SPECTRAL ANALYSIS OF CURRENT-NOISE DATA FOR BILAYER LIPID MEMBRANE

We report the spectral analysis of current-noise data obtained under voltage clamp condition in bilayer lipid membrane, using Lomb Scargle Periodogram. Such spectral analysis shows evidence of crossover from an uncorrelated phenomenon towards a state of long range correlation in time and space with the increase of applied bias. Based on these observations a probable mechanism of transmembrane charge conduction has been outlined in the light of self-organized criticality.

Mechanisms Determining the Dynamic Range of the Bullfrog Olfactory Receptor Cell

Spike discharges of single olfactory receptor cells (ORCs) were recorded with the whole cell patch-clamp method applied to slice preparation. In parallel, activities of transduction channels were measured under the voltage-clamp condition. When cells were stimulated by odorants, 54 out of 306 cells exhibited inward current responses (10 mM cineole in the puffer pipette). The amplitude of the inward current was dependent on the stimulus period, reflecting the time integration for the stimulus dose, and the relation could be fitted by the Hill equation. Under the current-clamp condition, current injection induced spike discharges. In cells showing repetitive firings, the firing frequency was dependent on the amount of injected current. The relation was fitted by the Michaelis-Menten equation showing saturation. When cells were responsive to the odorant and had abilities to discharge repetitive spikes, the depolarizing responses were accompanied by repetitive spikes. In those cells, the spike frequency was dose-dependent, expressing saturation similar to the result obtained by current injection. Since both transduction channel and spike generative steps expressed saturation in their dose dependences, we explored what step(s) actually determines saturation in ORC signaling processes. By examining dose-response relations of both the current and spikes in the same cells, saturating dose was found to be dependent largely on that of the transduction step. This suggests that the dynamic range is fundamentally determined by the transduction system. In addition, a simple model derived from the nonlinearity of the plasma membrane could explain that a critical level of dynamic range was, at least in part, modified by the membrane nonlinearity.

Modulation of High-Voltage Activated Ca2+ Channels in the Rat Periaqueductal Gray Neurons by μ-Type Opioid Agonist

Kim, Chang-Ju, Jeong-Seop Rhee, and Norio Akaike. Modulation of high-voltage activated Ca2+ channels in the rat periaqueductal gray neurons by μ-type opioid agonist. J. Neurophysiol. 77: 1418–1424, 1997. The effect of μ-type opioid receptor agonist, D-Ala2,N-MePhe4,Gly5-ol-enkephalin (DAMGO), on high-voltage-activated (HVA) Ca2+ channels in the dissociated rat periaqueductal gray (PAG) neurons was investigated by the use of nystatin-perforated patch recording mode under voltage-clamp condition. Among 118 PAG neurons tested, the HVA Ca2+ channels of 38 neurons (32%) were inhibited by DAMGO (DAMGO-sensitive cells), and the other 80 neurons (68%) were not affected by DAMGO (DAMGO-insensitive cells). The N-, P-, L-, Q-, and R-type Ca2+ channel components in DAMGO-insensitive cells shared 26.9, 37.1, 22.3, 7.9, and 5.8%, respectively, of the total Ca2+ channel current. The channel components of DAMGO-sensitive cells were 45.6, 25.7, 21.7, 4.6, and 2.4%, respectively. The HVA Ca2+ current of DAMGO-sensitive neurons was inhibited by DAMGO in a concentration-, time-, and voltage-dependent manner. Application of ω-conotoxin-GVIA occluded the inhibitory effect of DAMGO ∼70%. So, HVA Ca2+ channels inhibited by DAMGO were mainly the N-type Ca2+ channels. The inhibitory effect of DAMGO on HVA Ca2+ channels was prevented almost completely by the pretreatment of pertussis toxin (PTX) for 8–10 h, suggesting that DAMGO modulation on N-type Ca2+ channels in rat PAG neurons is mediated by PTX-sensitive G proteins. These results indicate that μ-type opioid receptor modulates N-type HVA Ca2+ channels via PTX-sensitive G proteins in PAG neurons of rats.

Effects of voltage perturbation of the lingual receptive field on chorda tympani responses to Na+ and K+ salts in the rat: implications for gustatory transduction.

