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1Properties of voltage‐dependent Na+currents were investigated in forty‐two dentate granule cells (DGCs) acutely isolated from the resected hippocampus of twenty patients with therapy‐refractory temporal lobe epilepsy (TLE) using the whole‐cell patch‐clamp technique.2Depolarizing voltage commands elicited large, rapidly activating and inactivating Na+currents (140 pS μm−2; 163 mmextracellular Na+) that were reduced in amplitude by lowering the Na+gradient (43 mmextracellular Na+). At low temperatures (8‐12 °C), the time course of Na+currents slowed and could be well described by the model of Hodgkin & Huxley.3Na+currents were reversibly blocked by tetrodotoxin (TTX) and saxitoxin (STX) with a half‐maximal block of 4.7 and 2.6 nm, respectively. In order to reduce series resistance errors, the Na+current was partially blocked by low toxin concentrations (10‐15 nm) in the experiments described below. Under these conditions, Na+currents showed a threshold of activation of about ‐50 mV, and the voltages of half‐maximal activation and inactivation were ‐29 and ‐55 mV, respectively.4The time course of recovery from inactivation could be described with a double‐exponential function (time constants, 3‐20 and 60‐200 ms). The rapid and slow time constants showed a distinct voltage dependence with maximal values around ‐55 and ‐80 mV, respectively. These properties contributed to a reduction of the Na+currents during repetitive stimulation that was more pronounced with higher stimulation frequencies and also showed a dependence on the holding potential.5In summary, the most striking features of DGC Na+currents were the large current density and the presence of a current component showing a slow recovery from inactivation. Our data provide a basis for comparison with properties of Na+currents in animal models of epilepsy, and for the study of drug actions in therapy‐refractory epilepsy.

1. The electrical properties of squid giant axons were examined by means of admittance bridges at frequencies from 0.5 to 300 kHz. A simple equivalent circuit was used to estimate the membrane capacity. 2. The calculated membrane capacities decreased monotonically over the whole frequency range. 3. At 100 kHz and higher frequencies the membrane capacity was independent of potential. 4. At frequencies greater than 20 kHz, exposure of the axons to saturated or 0.9 saturated solutions of n‐pentane (275‐306 micrometer) reduced the capacity per unit area by 0.1‐0.15 micro F cm‐2. 5. At 1 kHz the effect of the saturated pentane solutions depended on the membrane potential. In axons having potentials between ‐60 and zero mV the pentane solutions lowered the capacity, whereas for potentials between ‐160 and ‐60 mV they produced little or no change. 6. Saturated solutions of n‐hexane, n‐heptane and n‐octane exhibited qualitatively similar, but quantitatively smaller influences on the membrane capacity, the changes declining as the chain length increased. 7. Under voltage clamp, the peak inward and steady‐state outward currents were partially suppressed by the hydrocarbons. Saturated solutions of n‐pentane usually reduced the former (reversibly) by 60‐80% and the latter by 20‐40%. Solutions of n‐hexane, n‐heptane and n‐octane appeared to have successively less effect. Except in deteriorating axons, none of the hydrocarbons produced any consistent changes in the passive membrane resistance, the resting potential or in the reversal potential of the transient inward current. 8. Both the changes in the clamp currents and in the membrane capacity were largely, though not usually completely, reversible. In the hydrocarbon solution the axons deteriorated more rapidly than normal. 9. The responses of axons of Doryteuthis plei to the hydrocarbons were very similar to those of Loligo forbesi with the exception that for the former all observed changes were some five times faster. 10. The time courses of the peak inward and steady‐state outward currents on exposure of the axons to n‐pentane resembled the time course of the change in membrane capacity at 100 kHz. 11. The simplest interpretation of the high frequency capacity results is suggested to be that, as for lipid bilayers, the membranes become thicker through adsorption of the hydrocarbon.

The effects of the n‐alkanes propane to hexane, cyclopropane, cyclopentane and cyclohexane and carbon tetrachloride on the ionic currents and electrical capacity of the squid giant axon membrane have been examined. Both the peak inward and steady‐state outward currents were reduced reversibly by each substance, though propane at 1 atm had very little effect. The membrane capacity at 100 kHz was reduced by all substances except propane at 1 atm. Na currents were recorded in intracellularly perfused axons before and during exposure to the hydrocarbons and the records were fitted with equations similar to those proposed by Hodgkin & Huxley (1952). Shifts in the curves of the steady‐state activation and inactivation parameters (m infinity and h infinity) against membrane potential, changes in the peak heights of the activation and inactivation time constants (tau m and tau h) and reductions in the maximum Na conductance (gNa) have been tabulated. The effects of the various hydrocarbons and carbon tetrachloride on the parameters of the Hodgkin‐Huxley equations suggest that the suppression of the Na current by these substances originates from several different phenomena. The underlying physico‐chemical events are considered in the light of the observed capacity changes and of information on artificial pore‐containing membranes.

The effects of several n‐alkanols and n‐alkyl oxyethylene alcohols, methyl octanoate, glycerol 1‐monooctanoate and dioctanoyl phosphatidylcholine on the ionic currents and electrical capacity of the squid giant axon membrane have been examined. The peak inward current in voltage‐clamped axons was reduced reversibly by each substance. For n‐pentanol to n‐decanol the concentrations required to suppress the peak inward current by 50% were determined. From these data, it was estimated that the standard free energy per CH2 for adsorption to the site of action was ‐3.04 kJ mole‐1, as compared with ‐3.11 kJ mole‐1 for adsorption into phospholipid bilayers or an n‐alkane/aqueous solution interface. The membrane capacity at 100 kHz was not greatly by any of the test substances at concentrations which reduced the inward current by 50%. Na currents under voltage clamp were recorded in intracellularly perfused axons before, during and sometimes after exposure to the test substances and the records were fitted with equations similar to those proposed by Hodgkin & Huxley (1952). Shifts in the curves of the steady‐state activation and inactivation parameters (m infinity and h infinity) against membrane potential, changes in the peak heights of the activation and inactivation time constants (tau m and tau h) and reductions in the maximum Na conductance (gNa) have been tabulated. All of the test substances shifted the voltage dependence of the steady‐state activation in the depolarizing direction and lowered the peak time constants for both activation and inactivation. The origins of these effects, and of the differences in the present results from those of the hydrocarbons (Haydon & Urban, 1983), have been discussed in terms of the physico‐chemical properties of the two groups of substances and with reference to their effects on artificial membranes.

