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G Lynch

Publications and source records attributed to G Lynch.

At least 361 records · Page 20Linked to original sources
PubMed1987

Polyamines stimulate mitochondrial calcium transport in rat brain.

The effects of the polyamines spermine and spermidine on rat brain mitochondrial calcium transport were examined using a variety of techniques for measuring the kinetics of calcium uptake and the buffering capabilities of isolated mitochondria. Spermine both increased the rate of calcium accumulation and decreased the set-point to which isolated mitochondria buffer free calcium concentration. In the presence of physiological concentrations of sodium and magnesium, spermine lowered the extramitochondrial calcium level to approximately 0.3 microM, a value close to the resting intracellular calcium concentration. The effect of polyamines was concentration dependent, with a half-maximal effect of spermine observed at approximately 0.1-0.4 mM (respiratory substrate dependent), whereas spermidine was approximately 10 times less potent. Calcium transport by hippocampal mitochondria was stimulated markedly more by spermine than was calcium transport by mitochondria isolated from brainstem. The stimulatory effect of spermine was not due to an increase in the transport of respiratory substrates inside the mitochondria nor to an effect on the enzymes using these respiratory substrates. An examination of the effect of spermine on the kinetics of calcium uptake indicated that spermine increased calcium uptake maximally at low calcium concentrations. Beyond that level, the stimulatory effect of spermine decreases, and spermine can even inhibit calcium uptake. These results are in good agreement with previous reports on the effects of polyamines on calcium transport in mitochondria from peripheral tissue. They support the hypothesis that spermine increases the rate of calcium uptake by mitochondria by increasing the affinity of the uniporter for calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

L-phenylalanyl-L-glutamate-stimulated, chloride-dependent glutamate binding represents glutamate sequestration mediated by an exchange system.

Stimulation of glutamate binding by the dipeptide L-phenylalanyl-L-glutamate (Phe-Glu) was inhibited by the peptidase inhibitor bestatin, suggesting that the stimulation was caused by glutamate liberated from the dipeptide and not by the dipeptide itself. It further suggests that this form of glutamate binding should be reinterpreted as glutamate sequestration and that stimulation of binding both by dipeptides and after preincubation with high concentrations of glutamate is likely to be due to counterflow accumulation. Several other criteria indicate that most of glutamate binding stimulated by chloride represents glutamate sequestration: Binding is reduced when the osmolarity of the incubation medium is increased, when membranes incubated with [3H]glutamate are lysed before filtration, and when membranes are made permeable by transient exposure to saponin. Moreover, dissociation of bound glutamate after a 100-fold dilution of the incubation medium is accelerated about 50 times by the addition of glutamate to the dilution medium. This result would be anomalous if glutamate were bound to a receptor site; it suggests instead that glutamate is transported in and out of membrane vesicles by a transport system that preferentially mediates exchange between internal and external glutamate. Glutamate binding contains a component of glutamate sequestration even when measured in the absence of chloride. Sequestration is adequately abolished only after treating membranes with detergents; even extensive lysis, sonication, and freezing/thawing may be insufficient.

Aminopeptidases↗
PubMed1987

Ontogeny, compartmentation, and turnover of spectrin isoforms in rat central neurons.

A variant of a principal structural protein of erythrocytes, spectrin, is a major neuronal protein. Here we have examined the subcellular and regional distributions, subunit composition, ontogeny, and metabolism of spectrin in rat CNS. While all subcellular fractions, except the mitochondrial, expressed the previously characterized brain form of spectrin (fodrin, or alpha gamma-spectrin), limited brain regions contained, in cytoplasm, a second isoform immunologically related to erythrocyte alpha beta-spectrin. Both alpha gamma- and alpha beta-spectrin are primarily neuronal, as evidenced by immunocytochemistry. The spectrins are distributed between 2 distinct subneuronal compartments: a membrane-associated domain containing alpha gamma-spectrin in relatively constant amounts across brain regions, and a cytoplasmic domain containing both the alpha gamma and alpha beta isoforms in widely varying amounts across brain regions. Although forebrain has considerable alpha beta-spectrin, the diencephalon, mesencephalon, and brain stem are devoid of this isoform. Further evidence for spectrin compartmentation comes from its ontogeny. Membrane-associated alpha gamma-spectrin is present at birth at its adult levels, but cytoplasmic alpha beta-spectrin is expressed only following the second postnatal week. Similarly, the 4-fold difference in cytoplasmic alpha gamma-spectrin content across brain regions develops during the third postnatal week. In this compartment, both spectrin forms may be metabolized in vivo, at least in part, by calcium-activated proteolysis. The presence in mammalian neurons of 2 spectrin isoforms and their compartmentation into distinct domains suggests multiple functions for neuronal spectrin, one of which may be in the stabilization or maturation of forebrain neurons.

