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Biomedical subjects

G Lynch

Publications and source records attributed to G Lynch.

At least 271 records · Page 15Linked to original sources
PubMed1991

Inhibition of proteolysis protects hippocampal neurons from ischemia.

Intense proteolysis of cytoskeletal proteins occurs in brain within minutes of transient ischemia, possibly because of the activation of calcium-sensitive proteases (calpains). This proteolytic event precedes overt signs of neuronal degeneration, is most pronounced in regions of selective neuronal vulnerability, and could have significant consequences for the integrity of cellular function. The present studies demonstrate that (i) the early phase of enhanced proteolysis is a direct response to hypoxia rather than other actions of ischemia, (ii) it is possible to pharmacologically inhibit the in vivo proteolytic response to ischemia, (iii) inhibition of proteolysis is associated with a marked reduction in the extent of neuronal death, and (iv) protected neurons exhibit normal-appearing electrophysiological responses and retain their capacity for expressing long-term potentiation, a form of physiological plasticity thought to be involved in memory function. These observations indicate that calcium-activated proteolysis is an important component of the post-ischemic neurodegenerative response and that targeting this response may be a viable therapeutic strategy for preserving both the structure and function of vulnerable neurons.

Animals↗
PubMed1991

A brief period of hypoxia causes proteolysis of cytoskeletal proteins in hippocampal slices.

Breakdown products (BDPs) resulting from the partial proteolysis of spectrin were examined in hippocampal slices after periods of hypoxia lasting for 5 or 10 min. The concentration of a approximately 155 kDa BDP increased nearly twofold after 5 min of hypoxia; further increases were not seen with 10 min episodes or 10 min of hypoxia followed by reoxygenation. The hypoxia-induced proteolysis was blocked by prior infusion of a newly introduced inhibitor of calpain (calpain inhibitor I, 200 microM). Together with previously published data showing improved recovery of hippocampal slices from hypoxia in the presence of calpain inhibitors, these data suggest that activation of calpain may contribute significantly to the pathophysiology of ischemia.

Animals↗
PubMed1991

Modulation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/quisqualate receptors by phospholipase A2: a necessary step in long-term potentiation?

The effects of kainate (KA)-induced epileptic seizures on the binding properties of hippocampal glutamate receptors, on the modulation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/quisqualate receptor by phospholipase A2 (PLA2), and on the formation of long-term potentiation (LTP) were studied in hippocampal membranes and hippocampal slices. Systemic administration of KA (10 mg/kg; 15 hr survival) produced specific changes in the binding properties of the AMPA/quisqualate receptors and its regulation. Whereas the binding of various ligands to the N-methyl-D-aspartate receptors was not modified by KA treatment, there was a significant decrease in the maximal number of binding sites for [3H]AMPA. In addition, the increase in [3H]AMPA binding elicited by PLA2 treatment of hippocampal, but not cerebellar, membranes was markedly decreased after KA injection. LTP was also substantially reduced in area CA1 of hippocampal slices from KA-treated animals. The loss of LTP was not due to changes in postsynaptic responses elicited by the bursts that trigger the potentiation effect, thus suggesting that KA treatment disrupts processes that follow N-methyl-D-aspartate receptor activation. Systemic administration of KA was associated with calpain activation as the amount of spectrin breakdown products was increased severalfold in hippocampus but not in cerebellum. Pretreatment of telencephalic membranes with calpain greatly reduced the PLA2-induced increase in [3H]AMPA binding. The results provide evidence in favor of an essential role of PLA2 in the development of LTP and suggest that the order of activation of different calcium-dependent processes is critical for producing the final changes underlying LTP.

Animals↗
PubMed1991

A test of the spine resistance hypothesis for LTP expression.

Long-term potentiation (LTP) consists of an enhanced response to released transmitter by the quisqualate/AMPA subclass of glutamate receptors with little change in the slower currents generated by the NMDA receptor subclass. Recent computer simulations suggest that a decrease in the resistance of dendritic spines would selectively augment fast synaptic currents and this could produce the pattern of results found with LTP. The present experiments tested this hypothesis by asking whether non-NMDA responses slowed by low temperature to resemble NMDA responses could express LTP. Slow non-NMDA responses recorded at 25 degrees C did express LTP, indicating that the time courses of NMDA responses cannot explain why they do not express LTP. The results, therefore, do not support the hypothesis that spine resistance changes are responsible for the enhanced transmission.

Animals↗
PubMed1991

Proteolytic modification of neural cell adhesion molecule (NCAM) by the intracellular proteinase calpain.

The neural cell adhesion molecule, NCAM, is concentrated in synaptic regions and thus may contribute to the formation and maintenance of connections between brain cells. We present evidence that the cytoplasmic domain of NCAM can be experimentally modified by the intracellular calcium-dependent proteinase, calpain. This degradation could provide a mechanism for rapidly uncoupling and reorganizing synaptic contacts.

Animals↗
PubMed1991

Increased spectrin proteolysis in fibroblasts from aged and Alzheimer donors.

