Contributions of the sodium pump and ionic gradients to the membrane potential of a molluscan neurone

Binding of κ-Conotoxin-PVIIA to Open and Closed Shaker K-Channels Are Differentially Affected by the Ionic Strength

κ-Conotoxin-PVIIA (κ-PVIIA) is a potassium-channel blocking peptide from the venom of the fish-hunting snail, Conus purpurascens, which is essential for quick prey's excitotoxic immobilization. Binding of one κ-PVIIA to Shaker K-channels occludes the K+-conduction pore without additional conformational effects. Because this 27-residue toxin is +4-charged at neutral pH, we asked if electrostatic interactions play a role in binding. With Voltage-Clamp electrophysiology, we tested how ionic strength (IS) affects κ-PVIIA blockade to Shaker. When IS varied from ~0.06 to ~0.16 M, the dissociation constant for open and closed channels increased by ~5- and ~16-fold, respectively. While the association rates decreased equally, by ~4-fold, in open and closed channels, the dissociation rates increased 4-5-fold in closed channels but was IS-insensitive in open channels. To explain this differential IS-dependency, we propose that the bound κ-PVIIA wobbles, so that in open channels the intracellular environment, via ion-conduction pore, buffers the imposed IS-changes in the toxin-channel interface. A Brønsted-Bjerrum analysis on the rates predicts that if, instead of fish, the snail preyed on organisms with seawater-like lymph ionic composition, a severely harmless toxin, with >100-fold diminished affinity, would result. Thus, considerations of the native ionic environment are essential for conotoxins evaluation as pharmacological leads.

How to Build a Fast and Highly Sensitive Sound Detector That Remains Robust to Temperature Shifts

Frogs must have sharp hearing abilities during the warm summer months to successfully find mating partners. This study aims to understand how frog hair cell ribbon-type synapses preserve both sensitivity and temporal precision during temperature changes. Under room (∼24°C) and high (∼32°C) temperature, we performed in vitro patch-clamp recordings of hair cells and their afferent fibers in amphibian papillae of either male or female bullfrogs. Afferent fibers exhibited a wide heterogeneity in membrane input resistance (R_in) from 100 mΩ to 1000 mΩ, which may contribute to variations in spike threshold and firing frequency. At higher temperatures, most fibers increased their frequency of spike firing due to an increase in spontaneous EPSC frequencies. Hair cell resting membrane potential (V_rest) remained surprisingly stable during temperature increases, because Ca²⁺ influx and K⁺ outflux increased simultaneously. This increase in Ca²⁺ current likely enhanced spontaneous EPSC frequencies. These larger "leak currents" at V_rest also lowered R_in and produced higher electrical resonant frequencies. Lowering R_in will reduce the hair cells receptor potential and presumably moderate the systems sensitivity. Using membrane capacitance measurements, we suggest that hair cells can partially compensate for this reduced sensitivity by increasing exocytosis efficiency and the size of the readily releasable pool of synaptic vesicles. Furthermore, paired recordings of hair cells and their afferent fibers showed that synaptic delays shortened and multivesicular release becomes more synchronous at higher temperatures, which should improve temporal precision. Together, our results explain many previous in vivo observations on the temperature dependence of spikes in auditory nerves.SIGNIFICANCE STATEMENT The vertebrate inner ear detects and transmits auditory information over a broad dynamic range of sound frequency and intensity. It achieves remarkable sensitivity to soft sounds and precise frequency selectivity. How does the ear of cold-blooded vertebrates maintain its performance level as temperature changes? More specifically, how does the hair cell to afferent fiber synapse in bullfrog amphibian papilla adjust to a wide range of physiological temperatures without losing its sensitivity and temporal fidelity to sound signals? This study uses in vitro experiments to reveal the biophysical mechanisms that explain many observations made from in vivo auditory nerve fiber recordings. We find that higher temperature facilitates vesicle exocytosis and electrical tuning to higher sound frequencies, which benefits sensitivity and selectivity.

Chloride distribution in Aplysia neurones

1. The intracellular Cl(-) concentration (Cl(i)) and the membrane potential (E(m)) were measured in the medial pleural neurones of Aplysia under various experimental conditions designed to determine the Cl(-) conductance of the neurones and investigate the possibility of an active Cl(-) transport.2. The magnitude of the Cl(-) conductance of the cell depends on the experimental conditions.3. In normal sea water, large changes of E(m) produced by passing current across the cell membrane caused no change of Cl(i), suggesting that the Cl(-) conductance was low. Similarly, moderate changes of E(Cl) produced by decreasing Cl(o) or increasing Cl(i) had little or no effect on E(m).4. A high Cl(-) conductance was observed in high K(o) or very low Cl(o). It was greatly reduced if the external Ca(2+) was replaced by Co(2+), or in the presence of tubocurarine, or if the experiment was performed on an isolated cell soma. The high Cl(-) conductance is therefore attributed to the release of ACh and perhaps other transmitters from synaptic terminals.5. High concentrations of tetraethylammonium ions or procaine induced a depolarization of the cell, but a decrease of Cl(i). The rate of fall of Cl(i) was increased by lowering external K(+) or raising external Ca(2+), and was decreased by replacing external Ca(2+) by Co(2+).6. NH(4) (+) ions applied externally had effects similar to those of K(+) ions. In situations in which intracellular NH(4) (+) might be increased a fall in Cl(i) was observed.7. The changes of Cl(i) caused by TEA, procaine, or internal NH(4) (+) occur against the driving force for passive Cl(-) movements. They are still observed in isolated cell bodies, and cannot be attributed to the activation of synaptic channels.8. Some interpretations of these anomalous Cl(-) movements are discussed which could also account for the difference between E(Cl) and E(m) observed under normal conditions.

