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

1. The spontaneous activity and the membrane conductances to Na(+), Ca(2+) 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 Ca(2+) 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 Ca(2+) to abolish Ca(2+) currents, whereas the spontaneous action potentials of cell L-11 are abolished by external TTX, but are unaffected by external Ca(2+) removal.4. The membranes of both cells contain Na(+) and Ca(2+) inward currents. The specific Na(+) conductance of both cells is of similar magnitude, whereas the specific Ca(2+) 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 Ca(2+)-activated K(+) conductance of cell R-15 and cell L-11 was estimated using two methods. The Ca(2+)-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 Ca(2+)-free solution (to preclude Ca(2+) influx) or after internal EGTA injection (to prevent Ca(2+) accumulation) is about 23 times greater than this conductance in cell L-11. The Ca(2+)-activated K(+) conductance of cell R-15, estimated from local internal Ca(2+) injections in Ca(2+)-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 Ca(2+)-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 Ca(2+) conductance and the Ca(2+)-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.