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

1. 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 (I-DAP), 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. 2. 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 I-DAP than a single pulse. The I-DAP was absent when membrane potentials were more negative than -70 mV. In the range -70 to -50 mV, I-DAP amplitudes and durations increased as the membrane became more depolarized, with an activation threshold of -65.7 +/- 0.7 mV. 3. I-DAP with normal amplitude and duration could be evoked during the decay of a preceding I-DAP. As frequencies of depolarizing pulses rose from 2 to 20 Hz, the times to peak I-DAP amplitude were reduced but the amplitudes and durations did not change. 4. A consistent reduction in membrane conductance during the I-DAP 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 I-DAP, since the slope and duration of I-DAP with and without test pulses were similar. 5. The I-DAP 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 I-DAP, 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 I-DAP. 6. Both Na+ and Ca2+ currents failed to induce an I-DAP-like current during perifusion of slices with media containing high [K+](o) or TEA. However, the I-DAP 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 mu M TTX also reduced I-DAP by 55.5 +/- 9.0%. 7. 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 I-DAP, may also be involved.