Aging↗
PubMed1986

Induction of synaptic potentiation in hippocampus by patterned stimulation involves two events.

Electrical stimulation of axons in the hippocampus with short high-frequency bursts that resemble in vivo activity patterns produces stable potentiation of postsynaptic responses when the bursts occur at intervals of 200 milliseconds but not 2 seconds. When a burst was applied to one input and a second burst applied to a different input to the same target neuron 200 milliseconds later, only the synapses activated by the second burst showed stable potentiation. This effect was observed even when the two inputs innervated completely different regions of the postsynaptic cells; but did not occur when the inputs were stimulated simultaneously or when the second burst was delayed by 2 seconds. Intracellular recordings indicated that the first burst extended the decay phase of excitatory postsynaptic potentials evoked 200 milliseconds later. These results suggest that a single burst of axonal stimulation produces a transient, spatially diffuse "priming" effect that prolongs responses to subsequent bursts, and that these altered responses trigger spatially restricted synaptic modifications. The similarity of the temporal parameters of the priming effect and the theta rhythm that dominates the hippocampal electroencephalogram (EEG) during learning episodes suggests that this priming may be involved in behaviorally induced synaptic plasticity.

Animals↗
PubMed1986

Patterned stimulation at the theta frequency is optimal for the induction of hippocampal long-term potentiation.

Short, high frequency stimulation bursts (4 pulses at 100 Hz) were applied to Schaffer/commissural projections to the CA1 field of rat hippocampal slices at 0.1, 0.2, 1.0 or 2.0-s intervals to assess their efficacy in eliciting long-term potentiation (LTP). Bursts repeated at 2-s intervals induced very little LTP; shorter repetition intervals reliably elicited LTP, with the 200-ms repetition interval producing the greatest potentiation. A short-term potentiation effect, which was maximal 20 s after the last burst and decayed within 10 min, was affected differently by the stimulation parameters than was LTP, suggesting that the two phenomena are due to different processes. The results indicate that patterns of stimulation resembling spike discharge patterns of hippocampal neurons in animals in exploratory situations are effective in inducing LTP and suggest temporal constraints on the mechanisms involved in triggering synaptic plasticity.

Animals↗
PubMed1986

A comparative study of soluble calcium-dependent proteolytic activity in brain.

Recent studies have shown that soluble calcium activated proteases (calpains) in brain degrade proteins associated with the cytoskeleton and vary markedly in activity across regions and as a function of development. It was suggested that the observed differences in calpain activity reflect differences in the turnover rate of structural elements. The present study extends this analysis by measuring the properties and activity of calpain in representatives of the five classes of vertebrates with particular emphasis on the mammals. No evidence for proteolysis was found in soluble fractions of fish brains at neutral pH in the presence or absence of added calcium. A substantial calcium-independent proteolytic activity was found in amphibian brains--the effects of a variety of protease inhibitors indicated that it is also a neutral thiol (cysteine) protease. Reptilian brains exhibited both calcium-independent and calcium-dependent proteolytic activity. Virtually all proteolytic activity in birds (5 species) and mammals (9 species) measured at neutral pH was calcium-dependent. The endogenous substrates for the calcium activated proteases were very similar in several species of birds and mammals as were the effects of a variety of protease inhibitors. However, the activity of the enzyme, expressed per mg of soluble protein, was highly and negatively correlated with brain size in the mammals. The allometric expression for this relationship was similar to that found for the density of neurons in cerebral cortex as a function of absolute brain size. These results indicate that soluble proteolytic enzymes in brain are differentially expressed among classes of vertebrates and suggest that the turnover of cytoskeletal elements in birds and mammals differs in important ways from that found in fish and amphibians. The results obtained for mammals raise the possibility of a relationship between brain size and the rate at which structural elements are broken down and replaced in this vertebrate class.

