MACROSCOPIC AND SINGLE-CHANNEL CHLORIDE CURRENTS IN NEUROPIL 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, th eir rectification and their activity in the cell-attached configuratio n, showed the selectivity sequence I- > Br- greater than or equal to C l- > F- and were impermeable to SO42-. The four channels were blocked by NPPB, DPC, niflumic acid and DIDS and exhibited either three or fou r sublevel states. The outward rectifying 42 pS, 70 pS and 80 pS Cl- c hannels were classified as intermediate conductance Cl- channels and t hey 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 exper iments we calculated that the Cl- conductance amounts to one-third of the total membrane conductance. Reduction of this Cl- conductance by C l- channel inhibitors markedly depolarized the glial cell membrane. Th ese 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. (C) 1998 Elsevier Science B.V.