Calcium‐induced inactivation of calcium current causes the inter‐burst hyperpolarization of Aplysia bursting neurones.

A triphasic series of tail currents which follow depolarizing voltage‐clamp pulses in Aplysia neurones L2‐L6 was described in the preceding paper (Kramer & Zucker, 1985). In this paper, we examine the nature of the late outward component of the tail current (phase III) which generates the inter‐burst hyperpolarization in unclamped cells. The phase III tail current does not reverse between ‐30 and ‐90 mV, and is relatively insensitive to the external K+ concentration. In contrast, Ca2+‐dependent K+ current (IK(Ca)), elicited by intracellular Ca2+ injection, reverses near ‐65 mV, and the reversal potential is sensitive to the external K+ concentration. Addition of 50 mM‐tetraethylammonium (TEA) to the bathing medium causes a small increase in the phase III tail current. In contrast, IK(Ca) is completely blocked by addition of 50 mM‐TEA. The phase III tail current is suppressed by depolarizing pulses which approach ECa, is blocked by Ca2+ current antagonists (Co2+ and Mn2+), and is blocked by intracellular injection of EGTA. The phase III tail current is reduced by less than 10% after complete removal of extracellular Na+. These bursting neurones have a voltage‐dependent Ca2+ conductance which exhibits steady‐state activation at a membrane potential similar to the average resting potential of the unclamped cell (i.e. ‐40 mV). The steady‐state Ca2+ conductance can be inactivated by Ca2+ injection, or by depolarizing pre‐pulses which generate a large influx of Ca2+. The steady‐state Ca2+ conductance has a voltage dependence similar to that of the phase III tail current. The Ca2+‐dependent inactivation of the steady‐state Ca2+ conductance occurs in parallel with the phase III tail current; both have a similar sensitivity to Ca2+ influx, and both processes decay with similar rates after a depolarizing pulse. Hence, we propose that the phase III tail current is due to the Ca2+‐ dependent inactivation of a steady‐state Ca2+ conductance. The decay of IK(Ca) following simulated spikes or bursts of spikes is rapid (less than 1 s) compared to the time course of the phase III tail current and the inter‐burst hyperpolarization (tens of seconds). Thus, we conclude that IK(Ca) does not have a major role in terminating bursts or generating the inter‐burst hyperpolarization in these cells. We present a qualitative model of the ionic basis of the bursting pace‐maker cycle. The central features of the model are the voltage‐dependent activation and the Ca2+‐dependent inactivation of a Ca2+ current.

[1]  E. Kandel,et al.  An Anomalous form of Rectification in a Molluscan Central Neurone , 1964, Nature.

[2]  E. Kandel,et al.  MORPHOLOGICAL AND FUNCTIONAL PROPERTIES OF IDENTIFIED NEURONS IN THE ABDOMINAL GANGLION OF APLYSIA CALIFORNICA , 1967 .

[3]  A. Hodgkin,et al.  The effect of cyanide on the efflux of calcium from squid axons , 1969, The Journal of physiology.

[4]  H. Gainer Electrophysiological behavior of an endogenously active neurosecretory cell. , 1972, Brain research.

[5]  C. L. Stephens,et al.  Cyclic variation of potassium conductance in a burst‐generating neurone in Aplysia , 1973, The Journal of physiology.

[6]  F. Strumwasser Seventeenth Bowditch lecture. Neural and humoral factors in the temporal organization of behavior. , 1973, The Physiologist.

[7]  R. Meech Prolonged action potentials in Aplysia neurones injected with egta , 1974 .

[8]  H Wachtel,et al.  Negative Resistance Characteristic Essential for the Maintenance of Slow Oscillations in Bursting Neurons , 1974, Science.

[9]  H. Gainer,et al.  Requirements for bursting pacemaker potential activity in molluscan neurones , 1975, Nature.

[10]  H. Gainer,et al.  Studies on bursting pacemaker potential activity in molluscan neurons. I. Membrane properties and ionic contributions , 1975, Brain Research.

[11]  R. Meech,et al.  Potassium activation in Helix aspersa neurones under voltage clamp: a component mediated by calcium influx. , 1975, The Journal of physiology.

[12]  H. Gainer,et al.  Studies on bursting pacemaker potential activity in molluscan neurons. II. Regulation by divalent cations , 1975, Brain Research.

[13]  R. Eckert,et al.  A voltage‐sensitive persistent calcium conductance in neuronal somata of Helix. , 1976, The Journal of physiology.

[14]  D. Johnston Voltage clamp reveals basis for calcium regulation of bursting pacemaker potentials in Aplysia neurons , 1976, Brain Research.

[15]  S. Thompson Three pharmacologically distinct potassium channels in molluscan neurones. , 1977, The Journal of physiology.

[16]  A. Gorman,et al.  Changes in the intracellular concentration of free calcium ions in a pace‐maker neurone, measured with the metallochromic indicator dye arsenazo III. , 1978, The Journal of physiology.

