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J Neyton

Publications and source records attributed to J Neyton.

At least 19 recordsLinked to original sources
PubMed1995

Glycine-independent and subunit-specific potentiation of NMDA responses by extracellular Mg2+.

Extracellular Mg2+, which blocks NMDA channels in a voltage-dependent manner and increases the receptor's affinity for glycine, is shown here to potentiate NMDA responses at saturating glycine concentrations. This potentiation, induced by millimolar concentrations of Mg2+, is not mimicked by Ca2+ and Ba2+ and is voltage independent. The potentiation is variable in native receptors of cultured mouse central neurons; in recombinant receptors, it is "permitted" by the NR2B subunit and prevented by the NR1 splice variant containing an N-terminal insert. Mg2+ also induces a shift of the pH sensitivity of NMDA receptors. The similarity and nonadditivity of the effects of Mg2+ and spermine suggest that Mg2+ may be the physiological agonist acting at the subunit-specific spermine site.

Animals↗
PubMed1992

Properties of the mitochondrial peptide-sensitive cationic channel studied in planar bilayers and patches of giant liposomes.

A voltage-dependent cationic channel of large conductance is observed in phospholipid bilayers formed by the tip-dip method from proteoliposomes derived from mitochondrial membranes. It is blocked by peptide M, a 13 residue peptide having the properties of a mitochondrial signal sequence. To verify the reliability of the experimental approach, mitochondrial membranes from bovine adrenal cortex or porin-deficient mutant yeast were either fused to planar bilayers or incorporated in giant liposomes which were studied by patch clamp. Cationic channels were found with both techniques. They had the same conductance levels and voltage-dependence as those which have been described using the tip-dip method. Moreover, they were similarly blocked by peptide M. The voltage-dependence of block duration was analyzed in planar bilayer and tip-dip records. Results strengthen the idea that peptide M might cross the channel. Other mitochondrial channels were observed in planar bilayers and patch clamp of giant liposomes. Because they were never detected in tip-dip records, they are likely to be inactivated at the surface monolayer used to form the bilayer in this type of experiment.

Adrenal Cortex↗
PubMed1991

Multi-ion occupancy alters gating in high-conductance, Ca(2+)-activated K+ channels.

In this study, single-channel recordings of high-conductance Ca(2+)-activated K+ channels from rat skeletal muscle inserted into planar lipid bilayer were used to analyze the effects of two ionic blockers, Ba2+ and Na+, on the channel's gating reactions. The gating equilibrium of the Ba(2+)-blocked channel was investigated through the kinetics of the discrete blockade induced by Ba2+ ions. Gating properties of Na(+)-blocked channels could be directly characterized due to the very high rates of Na+ blocking/unblocking reactions. While in the presence of K+ (5 mM) in the external solution Ba2+ is known to stabilize the open state of the blocked channel (Miller, C., R. Latorre, and I. Reisin. 1987. J. Gen. Physiol. 90:427-449), we show that the divalent blocker stabilizes the closed-blocked state if permeant ions are removed from the external solution (K+ less than 10 microM). Ionic substitutions in the outer solution induce changes in the gating equilibrium of the Ba(2+)-blocked channel that are tightly correlated to the inhibition of Ba2+ dissociation by external monovalent cations. In permeant ion-free external solutions, blockade of the channel by internal Na+ induces a shift (around 15 mV) in the open probability--voltage curve toward more depolarized potentials, indicating that Na+ induces a stabilization of the closed-blocked state, as does Ba2+ under the same conditions. A kinetic analysis of the Na(+)-blocked channel indicates that the closed-blocked state is favored mainly by a decrease in opening rate. Addition of 1 mM external K+ completely inhibits the shift in the activation curve without affecting the Na(+)-induced reduction in the apparent single-channel amplitude. The results suggest that in the absence of external permeant ions internal blockers regulate the permeant ion occupancy of a site near the outer end of the channel. Occupancy of this site appears to modulate gating primarily by speeding the rate of channel opening.

Animals↗
PubMed1988

Acetylcholine-induced closure of gap junction channels in rat lacrimal glands is probably mediated by protein kinase C.

