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A R Kay

Publications and source records attributed to A R Kay.

At least 19 recordsLinked to original sources
PubMed1998

Noise from voltage-gated ion channels may influence neuronal dynamics in the entorhinal cortex.

Neurons of the superficial medial entorhinal cortex (MEC), which deliver neocortical input to the hippocampus, exhibit intrinsic, subthreshold oscillations with slow dynamics. These intrinsic oscillations, driven by a persistent Na+ current and a slow outward current, may help to generate the theta rhythm, a slow rhythm that plays an important role in spatial and declarative learning. Here we show that the number of persistent Na+ channels underlying subthreshold oscillations is relatively small (<10(4)) and use a physiologically based stochastic model to argue that the random behavior of these channels may contribute crucially to cellular-level responses. In acutely isolated MEC neurons under voltage clamp, the mean and variance of the persistent Na+ current were used to estimate the single channel conductance and voltage-dependent probability of opening. A hybrid stochastic-deterministic model was built by using voltage-clamp descriptions of the persistent and fast-inactivating Na+ conductances, along with the fast and slow K+ conductances. All voltage-dependent conductances were represented with nonlinear ordinary differential equations, with the exception of the persistent Na+ conductance, which was represented as a population of stochastic ion channels. The model predicts that the probabilistic nature of Na+ channels increases the cell's repertoire of qualitative behaviors; although deterministic models at a particular point in parameter space can generate either subthreshold oscillations or phase-locked spikes (but rarely both), models with an appropriate level of channel noise can replicate physiological behavior by generating both patterns of electrical activity for a single set of parameters. Channel noise may contribute to higher order interspike interval statistics seen in vitro with DC current stimulation. Models with channel noise show evidence of spike clustering seen in brain slice experiments, although the effect is apparently not as prominent as seen in experimental results. Channel noise may contribute to cellular responses in vivo as well; the stochastic system has enhanced sensitivity to small periodic stimuli in a form of stochastic resonance that is novel (in that the relevant noise source is intrinsic and voltage-dependent) and potentially physiologically relevant. Although based on a simple model that does not include all known membrane mechanisms of MEC stellate cells, these results nevertheless imply that the stochastic nature of small collections of molecules may have important effects at the cellular and network levels.

Animals↗
PubMed1998

Kinetic and stochastic properties of a persistent sodium current in mature guinea pig cerebellar Purkinje cells.

Whole cell voltage-clamp techniques were employed to characterize the sodium (Na) conductances in acutely dissociated, mature guinea-pig cerebellar Purkinje cells. Three phenomenological components were noted: two inactivating and a persistent component (I(P)(Na). All exhibited similar sensitivities to tetrodotoxin (TTX; IC50 approximately 3 nM). The inactivating Na current demonstrates two components with different rates of inactivation. The persistent component activates at a more negative membrane potential than the inactivating components and shows little inactivation during a 5-s pulse. The amplitude of the persistent Na conductance had a higher Q10 than the inactivating Na conductance (2.7 vs. 1.3). (I(P)(Na) rapidly activates (approximately 1 ms) and deactivates (< 0.2 ms) and like the fast component appears to be exclusively Na permeable. (I(P)(Na) is not a "window" current because its range of activation exceeds the small overlap between the steady-state activation and inactivation characteristics of the inactivating current. Anomalous tail currents were observed during voltage pulses above -40 mV after a prepulse above -30 mV. The tails rose to a maximum inward current with a time constant of 1.5 ms and decayed to a persistent inward current with a time constant of 20 ms. The tails probably arose as a result of recovery from inactivation through the open state. The noise characteristics of (I(P)(Na) were anomalous in that the measured variance was lower at threshold voltages than would be predicted by a binomial model. The form of the variance could be partially accounted for by postulating that the maximum probability of activation of the persistent current was less than unity. The noise characteristics of (I(P)(Na) are such as to minimize noise near spike activation threshold and sharpen the threshold.

Animals↗
PubMed1997

Trophic support of cultured spiral ganglion neurons by depolarization exceeds and is additive with that by neurotrophins or cAMP and requires elevation of [Ca2+]i within a set range.

Spiral ganglion neurons (SGNs) require both pre- and postsynaptic contacts to maintain viability. BDNF, NT-3, chlorphenylthio-cAMP, and depolarization (veratridine or elevated [K+]o) all promote survival of SGNs in vitro, depolarization being the most effective. Combining different trophic stimuli increases survival in an additive manner. Neurotrophins and depolarization maintain comparable soma size and neurite extension, but SGNs are shrunken in cAMP. Elevated [K+]o has a biphasic effect on SGN survival; survival improves as [K+]o is raised to 30 mM (30K) and falls as [K+]o is further increased; SGN survival in 80 mM [K+]o (80K) is poor relative to survival in 30K. These responses to elevated [K+]o are potentiated by an L-type channel agonist, whereas L-type Ca2+ channel blockers antagonize the trophic effect of depolarization. Four hours after depolarization, steady-state [Ca2+]i is elevated in SGNs in 30K and further elevated in SGNs in 80K. At 22 hr after depolarization, by which time death of neurons in 80K has begun, elevated [Ca2+]i levels in surviving neurons in 80K are not higher than those in neurons in 30K ( approximately 150-450 nM), suggesting that neurons with high [Ca2+]i are preferentially lost. Veratridine causes oscillatory increases in [Ca2+]i to 250-350 nM. Thus, [Ca2+]i is predictive of cell survival; [Ca2+]i elevated to 100-500 nM in a sustained or oscillatory manner permits SGN survival independent of exogenous neurotrophic factors. Higher [Ca2+]i is associated with cell death.

