexiform layers become discernible. Initially giving simple "on" responses, the cells progressively change their response characteristics and become "event" units. Subsequently, "dimming" units can be identified. Throughout larval life, response properties of these two types become more distinct from one another and approximate to those found in the adult. So do the arborization patterns of the dendritic trees of the ganglion cells. Two types of branching patterns are identifiable in Golgi preparations. Around metamorphic climax, a new type of ganglion cell appears, coinciding with the emergence of "sustained" units electrophysiologically. After metamorphosis, the retina still grows both in thickness (mainly in the inner plexiform layer) and diameter. The three unit types change such they they come to show pronounced inhibitory effects from the peripheral visual field on the receptive field and each unit type acquires a distinct pattern of endogenous discharge.

The synaptic organization of optic afferents in the amphibian tectum

Potentials in the amphibian tectum, evoked by stimulation of the optic nerve, were recorded extracellularly. Four discrete potentials, referred to as the m 1 , m 2 , u 1 and u 2 waves, occur at different latencies after stimulation. We have shown that these waves represent summed post-synaptic potentials generated by synchronous activation of four different groups of optic nerve fibres. The m 1 and m 2 waves are generated by two classes of myelinated optic nerve fibres, previously characterized as ‘dimming’ and ‘event’ fibres. The maximum amplitude of the m 2 wave occurs just below, and of the m 2 wave just above, cell layer 8 of P. Ramón. The u 1 and u 2 waves are generated by ‘edge’ and ‘convexity’ fibres, respectively. The maximum amplitude of the u 1 wave occurs near the surface of the tectum, and of the u 2 wave some 100 μm below it. Postsynaptic cell bodies for all four classes of optic fibre are located in layer 8.

Functional synaptic relations during the development of the retino-tectal projection in amphibians

Stimulation of the optic nerve elicited pronounced, monophasic potentials in the larval amphibian tectum. These potentials are identified as summed postsynaptic potentials generated by synchronous activation of optic nerve fibres. Functioning synaptic junctions are present in the rostrolateral portion of the tectum in early larval Xenopus . With development, the area on the tectum from which synaptic potentials are recorded expands caudally, until the whole tectal area can be synaptically activated just prior to metamorphic climax. These findings, taken in conjunction with other related studies, demonstrate that connexions between pre- and postsynaptic neurons in this system are not invariant but shift continuously throughout development.

Procion yellow M-4RS binding to neuronal membranes

Procion Yellow M-4RS, an intracellular neuronal marker, has been shown to bind covalently to the subcellular organelles of rat brain under physiologic conditions. Binding to purified proteins, nucleic acids and lipids also occurs under such conditions, the probable site of attachment being the primary amino group. Lipid membranes were highly impermeable to this dye. The fluorescence intensity of Procion Yellow M-4RS was shown to be viscosity-dependent. These results are consistent with the behavior of the dye when it is used for mapping neurons.

The evolution of the retinotectal map during development in Xenopus

The retinotectal projection was mapped eleetrophysiologically in tadpoles of Xenopus laevis . Recording was performed with the animals immersed in saline inside a transparent hemisphere. Visual responses could be recorded from the optic tectum from about stage 43 onwards. The visual map on the tectum of the tadpole was found to differ in several respects from that in the adult. The earliest responses recorded showed very large multi-unit receptive fields and no discernible retinotopic organization. From about stage 47 the map showed the adult type of order; nasal field projected rostrally, temporal field caudally, superior field medially and inferior field laterally. However, in tadpoles up to stage 63/64 the projection was markedly distorted in that nasal field was confined to the most rostral region of the tectum and theie was an expanded representation of the temporal pole of the field. In tadpoles the entire visual projection covers only the rostral one-half to two-thirds of the tectum. These results, in conjunction with the results of previous studies on the mode of growth of the retina and tectum, indicate a progressive shift of the retinotectal projection with development which may involve changing synaptic relations between retinal fibres and tectal cells.

Proceedings: The CLOOGE: a simple device for interspike interval analysis

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Dendritic-tree anatomy codes form-vision physiology in tadpole retina

In tadpole frog retina, the development of four classes of visual form detectors matched the growth of four types of ganglion cell dendritic trees. From this correlation of electrophysiology and anatomy we concluded that the structure of retinal ganglion cell dendritic trees provides the code for detection of visual shapes.

Effects of delayed reinforcement in a concurrent situation

Pigeons were trained to peck either of two response keys for food reinforcement on equated aperiodic schedules. Delays of reinforcement for pecks at one key reduced the relative frequency of pecking exponentially as a function of the delay interval. Similar functions were obtained when other dependent variables were plotted against the delay interval.