Physiology of obliquely striated muscle fibres withinGrillotia erinaceusmetacestodes (Cestoda: Trypanorhyncha)

The tentacular bulb ofGrillotia erinaceusmetacestodes consists of obliquely striated muscle fibres with obvious motor end-plates. In this study isometric tension recordings and intracellular microelectrodes have been used to record mechanical and electrical activity from single isolated bulbs. Bulbs were mechanically quiescent and displayed resting membrane polentials (RMP) in the region of −49 to −64 mV with a mean RMP of −56 mV (n= 60). The membrane potential varied with [K+]oin a manner consistent with the RMP being determined largely by the K+equilibrium potential. High K+solution (> 15 mM) caused membrane depolarization and contraction of the preparation with the contraction showing both phasic and tonic components. L-glutamate caused membrane depolarization, contraction of quiescent preparations and increased the amplitude of electrically evoked responses. In contrast, 5-HT, dopamine, histamine, adrenaline, GABA, noradrenaline and D-glutamate, at concentrations up to and including 10−3M, were without apparent affect, although acetylcholine, at relatively high concentrations (≥ 10−4M) slightly reduced the amplitude of field-evoked contractions.

The causal theory of the resting potential of cells

In this pedagogical article the causal theory of the resting potential of cells is presented, which for given extracellular ion concentrations predicts the intracellular ones simultaneously with the resting potential. In addition to the Na, K-pump, fixed charges on the membrane surfaces are taken into account. The equation determining the resting potential in the causal theory suggests a new explanation of the genesis of the resting potential. The usual criterion for an ion pump to be electrogenic is not relevant for the whole of the resting potential, and may therefore be misleading. The physical meaning of the Goldman-Hodgkin-Katz formula for the membrane potential as a diffusion potential is also explained and tested with numbers for the giant axon of the squid. A significant discrepancy between theory and experiment is found which calls for an experimental re-examination of the constitutive equations for passive potassium and sodium currents.

Millimeter waves thermally alter the firing rate of the Lymnaea pacemaker neuron

The effects of millimeter waves (mm-waves, 75 GHz) and temperature elevation on the firing rate of the BP-4 pacemaker neuron of the pond snail Lymnaea stagnalis were studied by using microelectrode techniques. The open end to a rectangular waveguide covered with a thin Teflon film served as a radiator. Specific absorption rates (SARs), measured in physiological solution at the radiator outlet, ranged from 600 to 4,200 W/kg, causing temperatures rises from 0.3 to 2.2 degrees C, respectively. Irradiation at an SAR of 4200 W/kg caused a biphasic change in the firing rate, i.e., a transient decrease in the firing rate (69 +/- 22% below control) followed by a gradual increase to a new level that was 68 +/- 21% above control. The biphasic changes in the firing rate were reproduced by heating under the condition that the magnitude (2 degrees C) and the rate of temperature rise (0.96 degrees C/s) were equal to those produced by the irradiation (for an SAR of 4,030 W/kg). The addition of 0.05 mM of ouabain caused the disappearance of transient responses of the neuron to the irradiation. It was shown that the rate of temperature rise played an important role in the development of a transient neuronal response. The threshold stimulus for a transient response of the BP-4 neuron found in warming experiments was a temperature rise of 0.0025 degrees C/s.

Mechanisms of hyperpolarization induced by two cytokines, hTNF alpha and hIL-1 alpha in neurons of the mollusc, Onchidium

The voltage and current responses induced by extracellular tumor necrosis factor (hTNF alpha) or interleukin-1 (hIL-1 alpha) on the Be-1 and Es-1 neurons of the Onchidium ganglia were examined. Pressure-ejected hTNF alpha or hIL-1 alpha produced an inhibitory, hyperpolarized effect in unclamped neurons. In the same neurons voltage-clamped at their resting potential levels, the same hTNF alpha or hIL-1 alpha elicited an outward current having a time course similar to that of the hyperpolarization, associated with a decreased membrane conductance. The hTNF alpha- or hIL-1 alpha-induced outward current did not reverse even at positive membrane potentials considerably above + 100 mV in the absence ouabain (a specific blocker of Na-pump). In the presence of ouabain, the hTNF alpha- or hIL-1 alpha-induced current was reduced over a wide range of membrane potential, so that the current reversed at about + 20 mV. Lowering the external Na+ concentration from 450 to 200 mM in the presence of ouabain, shifted the reversal potential from + 20 to 0 mV, to near the shift value of 20.8 mV predicted by the Nernst equation for a Na(+)-selective conductance. Neither an increase nor a decrease of extracellular K+, Cl- or Ca2+, however, significantly altered the current induced by hTNF alpha or hIL-1 alpha. These suggest that the hTNF alpha- or hIL-1 alpha-induced hyperpolarization or outward current response is mediated by two mechanisms, a decrease in Na+ conductance and activation of the Na-pump.

