Characteristics of sodium and calcium conductance changes produced by membrane depolarization in an Aplysia neurone.

Abstract

SUMMARY1. The time course and voltage dependence of Na and Ca conductance changes produced by depolarization of the soma of the neurone R15 in the abdominal ganglion of Aplysia juliana were examined at temperatures of 10-14 'C.2. During a maintained depolarization, Na currents turned on then decayed (inactivated). Inactivation was exponential with time constant rh. Activation (after correction for inactivation) was reasonably well described by the expression G' (t) = GNI(oo) (1 -exp [-tIrm])3 over a wide range of potentials. Half-maximal Na conductance occurred at a membrane potential of -8 mV and from -15 to -5 mV, a 5 mV change in membrane potential produced an e-fold change in steady-state Na conductance. 4. Steady-state inactivation of Na conductance (hNa(oo)) was voltage dependent with half-inactivation occurring at a membrance potential of -32 mV. Recovery from Na inactivation followed an exponential time course with a voltage-dependent time constant.5. During a maintained depolarization Ca currents activated then decayed (inactivated) more slowly than Na currents. The decay was exponential with time constant rH. The decay of Ca current was not an artifact produced by an outward current. The amplitude of calcium tail currents, produced by voltage steps back to eK at different times during the decay of Ic., decayed also with a time constant close to TH.6. Ca conductance (after correction for inactivation) could be described approximately by the expression G' (t) = G' (oo) ( 1-exp [-t/TM])P but it was necessary to vary p from 1 to 2 at different potentials. No value of p gave as good a fit to this model as that obtained for Na currents.7. TM and TH were voltage dependent. In the range of potentials from 0 to + 60 mV, TM varied from 9 to 5 msec and TH from 300 to 50 msec (13-5 'C).

Calcium‐mediated inactivation of the calcium conductance in caesium‐loaded giant neurones of Aplysia californica.

“…-40 mV) holding potentials (Hagiwara & Saito, 1959;Connor & Stevens, 1971;Neher, 1971;Standen, 1974). The depression of inward current by prior hyperpolarization of R-15 neurone reported by Adams & Gage (1979) may have been caused by incomplete suppression of the early potassium current by TEA and 4-aminopyridine in their experiments. Hyperpolarization during interpulse intervals failed to remove any of the inactivation exhibited by the pulse 2 calcium current (Fig.…”

Ca(2+)‐dependent inactivation of Ca2+ current in Aplysia neurons: kinetic studies using photolabile Ca2+ chelators.

“…Effects of cyclic AMP-dependent phosphorylation on recovery from inactivation The recovery from inactivation measured in twin-pulse experiments characteristically displays bi-exponential kinetics (Tillotson & Horn, 1978;Adams & Gage, 1979;Plant & Standen, 1981) with a fast time constant of tens to hundreds 513 of milliseconds and a slow time constant of seconds to tens of seconds. In the present experiments we focused on the slow phase of recovery, which should be less influenced by the late opening of Ca2+ channels during the long (200 ms) prepulse.…”

Arsenazo III transients and calcium current in a normally non‐spiking neuronal soma of crayfish.

“…However, a two-time-constant recovery from inactivation was not strongly supported because (1) the correlation coefficient for the fast phase (-0-81) is lower and (2) the related time constant (0-24 s) is very close to the test-pulse duration. These results can be compared with those obtained for Ca2+ current by other authors who concluded that two time constants existed using a similar two-pulse protocol: 0 42/4A4 s (Tillotson & Horn, 1978); _0,1/ _1 s (Adams & Gage, 1979); 0 12/9 4 s (Plant & Standen, 1981); 0-06/0 89 s (Ashcroft & Stanfield, 1982).…”

“…Recovery will then depend on the efficiency of the cell for controlling [Ca2+]1. Recovery occurs in the giant motoneurone with at least one time constant of about 1 s. It may be that the pulse duration used to obtain reliable Arsenazo III absorbance changes (300 ms) have prevented our observing a faster component (see Adams & Gage, 1979 Non-linearity of potential dependence As observed in other preparations (Hagiwara & Byerly, 1981;Ashcroft & Stanfield, 1982), the Ca2+ inward current shows a rectification, here above + 40 mV (Fig. 4A, dashed line).…”