Quantitative differences in the currents of bursting and beating molluscan pace-maker neurones

Abstract

1. The spontaneous activity and the membrane conductances to Na⁺, Ca²⁺ and K⁺ ions of the bursting pace-maker neurone R-15 and the repetitively discharging (beating) pace-maker neurone L-11 in the abdominal ganglion of the marine mollusc, Aplysia californica, were compared.

2. The bursting pace-maker R-15 can be converted to a beating pace-maker neurone by the removal of external Ca²⁺ or by the injection of EGTA intracellularly. Bursting pace-maker activity is not restored by changes in the resting potential.

3. Spontaneous action potentials of cell R-15 are reduced, but not abolished, by the addition of tetrodotoxin (TTX) to block Na⁺ currents or by the removal of external Ca²⁺ to abolish Ca²⁺ currents, whereas the spontaneous action potentials of cell L-11 are abolished by external TTX, but are unaffected by external Ca²⁺ removal.

4. The membranes of both cells contain Na⁺ and Ca²⁺ inward currents. The specific Na⁺ conductance of both cells is of similar magnitude, whereas the specific Ca²⁺ conductance is about half the Na⁺ conductance in R-15 cells and an order of magnitude smaller in L-11 cells.

5. The delayed K⁺ conductance of cell L-11 is about 1·2 times greater than this conductance in cell R-15. The transient K⁺ currents of the two cells are about the same magnitude.

6. The Ca²⁺-activated K⁺ conductance of cell R-15 and cell L-11 was estimated using two methods. The Ca²⁺-activated K⁺ conductance of cell R-15 estimated from the difference in the total outward current in normal external solution and the delayed K⁺ current in Ca²⁺-free solution (to preclude Ca²⁺ influx) or after internal EGTA injection (to prevent Ca²⁺ accumulation) is about 23 times greater than this conductance in cell L-11. The Ca²⁺-activated K⁺ conductance of cell R-15, estimated from local internal Ca²⁺ injections in Ca²⁺-free solution, is about 3 times greater than this conductance in cell L-11.

7. The leakage conductance of cell L-11 is about 1.3 times greater than this conductance in cell R-15. This conductance increases by a factor of about 2 in both cells in Ca²⁺-free external solutions containing 1 mM-EGTA, but is unchanged or is decreased slightly by injection of EGTA internally.

8. It is concluded that the Ca²⁺ conductance and the Ca²⁺-activated K⁺ conductance are appreciably greater in the bursting pace-maker neurone R-15 than in the beating pace-maker neurone L-11, whereas other voltage-dependent conductances to Na⁺ and K⁺ ions as well as the leakage conductance are quite similar. These quantitative differences provide a basis for understanding the different spontaneous activities of the two cells.

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