Effects on protein synthesis produced by pairing depolarization with serotonin, an analogue of associative learning in Aplysia

A form of associative plasticity in Aplysia, activity-dependent neuromodulation, involves the convergence of neuronal activity and the effects of a modulatory transmitter. To investigate the role of protein synthesis in associative plasticity, we examined the effects of a biochemical analogue of activity-dependent neuromodulation on the level of incorporation of labeled amino acid into proteins. To mimic associative training, abdominal ganglia were exposed to paired treatments of a depolarizing agent, elevated potassium, and a modulatory transmitter, serotonin. The effects of elevated potassium and serotonin applied alone were also examined. At least two proteins (nos. 9 and 17) were affected in a nonadditive way by the paired procedure. Incorporation of label into protein 9 was increased by the paired procedure but was not affected by either elevated potassium or serotonin. Incorporation of label into protein 17 was significantly affected by elevated potassium or serotonin, but the effect of the paired procedure was significantly less than the summed effects of elevated potassium and serotonin applied alone. These results indicate that changes in protein synthesis may be important in the induction of associative plasticities. Amino acid sequences of two peptides derived from protein 9 were obtained. Then, a partial cDNA clone for protein 9 was obtained by performing PCR with degenerate primers corresponding to portions of the sequences of the two peptides. The sequence of protein 9 is related to sequences previously reported for a family of genes comprising the stringent starvation protein of Escherichia coli, auxin-induced proteins of plants, and glutathione S-transferases of a number of organisms.

Lead ions close steady-state sodium channels in Helix neurons

Extracellularly applied Pb2+ (1-150 microM) induced an outward current (IPb) in intracellularly perfused snail neurons. The current-voltage relationship of the Pb(2+)-induced current was linear over the potential range of -100 to -40 mV with negative slope conductance. The Pb-induced current was strongly dependent on the Na+ gradient. The IPb in intra- or extracellular K+- and Cl(-)-free or -rich solutions was almost the same as in control external and internal salines. The negative slope of the I-V curve and the decreased conductivity during Pb2+ application suggested that IPb is owing to the blocking of the resting Na conductance. Data obtained from single-channel measurements also supported this conclusion. Patch-clamp data showed that the steady-state Na channel has a conductance of 14 pS and both closed and open time-distributions displayed single exponential character.

Thermal influences on nervous system function

The various effects of temperature change are only partially predictable. Temporal measures relevant to membrane activity, action potentials, synaptic transmission, and evoked potentials are all consistently increased with cooling and decreased by warming. However, the various measures of amplitude at different levels, and even within similar preparations, are contradictory: Some laboratories report increased amplitudes with cooling and others report decreased amplitudes under similar conditions. Emphasis is given to identifying factors that may resolve the differences. These include: (a) the rate of temperature change, (b) sites of cooling, stimulation and recording, (c) stimulus characteristics, and (d) fundamental differences in temperature sensitivities of different neural tissue. Other factors that may affect the ability to predict thermal influences on neural function from existing formulations are: relative ion permeabilities, metabolic ion pumps, the resting potential at the onset of cooling, and an animal's acclimated temperature at sacrifice.

Ouabain blocks some rapid concentration-induced clamp acetylcholine responses on Helix neurons

1. The effects of ouabain, a potent inhibitor of Na(+)-K+ ATPase, were determined on the transmembrane responses of internally dialyzed Helix neurons to rapid acetylcholine (ACh) application using the "concentration clamp" technique. 2. Ouabain selectively depressed "A"-type responses to ACh, which are due to a selective increase in membrane permeability to chloride. In contrast, the "B"-type responses, due primarily to an increase in monovalent cation permeability, was unaffected. 3. The blockade of the Cl- responses was not associated with a change of the reversal potential of the response. Ouabain depressed the maximal response without shifting the dose-response curve. 4. Ouabain caused an increase in the time constant of decay of the ACh current, but the value in the presence of ouabain was not different from that of a lower concentration of ACh determined so as to give a response of the same peak amplitude. Therefore, the effect of ouabain is not on the process of receptor desensitization directly.

GABAergic synaptic current in dissociated nucleus basalis of Meynert neurons of the rat

gamma-Aminobutyric acid (GABA)-mediated spontaneous inhibitory postsynaptic currents (IPSCs) were recorded from dissociated rat nucleus basalis of Meynert neurons which still had their synaptic boutons attached. The membrane currents were recorded by the whole-cell patch-clamp technique. Elevated extracellular K+ concentration and the addition of the calcium ionophore, A23187, enhanced the amplitude and frequency of spontaneous IPSCs. Ryanodine and Ca(2+)-free external solution containing EGTA or BAPTA markedly decreased the spontaneous IPSC activities. Spontaneous IPSC activities were reversibly reduced by baclofen and increased by phaclofen, indicating that the GABAB receptor regulates the release of GABA from nerve terminals and acts as a negative autoreceptor.

