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Biomedical subjects

G Lynch

Publications and source records attributed to G Lynch.

At least 289 records · Page 16Linked to original sources
PubMed1991

Complex interactions between polyamines and calpain-mediated proteolysis in rat brain.

Polyamine synthesis is induced by various extracellular signals, and it is widely held that this biochemical response participates in cell growth and differentiation. Certain of the triggers for synthesis in brain tissues also increase the breakdown of high-molecular-weight structural proteins, apparently by activating calcium-dependent proteases (calpains). The present experiments tested the possibility that calpain activity is modulated by polyamines. Spermine, spermidine, and putrescine all increased calcium-dependent proteolysis of [14C]casein by soluble fractions of rat brain. The order of potency was spermine greater than spermidine greater than putrescine, with apparent affinities of 30, 300, and 6,000 microM, respectively. Each of the three polyamines at physiological concentrations also potentiated the calcium-dependent breakdown of two endogenous high-molecular-weight structural proteins known to be substrates of calpain, in both supernatant and membrane fractions. The thiol protease inhibitor leupeptin, a known calpain inhibitor, also inhibited calcium-dependent proteolysis in the presence and absence of polyamines. The polyamines did not increase the activity of purified calpain I or calpain II determined with either [14C]casein or purified spectrin as the substrate, nor did they interfere with the inhibitory effects of calpastatin, an endogenous inhibitor of calpain. However, polyamines potentiated the stimulation of endogenous but not purified calpain activity produced by an endogenous calpain activator. These results suggest a role for polyamines in protein degradation as well as protein synthesis.

Animals↗
PubMed1991

Laparoscopic cholecystectomy: a hundred consecutive cases.

Laparoscopic cholecystectomy is a new, minimaly invasive technique for removing the gallbladder which has several advantages over the traditional laparotomy cholecystectomy. We reviewed our initial experience with 100 consecutive patients in whom laparoscopic cholecystectomy was attempted. The indications for operation were biliary colic, chronic cholecystitis, acute gallbladder and gallstone pancreatitis. Laparoscopic cholecystectomy was successfully performed in 87 patients. Anaesthesia time was 144 +/- 52 min. There was no mortality while overall morbidity was 14%. One patient had a retained common bile duct stone. Postoperative hospital stay was 4.1 +/- 2.2 days and the mean time to full activity in a random sample of 25 patients was 13.7 +/- 11.7 days. Laparoscopic cholecystectomy is a safe effective procedure which removes the gallbladder. We suggest that this technique be considered in all patients undergoing cholecystectomy.

Adolescent↗
PubMed1990

Effect of bromophenacyl bromide, a phospholipase A2 inhibitor, on the induction and maintenance of LTP in hippocampal slices.

The effect of bromophenacyl bromide (BPB), a phospholipase A2 (PLA2) inhibitor, on both the induction and the maintenance of long-term potentiation (LTP) was investigated in field CA1 of the hippocampal slice preparation. One hour of BPB application (50 microM) caused a large reduction in the magnitude of LTP induced by a theta burst stimulation (TBS) paradigm. BPB had no significant effect on either the degree of paired-pulse facilitation or the amount of pre-established LTP. Furthermore, the facilitation of postsynaptic responses occurring during TBS and in the first minute following TBS was not reduced by the PLA2 inhibitor. These results indicate that the inhibition of LTP produced by BPB is not due to an effect of the drug on a physiological event that triggers LTP. The data also suggest that PLA2 activation plays a critical role in the expression of LTP, but is not required for the maintenance of the potentiation.

Acetophenones↗
PubMed1990

Calpain inhibitors improve the recovery of synaptic transmission from hypoxia in hippocampal slices.

Two inhibitors of calcium activated proteases (calpains) were tested for their effects on hypoxia-induced synaptic dysfunction in hippocampal slices. Hypoxic episodes lasting for either one or two minutes beyond the point at which action potentials (fiber volleys) disappeared were used. Leupeptin and calpain inhibitor I had no reliable effects on the rate at which synaptic transmission declined during hypoxia or the time required for loss of action potentials, but both drugs did substantially improve the degree of recovery. Moreover, the percentage of slices meeting an arbitrary criterion for viability after hypoxic treatment was greatly increased by the drug treatment. These results point to the conclusion that proteolysis triggered by calcium influx during hypoxia contributes to pathophysiology.

Animals↗
PubMed1990

The effects of adenosine on the development of long-term potentiation.