Taste sensory responses from the chorda tympani nerve of the rat were recorded with the lingual receptive field under current or voltage clamp. Consistent with previous results (Ye, Q., G. L. Heck, and J. A. DeSimone. 1993. Journal of Neurophysiology. 70:167-178), responses to NaCl were highly sensitive to lingual voltage clamp condition. This can be attributed to changes in the electrochemical driving force for Na+ ions through apical membrane transducer channels in taste cells. In contrast, responses to KCl over the concentration range 50-500 mM were insensitive to the voltage clamp condition of the receptive field. These results indicate the absence of K+ conductances comparable to those for Na+ in the apical membranes of taste cells. This was supported by the strong anion dependence of K salt responses. At zero current clamp, the potassium gluconate (KGlu) threshold was > 250 mM, and onset kinetics were slow (12 s to reach half-maximal response). Faster onset kinetics and larger responses to KGlu occurred at negative voltage clamp (-50 mV). This indicates that when K+ ion is transported as a current, and thereby uncoupled from gluconate mobility, its rate of delivery to the K+ taste transducer increases. Analysis of conductances shows that the paracellular pathway in the lingual epithelium is 28 times more permeable to KCl than to KGlu. Responses to KGlu under negative voltage clamp were not affected by agents that are K+ channel blockers in other systems. The results indicate that K salt taste transduction is under paracellular diffusion control, which limits chemoreception efficiency. We conclude that rat K salt taste occurs by means of a subtight junctional transducer for K+ ions with access limited by anion mobility. The data suggest that this transducer is not cation selective which also accounts for the voltage and amiloride insensitive part of the response to NaCl.

Activation kinetics of the acetylcholine-gated potassium current in isolated atrial cells

Processes involved in activation of the acetylcholine (ACh) receptor-operated K+ current (IK) were examined in atrial cells isolated from guinea pig using a "concentration-clamp" technique. This approach allows for the intracellular perfusion and the rapid change of external solution (time constant 4 ms) with cell-attached condition under a single-electrode voltage-clamp condition. The ACh-induced IK increased in a sigmoidal fashion with increasing concentrations of ACh. The Ka value estimated from the concentration-response curve was 3 X 10(-7) M, and the Hill coefficient was 1.0. The activation phase was accelerated not only by increasing the concentration of ACh but also by elevating the temperature. Before activation of the current, there was a brief latent period after the application of ACh. The latent period was shortened considerably with the increase in ACh concentration and in temperature, i.e., 267 +/- 20 ms at 18 degrees C, 98 +/- 11 ms at 26 degrees C, and 44 +/- 6 ms at 37 degrees C for 10(-6) M ACh. These results suggest that the latent period of ACh response seems to be the time lag needed for the activation of K+ channels using remote sensor.

Action Potential in the Transverse Tubules and Its Role in the Activation of Skeletal Muscle

The double sucrose-gap method was applied to single muscle fibers of Xenopus. From the "artificial node" of the fiber, action potentials were recorded under current-clamping condition together with twitches of the node. The action potentials were stored on magnetic tape. The node was then made inexcitable by tetrodotoxin or by a sodium-free solution, and the wave form of the action potential stored on magnetic tape was imposed on the node under voltage-clamp condition (simulated AP). The twitch height caused by the simulated AP's was always smaller than the twitch height produced by the real action potentials, the ratio being about 0.3 at room temperature. The results strongly suggest that the transverse tubular system is excitable and is necessary for the full activation of twitch, and that the action potential of the tubules contributes to about 70 % of the total mechanical output of the normal isotonic twitch at 20°C. Similar results were obtained in the case of tetanic contraction. At a temperature near 10°C, twitches produced by the simulated AP were not very different (85 % of control amplitude) from the twitches caused by real action potentials. This indicates that the excitability of the tubules becomes less necessary for the full activation of twitch as the temperature becomes lower.

Analysis of K Inactivation and TEA Action in the Supramedullary Cells of Puffer

Under the voltage clamp condition, the K inactivation was analyzed in cells bathed in the isosmotic KCl Lophius-Ringer solution. After conditioning hyperpolarization, the cells respond to depolarizations with increased K permeability, which in turn is decreased during maintained depolarizations. The steady-state levels of the K inactivation as a function of the membrane potential are related by an S-shaped curve similar to that which describes the steady-state Na inactivation in the squid giant axon. TEA reduced the K conductance by a factor which is independent of the potential, and without a shift of the inactivation curve along the voltage axis. The rapid phase of the K activation is less susceptible to TEA than the slow phase of the K activation. Hyperpolarizing steps remove the K inactivation, the rate of the removal being faster the larger the hyperpolarization from the standard potential of about -60 mv.