The effects of diethyl ether, methoxyflurane, halothane, dichloromethane and chloroform on the ionic currents and electrical capacity of the squid giant axon have been examined. The peak inward current in voltage‐clamped axons was reduced reversibly by each substance. Sodium currents under voltage clamp were recorded in intracellularly perfused axons before, during, and sometimes after exposure to the test substances, and the records were fitted with equations similar to those proposed by Hodgkin & Huxley (1952). Shifts in the dependence of the steady‐state activation and inactivation parameters (m infinity and h infinity) on membrane potential, reductions in the peak heights of the activation and inactivation time constants (tau m and tau h) and decreases in the maximum Na conductance (gNa) have been tabulated. For each of the anaesthetics the steady‐state inactivation curve was shifted in the hyperpolarizing direction though less markedly than for the hydrocarbons. The steady‐state activation curve was in each instance shifted in the depolarizing direction, as for the alcohols and other surface active substances. In common with both the hydrocarbons and the surface active substances the peak time constants were invariably reduced. The membrane capacity at 100 kHz was affected significantly only by methoxyflurane, where decreases of ca. 9% were observed for 3 mM solutions. The extent to which the results can be accounted for in terms of the perturbation of membrane lipid has been discussed.

The admittance of the squid giant axon membrane has been measured, using an intracellular electrode, at frequencies up to 40 MHz. The existence of a radio frequency dispersion, previously detected with extracellular electrodes (Cole, 1976) and attributed to the Schwann cell layer, has been confirmed and followed to higher frequencies. For a comparable method of analysis, membrane parameters similar to those given by Cole (1976) have been calculated. The radio frequency dispersion has a centre frequency at approximately 1.8 MHz, and the properties of a parallel combination of a 28 nF cm‐2 capacity and a 3.3 omega cm2 resistance. When the axon membrane capacity is calculated, taking into account the radio frequency dispersion, as described above, the capacity remains frequency dependent throughout the range studied. If it is assumed that at high frequencies the axolemma capacity becomes constant at approximately the value for a lipid bilayer, a radio frequency dispersion is found which cannot be accounted for in terms of a simple equivalent circuit with two passive components, but appears to arise from a network with a distribution of relaxation times. This result could be consistent with the morphology of the Schwann cell layer. The radio frequency dispersion referred to in (4) can be described reasonably well by a circuit with two dispersions having centre frequencies of 250 kHz and 3.2 MHz respectively. The corresponding axolemma capacity (100‐500 kHz) would be approximately 0.6 microF cm‐2. It is argued that between 50 and 100 kHz the geometrical capacity arising from the non‐polar regions of the membrane is a major contributor to the axon membrane capacity, and that capacity variations arising from compositional changes in the lipid bilayer are best monitored in this frequency range.

A number of small organic molecules with general anaesthetic action have been examined for their effects on the voltage‐dependent potassium current of the squid giant axon. They include representatives of the three classes of anaesthetics examined in previous studies on the sodium current (Haydon & Urban, 1983a, b, c), i.e. the non‐polar molecules n‐pentane, cyclopentane and CCl4, several n‐alkanols and the inhalation anaesthetics chloroform, halothane, diethyl ether and methoxyflurane. Potassium currents under voltage clamp were recorded in intact and in intracellularly perfused axons before, during and after exposure to the test substances, and the records were fitted with equations similar to those proposed by Hodgkin & Huxley (1952). Shifts in the curves of the steady‐state activation against membrane potential and reductions in the potassium conductance at 60 or 70 mV membrane potential have been tabulated. On the same intact axons, all the anaesthetics with the exception of methoxyflurane reduced potassium currents less than sodium currents by about a factor of two or more. For the n‐alkanols, butanol to decanol, the concentrations required to reduce the potassium current at 60 mV membrane potential by 50% were determined. For n‐butanol to n‐heptanol, the standard free energy per CH2 for adsorption to the site of action was estimated to be ‐2.91 kJ mol‐1 as compared with ‐3.04 kJ mol‐1 for reduction of the sodium current. The magnitude of the free energy decreased for alkanols with longer chain lengths. At anaesthetic concentrations that reduce the sodium current by 50%, the hydrophobic substances n‐pentane and cyclopentane reduced the maximal sodium conductance, gNa, and the potassium conductance at 70 mV, gK70, equally by about a third, while the n‐alkanols reduced both parameters by less than 10%. By contrast, diethyl ether and methoxyflurane were more effective in reducing the maximal potassium conductance. All of the test substances examined, except n‐pentane and n‐hexane, shifted the voltage dependence of the potassium steady‐state activation in the depolarizing direction. A broad qualitative correlation was found between the shifts in the activation curves for sodium and potassium currents but, quantitatively, the agreement between the two shifts was poor. In n‐decanol and methoxyflurane solutions, the voltage‐clamped potassium currents exhibited pronounced inactivation‐like behaviour. These currents can be fitted by the Hodgkin‐Huxley formalism if an inactivation term analogous to the sodium current inactivation is added.(ABSTRACT TRUNCATED AT 400 WORDS)