A nervous system cannot work by excitation alone. Neurons need ways to encourage other neurons to fire, but they also need ways to restrain them, sharpen their timing, filter competing signals, and prevent activity from spreading indiscriminately through a network. In the vertebrate brain, much of that inhibitory work depends on a small molecule called GABA, short for gamma-aminobutyric acid. GABA is commonly described as the brain's main inhibitory neurotransmitter, and that is a useful starting point. But it can also make inhibition sound simpler than it really is. GABA can make a neuron less likely to fire, change how strongly it responds to incoming signals, influence when it fires, or even increase activity elsewhere in a circuit by inhibiting another inhibitory neuron. The deeper story is not simply that GABA acts as a brake. It is that nervous systems use inhibition as one of the basic tools for organizing activity.
The chemistry begins with an interesting relationship. GABA is synthesized from glutamate, another neurotransmitter that usually plays the opposite role in the vertebrate central nervous system. An enzyme called glutamic acid decarboxylase modifies glutamate to produce GABA, which can then be packaged into tiny membrane-bound vesicles inside a neuron and released at synapses. After release, GABA can be taken back up by specialized transport proteins and recycled or broken down. Two of the most important neurotransmitters in the brain, one usually associated with excitation and the other with inhibition, are therefore separated by only a short biochemical step. Their very different effects arise not because the molecules carry instructions like "excite" or "inhibit," but because different receptors, ion gradients, cells, and circuits respond to them differently.
