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R Malinow

Publications and source records attributed to R Malinow.

At least 37 records · Page 2Linked to original sources
PubMed1998

Use-dependent AMPA receptor block in mice lacking GluR2 suggests postsynaptic site for LTP expression.

The mechanisms responsible for enhanced transmission during long-term potentiation (LTP) at CA1 hippocampal synapses remain elusive. Several popular models for LTP expression propose an increase in 'use' of existing synaptic elements, such as increased probability of transmitter release or increased opening of postsynaptic receptors. To test these models directly, we studied a GluR2 knockout mouse in which AMPA receptor transmission is rendered sensitive to a use-dependent block by polyamine compounds. This method can detect increases during manipulations affecting transmitter release or AMPA receptor channel open time and probability, but shows no such changes during LTP. Our results indicate that the recruitment of new AMPA receptors and/or an increase in the conductance of these receptors is responsible for the expression of CA1 LTP.

Action Potentials↗
PubMed1997

Mechanisms of potentiation by calcium-calmodulin kinase II of postsynaptic sensitivity in rat hippocampal CA1 neurons.

Mechanisms of potentiation by calcium-calmodulin kinase II of postsynaptic sensitivity in rat hippocampal CA1 neurons. J. Neurophysiol. 78: 2682-2692, 1997. Preactivated recombinant alpha-calcium-calmodulin dependent multifunctional protein kinase II (CaMKII*) was perfused internally into CA1 hippocampal slice neurons to test the effect on synaptic transmission and responses to exogenous application of glutamate analogues. After measurement of baseline transmission, internal perfusion of CaMKII* increased synaptic strength in rat hippocampal neurons and diminished the fraction of synaptic failures. After measurement of baseline responses to applied transmitter, CaMKII* perfusion potentiated responses to kainate but not responses to N-methyl--aspartate. Internal perfusion of CaMKII*potentiated the maximal effect of kainate. Potentiation by CaMKII* did not change the time course of responses to kainate, whereas increasing response size by pharmacologically manipulating desensitization or deactivation rate constants significantly altered the time course of responses. Nonstationary fluctuation analysis of responses to kainate showed a decrease in the coefficient of variation after potentiation by CaMKII*. These data support the hypothesis that CaMKII increases postsynaptic responsiveness by increasing the available number of active alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate channels and suggests that a similar process may occur during the expression of long-term potentiation.

Animals↗
PubMed1996

Maturation of a central glutamatergic synapse.

Whole-cell recordings from optic tectal neurons in Xenopus tadpoles were used to study the maturation of a glutamatergic synapse. The first glutamatergic transmission is mediated only by N-methyl-D-aspartate (NMDA) receptors and is silent at resting potentials. More mature synapses acquire transmission by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors. This maturational program is mimicked by postsynaptic expression of constitutively active calcium-calmodulin-dependent protein kinase II (CaMKII). Newly formed synapses may be silent unless sufficient depolarization is provided by coincident activity that could activate postsynaptic CaMKII, resulting in the appearance of AMPA responses.

Animals↗
PubMed1995

Activation of postsynaptically silent synapses during pairing-induced LTP in CA1 region of hippocampal slice.

Long-term potentiation (LTP) is an enhancement of synaptic strength that can be produced by pairing of presynaptic activity with postsynaptic depolarization. LTP in the hippocampus has been extensively studied as a cellular model of learning and memory, but the nature of the underlying synaptic modification remains elusive, partly because our knowledge of central synapses is still limited. One proposal is that the modification is postsynaptic, and that synapses expressing only NMDA (N-methyl-D-aspartate) receptors before potentiation are induced by LTP to express functional AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors. Here we report that a high proportion of synapses in hippocampal area CA1 transmit with NMDA receptors but not AMPA receptors, making these synapses effectively non-functional at normal resting potentials. These silent synapses acquire AMPA-type responses following LTP induction. Our findings challenge the view that LTP in CA1 involves a presynaptic modification, and suggest instead a simple postsynaptic mechanism for both induction and expression of LTP.

Animals↗
PubMed1995

Vaccinia virus transfection of hippocampal slice neurons.

Here we describe a technique that uses a recombinant vaccinia virus to transfect neurons in rat hippocampal slices. This technique allows the use of molecular biological manipulations on neuronal tissue while maintaining intact synaptic function. This method should be useful in testing specific hypotheses regarding the role of synaptic proteins.

Animals↗
PubMed1994

Potentiated transmission and prevention of further LTP by increased CaMKII activity in postsynaptic hippocampal slice neurons.

Calcium-calmodulin-dependent protein kinase II (CaMKII) is a necessary component of the cellular machinery underlying learning and memory. Here, a constitutively active form of this enzyme, CaMKII(1-290), was introduced into neurons of hippocampal slices with a recombinant vaccinia virus to test the hypothesis that increased postsynaptic activity of this enzyme is sufficient to produce long-term synaptic potentiation (LTP), a prominent cellular model of learning and memory. Postsynaptic expression of CaMKII(1-290) increased CaMKII activity, enhanced synaptic transmission, and prevented more potentiation by an LTP-inducing protocol. These results, together with previous studies, suggest that postsynaptic CaMKII activity is necessary and sufficient to generate LTP.

2-Amino-5-phosphonovalerate↗
PubMed1994

Visualizing hippocampal synaptic function by optical detection of Ca2+ entry through the N-methyl-D-aspartate channel.