In previous work we found that a brief period of hypoxia occurring within 1-2 min of high-frequency stimulation disrupts the development of long-term potentiation (LTP) in the CA1 field of rat hippocampal slices. We now report that extracellular application of adenosine if applied within 1 min but not 5 min after high-frequency stimulation similarly prevents LTP. Adenosine was ineffective if DPCPX (1,3-dipropyl-8-cyclopentyl-xanthine), a selective antagonist of adenosine A1 receptors, was present. The post-stimulation application mode excludes the possibility that adenosine interferes with NMDA receptor activation and its role in initiating LTP. It suggests instead that changes in intracellular control systems linked to adenosine receptors can during a brief vulnerable period interrupt the biochemical processes leading to the expression of long-term potentiation.

Adenosine↗
PubMed1990

Development of hippocampal long-term potentiation is reduced by recently introduced calpain inhibitors.

The effects of two recently synthesized inhibitors of calpains, calpain inhibitor I (CiI) and calpain inhibitor II (CiII) were tested on the development of long-term potentiation (LTP) in region CA1 of rat hippocampus. Slices maintained in 100 microM of CiI or CiII showed an initial degree of potentiation after theta burst stimulation that, in contrast to controls, slowly decayed across time. The effects of CiI and CiII appeared to be independent of possible actions on the physiological mechanisms that take place during the induction stage of LTP. Since these inhibitors are more potent and specific than leupeptin in blocking calpain activity, their effects on LTP can be more convincingly ascribed to a selective blockade of the calcium-sensitive protease. Accordingly, the results favor the idea that a proteolytic event of the kind found after N-methyl-D-aspartate receptor activation is an intermediary step in the development of LTP.

Animals↗
PubMed1990

Regional distribution of mRNA for a putative kainate receptor in rat brain.

A cDNA clone which encodes a putative kainate receptor was prepared with the polymerase chain reaction; from this, 'antisense' RNA fragments were produced and used to evaluate the distribution of mRNA for the kainate-receptor in rat brain with in situ hybridization techniques. The data demonstrate marked regional differences in the abundance of kainate receptor mRNA with highest levels present in the granule and pyramidal cell layers of hippocampus, the Purkinje cell layer of the cerebellum, and all cell layers of the olfactory bulb.

Animals↗
PubMed1990

Regional differences in the activation of synaptosomal mitochondrial Ca2+ uptake by spermine in rat brain.

Several properties of Ca2+ uptake by isolated synaptosomal mitochondria were characterized by using a repetitive Ca2+ loading technique. Synaptosomal mitochondria maintained extramitochondrial Ca2+ concentration at submicromolar levels when challenged with successive additions of small amounts of Ca2+. Ca2+ uptake under these conditions was markedly stimulated by the presence of spermine, a polyamine found in high concentrations in brain. Moreover, mitochondria isolated from telencephalic areas of rat brain were activated to a greater extent by spermine than were mitochondria from non-telencephalic brain regions. The present results support the idea that brain mitochondria could play a significant role in limiting the intraneuronal rise in Ca2+ that follows stimulation or injury. In addition, telencephalic mitochondria exhibit more flexibility in the regulation of Ca2+ uptake than do mitochondria from phylogenetically older non-telencephalic brain regions and this could be related to differences in Ca2+ influx mechanisms in these brain regions.

Animals↗
PubMed1990

The glycine site modulates NMDA-mediated changes of intracellular free calcium in cultures of hippocampal neurons.

Recent evidence indicates that the N-methyl-D-aspartate (NMDA) receptor-channel complex contains a glycine subunit whose activation may be necessary for channel operation. It has been previously shown that stimulation of the NMDA receptor leads to an increase in intracellular ionic Ca2+ [( Ca2+]i); therefore, we examined the role of the NMDA receptor-associated glycine site in modulating [Ca2+]i using the fluorescent dye Fura II in hippocampal neuron cultures. A 3-s pulse of 200 microM NMDA resulted in a mean [Ca2+]i increase of 363 nM above the average resting concentration of 122 nM. Perfusion of the glycine site antagonist 7-chlorokynurenate (Cl-Kyn) essentially eliminated the NMDA-induced alteration in [Ca2+]i. Either 40 microM glycine or 50 microM D-serine completely reversed the effect of Cl-Kyn, indicating that the drug was acting at the glycine site. The NMDA receptor antagonists 2-amino-5-phosphonovalerate (AP5) and ketamine, which bind to the glutamate recognition site and the ion channel, respectively, also blocked the NMDA-mediated [Ca2+]i response; however, glycine or D-serine did not reverse this effect. These data show that the glycine binding site coupled to the NMDA receptor modulates the NMDA-mediated increase in [Ca2+]i. Antagonists of the glycine site provide a new tool to investigate and possibly control neuroplasticity and neurotoxicity related to the NMDA receptor complex.

