Persistent inward currents in cultured Retzius cells of the medicinal leech

Abstract

Current-clamp studies of cultured leech Retzius cells revealed inward rectification in the form of slow voltage sags in response to membrane hyperpolarization. Sag responses were eliminated in Na⁺-free saline and blocked by Cs⁺, but not Ba²⁺. Voltage clamp experiments revealed a Cs⁺-sensitive inward current activated by hyperpolarization negative to −70 mV. Cs⁺ decreased the frequency of spontaneous impulses in Retzius cells of intact ganglia. Plateau potentials were evoked in Retzius cells following block of Ca²⁺ influx with Ni²⁺ and suppression of K⁺ currents with internal tetraethylammonium. Plateau potentials continued to be expressed with Li⁺ as the charge carrier, but were eliminated when Na⁺ was replaced with N-methyl-d-glucamine. A persistent Na⁺ current with similar pharmacology that activated positive to −40 mV and reached its peak amplitude near −5 mV was identified in voltage-clamp experiments. Inactivation of the persistent Na⁺ current was slow and incomplete. The current was revealed by slow voltage ramps and persisted for the duration of 5-s voltage steps. Persistent Na⁺ current may underlie Na⁺-dependent bursting recorded in neurons of intact ganglia exposed to Ca²⁺-channel blockers.