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C F Stevens

Publications and source records attributed to C F Stevens.

At least 19 recordsLinked to original sources
PubMed1992

Adenosine decreases neurotransmitter release at central synapses.

Adenosine, at concentrations ranging from 5 to 100 microM, decreases the efficacy of transmission at the perforant path synapses on dentate granule cells. We have used whole cell recording from these cells in slices to determine the mechanism of the reduced synaptic strength. We find that size of miniature excitatory postsynaptic currents (mepscs) is unaffected by adenosine at concentrations up to 100 microM, an observation that indicates adenosine's mode of action is not through a decreased postsynaptic sensitivity to neurotransmitter. A quantal analysis indicates, however, that the quantity of neurotransmitter released is sufficiently diminished by adenosine to account entirely for the adenosine-produced decrease in synaptic strength. Application of 3-isobutyl-1-methylxanthine (IBMX), a drug that antagonizes the effects of endogenous adenosine, produces an increase in synaptic strength. This observation suggests that the resting level of adenosine in our slices is appreciable, and an analysis of the adenosine dose-response relation is consistent with endogenous adenosine levels of about 10 microM. IBMX application produces only slight changes in the amplitude of mepscs, whereas a quantal analysis demonstrates that the drug significantly increases the amount of neurotransmitter released. Thus IBMX acts as an "anti-adenosine" in our experiments. In some experiments we have been able to record excitatory and inhibitory synaptic currents produced by the same perforant path stimulus. In these instances we find that inhibitory transmission is unaffected by concentrations of adenosine that produce a marked decrease in the strength of excitatory synapses.

1-Methyl-3-isobutylxanthine↗
PubMed1992

Deficient hippocampal long-term potentiation in alpha-calcium-calmodulin kinase II mutant mice.

As a first step in a program to use genetically altered mice in the study of memory mechanisms, mutant mice were produced that do not express the alpha-calcium-calmodulin-dependent kinase II (alpha-CaMKII). The alpha-CaMKII is highly enriched in postsynaptic densities of hippocampus and neocortex and may be involved in the regulation of long-term potentiation (LTP). Such mutant mice exhibited mostly normal behaviors and presented no obvious neuroanatomical defects. Whole cell recordings reveal that postsynaptic mechanisms, including N-methyl-D-aspartate (NMDA) receptor function, are intact. Despite normal postsynaptic mechanisms, these mice are deficient in their ability to produce LTP and are therefore a suitable model for studying the relation between LTP and learning processes.

Animals↗
PubMed1992

Modulation of synaptic efficacy in field CA1 of the rat hippocampus by forskolin.

Activation of cAMP-dependent protein kinase (kinase A) has recently been shown to enhance responses evoked by stimulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in cultured hippocampal pyramidal neurons. Here we report results of experiments designed to determine if activation of the cAMP cascade potentiates synaptic strength in field CA1 of rat hippocampal slices. We find that bath application of the direct adenylate cyclase activator forskolin (50 microM) enhances the field excitatory postsynaptic potential (EPSP) slope and population spike amplitude evoked by stimulation of Schaffer/commissural afferents. This effect is potentiated by the phosphodiesterase inhibitor and adenosine receptor antagonist 3-isobutyl-1-methylxanthine (IBMX). The enhancement produced by forskolin is suppressed in the presence of adenylate cyclase inhibitors and is not mimicked by the inactive forskolin analogue 1,9-dideoxyforskolin, indicating that, indeed, activation of adenylate cyclase mediates the effects of forskolin in field CA1. Our observations support the idea that changes in intracellular cAMP levels can modulate synaptic efficacy of excitatory glutamatergic synapses in the mammalian hippocampus.

1-Methyl-3-isobutylxanthine↗
PubMed1992

Cloning of a putative glutamate receptor: a low affinity kainate-binding subunit.

Kainate, a glutamate receptor agonist, is a potent neuroexcitatory agent that produces epileptiform activity and selective neuronal degeneration. Binding studies using neuronal membrane homogenates or brain sections have identified sites having either high or low affinity for [3H]kainate. Here we report the cloning of a gene, GluR7, with approximately 75% sequence identity with the previously cloned GluR5 and GluR6 subunit genes. Transcripts of the GluR7 gene are evident in brain areas that bind [3H]kainate and are susceptible to kainate-induced neurotoxicity. We have performed ligand binding studies with membranes of transfected HeLa cells expressing GluR6 or GluR7 subunits. Our data show that the GluR6 and GluR7 subunits have a rank order of agonist affinity (domoate greater than kainate much greater than L-glutamate, quisqualate much greater than AMPA, NMDA) and a dissociation constant for kainate (95 and 77 nM, respectively) characteristic of the low affinity kainate-binding sites described in the brain.