Effect of extracellular potassium on ouabain-sensitive consumption of high-energy phosphate by crayfish giant axons: a study of the energy requirement for transport in the steady state

Crayfish axons exposed to a high or low extracellular K+ concentration ([K+]o) maintain intracellular Na+ and K+ concentrations constant, for up to 3 h, by adjusting both the Na+/K+ transport "coupling ratio" and turnover rate in compensation for changes in ion fluxes due to altered electrochemical gradients. These findings give rise to the prediction that the steady-state consumption of high-energy phosphate (approximately P) [ATP and phospho-L-arginine (Arg-P)] is inversely proportional to the [K+]o, i.e., directly proportional to the product of membrane conductance and magnitude of the transmembrane electrochemical gradients for Na+ and K+. This investigation was designed to test this hypothesis. The [K+]o did not influence total approximately P consumption (Q approximately P) of the axon. For a [K+]o between 0.5 and 21.6 mM, Q approximately P averaged 52.8 +/- 4.7%/h (n = 44) of the initial [ATP] + [Arg-P]. Unlike total Q approximately P, the ouabain-sensitive portion of Q approximately P was markedly influenced by [K+]o. In 0.5 mM K+o, ouabain poisoning reduced Q approximately P to 8%/h, a result indicating that 85% of the total Q approximately P was ouabain sensitive. For 1.35 mM K+o, the ouabain-sensitive portion was 66%; at 5.4 mM K+o, 45%; and at 13.5 mM K+o, 41%. There was a small but significant increase in the ouabain-sensitive Q approximately P at 21.6 mM K+o, compared with Q approximately P at 5.4 mM K+o. The pattern of effect of [K+]o on Q approximately P was similar to its effect on the electrical power content of the Na+ and K+ electrochemical gradients. In contrast to the generally accepted Na+ flux (JNa)/approximately P stoichiometry of 3, an actual ratio of JNa/approximately P stoichiometry of approximately 33:1 was calculated for the experiments reported here, a result suggesting that cells in a zero-membrane current steady state utilize efficient energy conservation mechanisms that may not operate under non-steady-state conditions.

Calcium in phase control of the Bulla circadian pacemaker

The circadian pacemaker in the retina of the eye of the marine snail Bulla gouldiana was examined using the whole eye in vitro preparation. Phase-response curves were generated to 6-h pulses of a low calcium EGTA solution and to a hyperpolarizing low potassium-low sodium solution. Both treatments yielded similar phase response curves with phages delays in the late subjective night/early subjective day and phase advances in the late subjective day. The similarity of the phase response curves to hyperpolarizing and low calcium solutions and the absence of additivity when both treatments are combined raises the possibility that both treatments affect the underlying pacemaker through a common mechanism. The persistence of phase shifts to low calcium pulses delivered in the presence of depolarizing light suggests that hyperpolarization is not required for low calcium phase shifting. However, it is possible that both treatments act by reducing a transmembrane calcium flux which is postulated to result from the periodic depolarization of the pacemaker cell membrane during the subjective day. Since a transmembrane calcium flux is known to be essential for both light and depolarization-induced phase shifting, we discuss the hypothesis that calcium fluxes play a pivotal role in the entrainment pathway of the circadian pacemaker.

Effects of temperature alterations on population and cellular activities in hippocampal slices from mature and immature rabbit

Effects of temperature on population spike and cellular activities have been assessed in the CA1 region of hippocampal slices from mature and immature rabbit. In field potential recordings, population spike amplitude was maximal at near 30 degrees C for both mature and immature tissue, and fell off as temperature was either raised (to a maximum of 44 degrees C) or lowered (to a minimum of 20 degrees C). With cooling below 30 degrees C, population spikes decreased in amplitude and became broader; stimuli always elicited some response, and changes due to cooling were reversible. With increases in temperature, however, irreversible decrease and/or loss of population spikes occurred when tissue was warmed beyond 43 degrees C. Input-output curves established for mature and immature slices indicated that, at all temperatures, population spike amplitude grew more rapidly with small increases in stimulus intensity in immature slices as compared to mature slices. Intracellular recordings were made from CA1 pyramidal cells in mature and immature hippocampal slices. For both mature and immature tissues, moderate warming (to 40 degrees C) produced membrane hyperpolarizations in many cells, especially in the mature hippocampus. Increasing temperature beyond 40 degrees C led to marked depolarizations in a number of cells, a depolarization that was irreversible, particularly in mature neurons. Cooling generally produced a depolarizing shift in membrane potential and an accompanying increase in input resistance; these effects, however, were reversible. Temperature changes in both warming and cooling directions had effects on repetitive firing patterns in both mature and immature neurons. In particular, spike trains elicited by a constant current pulse at a given membrane potential became shorter. The effects of cooling on this cell parameter were reversible, but warming-induced changes were usually permanent. Irreversibility of the warming effects was more pronounced in cells from mature than from immature hippocampus. As reported previously, cooling produced marked spike broadening and changes in synaptic potentials in both mature and immature neurons. These studies confirm previously reported temperature sensitivities of neuronal properties in hippocampal slices. On the basis of these data, and reports from other laboratories, it is clear that relatively small changes in temperature can have rather dramatic effects on properties of single cells and cell populations. Such temperature sensitivity is critical in evaluating data obtained from in vitro slice preparations.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane currents induced by pentylenetetrazol in identified neurons of Helix pomatia