Fura-2 and imaging technology were used to detect intracellular Ca2+ changes in CA1 pyramidal cells in hippocampal slices. During focal synaptic stimulation, one or more highly localized regions of Ca2+ elevation (hot spots) were detected in the dendrites. Ca2+ spread from the center of hot spots with properties consistent with diffusion. Several lines of evidence indicate that these hot spots were due to Ca2+ entry through N-methyl-D-aspartate synaptic channels. The spatial and temporal resolution of the method was sufficient to detect the response of single hot spots to single stimuli, thus providing a real-time method for monitoring local synaptic activity. Using this method, we show that synapses on the same dendrite differ in their probability of response and in their facilitation properties.

2-Amino-5-phosphonovalerate↗
PubMed1993

The probability of transmitter release at a mammalian central synapse.

When an action potential reaches a synaptic terminal, fusion of a transmitter-containing vesicle with the presynaptic membrane occurs with a probability (pr) of less than one. Despite the fundamental importance of this parameter, pr has not been directly measured in the central nervous system. Here we describe a novel approach to determine pr, monitoring the decrement of NMDA (N-methyl-D-aspartate)-receptor mediated synaptic currents in the presence of the use-dependent channel blocker MK-801 (ref. 2). On a single postsynaptic CA1 hippocampal slice neuron, two classes of synapses with a sixfold difference in pr are resolved. Synapses with low pr contribute to over half of transmission and are more sensitive to drugs enhancing transmitter release. Switching between these two classes of synapses provides the potential for large changes in synaptic efficacy and could underlie forms of activity-dependent plasticity.

Animals↗
PubMed1993

Measuring the impact of probabilistic transmission on neuronal output.

We have investigated the impact of stochastic transmission on the input-output relations of neurons in hippocampal slices. A synaptic input that fires a cell has a significant trial-to-trial variability in amplitude, reflecting the probabilistic release of transmitter. By measuring miniature excitatory postsynaptic currents, we estimate that synchronous release of a few vesicles can fire a CA1 cell. The firing threshold and variability can be physiologically modulated. Different cell types have distinct firing thresholds and variabilities. Long-term potentiation (LTP) decreases trial-to-trial variability. If after LTP, the stimulus is reduced to produce a threshold response, the variability returns to that observed before LTP. Thus, for a threshold input, the trial-to-trial variability is maintained with LTP. This may be important for the proper functioning of a plastic nervous system.

Action Potentials↗
PubMed1992

Persistent signalling and changes in presynaptic function in long-term potentiation.

Long-term potentiation (LTP) is an example of a persistent change in synaptic function in the mammalian brain, thought to be essential for learning and memory. At the synapse between hippocampal CA3 and CA1 neurons LTP is induced by a Ca2+ influx through glutamate receptors of the NMDA (N-methyl-D-aspartate) type (see Collingridge et al 1992, this volume). How does a rise in [Ca2+]i lead to enhancement of synaptic function? We have tested the popular hypothesis that Ca2+ acts via a Ca(2+)-dependent protein kinase. We found that long-lasting synaptic enhancement was prevented by prior intracellular injection of potent and selective inhibitory peptide blockers of either protein kinase C (PKC) or Ca2+/calmodulin-dependent protein kinase II (CaMKII), such as PKC(19-31) or CaMKII(273-302), but not by control peptides. Evidently, activity of both PKC and CaMKII is somehow necessary for the postsynaptic induction of LTP. To determine if these kinases are also involved in the expression of LTP, we impaled cells with microelectrodes containing protein kinase inhibitors after LTP had already been induced. Strikingly, established LTP was not suppressed by a combination of PKC and CaMKII blocking peptides, or by intracellular postsynaptic H-7. However, established LTP remained sensitive to bath application of H-7. Thus, the persistent signal may be a persistent kinase, but if so, the kinase cannot be accessed within the postsynaptic cell. Evidence for a presynaptic locus of expression comes from our studies of quantal synaptic transmission under whole-cell voltage clamp. We find changes in synaptic variability expected to result from enhanced presynaptic transmitter release, but little or no increase in quantal size. Furthermore, miniature synaptic currents in hippocampal cultures are increased in frequency but not amplitude as a result of a glutamate-driven postsynaptic induction. The combination of postsynaptic induction and presynaptic expression necessitates a retrograde signal from the postsynaptic cell to the presynaptic terminal.

Animals↗
PubMed1992

Direct measurement of quantal changes underlying long-term potentiation in CA1 hippocampus.

The modification responsible for the long-term synaptic potentiation (LTP) that follows a brief conditioning period is not known. To elucidate this change, we have resolved quantal levels of transmission before and after induction of LTP. We find an increase both in the number of quanta released and in quantal amplitude, consistent with combined pre- and postsynaptic modifications. On average, about 60% of LTP can be accounted for by presynaptic enhancement. The increase in either quantal amplitude or quantal content varies significantly among different experiments, but is inversely correlated with its initial value. These results may help to reconcile the different views concerning the site of LTP expression.

Animals↗
PubMed1991

Transmission between pairs of hippocampal slice neurons: quantal levels, oscillations, and LTP.

Long-term potentiation (LTP) of synaptic transmission after coincident pre- and postsynaptic activity is considered a cellular model of changes underlying learning and memory. In intact tissue, LTP has been observed only between populations of neurons, making analysis of mechanisms difficult. Transmission between individual pre- and postsynaptic hippocampal cells was studied, suggesting quantal amplitude distributions with little variability in quantal size. LTP between such pairs is manifested by large, persistent, and synapse-specific potentiation with a shift in amplitude distribution that suggests presynaptic changes. Oscillations in amplitude of transmission, apparently of presynaptic origin, are common and can be triggered by LTP.

Animals↗