Amphibians↗
PubMed1986

Jump function following nerve division.

This paper presents the results of an experimental study of the conduction of nerve impulses across the suture line of freshly repaired nerves. The sciatic nerves of ten New Zealand white rabbits were divided and repaired using microsurgical techniques. The conduction impulses from a nerve stimulator across the repair was compared with the conduction before division. In 7 out of 10 nerves the impulse was successfully transmitted using a current comparable with that required before division. These findings provide an explanation for the fact that nerve function can sometimes be detected after trauma when the nerves are subsequently shown to have been transected. The implications are discussed.

Animals↗
PubMed1986

Studies on retrograde and anterograde amnesia of olfactory memory after denervation of the hippocampus by entorhinal cortex lesions.

The effect of hippocampal denervation on olfactory memory in rats was tested after interrupting the lateral olfactory tract projections at the level of the entorhinal cortex. When lesioned animals were trained to learn new odors, they showed no evidence of retention 3 h after acquisition. These results confirm earlier data on rapid forgetting in rats after hippocampal deafferentation and are in parallel to the anterograde amnesia typically found in humans with hippocampal damage. On the other hand, preoperatively learned information was minimally impaired after hippocampal deafferentation even if it was acquired within less than 1 h before the lesion. This finding differs from reports on humans as well as monkeys with hippocampal damage where memories formed during a critical time span of months or even years before the lesion are found to be impaired. This may suggest that the consolidation process in humans and rodents has different time scales or that the roles of the human and the rat hippocampal structure in memory formation are somewhat different.

Amnesia↗
PubMed1986

Induction of glutamate binding sites in hippocampal membranes by transient exposure to high concentrations of glutamate or glutamate analogs.

The number of Na+-independent, Cl--dependent glutamate binding sites in rat hippocampal membranes is increased two- to fourfold after pre-exposing isolated membranes or hippocampal slices to high concentrations (0.1-10 mM) of L-glutamate or of glutamate analogs with high affinity for this binding site, such as quisqualate, homocysteate, or aminoadipate. N-Methylaspartate and kainate are ineffective. A similar binding increase is induced by transient exposure to the dipeptide tyrosylglutamate. The newly induced binding sites appear to be identical with pre-existing Cl--dependent binding sites by several criteria: They have a similar pharmacological profile, they are sensitive to low concentrations of Na+, and the number of sites can be further increased by transient exposure to micromolar calcium concentrations. Moreover, binding of [3H]APB, a ligand selective for the Cl--dependent glutamate binding sites, is also increased after glutamate preincubation. The induction of binding sites by high glutamate concentrations, described herein, is calcium-independent, not inhibited by leupeptin and, therefore, different from the previously described activation of binding sites by a calcium-sensitive protease. The high concentration of ligand needed to induce increased binding suggests the presence in hippocampal membranes of a binding site with low, millimolar affinity that is functionally related to the known high-affinity binding sites. Several interpretations of the observed effects and their implications for the possible relationship between the binding site and the synaptic receptor are discussed.

2-Amino-5-phosphonovalerate↗
PubMed1986

Induction of ornithine decarboxylase as a possible mediator of seizure-elicited changes in genomic expression in rat hippocampus.

Small electrolytic lesions placed in the hilus of the dentate gyrus have been shown to induce behavioral seizures, an elevation in the concentration of the opioid peptide enkephalin, and an increase in the transcription of the gene coding for the peptide precursor of enkephalin. Since polyamines and ornithine decarboxylase (ODC), the rate-limiting enzyme in their synthesis, have been shown to play critical roles in the growth and differentiation of several types of tissue, we tested for changes in ODC activity at various times following the initiation of seizures. ODC activity is significantly increased 3 hr after the lesions, reaches maximal (50-fold) levels about 12 hr later, and returns to control values after 48 hr. The increase occurs in both hippocampi following unilateral electrolytic lesions, is blocked by treatments that suppress limbic seizures, and does not occur after lesions that fail to elicit seizures; accordingly, we conclude that the increase in ODC activity results from epileptiform activity rather than some other consequence of the hilar lesion (e.g., deafferentation). The increase in ODC activity precedes the increase in the amount of mRNA coding for the enkephalin prohormone, which, in turn, precedes the increase in enkephalin levels. These results are consistent with the hypothesis that the early induction of ODC following the initiation of seizures leads to an alteration in genomic expression, which, in turn, changes neuropeptide levels. Adult brains thus appear to possess trophic responses of a type found in a variety of developing cell types and organs, and the possibility exists that these are involved in the control of seizure susceptibility.