Animals↗
PubMed1997

Imaging free zinc in synaptic terminals in live hippocampal slices.

Some glutamatergic synapses in the mammalian central nervous system exhibit high levels of free ionic zinc in their synaptic vesicles. The precise role of this vesicular zinc remains obscure, despite suggestive evidence for zinc as a neuromodulator. As a step towards elucidating the role of free zinc in the brain we have developed a method for imaging zinc release in live brain slices. A newly synthesized zinc-sensitive fluorescent probe, N-(6-methoxy-8-quinolyl)-p-carboxybenzoylsulphonamide (TFLZn), was used to monitor intracellular zinc in live rat hippocampal slices. The dye loaded into the zinc-rich synaptic vesicles of the mossy fibre terminals in the hippocampal formation. Direct electrical stimulation of the mossy fibre pathway diminished the fluorescence in the mossy fibre terminals, consistent with a stimulus-dependent zinc release. The synaptic release of zinc was followed by the rapid replenishment of the zinc levels in vesicles from an as yet unidentified intracellular zinc source. Furthermore, we present evidence that zinc may play a role in a form of long-term potentiation exhibited by the mossy fibre pathway.

Animals↗
PubMed1995

A bifurcation analysis of neuronal subthreshold oscillations.

The conditions under which a noninactivating sodium current and either a potassium current or an inwardly rectifying cation current can generate subthreshold oscillations were analyzed using nonlinear dynamical techniques applied to a neuronal model consisting of two differential equations. Mathematical descriptions of the membrane currents were derived using voltage-clamp data collected from entorhinal cortical neurons. A bifurcation analysis was performed using applied current as the control parameter to map the range of magnitudes of the sodium, potassium/cation, and leakage conductances over which subthreshold oscillations exist. The threshold of the potassium/cation current was an important determinant of the robustness of oscillatory behavior. The activation time constant of the potassium/cation current largely determined the frequency range of emergent oscillations. This result implicates the slow inward rectifier or an as yet undescribed slow outward current in entorhinal cortical oscillations; the latter explanation, while more speculative, is more consistent with the pharmacological properties of subthreshold oscillations and gives oscillations over a larger current range. The shallowness of the sodium activation curve confined emergent oscillations to rise gradually rather than abruptly and extended the current range over which the model oscillated.

Animals↗
PubMed1995

Errors in persistent inward currents generated by space-clamp errors: a modeling study.

1. The effects of imperfect space clamp on inactivating inward currents were examined with the use of a "ball-and-stick" neuronal model with uniform active and passive membrane properties. With poor space clamp, both transient and steady-state (persistent) components were distorted. The ratio of steady-state to peak current (i(s)/p), measured at the soma, was sometimes smaller but usually larger than would be the case with uniform space clamp. For a fast Na+ current, the anomalous persistent component was largest for large electrotonic lengths, low-conductance densities, and voltage-clamp potentials near the threshold of the current. Under some conditions, steady-state current could take one of two values, depending on the holding potential. 2. Membrane potential as a function of distance was examined, revealing a steady-state voltage gradient in which distal portions of the neuron were more positive than in the passive case, and often more positive than the command potential itself. These reversed voltage gradients, caused by the uncontrolled "window" Na+ current at remote electrotonic distances, produced steady-state axial current flow into the soma, thereby increasing the persistent current measured somatically. 3. The time at which the current peaked (tp) was sensitive to imperfections in the space clamp. This phenomenon made somatic membrane current and axial current at tp sensitive to the fidelity of space clamp as well. The ratio of steady-state axial current to that at t = tp was a good predictor of the degree of distortion of i(s)/p.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics↗
PubMed1994

Hyperpolarization-activated Na(+)-K+ current (Ih) in neocortical neurons is blocked by external proteolysis and internal TEA.