In rat lacrimal gland cells, application of acetylcholine (ACh) opens Ca-dependent channels and closes gap junction channels. We have shown previously that the increase in intracellular calcium concentration induced by ACh, is not required for the closure of gap junctions. We have examined the effects of activators of protein kinase C on gap junction conductance measured with the double patch-clamp technique. This conductance was markedly reduced by incubating the cell pairs for a few minutes with 100 nM phorbol dibutyrate (PdBu). Two membrane permeant analogues of diacylglycerol, OAG (1-oleoyl-2-acetyl-glycerol) and DOG (1,2-dioleoyl-glycerol) also induced a closure of gap junction channels. This effect was observed in the concentration range 10-100 microM when the diacylglycerol was added on intact cells, and at 75 microM when it was applied on dialysed cells. The cell uncoupling was not mediated by phosphatidate, a degradation product of OAG, nor by a phospholipase A2-induced increase in arachidonate concentration. The OAG-induced closure of gap junction channels reversed spontaneously upon prolonged exposure (more than 90 min at 37 degrees C) to 25 microM OAG. After a prolonged OAG treatment, the ability of ACh to uncouple the cells was markedly reduced. ACh induced uncoupling was modulated to some extent by intracellular Ca and had an absolute requirement for Mg. These results indicate that ACh-induced closure of gap junction channels may be mediated by PKC.

Acetylcholine↗
PubMed1988

Potassium blocks barium permeation through a calcium-activated potassium channel.

Single high-conductance Ca2+-activated K+ channels from rat skeletal muscle were inserted into planar lipid bilayers, and discrete blocking by the Ba2+ ion was studied. Specifically, the ability of external K+ to reduce the Ba2+ dissociation rate was investigated. In the presence of 150 mM internal K+, 1-5 microM internal Ba2+, and 150 mM external Na+, Ba2+ dissociation is rapid (5 s-1) in external solutions that are kept rigorously K+ free. The addition of external K+ in the low millimolar range reduces the Ba2+ off-rate 20-fold. Other permeant ions, such as Tl+, Rb+, and NH4+ show a similar effect. The half-inhibition constants rise in the order: Tl+ (0.08 mM) less than Rb+ (0.1 mM) less than K+ (0.3 mM) less than Cs+ (0.5 mM) less than NH4+ (3 mM). When external Na+ is replaced by 150 mM N-methyl glucamine, the Ba2+ off-rate is even higher, 20 s-1. External K+ and other permeant ions reduce this rate by approximately 100-fold in the micromolar range of concentrations. Na+ also reduces the Ba2+ off-rate, but at much higher concentrations. The half-inhibition concentrations rise in the order: Rb+ (4 microM) less than K+ (19 microM) much less than Na+ (27 mM) less than Li+ (greater than 50 mM). The results require that the conduction pore of this channel contains at least three sites that may all be occupied simultaneously by conducting ions.

Animals↗
PubMed1988

Discrete Ba2+ block as a probe of ion occupancy and pore structure in the high-conductance Ca2+ -activated K+ channel.

In this study, high-conductance Ca2+-activated K+ channels from rat skeletal muscle were incorporated into planar phospholipid bilayers, and discrete blockade of single channels by Ba2+ was studied. With 150 mM K+ held constant in the internal solution, increasing external K+ over the range 100-1,000 mM raises the rate of Ba2+ dissociation. This "enhancement effect," which operates at K+ concentrations 3-4 orders of magnitude higher than those required for the "lockin" effect described previously, depends on applied voltage, saturates with K+ concentration, and is not observed with Na+. The voltage dependence of the Ba2+ off-rate varies with external K+ in a way suggesting that K+, entering the channel from the external side, forces Ba2+ dissociation to the internal solution. With K+ held fixed in the external solution, the Ba2+ off-rate decreases as internal K+ is raised over the range 0-50 mM. This "lock-in" effect is similar to that seen on the external side (Neyton and Miller, 1988), except that the internal lock-in site is of lower affinity and shows only a fivefold preference for K+ over Na+. All the results taken together argue strongly that this channel's conduction pathway contains four sites of very high affinity for K+, all of which may be simultaneously occupied under normal conducting conditions. According to this view, the mutual destabilization resulting from this high ionic occupancy leads to the unusually high conductance of this K+-specific channel.

Animals↗
PubMed1986

Acetylcholine modulation of the conductance of intercellular junctions between rat lacrimal cells.

