A furosemide-sensitive K+-Cl- cotransporter counteracts intracellular Cl- accumulation and depletion in cultured rat midbrain neurons

Efficacy of postsynaptic inhibition through GABA(A) receptors in the mammal ian brain depends on the maintenance of a Cl- gradient for hyperpolarizing Cl- currents. We have taken advantage of the reduced complexity under which Cl- regulation can be investigated in cultured neurons as opposed to neuro ns in other in vitro preparations of the mammalian brain. Tight-seal whole- cell recording of spontaneous GABA(A) receptor-mediated postsynaptic curren ts suggested that an outward Cl- transport reduced dendritic [Cl-](i) if th e somata of cells were loaded with Cl- via the patch pipette, We determined dendritic and somatic reversal potentials of Cl- currents induced by focal ly applied GABA to calculate [Cl-](i) during variation of [K+](o) and [Cl-] in the patch pipette, [Cl-](i) and [K+](o) were tightly coupled by a furos emide-sensitive K+-Cl- cotransport. Thermodynamic considerations excluded t he significant contribution of a Na+-K+-Cl- cotransporter to the net Cl- tr ansport. We conclude that under conditions of normal [K+](o) the K+-Cl- cot ransporter helps to maintain [Cl-](i) at low levels, whereas under patholog ical conditions, under which [K+](o) remains elevated because of neuronal h yperactivity, the cotransporter accumulates Cl- in neurons, thereby further enhancing neuronal excitability.