The long term stability and function of neuronal networks is dependent on a maintained balance between excitatory and inhibitory synaptic transmission. The major inhibitory neurotransmitter in the CNS is GABA, which is converted from glutamate by the enzyme Glutamic Acid Decarboxylase (GAD). Similar to glutamate receptors (GluRs), GABA receptors are ionotropic (GABA-A) or metabotropic (GABA-B). GABA-A receptors are heteropentameric ligand-gated ion channels that selectively permit the influx of chloride and bicarbonate ions to decrease membrane excitability. Extremely heterologous, with at least nineteen known subunit genes, GABA-A receptors mediate the majority of fast synaptic inhibition. Metabotropic GABA-B receptors are G protein-coupled heterodimers of GABA-B R1 and GABA-B R2. They are expressed on both the presynaptic and postsynaptic terminals where they inhibit neurotransmitter release and induce cell membrane hyperpolarization, respectively.