Cyclic variation of potassium conductance in a burst‐generating neurone in Aplysia

Abstract

SUMMARY1. The hyperpolarization between bursts in the R 15 cell of Aplysia is accompanied by an increase in membrane slope conductance.2. The post-burst hyperpolarization can be observed with ouabain, lithium, or potassium-free solution if artificial inward current is applied. The hyperpolarization can be observed with dinitrophenol or cooling to 10°C, with no injected current. Thus, the hyperpolarization apparently is not due to the cyclic activity of an electrogenic pump.3. A reversal potential for the post-burst hyperpolarization can be demonstrated by passage of inward current during the inter-burst period. The reversal of direction of the potential depends on recent occurrence of a burst.4. The reversal potential varies with external potassium concentration, but not with chloride or sodium.5. The post-burst hyperpolarization is not blocked by external tetraethylammonium at a concentration which greatly prolongs the action potentials.6. During the onset of spike blockage by, and recovery from, calciumfree + tetrodotoxin saline, the bursts of action potentials appear to be driven by endogenous waves of membrane potential.7. The hyperpolarizing phase of the waves in calcium-free + tetrodotoxin medium is accompanied by an increased slope conductance.8. A reversal potential can be demonstrated for the hyperpolarization following a wave in calcium-free + tetrodotoxin medium by applying inward current during the interwave period.9. The waves in calcium-free + tetrodotoxin medium are blocked by ouabain but can be reinstated by artificial hyperpolarization.10. The post-burst hyperpolarization and the post-wave hyperpolarization appear to result from a periodic increase in membrane conductance, primarily to potassium ions. DOUGLAS JUNGE AND CATHY L. STEPHENS

Ionic requirements for membrane oscillations and their dependence on the calcium concentration in a molluscan pace‐maker neurone

“…Fig. 15 There is a variety of evidence to suggest that slow, spontaneous oscillations of membrane potential can occur in the absence of action potential discharge under certain conditions (Strumwasser, 1968(Strumwasser, , 1973Mathieu & Roberge, 1971;Junge & Stephens, 1973; Barker & Gainer, 1975a;Gola, 1976). Separation of action potential discharge from the slow membrane potential oscillation has been achieved by a high external concentration of TTX (Mathieu & Roberge, 1971;Barker & Gainer, 1975b;Gola, 1976), by a partial removal ofexternal Na+ ions (Mathieu, Roberge & Gulrajani, 206 IONIC REQUIREMENTS FOR MEMBRANE OSCILLATIONS 207 1976), by a combination ofhigh external TTX and Ca2+-free conditions (Strumwasser, 1968(Strumwasser, , 1973Junge & Stephens, 1973), or by a combination of TTX, Ca2+-free conditions and external Co2+ (Gola, 1976).…”

“…Such cells form a convenient system for the study of the electrical oscillations which underlie pace-maker activity, but their mechanism is still poorly understood. Studies on the pace-maker neurones found in molluscan ganglia suggest that there is a slow inward current during the depolarizing phase of pace-maker activity, involving a voltage-dependent change in a channel that is insensitive to external tetrodotoxin, (Gola, 1974(Gola, , 1976Wilson & Wachtel, 1974;Smith, Barker & Gainer, 1975;Eckert & Lux, 1976;Johnson, 1976) and that during the hyperpolarizing phase of the cycle there is an outward current which is probably carried by K+ ions (Junge & Stephens, 1973; Gola, 1974;Meech, 1974Meech, , 1979Wilson & Wachtel, 1974;Smith et al 1975;Eckert & Lux, 1976;Gorman & Thomas, 1978). A variety of currents have been identified in molluscan neurones (Adams, Smith & Thompson, 1980), but there is no clear agreement about which is crucial for pace-maker activity.…”

Changes in the intracellular concentration of free calcium ions in a pace‐maker neurone, measured with the metallochromic indicator dye arsenazo III.

“…(pace-maker potential) of the soma membrane (Alving, 1968) and that the membrane hyperpolarization which separates bursts and determines their frequency is likely to be due to an increase in K+ permeability (Junge & Stephens, 1973). It is known that pace-maker potentials can occur in the absence of action potential discharge (Strumwasser, 1968) and therefore may involve a separate mechanism, that pacemaker potentials can also occur after blocking the Na+ pump (Carpenter, 1973) and that the post burst hyperpolarization is not due to a cyclic variation of an electrogenic component of the Na+ pump (Junge & Stephens, 1973).…”

“…It is known that pace-maker potentials can occur in the absence of action potential discharge (Strumwasser, 1968) and therefore may involve a separate mechanism, that pacemaker potentials can also occur after blocking the Na+ pump (Carpenter, 1973) and that the post burst hyperpolarization is not due to a cyclic variation of an electrogenic component of the Na+ pump (Junge & Stephens, 1973). Finally, the currentvoltage relation for pace-maker cells exhibits a negative resistance region in the range of the pace-maker oscillation which has been related to a slow inward current (Gola, 1974;Wilson & Wachtel, 1974) caused by an influx of Na+ ions (Smith, Barker & Gainer, 1975) or by Ca2+ ions (Eckert & Lux, 1976) or by both (Johnston, 1976).…”

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

“…It is difficult to reduce the external Ca2+ concentration below about 4/M in ASW solution (see Methods) and this value is likely to be much greater in the bath near the cell membrane. It is not surprising that the removal of external Ca2+ without the use of a Ca2+ chelator had very little effect on the discharge behaviour of cell R-15 in previous studies (see Junge & Stephens, 1973). The normal bursting pattern of the cell was restored when Ca2+ was returned to the bathing solution (Fig.…”