Bursting activity in leech Retzius neurons induced by low external chloride

We have investigated the bursting activity of Retzius neurons in the centra l nervous system of the leech Hirudo medicinalis as induced in Cl--free sal ine by measuring membrane potential, membrane current and the intracellular calcium concentration ([Ca2+](i)), using fura-2 or Oregon-Green(488)-B apt a-1. The Retzius neurons changed their low tonic firing to rhythmical burst ing activity when the extracellular Cl- concentration ([Cl-](o)) was lowere d to 1 mM or less. In Cl--free saline (Cl- exchanged by gluconate), burstin g was accompanied by a rise in intracellular Ca2+ in both cell body and axo n, which oscillated in synchrony with the bursts. The Ca2+ transients depen ded on the amplitude and duration of the depolarization underlying the burs t, and were presumably due to Ca2+ influx through voltage-dependent Ca2+ ch annels. In Ca2+-free, EGTA-buffered saline or in the presence of Ca2+ chann el blockers verapamil (1 mM) or diltiazem (500 muM) the depolarizations und erlying the bursts in Cl--free saline were enhanced in amplitude and durati on. Bursting was not affected by depleting the intracellular Ca2+ stores wi th cyclopiazonic acid. The depolarization in Cl-- and Ca2+-free saline did not evoke intracellular Ca2+ changes. The burst-underlying membrane depolar ization induced by Cl- removal was found to be due to a Na+-dependent persi stent inward current and could be inhibited by saxitoxin (25-50 muM). The r esults suggest that a persistent Naf current is generated in Cl--free salin e and induces the depolarization underlying rhythmic activity, and that pre sumably Ca2+-induced K+ currents modulate the bursting behaviour.