Depolarizing afterpotentials and burst production in molluscan pacemaker neurons

Whole‐cell patch clamp recordings were obtained from sixty‐five rat supraoptic nucleus (SON) neurones in brain slices to investigate ionic mechanisms underlying depolarizing after‐potentials (DAPs). When cells were voltage clamped around ‐58 mV, slow inward currents mediating DAPs (Idap), evoked by three brief depolarizing pulses, had a peak of 17 ± 1 pA (mean ± s.e.m.) and lasted for 2.8 ± 0.1 s. No significant differences in the amplitude and duration were observed when one to three preceding depolarizing pulses were applied, although there was a tendency for twin pulses to evoke larger Idap than a single pulse. The Idap was absent when membrane potentials were more negative than ‐70 mV. In the range ‐70 to ‐50 mV, Idap amplitudes and durations increased as the membrane became more depolarized, with an activation threshold of ‐65.7 ± 0.7 mV. I dap with normal amplitude and duration could be evoked during the decay of a preceding Idap. As frequencies of depolarizing pulses rose from 2 to 20 Hz, the times to peak Idap amplitude were reduced but the amplitudes and durations did not change. A consistent reduction in membrane conductance during the Idap was observed in all SON neurones tested, and averaged 34.6 ± 3.3%. Small hyperpolarizing pulses used to measure membrane conductances appeared not to disturb major ionic mechanisms underlying Idap, since the slope and duration of Idap with and without test pulses were similar. The Idap had an averaged reversal potential of ‐87.4 ± 1.6 mV, which was close to the K+ equilibrium potential. An elevation in [K+]O reduced or abolished the Idap, and shifted its reversal potential toward more positive levels. Perifusion of slices with 7.5–10 mm TEA, a K+ channel blocker, reversibly suppressed the Idap. Both Na+ and Ca2+ currents failed to induce an Idap‐like current during perifusion of slices with media containing high [K+]o or TEA. However, the Idap was abolished by replacing external Ca2+ with Co2+, or replacing 82% of external Na+ with choline or Li+. Perifusion of slices with media containing 1–2 μm TTX also reduced Idap by 55.5 ± 9.0 %. These results suggest that the generation of DAPs in SON neurones mainly involves a reduction in outward K+ current(s), which probably has little or no inactivation and can be inhibited by [Ca2+]i transients, due to Ca2+ influx during action potentials and Ca2+ release from internal stores. Na+ influx might provide a permissive influence for Ca2+‐induced reduction of K+ conductances and/or help to raise [Ca2+]i via reverse‐mode Ca2+‐Na+ exchange. Other conductances, making minor contributions to the Idap, may also be involved.

supporting

confidence: 52%

Reduced outward K⁺ conductances generate depolarizing after–potentials in rat supraoptic nucleus neurones

Hatton²

1997

“…are reduced when Na+ or Ca2+ are removed from the bathing medium (Thompson, 1976). It has also been suggested that d.a.p.s provide a mechanism for sustaining repetitive firing, and are critical for the depolarizing phase of bursting pace-maker activity (Thompson & Smith, 1976).…”

Section: Introductionmentioning

confidence: 99%

Calcium‐dependent inward current in Aplysia bursting pace‐maker neurones.

Krämer¹,

Zucker²

1985

124

SUMMARY1. Depolarizing voltage-clamp pulses elicit a triphasic series of tail currents (phase I, II and III) in Aplysia burst-firing neurones L2-L6. The sequence and time course of the tail currents resemble slow changes in membrane potential which follow bursts in the unclamped cell.2. The phase II tail current is an inward current with a time course similar to that of the depolarizing after-potential (d.a.p.) which follows bursts in the unclamped cell.The phase II 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.3. The phase II tail current is not blocked by agents which block Na+-dependent action potentials, the Na+{-Ca2+ exchange pump, or the Na+-K+ exchange pump. The phase II tail current is not blocked by the elimination of large outward K+ currents which can lead to extracellular K+ accumulation. Thus, the phase II tail current is not generated by any of these processes.4. The phase II tail current is reduced by about 60 % following substitution of tetramethylammonium (TMA+) for external Na+, but is unaffected by reducing external Cl-.5. The phase II tail current is distinct from a persistent inward Ca2+ current which underlies the negative resistance region of the steady-state current-voltage relation of bursting cells. The persistent inward current is only slightly reduced by TMA+ substitution for Na+, and is enhanced by EGTA injection.6. Injection of Ca2+ into Aplysia bursting cells elicits a biphasic (inward-outward)current. The inward current can be observed in isolation after blocking the outward component (Ca2+-activated K+ current) with 50 mM-external tetraethylammonium. 7. The Ca2+-elicited inward current has a reversal potential near -22 mV, and is non-selective for Na+, K+ and Ca2+. The reversal potential is unaffected by changes in Cl-and pH. The Ca2+-activated conductance is apparently voltage independent.8. We propose that the phase II tail current, and hence the d.a.p., is due to the Ca2+-dependent activation of a voltage-independent non-specific cationic conductance. This conductance participates in generating the depolarizing phase of bursting pace-maker activity.

“…DAP sizes depend on the number of preceding spikes and can be reduced by membrane hyperpolarization (Andrew & Dudek, 1984a;Andrew, 1987). Removal of extracellular Ca2+, blockade of membrane Ca2+ channels or chelation of intracellular free Ca2+ all abolish DAPs , suggesting that DAPs in SON cells, like those identified in invertebrate bursting pacemaker (Thompson & Smith, 1976;Kramer & Zucker, 1985) and mammalian (White, Lovinger & Weight, 1989) neurones require Ca2+ influx for their generation. However, ionic mechanisms underlying these potentials are not yet well understood, although DAPs in SON neurones are recognized to be important in the formation of phasic or burst firing patterns that promote vasopressin release from the neurohypophysis (see Hatton, 1990, for review).…”

mentioning

confidence: 97%

Ca2+ release from internal stores: role in generating depolarizing after‐potentials in rat supraoptic neurones.

Hatton

1997