After systemic application of pentylenetetrazol (PTZ), mammalian as well as molluscan neurons generate epileptic paroxysmal depolarization shifts. For a further analysis of these potential oscillations the membrane currents induced by local application of PTZ onto identified neurons of Helix pomatia were investigated. Different types of responses were obtained at membrane potentials negative and positive to ca. -30 mV. At holding potentials more negative than -30 mV, PTZ as a rule evoked an inward current, sometimes preceded by a brief outward current. In a few experiments only a solitary outward current was found. The amplitudes of the inward and outward currents increased towards more negative potentials. The inward current was associated with a decrease and the outward current with an increase in membrane resistance. Besides these findings pharmacological and ion substitution experiments indicate that the inward current represents an unspecific current. At holding potentials more positive than -30 mV, PTZ evoked a sequence of currents which was the same in all neurons. This stereotyped current sequence consisted of (i) an early inward current, (ii) an intermediate outward current, and (iii) a late long-lasting inward current. The amplitudes of all these components increased towards more positive potentials with the outward current being particularly enhanced. The early inward current and the following outward current were associated with a decrease and the late inward current with an increase of the membrane resistance. Besides these pharmacological and ion substitution experiments suggest that the early inward current represents a mixed sodium and calcium current, the intermediate outward current a calcium activated potassium current. The late inward current is assumed to be due to a decreased potassium conductance. On the basis of the present results, it may be concluded that the unspecific inward current in the negative potential range is involved in the initiation and the calcium dependent potassium current in the termination of spontaneously occurring paroxysmal depolarization shifts.

Rat adrenal zona fasciculata cell electrophysiological responses to ACTH and gamma-MSH vary with age

A comparison was made of the resting membrane potential (MP) as well as the MP responses to K+, ACTH and gamma-MSH of superfused adrenocortical, zona fasciculata cells from young (Y) and old (O) male rats. The resting MP of these cells did not vary with age. However, the MP responses to altered extracellular [K+] varied significantly with age. A prolonged biphasic depolarization was observed following ACTH administration; these responses were significantly reduced in O cells. In contrast, stimulation with gamma-MSH did not consistently elicit depolarization in the Y but caused a consistent and prominent depolarization in O cells. These findings suggest that aging is associated with changes in primary membrane events which could explain the reduced ACTH-stimulated steroidogenesis associated with age. Elevated cellular responses to gamma-MSH may contribute to a maintenance of, or increase in, circulating corticosterone levels in aged rats.

Copper activates a unique inward current in molluscan neurones

1. Reidentifiable Aplysia neurones were current and voltage clamped in vitro using standard microelectrode techniques. 2. Bath or focal application of Cu2+ at concentrations of 1-100 microM produced a rapid and reversible depolarization of the somal, but not the axonal, membrane potential. The depolarization was accompanied by an increased membrane conductance and activation of an inward current (ICu) which could not be activated by intracellular ionophoretic injection of Cu2+. 3. ICu is carried, in part, by Na+ because the reversal potential of ICu was shifted in a Nernstian fashion by decreasing the extracellular Na+ concentration. The reversal potential of ICu was not affected by removal of extracellular Ca2+ or K+. 4. ICu does not result from (1) activation of known chemically or voltage-gated Na+ conductances, (2) inhibition of the Na+-K+-ATPase or (3) a generalized increase in membrane permeability resulting from lipid peroxidation. 5. A similar inward current was activated by AgNO3 (100 microM) and HgCl2 (100 microM).

Intracellular K+, Na+ and Cl- concentrations and membrane potential in human monocytes

The relationship between the resting membrane potential and the intracellular ionic concentrations in human monocytes was investigated. Cell volume, cell water content, and amount of intracellular K+, Na+, and Cl- were measured to determine the intracellular concentrations of K+ (Ki), Na+ (Nai) and Cl- (Cli) of monocytes, and of lymphocytes and neutrophils. Values found for monocytes were similar to those for neutrophils, i.e., cell volumes were 346 and 345 micron3, respectively, cell water content 78%, and Ki, 128 and 125, Nai, 24 and 26, and Cli, 102 and 103 mmol/l cell water, respectively. Lymphocytes, however, had different values: 181 micron3 cell volume, 77% cell water content, and for Ki, Nai, and Cli, 165, 37, and 91 mmol/l cell water, respectively. The resting membrane potential of cultured human monocytes (range -30 to -40 mV), determined by measurement of the peak potential occurring within the first milliseconds after microelectrode entry, was most dependent on extracellular K+, followed by Cl-, and Na+. The membrane permeability ratio of Cl- to K+ was estimated by use of the constant field equation to be 0.23 (range 0.22 to 0.30).

