GABA (also known as <em>gamma-Aminobutyric acid</em>) is a substance of great relevance in the Central Nervous System since it is the most important inhibitory neurotransmitter. GABA is released from presynaptic neurons to reduce the excitability of postsynaptic neurons, inhibiting them.
GABA inhibits the neurons because, when this neurotransmitter acts as a ligand in the GABAa receptors of the postsynaptic neurons, chloride channels open, allowing the flow of chloride ions. <u>Chloride is highly concentrated in the extracellular fluid, while it is poorly concentrated in the inside of the cell - this generates a concentration gradient</u>. When the chloride channels open, this ion will enter the cell to even out the concentrations between the two fluids. The inhibitory effect occurs because chloride is an anion, meaning that is negatively-charged and is entering a fluid that is already very negative in comparison to the extracellular fluid - this <u>makes the intracellular fluid more and more negative</u>, and this phenomenon is called <u>hyperpolarization</u>. To fire action potentials as excitatory neurons do, the cell needs to depolarize (become less negative), not hyperpolarize. This is why the opening of chloride channels is crucial to reducing the excitability of a neuron and why GABA works as an inhibitory neurotransmitter.
