Macroscopic and single-channel chloride currents in neuropile glial cells of the leech central nervous system

In patch-clamp experiments we characterized four Cl- channels (42 pS, 70 pS, 80 pS and 229 pS) underlying the large Cl- conductance of leech neuropile glial cells. They differed with respect to their gating, their rectification and their activity in the cell-attached configuration, showed the selectivity sequence I->Cl->/=Br->F- and were impermeable to SO42-. The four channels were blocked by NPPB, DPC, niflumic acid and DIDS and exhibited either three or four sublevel states. The outward rectifying 42 pS, 70 pS and 80 pS Cl- channels were classified as intermediate conductance Cl- channels and they could contribute to the high Cl- conductance of the glial membrane, which stabilizes the glial membrane potential. The inward rectifying 229 pS Cl- channel is very similar to vertebrate high conductance Cl- channels, which are assumed to be part of an emergency system that is activated under pathophysiological conditions. In voltage-clamp experiments we calculated that the Cl- conductance amounts to one-third of the total membrane conductance. Reduction of this Cl- conductance by Cl- channel inhibitors markedly depolarized the glial cell membrane. These prominent depolarizations depended on Na+ influx and in most cases the glial cells failed to regulate their membrane potential following wash-out of the inhibitors.