Two isoforms of the cation-Cl - cotransporter family are expressed in neurons and modulate neurotransmission. NKCC1, a Na + -K + -2Cl - cotransporter, by raising internal Cl - , is responsible for excitatory GABAergic activity in immature brain and in adult sensory neurons. KCC2, a neuronal-specific isoform of the K + -Cl - cotransporter, by lowering internal Cl - , is critical for inhibitory GABA responses in mature central nervous system neurons.