Adrenalectomy↗
PubMed1985

Distribution of calpain I, an enzyme associated with degenerative activity, in rat brain.

The calcium-activated protease calpain I was localized in rat brain by immunocytochemistry. Calpain I-like immunoreactivity (CLI) was prominent in several structures in which degeneration is an ongoing feature, e.g. spinal motoneurons, olfactory nerve. Also noteworthy was the presence of CLI in regions susceptible to age-related pathologies, e.g. cerebellar Purkinje cells, substantia nigra and subiculum. This distribution suggests that calpain I may be involved with both normal and pathological neuronal degeneration.

Animals↗
PubMed1985

Olfactory discrimination learning is blocked by leupeptin, a thiol protease inhibitor.

Rats were trained on successive two-odor discriminations with the cues randomly located in an 8-arm radial maze. After several days of training using different odor pairs, the thiol protease inhibitor leupeptin was infused into the ventricles and testing continued. Leupeptin caused a pronounced, dose-dependent and reversible deficit in performance in this task. Previous studies have shown that these drug concentrations do not influence spontaneous activity, feeding and drinking, or the acquisition and retention of avoidance conditioning. The results are interpreted as supporting the hypothesis that a calcium-sensitive proteinase is involved in certain forms of memory that require modification of telencephalic circuitries.

Animals↗
PubMed1985

Regional distribution of soluble calcium activated proteinase activity in neonatal and adult rat brain.

Calcium dependent proteolytic activity in the soluble fraction of various rat brain regions was assayed using 14C-radiolabelled denatured casein as a substrate. Two forms of activity, distinguishable by their calcium requirement for half maximal activation (5 and 80 microM), were found; both were blocked by sulfhydryl alkylating agents and thiol proteinase inhibitors. Preincubation at 58 degrees C for 10 min also eliminated the high threshold activity. These characteristics are identical to those reported for calcium activated neutral proteinases ('calpains') found in other tissues. Calpain activity varied markedly across brain regions. The greatest values for the high threshold enzyme were found in pons-medulla followed by cerebellum/mesencephalon and finally the telencephalon. The low threshold enzyme had low levels of activity throughout the brainstem and diencephalon and was barely detectable in telencephalic structures. In contrast, a previously described endogenous inhibitor of calpain, 'calpastatin', was found not to vary in its activity across brain regions. Calpain activity was high in the prenatal brain, but while the hindbrain maintained high levels of activity into adulthood, the activity in the forebrain dropped 80% during the early postnatal period. The differences between forebrain and hindbrain activity levels were evident during the first 5 days of the neonatal period, suggesting that glial cell differentiation is not responsible for the regional variations found in the adult. These results are discussed with regard to the possibility that the turnover of anatomical structures differs between brain regions.

Animals↗
PubMed1985

Pharmacological dissociation of memory: anisomycin, a protein synthesis inhibitor, and leupeptin, a protease inhibitor, block different learning tasks.

Inhibition of protein synthesis by anisomycin for a short duration impairs memory of a one-trial inhibitory avoidance task in rats. Memory of escape conditioning involving eight trials is disrupted only if the duration of protein synthesis is prolonged by repeated injections. In marked contrast, olfactory memory of rats trained on two odor discriminations is not affected by anisomycin even if the duration of inhibition is prolonged and the number of trials is reduced to a minimum. In previous work, leupeptin, a thiol proteinase inhibitor, was shown to impair olfactory discrimination learning, but left inhibitory and avoidance conditioning intact. Together, these results provide a pharmacological double dissociation of memory, and suggest that the same chemistries, or mixtures of chemistries, may not be involved in all types of memory.

Animals↗