Amino Acid Sequence↗
PubMed1991

A Drosophila mutant defective in extracellular calcium-dependent photoreceptor deactivation and rapid desensitization.

CALCIUM is involved in the adaptation of vertebrate photoreceptors to light and may have a similar role in invertebrate phototransduction. But the molecular mechanisms mediating this stimulus-dependent regulation are not well understood in any G protein-coupled transduction system. We have developed a preparation of isolated Drosophila photoreceptors that has allowed us to carry out an electrophysiological characterization of the light-activated response in these sensory neurons using patch-clamp techniques. We report here that extracellular calcium entering through the light-activated conductance is a key regulator of both the activation and deactivation phases of the phototransduction cascade, and that inaC mutant photoreceptors are specifically defective in the calcium-dependent deactivation mechanism. These data suggest that the light-dependent calcium influx inactivates this cascade through a biochemical pathway that requires the inaC gene product, and that this mechanism represents a molecular basis for stimulus-dependent regulation of visual transduction in Drosophila photoreceptors.

Animals↗
PubMed1991

Enhancement of the glutamate response by cAMP-dependent protein kinase in hippocampal neurons.

Receptor channels activated by glutamate, an excitatory neurotransmitter in the mammalian brain, are involved in processes such as long-term potentiation and excitotoxicity. Studies of glutamate receptor channels expressed in cultured hippocampal pyramidal neurons reveal that these channels are subject to neuromodulatory regulation through the adenylate cyclase cascade. The whole-cell current response to glutamate and kainate [a non-NMDA (N-methyl-D-aspartate) receptor agonist] was enhanced by forskolin, an activator of adenylate cyclase. Single-channel analysis revealed that an adenosine 3',5'-monophosphate-dependent protein kinase (PKA) increases the opening frequency and the mean open time of the non-NMDA-type glutamate receptor channels. Analysis of synaptic events indicated that forskolin, acting through PKA, increased the amplitude and decay time of spontaneous excitatory postsynaptic currents.

Adenosine Triphosphate↗
PubMed1991

Excitatory and inhibitory autaptic currents in isolated hippocampal neurons maintained in cell culture.

Individual rat hippocampal neurons, grown in isolation from other neurons on small spots of permissive substrate, were studied in order to characterize the electrical properties of the synapses that such cells formed with themselves (autapses). Excitatory (probably glutamatergic) or inhibitory (probably type A gamma-aminobutyratergic) autapses were frequently found. Excitatory autaptic currents reversed near the potential expected for monovalent cations were blocked by the glutamatergic antagonist kynurenic acid, and possessed a slow component with the pharmacological profile of N-methyl-D-aspartate-type channels. These currents also exhibited trial-to-trial statistical fluctuations in their amplitudes, this being well-described by quantal analysis. Inhibitory autaptic currents reversed at hyperpolarized potentials, as expected for chloride-permeable pores and were blocked by picrotoxin, a type A gamma-aminobutyric receptor antagonist. It is concluded that autaptic currents in culture are identical to those found at synapses.

2-Amino-5-phosphonovalerate↗
PubMed1991

Role of a key cysteine residue in the gating of the acetylcholine receptor.

We have examined changes in single-channel behavior that result from conservative amino acid substitutions at the Cys230 residue in the putative first transmembrane region (M1) of the murine nicotinic acetylcholine receptor. Mutations made in the gamma subunit altered the energy barrier for a single closing rate constant in proportion to the size of the substituted side chain. One of these substitutions, when made in the alpha subunits, had no effect on gating. No mutations altered permeation. We conclude that the region surrounding the M1 Cys is involved in the gating of the nicotinic acetylcholine receptor and that the gamma subunit contributes significantly to the control of channel closure.

Animals↗
PubMed1990

Influence of the gamma subunit and expression system on acetylcholine receptor gating.

We have developed a partial kinetic theory for the gating of murine nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes and have used this theory to characterize the role of the gamma subunit in single-channel behavior. Permeation and gating were found to be largely unaffected in AChRs produced in oocytes when the gamma subunit transcript was omitted from microinjections of AChR subunit RNAs. In contrast, marked changes in gating kinetics resulted when even very conservative single amino acid substitutions were introduced into the gamma subunit, indicating that the gamma subunit can have a large effect on AChR gating. We also found that channel openings were much prolonged when murine AChRs were expressed in BC3H-1 cells.

