Non-Neuronal GABA Systems: An Overview

Free amino acids are substrates for numerous metabolic pathways in mammals and, as such, they were once considered to be of primarily metabolic relevance. Therefore, the discovery of γ-aminobutyric acid (GABA) and its specific inhibitory action in the central nervous system (CNS) 40 years ago [6,65,73] evoked significant attention. Comprehensive studies during the following two decades revealed that GABA is a major transmitter of synaptic inhibition in the vertebrate CNS [52]. In 1979, Jessen et al. provided partial evidence to show that this role is not restricted to the CNS, but that GABA may also be a neurotransmitter in the peripheral nervous system (PNS), namely, in the myenteric plexus of the intestine [45]. However the general recognition of GABA’s central and peripheral neurotransmitter function, led, until recently, to the neglect of its possible extraneuronal functions. This chapter is, therefore, an attempt to summarize the recent knowledge of non-neuronal GABA systems in mammals and to emphasize that GABA may play a specific mediator role in a number of non-neuronal tissues.