Permeability of endoneurial capillaries to K, Na and Cl and its relation to peripheral nerve excitability

The permeability coefficient-surface area products (PA) of frog sciatic nerve endoneurial capillaries to K, Na and Cl were measured with an in situ perfusion technique and found to be 40.3, 24.6 and 32.8 X 10(-5) ml . g-1 . s-1, respectively. PAs to [14C]sucrose and 42K, when measured simultaneously, and their ratio were independent of perfusate K concentration (0.1-10.0 mM). Simultaneous measurements with 36Cl and 42K indicated that the Cl/K permeability ratio was significantly smaller than the mobility ratio of these two ions in free solution. On the other hand, comparable experiments with 22Na and 42K revealed that the K/Na permeability ratio was not significantly different from its respective mobility ratio. Thus, these results provide no evidence of facilitated transport of K by endoneurial capillaries, and suggest that K, Na and Cl traverse the endoneurial capillary wall by a paracellular route which is weakly selective for cations. The minimum extracellular K concentration (Ke) capable of producing a depolarization conduction block in frog sciatic nerve was between 12.5 and 15.0 mM. When the vasculature of this nerve was perfused with a hyperkalaemic (20.0 mM) Ringer solution, a conduction block developed in 7.9 min. Comparison of this time with the theoretically predicted rate of change of endoneurial Ke (induced by a comparable change of intravascular K concentration) indicated that an increase of endoneurial Ke is transmitted directly to the paranodal spaces of nerve fibres so as to immediately influence axonal excitability.(ABSTRACT TRUNCATED AT 250 WORDS)

Short-term effects of PTH on cultured rat osteoblasts: changes in membrane potential

The introduction of parathyroid hormone [bPTH (1-34)], 10(-8) M, into the medium of cultured rat osteoblasts results in rapid (less than 1 min) depolarization of the osteoblast membranes. Conventional and pH-sensitive microelectrodes were used to assess the mechanism underlying this change. PTH depolarized cell membrane independently of steady-state membrane potential (Vm). Blocking K+ conductance (Ba2+) and Ca2+-dependent K+ conductance (quinine) depolarized Vm by +13.1 +/- 4.6 (n = 6) and +14.8 +/- 6.7 mV (n = 6), respectively, and both abolished the effect of PTH on Vm. The rate of depolarization was reduced in low-Ca2+ medium. PTH inhibited low Na+-induced cell hyperpolarization, but intracellular pH was not altered by hormone addition. PTH-induced depolarization occurred even when the Na+-K+ pump was blocked with ouabain. A second slower response was seen in cells having a Vm lower than -60 mV, with an increase in negativity 5-15 min after hormone application. The results indicate that PTH rapidly modifies Vm by changes of K+ conductance, which may be the first step in hormonal stimulus-response coupling, and induces delayed, long-term changes in cell status.

Effects of sea water temperature on bursting pacemaker activity in cell R15 in the intact Aplysia

Thermosensitivity of bursting pacemaker activity (BPA) in neuron R15 of Aplysia was compared in an intact preparation and after excision of the ganglion containing R15. We noted no differences indicative of an in situ antagonism of cooling-induced suppression of BPA. Interruption of circulation hastened hemolymph cooling during exposure of the animal to cold sea water, suggesting mechanisms to protect R15 and other cells from transient cooling of the animal.

The effects of temperature on synaptic transmission in hippocampal tissue slices

Fully submerged rat hippocampal tissue slices were exposed to temperature changes, and the effects on CA1 pyramidal cell electrophysiology studied. Raising the temperature from 29 to 33 or 37 degrees C simultaneously increased the focal-excitatory postsynaptic potentials and decreased the population spikes. These changes were largely reversible for slices warmed to 33 degrees C, but not for slices warmed to 37 degrees C. During warming transiently increased excitatory transmission was observed; the degree of increased transmission was related to the rate of temperature rise. It is postulated that neuronal membrane hyperpolarization with warming is responsible for several of the effects seen.

Ionic dependence of early adaptation in the crustacean stretch receptor

In the present study we have examined the effects of changes in potassium and calcium concentration on the early adaptation of the slowly adapting stretch receptor of the crayfish using intracellular recordings including the potential clamp technique. This was because previous studies had suggested that the early adaptative decline of the receptor potential may be attributed mainly to ionic mechanisms involved in the transducer process. During prolonged exposure to K-free saline the cell depolarized; the early adaptive fall of the receptor potential was reduced and finally the response became almost rectangular. These effects developed more rapidly if the concentration of Ca was reduced in the K-free saline. It was shown by injection of current that the effects were not potential dependent. Removal of Ca reduced the amplitude of both the dynamic and static phase of the receptor potential. Isotonic Ca-saline suppressed the static phase of the receptor potential and prolonged exposure completely abolished the response. Potential clamp experiments demonstrated that in the Ca-free saline the passive membrane conductance increased; the static phase of the receptor current increased while the peak current decreased somewhat. In the K-free and Ca-free saline both phases of the receptor current increased. The present results support earlier findings that the major part of the early adaptive fall of the receptor potential is caused by an outward K+ current. Ca2+ modifies the adaptive fall and the static phase, most likely by activation of a Ca2+-dependent K+ current and/or by inactivation of the Na+ current.