Since calcium homeostasis is altered in cultured skin fibroblasts from aged and Alzheimer donors, the present study examined the degradation of spectrin, a substrate of the calcium dependent protease calpain. Spectrin proteolysis was estimated as the percentage of spectrin breakdown products (e.g., 150 + 155 kDa bands) per total spectrin immunoreactivity. In the baseline condition (e.g., unstimulated fibroblasts), spectrin breakdown was 53% greater in cells from aged donors when compared to cells from either young or Alzheimer donors. Compared to unstimulated cells, serum increased spectrin breakdown in cells from aged (22.4%) or Alzheimer (92.1%) donors but was ineffective in cells from young donors. Thus, when compared to young donors (100%), serum stimulation increased spectrin proteolysis by 183.9% (aged) or 231.7% (Alzheimer) after serum stimulation. Treatment of unstimulated cells with carbonyl cyanide 4-trifluoromethoxy-phenylhydrazone (FCCP), an uncoupler of mitochondrial function, increased spectrin degradation by 360.6% (young), 242.4% (aged) or 239.7% (Alzheimer) when compared to unstimulated cells of the same group. The combination of FCCP and serum stimulation enhanced spectrin breakdown in cells from aged (123.6%) and Alzheimer (154.0%) donors when compared to young cells (100%). Thus, changes in the regulation of calcium dependent proteases may contribute to decreased cell spreading and may play a role in the altered cytoskeletal dynamics characteristic of Alzheimer's disease.

Aged↗
PubMed1991

Temporal and hippocampal metabolic rate during an olfactory memory task assessed by positron emission tomography in patients with dementia of the Alzheimer type and controls. Preliminary studies.

Positron emission tomography with fludeoxyglucose F 18 was used to assess cortical metabolic rate during an olfactory memory task in six patients with dementia of the Alzheimer type. Metabolic activity was compared with that of both age-matched controls performing the olfactory task and controls resting with their eyes closed. Patients had lower metabolic rates in the anterior portion of the medial-temporal cortex than did controls, and the difference was greatest between patients and the controls performing the memory task. This region is known to receive a large olfactory input and has been implicated in the encoding of human memory. Normal controls resting with their eyes closed had significantly lower metabolic rates in this area when compared with normal controls performing the task. Our results are consistent with those of earlier reports of temporal lobe decreases in metabolic rate and extend them through the examination of areas salient to the behavioral loss.

Aged↗
PubMed1991

Selective effects of aniracetam across receptor types and forms of synaptic facilitation in hippocampus.

Aniracetam reversibly increased synaptic responses mediated by the AMPA but not the NMDA subclass of glutamate receptors in hippocampus and was considerably more potent than structurally similar nootropics. The drug had greater effects on field excitatory postsynaptic potentials (EPSPs) in the dentate gyrus and CA1 region than it did in the CA3 region, suggesting that it differentiates between variants of the AMPA receptor. Ligand binding to glutamate receptors in synaptosomal membrane fractions was minimally changed by aniracetam. Finally, the percent facilitation produced by aniracetam in the CA1 region was not reduced by any of three treatments (4-aminopyridine, changes in extracellular calcium concentrations, paired-pulse stimulation) that affect release but, in accord with a previous report, was substantially decreased by long-term potentiation. These results support the conclusion that aniracetam selectively increases the conductance of a subgroup of synaptic AMPA receptors in hippocampus and suggest that receptor changes underlie the expression of long-term potentiation.

4-Aminopyridine↗
PubMed1991

Evidence that changes in spine neck resistance are not responsible for expression of LTP.

From modeling studies it is known that changes in spine neck resistance can influence the shape of the non-linear curve relating synaptic current to synaptic conductance if the resistance of the neck approaches the synaptic input resistance. Such work also indicates that the effects of resistance will be much more pronounced for fast rather than slow synaptic currents. Accordingly, a reduction in neck resistance could produce an increase in the rapid responses generated by the quisqualate/AMPA class of glutamate receptors while only minimally affecting the slower NMDA receptor-mediated responses and thus account for the pattern of changes known to be associated with long-term potentiation (LTP). This hypothesis predicts that large reductions in synaptic conductance should have disproportionate effects on potentiated versus control responses. This was tested by using field potential recordings of synaptic currents in CA1 pyramidal cells in hippocampal slices in response to stimulation of Schaffer/commissural inputs that either received LTP-inducing stimulation or did not. Two manipulations were used to systematically reduce synaptic conductances: reductions of extracellular Ca++ and partial blockade of postsynaptic receptors. Reductions of synaptic field potentials by 40-75% by either method at control synapses were accompanied by equivalent reductions at previously potentiated synapses. These results suggest that LTP expression is not due to a change in the curves relating synaptic current to synaptic conductance as would be predicted by the spine resistance hypothesis.

Animals↗
PubMed1991

Compartmentation and glycoprotein substrates of calpain in the developing rat brain.

An activated form of calpain I associates with telencephalic membranes in a developmentally regulated fashion during early postnatal ontogeny. During this period, the cytoskeletal component spectrin is available and appears to be differentially susceptible to calpain-mediated cleavage. Lectin blotting techniques demonstrated that the leupeptin-sensitive action of calpain is primarily directed toward large proteins which are glycoconjugate in nature; neuronal cell adhesion molecules are among the glycoproteins whose associations with the telencephalic membranes decrease due to calpain activity. These data suggest that cytoplasmic calpain is translocated to the membrane during early brain development in order to act on the cytoskeletal and adhesive structures responsible in part for neuronal shape and function.