2-Amino-5-phosphonovalerate↗
PubMed1990

Distribution of calcium-activated protease calpain in the rat brain.

Calpain is a calcium-activated neutral protease that degrades a number of cytoskeletal proteins. It may participate in the maintenance of the cytoskeleton and in the rapid turnover of structural proteins associated with synaptic plasticity. Calpain may also be involved in the neurodegeneration that accompanies aging and age-related diseases. To aid in the interpretation of disease-related alterations in staining patterns, the present study examined calpain's normal distribution in the mammalian brain and spinal cord. A monoclonal antibody was employed with the avidin-biotin-peroxidase immunocytochemical technique on samples of rat tissue. Glia (astrocytes, microglia) and virtually all neurons were immunopositive, although neuronal processes exhibited varying staining patterns. The axonal staining pattern depended upon either the origin or destination of the process: those axons remaining within the brain (e.g., corpus callosum) were only lightly immunoreactive, whereas spinal cord and peripheral axons (trigeminal nerve) were more darkly labeled. The architecture of the dendritic tree determined the dendritic staining pattern: neurons with prominent apical and basal dendritic trees (e.g., pyramidal cells) were immunolabeled along their entire extent; labeling of multipolar cells (e.g., hilar cells of dentate gyrus) was limited to the proximal dendrites. The ubiquitous distribution of calpain argues against a primary role for the enzyme in the regional pattern of neuronal death seen in Alzheimer's disease. An alteration in the concentration, localization, or inhibition of the enzyme could, however, lead to the abnormal accumulations of cytoskeletal elements seen with the disease.

Animals↗
PubMed1990

Stable depression of potentiated synaptic responses in the hippocampus with 1-5 Hz stimulation.

Adult rats with two chronic stimulating electrodes in the Schaffer collateral/commissural system of the hippocampus and one recording electrode in the stratum radiatum (apical dendrites) of field CA1 were administered high-frequency stimulation (10 brief bursts at theta frequency) to produce long-term potentiation (LTP). 'Low frequency' stimulation (100 pulses at 1 Hz alone or followed by 250 pulses at 5 Hz) delivered 5-15 min later had no effect on LTP in 18% of the rats, caused a transient reversal in 18% of the group, but produced an apparent reversal of LTP for the remainder of a 1 h test session in 64% of the animals. LTP did not recover in animals tested 24 h later, at which point a second episode of high-frequency stimulation but without subsequent low-frequency stimulation was administered. This produced an LTP effect that persisted for a 1 h test session in 94% of the cases and that was still present in 86% of the animals tested 24 h later. Low-frequency stimulation applied prior to induction of LTP had no lasting effects on evoked responses not did it affect responses to a control stimulating electrode in those cases in which it reversed LTP. Possible implications of these results for hypotheses concerning the substrates of LTP and mechanisms of forgetting are discussed.

Action Potentials↗
PubMed1990

Anoxia reveals a vulnerable period in the development of long-term potentiation.

Transient anoxia occurring 1-2 min after high-frequency stimulation selectively prevented the stable expression of long-term potentiation (LTP). Anoxia occurring after this brief vulnerable period did not reverse LTP. Experiments on the duration of anoxia necessary to block LTP expression indicated that simply reducing synaptic transmission was insufficient but that membrane depolarization was not required. The effects of anoxia on LTP were blocked by antagonists of A1 adenosine receptors. It is concluded that LTP develops in about one minute and that the chemistries operating in this period are easily disrupted by an event triggered by adenosine receptors.

Action Potentials↗
PubMed1990

Simulation of paleocortex performs hierarchical clustering.

Simulations were performed of layers I and II of olfactory paleocortex, as connected to its primary input structure, olfactory bulb. Induction of synaptic long-term potentiation by means of repetitive sampling of inputs caused the simulation to organize encodings of learned cues into a hierarchical memory that uncovered statistical relationships in the cue environment, corresponding to the performance of hierarchical clustering by the biological network. Simplification led to characterization of those parts of the network responsible for the mechanism, resulting in a novel, efficient algorithm for hierarchical clustering. The hypothesis is put forward that these corticobulbar networks and circuitry of similar design in other brain regions contain computational elements sufficient to construct perceptual hierarchies for use in recognizing environmental cues.