The conductance of intercellular junctions between rat lacrimal cells was studied with the double whole-cell tight-seal recording technique. This conductance decreases spontaneously with time as a result of the double-cell dialysis. The rate of this 'spontaneous' uncoupling is unaffected by changing the internal Ca concentration, [Ca]i, between 10(-8) M and 10(-6) M. This rate of uncoupling is greatly increased when [Ca]i is approximately 10(-5) M, and this effect does not involve changes in the internal proton concentration. When [Ca]i is weakly buffered in one of the two cells, 1-2 microM-acetylcholine (ACh) both activates Ca-dependent channels in that cell (Marty, Tan & Trautmann, 1984) and uncouples the two cells. The uncoupling is not synchronous with the increase in [Ca]i as reflected by the Ca-dependent currents. When [Ca]i is strongly buffered in both cells, ACh fails to activate Ca-dependent currents, but it can still uncouple the cells. This ACh-induced uncoupling is often preceded by a transient enhancing of the coupling. In conclusion, ACh has several distinct effects on lacrimal cells: activation of Ca-dependent channels in the plasma membrane, closure of junctional channels involving a Ca-independent mechanism, and sometimes, an increase in the junctional coupling by a Ca-independent mechanism.

Acetylcholine↗
PubMed1986

Physiological modulation of gap junction permeability.

In many tissues cells communicate directly through arrays of intercellular channels which are organized to form gap junctions. These channels are permeant to inorganic ions as well as to small hydrophilic molecules up to Mr 2000. The electrical and chemical coupling provided by such junctions is under the control of intracellular and, in many cases, extracellular substances. The latter (hormones or neurotransmitters) function via the activation of intracellular second messengers. These can rapidly affect the state of opening of the junctions, or induce long-term modulation of the coupling. What are the second messengers and how do they control the functional state of the junctions? These questions' remain largely unanswered, although several internal molecules are thought to be involved in these modulations (e.g. Ca2+, H+ or cyclic AMP). The double patch-clamp technique which enables control of both the intracellular milieu and high resolution measurement of transjunctional currents, has recently been applied to study these problems. In particular, it is now possible to examine at the single channel level how junctional conductance is modulated in terms, for example, of the number of open channels or channel elementary properties.

Animals↗
PubMed1985

Quisqualate and L-glutamate inhibit retinal horizontal-cell responses to kainate.

Currents elicited by L-glutamate and the related agonists quisqualate and kainate were analyzed under voltage clamp in isolated goldfish horizontal cells, using the whole-cell recording configuration of the patch-clamp method [Hamill, O.P., Marty, A., Neher, E., Sakmann, B. & Sigworth, F. J. (1981) Pflügers Arch. 391, 85-100]. These currents resulted from an increase in cationic conductance and were indistinguishable from one another in terms of reversal potential (approximately equal to 0 mV) and apparent elementary conductance (2-3 pS). The power-density spectra of the noise increases produced by each agonist were fit by the sum of two Lorentzian curves having similar cutoff frequencies (tau 1 approximately equal to 5 msec, tau 2 approximately equal to 1 msec), but the relative power of these components were different for quisqualate and glutamate than for kainate. Moreover, the responses to high doses of either quisqualate or glutamate rapidly faded, whereas the responses to kainate did not. Finally, quisqualate and glutamate produced an inhibition of responses to kainate which appeared to be uncompetitive. Kainate, quisqualate, and in our preparation, glutamate appear to activate channels different than those activated by N-methyl-D-aspartate in other preparations. At least some of the effects of quisqualate and glutamate appear to be mediated by receptors bound by kainate.

Animals↗
PubMed1984

Decrease of gap junction permeability induced by dopamine and cyclic adenosine 3':5'-monophosphate in horizontal cells of turtle retina.

The axon terminals of the H1 horizontal cells of the turtle retina are electrically coupled by extensive gap junctions. Dopamine (10 nM to 10 microM) induces a narrowing of the receptive field profile of the H1 horizontal cell axon terminals, increases the coupling resistance between them, and decreases the diffusion of the dye Lucifer Yellow in the network formed by the coupled axon terminals. These actions of dopamine involve the activation of D1 receptors located on the membrane of the H1 horizontal cell axon terminals proper. Increases of the intracellular cyclic AMP concentration induced by either stimulating the adenylate cyclase activity with forskolin or inhibiting the phosphodiesterase activity with isobutylmethylxanthine, theophylline, aminophylline, or compound RO 20-1724 elicit effects similar to those of dopamine on the receptive field profile of the H1 horizontal cell axon terminals, on their coupling resistance, and on the diffusion of Lucifer Yellow in the axon terminal network. It is concluded that dopamine decreases the permeability of the gap junctions between the axon terminals of the H1 horizontal cells of the turtle retina and that this action probably involves cyclic AMP as a second messenger.