Animals↗
PubMed1991

Computation of frequency-to-spatial transform by olfactory bulb glomeruli.

A physiological simulation of 2.5% of the input and inhibitory neurons and 25% of the primary mitral/tufted cells in a single mammalian olfactory bulb glomerulus was constructed. This physiological simulation used the integrate-and-fire paradigm with realistic activation curves and synaptic delays. The dendritic integration incorporated non-linear interactive effects of individual cell excitatory and inhibitory post-synaptic potentials (PSPs) from both axodendritic and dendro-dendritic synaptic contacts. Refractory periods for granule-cell inhibition of mitral/tufted cell activity lead to relatively fixed-frequency rhythmic activity in the glomerulus, independent of the input frequency from the olfactory nerve. Though the frequency of mitral/tufted cell firing in bulb was approximately independent of input frequency, the number of cells active in the glomerulus was a roughly-linear function of input frequency to the glomerulus, indicating the mechanism's ability to function as a frequency-to-spatial encoder.

Animals↗
PubMed1991

Higher olfactory processes: perceptual learning and memory.

The past year has seen several important findings emerge from studies of higher olfactory processes. The identification of synaptic long-term potentiation in the olfactory cortex, induced via repetitive burst stimulation at the theta rhythm, and physiological activity patterns associated with learning, some of which mimic long-term potentiation induction patterns, have suggested relationships between rhythmic activity, behavioral learning and synaptic plasticity. In addition, the construction of computational models of the olfactory bulb and cortex have generated testable behavioral and physiological predictions which have been supported by experimental evidence.

Animals↗
PubMed1991

Hippocampus and olfactory discrimination learning: effects of entorhinal cortex lesions on olfactory learning and memory in a successive-cue, go-no-go task.

Three experiments assessed the effect of entorhinal cortex lesions on olfactory learning and memory using a successive-cue olfactory discrimination paradigm. In contrast to the results of other studies that used a simultaneous-cue paradigm, lesions of the entorhinal cortex facilitated rats' acquisition of individual odor discrimination problems, with no impairment in memory for the individual odors across both short (24-hr) and long (65-day) retention intervals and despite limited training. When considered together with previous observations of facilitation or impairment in learning after damage to the hippocampal system, the present data suggest that the hippocampus is preferentially involved in encoding relations among multiple stimuli. By this account, facilitation of performance is due to an interaction between hippocampal system dysfunction and task conditions that hinder direct comparisons among cues.

Animals↗
PubMed1991

Fibronectin binding by brain synaptosomal membranes may not involve conventional integrins.

Synaptosomal plasma membranes (SPMs) from adult rat brain tested for fibronectin binding and antigenicity toward antibodies against integrin-type fibronectin receptors. Binding (1-10 pmol mg-1 protein) was considerably greater in SPMs than in homogenates for hippocampus and neocortex but not for brain stem. The tetrapeptide L-arginyl-glycyl-L-aspartyl-L-serine blocked up to 90% of the binding indicating that integrin-type receptors are present. Antibodies against two sub-classes of integrins detected high Mr integrin subunits in homogenates but not in SPMs. However, antibodies against hamster ovarian fibronectin receptor reacted intensely with polypeptides of 55,000 and 40,000 Mr that were concentrated 20 to 40-fold in SPMs compared to homogenates. These peptides may constitute a new class of matrix receptor involved in synaptic adhesion.

Animals↗
PubMed1991

Antibodies to the alpha v beta 3 integrin label a protein concentrated in brain synaptosomal membranes.

Immunochemical methods were used to test whether vitronectin receptors exist in synaptosomal membranes (SPMs) and hence are positioned to play a role in synaptic adhesion. Antibodies against the alpha v beta 3 integrin detected proteins in brain homogenates that correspond to conventional integrin subunits. Conversely, these antigens were not found in SPMs prepared from the same brain tissue. The antibodies did, however, express strong immunoreactivity towards a 27 kDa polypeptide that was greatly concentrated in SPMs from major brain regions and that was not found in tissues other than brain. This is an example of an integrin epitope contained in a synaptic polypeptide that is too small to be a conventional matrix receptor, thus, suggesting the possibility that synaptic adhesion involves unusual proteins.

Amino Acid Sequence↗
PubMed1991

Evidence that matrix recognition contributes to stabilization but not induction of LTP.

Slices of hippocampus were incubated with Arg-Gly-Asp (RGD) peptides known to block members of the integrin class of matrix receptors. Though the peptides caused no detectable difference in the amount of long-term potentiation (LTP) expressed in the CA1 field 1-2 min after induction with high frequency stimulation, they did produce a reversible, dose dependent decay of LTP over a period of 40 min. This effect was not obtained with various non-RGD control peptides. These results suggest that stabilization of LTP requires adhesive interactions via specific matrix recognition sites, whereas induction and expression do not.

Amino Acid Sequence↗