Animals↗
PubMed1990

Increased spectrin proteolysis in the brindled mouse brain.

Proteolytically generated fragments of the microfilament anchoring protein brain spectrin were found to accumulate in brindled mouse brain. Proteolysis was most extensive in brain regions possessing high concentrations of N-methyl-D-aspartate (NMDA) receptors (e.g. cortex, striatum, hippocampus). The brindle mutation affects copper homeostasis and thus a variety of copper-dependent enzymes needed in intermediary metabolism. The altered mitochondria of these mice are suggested to less efficiently buffer NMDA receptor-gated calcium fluxes, thus promoting activation of calcium-activated proteases and subsequent degradation of the spectrin meshwork.

Aging↗
PubMed1990

Synaptic modulation of N-methyl-D-aspartate receptor mediated responses in hippocampus.

Low magnesium medium and the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (D-AP5) were used to analyze the effect of several manipulations on the component of excitatory postsynaptic potentials (EPSPs) mediated by activation of NMDA receptors in area CA1 of hippocampal slices. The D-AP5 sensitive component of synaptic responses was characterized by a marked sensitivity to changes in extracellular magnesium and calcium concentrations. In both cases the changes in D-AP5 sensitive responses were considerably larger than those in non-NMDA-dependent potentials. Similarly, frequency facilitation, which is due to a transient increase in release, was accompanied by a greater enhancement of NMDA than non-NMDA receptor-mediated components. The degree of paired-pulse facilitation observed with D-AP5 sensitive responses was magnesium-dependent between concentrations of 0.05 and 0.5 mM, an effect not seen with control potentials. Intracellular injections of hyperpolarizing current pulses differentially affected NMDA and non-NMDA receptor-mediated components. Taken together, these results indicate that changes in the amount of transmitter release may affect to a greater degree NMDA than non-NMDA receptor-mediated components of synaptic responses, probably because of the voltage-sensitive blockade by magnesium of the NMDA receptors. In contrast, induction of long-term potentiation (LTP) by high frequency stimulation produced a larger increase in non-NMDA as opposed to NMDA receptor-dependent responses, a result that does not support the idea that an increase in transmitter release is responsible for LTP.

2-Amino-5-phosphonovalerate↗
PubMed1990

Glycine site associated with the NMDA receptor modulates long-term potentiation.

Recent work has shown that kynurenic acid and several quinoxaline derivatives act as non-competitive NMDA receptor antagonists by binding to the glycine site associated with this receptor. In this study, we have tested the effect of the most potent and selective of these compounds, 7-chlorokynurenic acid (Cl-Kyn), on the induction of long-term potentiation, an event known to involve activation of NMDA receptors. It was found that 30 microM Cl-Kyn reversibly abolished the development of both short-term and long-term potentiation in the CA1 region of hippocampal slices. The effectiveness of Cl-Kyn matched its ability to inhibit 3H-glycine binding and the association of 3H-TCP with the NMDA receptor in binding experiments (Ki 0.7-1 microM). Weak interactions of Cl-Kyn with AMPA receptor sites were observed and may account for a partial, reversible reduction in the epsp. However, blockade of long-term potentiation by Cl-Kyn was completely reversed by simultaneous application of the glycine site agonist D-serine and thus must be attributed to its interaction with the glycine site. These results indicate that the glycine site coupled to the NMDA receptor potently modulates channel function during physiological events related to synaptic activation.

Adaptation, Physiological↗
PubMed1990

Long-term potentiation of monosynaptic EPSPs in rat piriform cortex in vitro.

Induction of long-term potentiation (LTP) by burst stimulation patterned after the limbic system theta rhythm was studied in slices of piriform cortex. Monosynaptic responses were evoked by stimulation of afferent fibers of the lateral olfactory tract (LOT) or the intrinsic associational (ASSN) feedback system. LTP was difficult to elicit at LOT synapses in the presence of 2.5 mM extracellular Mg2+, and when it was induced potentiation increased for 20-30 min after burst stimulation before stabilizing. The probability of inducing LTP was increased when the extracellular Mg2+ concentration was reduced to 50 microM. In ASSN synapses LTP developed in about 1 min after burst stimulation and then remained stable. ASSN system LTP was more readily induced in slices from caudal than rostral piriform. Induction of LTP at both LOT and ASSN synapses was blocked by D-2-amino-5-phosphonopentanoate, indicating that NMDA receptor activation was required. Neither system exhibited the decremental short-term potentiation effect observed after burst stimulation of inputs to the CA1 field of hippocampus.

2-Amino-5-phosphonovalerate↗