The neutral amino acid, 4-aminobutanoic acid (GABA), is an inhibitory transmitter in the central nervous system (CNS). Furthermore, GABA is involved as a neurotransmitter and/or a paracrine effector in the regulation of a variety of physiological mechanisms in the periphery. Some of these latter functions may be under central GABA control, whereas others are managed by local GABA neurons. A large percentage, perhaps the majority, of central neurons are under GABA control. The complex mechanisms underlying the GABA-mediated neurotransmission have been extensively studied using a broad spectrum of electrophysiological, neurochemical, pharmacological, and, in recent years, molecular biological techniques (Curtis and Johnston, 1974; Krnjevic, 1974; Olsen and Venter, 1986; Redburn and Schousboe, 1987; Bowery and Nistico, 1989; Biggio and Costa, 1990; Bowery et al., 1990; Schousboe et al., 1992a; Doble and Martin, 1996; Krogsgaard-Larsen et al., 1997).