GABA receptor multiplicity. Visualization of different receptor types in the mammalian CNS.

Receptors for GABA in the mammalian brain are not homogeneous. A clear separation exists between receptors which recognize the antagonist bicuculline and a population which does not. These two classes have been designated GABAA and GABAB sites respectively. Within the GABAA category there may also be many subtypes which exhibit subtle pharmacological differences. Numerous centrally-active agents influence GABAA site function generally via an allosteric interaction. By comparison very few substances currently available interact with GABAB sites. The GABAA and GABAB sites show many contrasting characteristics not least of which is their distribution pattern within the rat brain. Autoradiographic analysis has indicated that although both receptors may be present within many regions, in some areas only one type is present. For example, GABAA sites only are present in the lamina molecularis of the olfactory bulb and granule cell layer of the cerebellum whereas GABAB sites are present in the interpeduncular nucleus without any evidence of GABAA sites. In the spinal cord GABAB sites are unevenly distributed with high densities in laminae I-IV. GABAA sites are more uniformly distributed throughout the dorsal and ventral horns. In conclusion, there is now good evidence for multiple GABA receptors and the way is open to determine the functional significance of the GABAB receptor in relation to the now classical GABAA site.