Post-stimulus hyperpolarization in retzius nerve cells of leech, Haemopis sanguisuga

Abstract

Abstract 1. 1. Post-stimulus hyperpolarization (PSH) that occurred in response to direct or indirect (across the electrotonic junction) depolarizing stimulation has been studied in Retzius neurons of horse leech, Haemopis sanguisuga. 2. 2. The amplitude and time course of PSH were dependent on the duration of depolarizing stimulation and passive membrane properties of Retzius cells. The recovery of membrane potential during PSH has an approximately exponential time course and consisted of two phases: the early (fast) one, which has been associated with a decrease in membrane input and transfer resistance and the late (slow) one. 3. 3. Na-free saline (with TRIS as substituing cation) and K-free saline augmented the amplitude of PSH and abolished its late phase. 4. 4. Ouabain completely blocked the late phase and greatly diminished the early one. Verapamil almost completely abolished PSH. 5. 5. Increased external Ca concentrations slightly augmented the amplitude and duration of PSH. 6. 6. Application of strong inward current reversed the PSH. The PSH reversal potential varied with extracellular K concentrations, approx. 40 mV per ten-fold change in external K+. 7. 7. It was concluded that PSH in Retzius nerve cells is mediated by two mechanisms: the prolonged increase in membrane K conductance and increased activity of the electrogenic sodium pump.