[17]  R. Meech Membrane potential oscillations in molluscan "burster" neurones. , 1979, The Journal of experimental biology.

[18]  I Kupfermann,et al.  Modulatory actions of neurotransmitters. , 1979, Annual review of neuroscience.

[19]  W. B. Adams,et al.  Synaptic and hormonal modulation of a neuronal oscillator: a search for molecular mechanisms. , 1979, The Journal of experimental biology.

[20]  A. Gorman,et al.  External and internal effects of tetraethylammonium on voltage-dependent and Ca-dependent K+ currents components in molluscan pacemaker neurons , 1979, Neuroscience Letters.

[21]  H. Wachtel,et al.  Two reciprocating current components underlying slow oscillations in Aplysia bursting neurons , 1980, Brain Research Reviews.

[22]  J. Byrne,et al.  Molluscan nerve cells, from biophysics to behavior , 1980 .

[23]  D. Johnston Voltage, temperature and ionic dependence of the slow outward current in Aplysia burst‐firing neurones. , 1980, The Journal of physiology.

[24]  R. Zucker,et al.  Aequorin response facilitation and intracellular calcium accumulation in molluscan neurones , 1980, The Journal of physiology.

[25]  R. Meech,et al.  Effect of measured calcium chloride injections on the membrane potential and internal pH of snail neurones. , 1980, The Journal of physiology.

[26]  D. J. Adams,et al.  Ionic currents in molluscan soma. , 1980, Annual review of neuroscience.

[27]  A. Gorman,et al.  Potassium conductance and internal calcium accumulation in a molluscan neurone , 1980, The Journal of physiology.

[28]  R. Zucker Cytoplasmic alkalization reduces calcium buffering in molluscan central neurons , 1981, Brain Research.

[29]  A. Gorman,et al.  Intracellular calcium and the control of neuronal pacemaker activity. , 1981, Federation proceedings.

[30]  A. Gorman,et al.  Effects of tetraethylammonium on potassium currents in a molluscan neurons , 1981, The Journal of general physiology.

[31]  R. Eckert,et al.  Calcium‐mediated inactivation of the calcium conductance in caesium‐loaded giant neurones of Aplysia californica. , 1981, The Journal of physiology.

[32]  D. A. Brown,et al.  Intracellular Ca2+ activates a fast voltage-sensitive K+ current in vertebrate sympathetic neurones , 1982, Nature.

[33]  A. Gorman,et al.  Ionic requirements for membrane oscillations and their dependence on the calcium concentration in a molluscan pace‐maker neurone , 1982, The Journal of physiology.

[34]  A. Gorman,et al.  Quantitative differences in the currents of bursting and beating molluscan pace‐maker neurones , 1982, The Journal of physiology.

[35]  R. Eckert,et al.  Residual calcium ions depress activation of calcium-dependent current. , 1982, Science.

[36]  H Lecar,et al.  Single calcium-dependent potassium channels in clonal anterior pituitary cells. , 1982, Biophysical journal.

[37]  K L Magleby,et al.  Properties of single calcium‐activated potassium channels in cultured rat muscle , 1982, The Journal of physiology.

[38]  S. Thompson,et al.  Calcium buffering and slow recovery kinetics of calcium‐dependent outward current in molluscan neurones. , 1983, The Journal of physiology.

[39]  D. A. Brown,et al.  Calcium‐activated outward current in voltage‐clamped hippocampal neurones of the guinea‐pig. , 1983, The Journal of physiology.

[40]  J H Halsey,et al.  Reversible changes in the intracellular potassium ion activities and membrane potentials of Aplysia L2-L6 neurones in response to normoxia and hypoxia. , 1983, The Journal of experimental biology.

[41]  R. Eckert,et al.  Calcium tail currents in voltage‐clamped intact nerve cell bodies of Aplysia californica. , 1983, The Journal of physiology.

[42]  I. Levitan,et al.  Serotonin increases an anomalously rectifying K+ current in the Aplysia neuron R15. , 1983, Proceedings of the National Academy of Sciences of the United States of America.

[43]  S. Hagiwara,et al.  The calcium channel , 1983, Trends in Neurosciences.

[44]  L. Satin Sodium-dependent calcium efflux from single Aplysia neurons , 1984, Brain Research.

[45]  D. Lewis Spike aftercurrents in R15 of Aplysia: their relationship to slow inward current and calcium influx. , 1984, Journal of neurophysiology.

[46]  W. B. Adams,et al.  Slow depolarizing and hyperpolarizing currents which mediate bursting in Aplysia neurone R15. , 1985, The Journal of physiology.

[47]  W. B. Adams,et al.  Voltage and ion dependences of the slow currents which mediate bursting in Aplysia neurone R15. , 1985, The Journal of physiology.

[48]  R S Zucker,et al.  Calcium‐dependent inward current in Aplysia bursting pace‐maker neurones. , 1985, The Journal of physiology.