Axon-to-axon transmission in tullidora (buckthorn) neuropathy

Oral administration of ether extracts of the tullidora ( Karwinskia humboldtiana ) fruit, which contains an identified neurotoxin, produced flaccid hind limb paralysis in cats after a latency of 4 to 7 weeks. Acute experiments were conducted after the paralysis was evident. Spinal roots of lumbar and sacral segments were transected as close as possible to the spinal cord and divided into several filaments. Stimulation of some filaments distal to the transection evoked action potentials in other filaments (axon-to-axon transmission or cross talk) after a latency of at least 8 ms. Cross-talk responses frequently consisted of multiple discharges. Axon-to-axon transmission was seen only between motor axons and disappeared when hind limb nerves were transected 10 to 15 cm from the spinal cord. Twin pulses were applied to a filament at various intervals; the pulse intensity was adjusted so that the conditioning pulse was subthreshold to elicit cross talk, but the test pulse frequently elicited it (temporal facilitation). In three fully studied fibers the facilitation was prolonged to 50 to 80 ms. In some cases, no cross talk was evoked in a given filament by individual stimulation of two other filaments, but simultaneous stimulation of the same filaments did evoke cross talk (spatial facilitation). Series of periodic bursts of activity spontaneously occurred in those axons responding with multiple discharges to single stimulation of other axons. At low temperatures (about 30 degrees C) the stimulus could trigger essentially similar series of bursts. Single motoneurons were intracellularly stimulated by brief depolarizing pulses. The action potential elicited by the stimulus was followed after several msec by a secondary train of discharges generated at the periphery ("back firing").

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic and spectral mechanisms of the off response to light in hyperpolarizing photoreceptors of the clam, Lima scabra

Intracellular recordings were made from distal photoreceptor cells of the file clam Lima scabra in order to examine the ionic and spectral mechanisms which underly the response to light decrement. These receptors are primary sensory neurons that generate nerve impulses in the optic nerve upon light termination without benefit of synaptic interconnections between photoreceptor cells. Microelectrode measurements were made on these cells. Membrane conductance changes were assessed by measuring membrane voltage changes elicited under different conditions while passing constant-current pulses through the microelectrode from an active bridge amplifier. Responses of membrane potential in light and darkness in different concentrations of external potassium ions were fitted to a simplified form of the constant field equation. This analysis allowed an estimation of internal potassium activity (281 mM) as well as changes in PNa/PK in darkness and light. PNa/PK changed from 0.09 in darkness to 0.03 at the peak of the light response. A persistent decrease in membrane conductance at the termination of light is associated with a depolarization that overshoots the dark resting membrane potential. This transient depolarization is dependent on the intensity and duration of the preceding period of light. The amplitude of the dark-dependent depolarization is related to the absorbance of light during the preceding period of light by a long wavelength intermediate of rhodopsin bleaching (metarhodopsin). The frequency of discharge of action potentials with rapid kinetics which occurs following light is proportional to the amplitude of the after depolarizing response. The delay to onset of the discharge is inversely proportional to the amplitude of the after depolarizing response. The sensitivity (response/photon) of distal cells can be modified by background light which passes through a screening pigment found in cells that surround the eye. These data, taken together, provide an explanation for the persistent discharge of action potentials which occurs on termination of light in these cells as well as the visual cells of other gastropod mollusks.

From sea lemons to c-waves

This review of retinal pigment epithelial (RPE) physiology pays tribute to Anthony L. F. Gorman, who introduced the author to the giant neuron of Anisodoris nobilis (the sea lemon) and cellular neurobiology. The RPE is an epithelial monolayer with tight junctions, which controls the environment of the photoreceptor outer segments. The apical and basal membranes have different electrical properties and generate a standing potential across the eye. The RPE helps maintain adhesion between the retina and the wall of the eye. Adhesion is weakened by cyanide, low pH or low calcium, but enhanced by ouabain or acetazolamide. The RPE transports water from the subretinal space toward the choroid. This water movement is inhibited by hypoxia or cyanide but enhanced by ouabain or acetazolamide. The c-wave of the electroretinogram is a composite of a cornea-positive wave produced by hyperpolarization of the apical RPE membrane and a cornea-negative wave produced by the Muller cells, both in response to the fall in extracellular potassium that follows illumination of the photoreceptors. The "light response" of the standing potential is produced by depolarization of the basal membrane of the RPE. These examples illustrate how principles of cellular neurophysiology can be applied to questions of clinical relevance.

Effect of sodium concentration and of atropine on the contractile response of the guinea-pig ileum to potassium ions

The possibility that the guinea-pig ileum's contractile response to K+-rich solutions is partly mediated by acetylcholine release from the intramural nervous tissue was examined by studying the inhibition of that response by atropine at different values of [Na+]o. In a medium in which the NaCl was replaced by iso-osmotic glucose, the response to high [K+]o was not greatly affected, while the responses to acetylcholine and to other agonists were significantly reduced. In control medium (149 mM Na+), atropine (10(-7) M) partly inhibited the responses to K+-rich solutions and to agonists such as histamine, 5-hydroxytryptamine and bradykinin. When [Na+]o was reduced to 12 mM, by iso-osmotic substitution of glucose for NaCl, the response to high K+ was no longer inhibited by atropine, which still partly inhibited the contractions elicited by the three agonists and totally blocked the response of acetylcholine. It is proposed that atropine's inhibition of the response to high K+ and to agonists is not due to its specific anti-muscarinic effect, but to an unspecific action, which in the case of the agonists is independent of [Na+]o. In addition, the inhibition of the response to high K+ would results from a different Na+-dependent mechanisms, possibly involving the stimulation of the Na-K pump by atropine. This is supported by the observation that this drug partly relaxed ileum preparations that were contracted by ouabain.