Animals↗
PubMed1982

gamma-Aminobutyric acid antagonists decrease junctional communication between L-horizontal cells of the retina.

The antagonists of gamma-aminobutyric acid, bicuculline and picrotoxin, were found to narrow the receptive field profile of the large field horizontal cell (L1HC) in the turtle retina when added to the perfusion medium in micromolar concentrations. The coupling resistance between neighboring L1HCs was increased by bicuculline or picrotoxin. Under control conditions, the dye Lucifer yellow injected into one L1HC diffused into a large number of neighboring L1HCs; bicuculline or picrotoxin greatly restricted dye passage between these same cells. We conclude that antagonists of gamma-aminobutyric acid decrease the conductance of gap junctions between L1HCs.

Animals↗
PubMed1981

Involvement of small-field horizontal cells in feedback effects on green cones of turtle retina.

Light stimuli depolarize green cones of turtle retina through a circuit involving a feedback connection from luminosity horizontal cells (L-HC) to green cones. In turtle retina two types of L-HC have been distinguished: large-field L-HC and small-field L-HC. The spatial properties of the feedback depolarizations of green cones were compared with those of both large- and small-field L-HC. Green cones were found to be more effectively depolarized by relatively small spots of red light than by large red annuli. Moreover, red light stimulation of the periphery of the receptive field could reduce the depolarizing influence of central red stimuli. These spatial properties greatly differ from those of the large-field L-HC, whereas they strongly resemble those of the small-field L-HC. These results suggest that the small-field L-HC mediate the feedback action on green cones.

Animals↗
PubMed1981

Center-surround antagonistic organization in small-field luminosity horizontal cells of turtle retina.

1. The spatial properties of the small-field luminosity horizontal cell (L2-HC) were investigated in the retina of the turtle by using circular or annular light stimuli of varying dimensions. 2. The amplitude of the hyperpolarizing response induced by dim light spots decreased when the diameter of the spot was increased beyond about 1,000 micrometer. 3. With bright-light stimuli the peak amplitude of the light responses increased monotonically when increasing the illuminated area, but a delayed antagonistic effect appeared with spots of more than 1,000-micrometer diameter. 4. Depolarizing responses were observed if the periphery of the receptive field was stimulated with annuli of light in the presence of central background illumination. 5. The interaction of the inputs converging on the L2-HC from the peripheral and central regions of the receptive field was found to be nonlinear. The same peripheral stimulation could result either in an enhancement or in a depression of the central response according to the intensity of the central illumination. 6. By comparing the receptive-field properties of the L2-HC and the red cones a model is proposed that explains the antagonistic surround mechanism of the L2-HC on the basis of the antagonistic peripheral mechanism in cones. '

Animals↗
PubMed1980

Feed-back modulation of cone synapses by L-horizontal cells of turtle retina.

Light stimulation of the periphery of the receptive field of turtle cones can evoke both transient and sustained increases of the cone Ca2+ conductance, which may become regenerative. Such increase in the cone Ca2+ conductance evoked by peripheral illumination results from the activation of a polysynaptic pathway involving a feed-back connexion from the L-horizontal cells (L-HC) to the cones. Thus the hyperpolarization of a L-HC by inward current injection can evoke a Ca2+ conductance increase in neighbouring cones. The cone Ca2+ channels thus activated are likely located at its synaptic endings and probably intervene in the cone transmitter release. Therefore the feed-back connexion between L-HC and cones by modifying the Ca2+ conductance of cones could actually modulate the transmitter release from cone synapses. Such feed-back modulation of cone synapses plays a role in the organization of the colour-coded responses of the chromaticity type-horizontal cells and probably of other second order neurones, post-synaptic to the cones. The mechanisms operating the feed-back connexion from L-HC to cones are discussed.

Animals↗