1. After 1 day in culture, neurons derived from the neonatal cerebral cortex of the rat exhibited a slowly activating current gated by hyperpolarizing voltage clamp pulses. The current was blocked by extracellular Cs+ and unaffected by extracellular Ba2+, and was permeable to both Na+ and K+ (PNa/PK = 0.29). Its form and pharmacology are consistent with a current termed Ih in other preparations. 2. Ih was absent from cells acutely dissociated from both the neonatal and mature cerebral cortex, despite the use of low enzyme concentrations. The sensitivity of Ih to extracellular proteolysis was demonstrated by superfusing the cells with trypsin (1 mg/ml) while monitoring the presence of Ih in the whole cell mode of recording. Ih was rapidly abolished (t1/2 approximately 5 min at 22 degrees C) by proteolysis and exhibited no shifts in its range of activation or changes in its activation kinetics during the course of the digestion. 3. Intracellular tetraethylammonium (TEA), at a concentration of < 15 mM was shown to block Ih completely, while extracellular TEA had no effect on the current. This suggests that the inner vestibule of Ih may be structurally related to that of potassium channels.

Animals↗
PubMed1993

A heart-like Na+ current in the medial entorhinal cortex.

Acutely dissociated neurons from the superficial layers of the medial entorhinal cortex of the rat were studied under voltage clamp using the whole-cell patch-clamp configuration. Neurons from the medial entorhinal cortex exhibit a tetrodotoxin (TTX)-resistant Na+ current (ITTX-R; IC50 approximately 146 nM), in addition to the normal TTX-sensitive Na+ current (ITTX-S; IC50 approximately 6 nM). ITTX-R was found in both putative stellate and putative pyramidal neurons from the medial entorhinal cortex. ITTX-R is kinetically indistinguishable from ITTX-S, but can be distinguished from ITTX-S based on its enhanced sensitivity to block by Cd2+, La3+, and Zn2+. ITTX-R is kinetically and pharmacologically similar to the TTX-resistant Na+ current found in cardiac muscle.

Animals↗
PubMed1992

An intracellular medium formulary.

Whole-cell patch-clamp recordings allow diffusible intracellular ions and molecules to be replaced by the contents of the recording pipette. In this review, the formulation of intracellular media is considered with a view to improving the stability of recordings and emulating the intracellular environment.

Animals↗
PubMed1991

Inactivation kinetics of calcium current of acutely dissociated CA1 pyramidal cells of the mature guinea-pig hippocampus.

1. The process of inactivation of the Ca2+ current of acutely dissociated pyramidal cells from the CA1 subfield of mature guinea-pig hippocampus was characterized. The decline of the current after rapid activation could be approximated well by the sum of two exponentials (time constants approximately 200 ms and 2 s) and a constant offset. 2. The time constants of inactivation exhibited a voltage dependence consistent with a voltage-dependent mechanism. However, under conditions which normally counteract Ca(2+)-dependent inactivation (viz. intracellular bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) and external Ba2+) all three showed a U-shaped inactivation curve, characteristic of Ca(2+)-dependent inactivation. 3. The rate of inactivation was found to increase with current at a given voltage; however, increasing external divalent ion concentrations did not accelerate inactivation. 4. Calcium imaging experiments, using the Ca(2+)-sensitive probe, Fura-2, were performed to estimate the accumulation of Ca2+ in the presence of 10 mM-intracellular BAPTA. Under these conditions voltage steps which induced maximal Ca2+ currents lead to free Ca2+ concentrations of less than 500 nM in the bulk of the cytoplasm. 5. Elevation of the intracellular free Ca2+ concentration to above 1 microM suppressed all the components of the Ca2+ current. However, even at a concentration of 3 microM-Ca2+ the U-shaped inactivation curve persisted. 6. Substitution of Ca2+ for Ba2+ led to an acceleration of inactivation through an increase in the proportion of the fast process of inactivation and an acceleration of both the fast and slow rates of inactivation. 7. During the slow decline of Ca2+ current ('run-down') the proportion of all three components remained approximately constant and there was little change in the rate of inactivation. 8. On the basis of the results I suggest that inactivation results fro a dual process of voltage- and Ca(2+)-dependent inactivation. Ca(2+)-dependent inactivation seems to result from the accumulation of Ca2+ close to the channel mouth. 9. The macroscopic properties of the Ca2+ channel are consistent with the existence of one channel type in the CA1 pyramidal cells.

Animals↗
PubMed1990

Brain magnetic resonance imaging with contrast dependent on blood oxygenation.

Paramagnetic deoxyhemoglobin in venous blood is a naturally occurring contrast agent for magnetic resonance imaging (MRI). By accentuating the effects of this agent through the use of gradient-echo techniques in high fields, we demonstrate in vivo images of brain microvasculature with image contrast reflecting the blood oxygen level. This blood oxygenation level-dependent (BOLD) contrast follows blood oxygen changes induced by anesthetics, by insulin-induced hypoglycemia, and by inhaled gas mixtures that alter metabolic demand or blood flow. The results suggest that BOLD contrast can be used to provide in vivo real-time maps of blood oxygenation in the brain under normal physiological conditions. BOLD contrast adds an additional feature to magnetic resonance imaging and complements other techniques that are attempting to provide positron emission tomography-like measurements related to regional neural activity.

Animals↗