Animals↗
PubMed1990

Two different ways evolution makes neurons larger.

As evolution makes larger brains it also increases the size of many of the individual neurons that make up the brain. How neurons are made larger can give clues about design principles of the brain's circuits. One way of making a larger neuron is called conservative scaling. If evolution magnifies a particular type of neuron by a factor of two-that is, each dendrite is made twice as long-then the neuron is scaled conservatively if the magnified neuron has dendrites with 4 times the diameter of their unscaled counterparts. This type of scaling leaves the passive cable properties of the neuron unchanged and so maintains a balance in effectiveness between proximal and distal dendritic inputs. One might imagine that, for some types of circuits, maintaining such a balance would be necessary to use just the same neuronal interconnections in both large and small brains. We have compared dentate granule cells and CA1 pyramidal neurons in cat and human to establish how these cell types are, in fact, scaled. Both cell types are larger in human than in cat, even though their general form is conserved. Pyramidal neurons scale conservatively, but dentate granule cells do not. The CA1 circuits seem, then, to require conservation of the passive cable properties of their elements, whereas dentate does not. We suggest that the reason CA1 neurons scale conservatively is that, for this region, each individual synaptic input exerts a significant effect on the cell's output, whereas in dentate the neuronal output represents the average of a large number of anonymous individual inputs.

Animals↗
PubMed1990

Failure to reverse long-term potentiation by coupling sustained presynaptic activity and N-methyl-D-aspartate receptor blockade.

The proposal that long-term potentiation (LTP) is a mechanism underlying memory in the mammalian brain rests on a number of properties of LTP that parallel characteristics of memory defined by behavior. A prominent feature of behaviorally defined memory is its reversibility. LTP is induced at synapses that correlate in their activity, and the signal for induction is calcium influx through N-methyl-D-aspartate (NMDA) receptor channels. By analogy to the reversibility of behaviorially defined memory, uncorrelated synaptic activity might be expected to reverse LTP, an anti-Hebbian effect called long-term depression, which has only recently been described in the hippocampus [Stanton, P. K. & Sejnowski, T. J. (1989) Nature (London) 339, 215-218]. Because the extent to which synaptic activity is correlated is represented by postsynaptic calcium concentrations, it seemed likely to us that long-term depression is related to the failure of calcium to pass through the NMDA channel. One way to block the calcium influx that signals correlated synaptic activity is with the NMDA receptor antagonist D-(-)-2-amino-5-phosphonovalerate. We performed a series of experiments in rat hippocampal slices designed to estimate the amount of synaptic depression per afferent test pulse under these conditions. Schaffer collateral-commissural afferents to field CA1 were repetitively stimulated in the presence of 2-amino-5-phosphonovalerate. No enduring synaptic depression nor reversal of LTP could be detected. We conclude that some other mechanism underlies long-term depression in the hippocampus.

2-Amino-5-phosphonovalerate↗
PubMed1990

Computational implications of NMDA receptor channels.

We have summarized the quantitative relations developed so far for the description of NMDA receptor function. One of the most important gaps in our knowledge relates to desensitization. A full quantitative treatment of computational uses of NMDA receptor channels must await a formalization of this process and also a more detailed examination of the occupation of closed states of the receptor whose binding sites are occupied. As this information becomes available and the role of NMDA receptors in the function of brain circuits is further explored, we should be able to define accurately this second computational mode.

Animals↗
PubMed1990

A quantitative description of NMDA receptor-channel kinetic behavior.

Currents evoked in neurons of the vertebrate CNS by the glutamate agonist N-methyl-D-aspartate (NMDA) exhibit a marked voltage dependence in the presence of extracellular Mg. At the single-channel level, the addition of external Mg alters single-channel openings from long-lived events to many very short events grouped into bursts of openings. These bursts apparently result from short interruptions of current flow during periods when the channel is in the open configuration. Single-channel currents evoked by NMDA have been studied in outside-out patches of membrane taken from hippocampal CA 1 neurons grown in dissociated cell culture. The effects of changing external Mg concentration and holding potential on the single-channel parameters of open time, closed time, and burst length have been successfully described assuming a 3- or 4-state model with 1 open state, 1 or 2 "blocked" states, and 1 absorbing closed state. Evaluation of the blocking rates over Mg concentrations from 0.2-200 microM indicate that a single "blocking" mechanism cannot account for the short closed states and that a second voltage-dependent but Mg-independent "blocked" state is necessary to explain the data especially at low Mg concentrations.

Animals↗