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 Idapwith 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 m m TEA, a K + channel blocker, reversibly suppressed the I dap . 6 Both Na + and Ca 2+ 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 Ca 2+ with Co 2+ , or replacing 82% of external Na + with choline or Li + . Perifusion of slices with media containing 1–2 μ 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 [Ca 2+ ] i transients, due to Ca 2+ influx during action potentials and Ca 2+ release from internal stores. Na + influx might provide a permissive influence for Ca 2+ ‐induced reduction of K + conductances and/or help to raise [Ca 2+ ] i via reverse‐mode Ca 2+ ‐Na + exchange. Other conductances, making minor contributions to the I dap , may also be involved.