GABA
GABA, also known as gamma-Aminobutyric acid, is the main inhibitory neurotransmitter.

Chemistry
GABA can be classified as an amino acid; a carbon chain with an amine group at one end and then a carboxylic acid at the other. However, as it is not a alpha amino acid, it does not form proteins.
Inhibitory Response
GABA inhibits various synapses in the brain, causing the opening of ion channels to allow the flow of either negatively charged chloride ions into the cell or positively charged potassium ions out of the cell. This action results in a negative change in the transmembrane potential, usually causing hyperpolarization.