ABSTRACT
In the adult nervous system, gamma-aminobutyric acid (GABA) is an inhibitory transmitter that regulates neuronal excitability (Barker and Nicoll, 1972; Roberts, 1986). However, in the developing nervous system, GABA is the first neurotransmitter to be expressed, and its initial role is excitatory. The release of GABA causes an elevation of ionic calcium at the synaptic site (Yuste and Katz, 1991), which is essential to the differentiation and stabilization of synapses at the time that excitatory glutamatergic connections are still immature (Hosokawa et al., 1994; for review, see Cherubini et al., 1991). GABA also plays a role as a neurotrophic factor (Spoerri, 1987; for review, see Lauder, 1993) in supporting neuronal proliferation, growth, and migration (Antonopoulos et al., 1997; Behar et al., 1998). The early developmental expression of GABA or its synthesizing enzyme, glutamic acid decarboxylase (GAD), is transitory and usually fades before the differentiation of the mature pattern of GABAergic neurons (W.-J. Gao et al., 1999; also, for review, see Sandell, 1998).