Increasing external K+ blocks phase shifts in a circadian rhythm produced by serotonin or 8-benzylthio-cAMP

Serotonin (5-HT) phase shifts the circadian rhythm from the isolated eye of Aplysia. The discovery of the mechanisms involved in phase shifting by 5-HT may help elucidate the nature of the circadian oscillator. We have found that 5-HT appears to phase shift by causing a change in membrane K+ conductance. Solutions containing zero K+(0-K+) phase shift the rhythm and the phase response curve (PRC) for 0-K+ is similar to one previously obtained for 5-HT. The similarity in PRCs for 0-K+ and 5-HT suggested that these treatments may be phase shifting the rhythm through a common mechanism. The nonadditivity of phase shifting by 0-K+ and 5-HT supports this suggestion. A common mechanism of action of 5-HT and 0-K+ might be effects on membrane potentials. The possible involvement of a membrane potential change in mediating the effect of 5-HT and the lack of an effect of large reductions in Na+, Cl-, and Ca2+ ions on phase shifting by 5-HT led us to examine the role of K+ ions in phase shifting by 5-HT. A change in K+ conductance may mediate the effects of 5-HT on the rhythm because HiK (30mM) solutions blocked the phase shift normally produced by 5-HT. The conductance change produced by 5-HT may be an increase in K+ conductance which would produce a hyperpolarization and not a decrease in K+ conductance which would produce a depolarization since depolarizing treatments, HiK (30-110mM), had no effect on the rhythm at the phase where 5-HT produces its largest phase shifts. Since we previously found that the effects of 5-HT appear to be mediated by cAMP, we examined whether HiK solutions could block the effects of 8-benzylthio-cAMP on the rhythm. HiK (40mM) blocked the phase shifts normally produced by 8-benylthio-cAMP. Our working hypothesis for the 5-HT phase-shifting pathway based on these results is 5-HT leads to increased cAMP leads to elevates K+ conductance leads to membrane hyperpolarization leads to phase shifts the rhythm.

Temperature acclimation: effects on membrane physiology of an identified snail neuron

The neuronal basis for thermal acclimation was examined by comparing the short- and long-term effects of temperature change on the physiological properties of an identified neuron in the isolated ganglion of Hexis aspersa. Using intracellular electrophysiological techniques, we found that the frequency of spontaneous action potentials and excitability of neurons from warm-acclimated animals was depressed by abruptly cooling from 20 to 5 degrees C. After a 2-wk period of acclimation to 5 degrees C, the levels of spontaneous activity and excitability were comparable to those of warm-acclimated neurons at 20 degrees C. Conversely, abrupt warming of neurons from cold-acclimated animals greatly increased the frequency of spontaneous activity, but after acclimation to 20 degrees C the frequency decreased. Although the duration of the action potential and the cell's electrogenic Na-K pump were temperature sensitive, thermal acclimation had no obvious effects on these parameters. Membrane permeability to Na and PNa/PK decreased with cooling, whereas PRb/PK and PCs/PK increased. Warming had the opposite effect on the relative alkali cation permeability (PX/PK). With acclimation PX/PK underwent compensatory changes.

Excitatory and inhibitory effects of histamine on molluscan neurons

Histamine elicited depolarization (excitation) in some neurons and hyperpolarization (inhibition) in other neurons of the central nervous system of the marine mollusc, Onchidium verruculatum. The histamine sensitive region was along the axon at some distance from the soma. H1-receptor blockers (SA-97 and mepyramine) suppressed the excitatory (H1) response without affecting the inhibitory (H2) response, while H2-receptor blockers (burimamide and metiamide) suppressed the H2-response without affecting the H1-response. The H1-response was associated with a marked increase in membrane conductance and was blocked by removal of the external Na. The H2-response consisted of a hyperpolarization without much change in conductance, compared with the hyperpolarization of same amplitude produced by glutamate in the same neuron. Passive polarization of the membrane and reduction of Cl concentrations to 1/5-1/25 caused no significant change in H2-response. The H2-response was slightly suppressed in K-free saline. Thus, it seems difficult to account for the hyperpolarization only by an increase in K or Cl conductance. Complete removal of Na and addition of ouabain blocked the H2-response, suggesting a contribution of an electrogenic Na-pump to the hyperpolarization. However, in 20 mM Na saline with or without K, histamine still caused clear hyperpolarization. In this solution, the histamine response was not affected by ouabain. Although it is difficult to exclude the possibility that an increase in K conductance may be responsible for the hyperpolarization, it is tentatively proposed as a hypothesis that the H2-response involved some active transport mechanism, different from a ouabain-sensitive electrogenic Na-pump.