Gamma-Aminobutyric acid (γ-Aminobutyric acid) (GABA) is the chief inhibitory neurotransmitter in the mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system. In humans, GABA is also directly responsible for the regulation of muscle tone. While GABA is an inhibitory transmitter in the mature brain, its actions were thought to be primarily excitatory in the developing brain. The gradient of chloride was reported to be reversed in immature neurons, with its reversal potential higher than the resting membrane potential of the cell; activation of a GABA-A receptor thus leads to efflux of Cl?ions from the cell (that is, a depolarizing current). The differential gradient of chloride in immature neurons was shown to be primarily due to the higher concentration of NKCC1 co-transporters relative to KCC2 co-transporters in immature cells. GABA itself is partially responsible for orchestrating the maturation of ion pumps. GABAergic interneurons mature faster in the hippocampus and the GABA signalling machinery appears earlier than glutamatergic transmission. Thus, GABA was considered the major excitatory neurotransmitter in many regions of the brain before the maturation of glutamatergic synapses.