Benzodiazepine receptors are coupled to a subpopulation of gamma-aminobutyric acid (GABA) receptors: evidence from a quantitative autoradiographic study.

The colocalization and interaction of gamma-aminobutyric acid (GABA) and benzodiazepine (BZ) receptors in the rat brain were characterized using standardized, quantitative, light microscopic autoradiographic methods. In serial sections, striking differences were observed in the distribution of high affinity GABA and BZ receptors in areas such as the cerebral cortex, globus pallidus, thalamus, hypothalamus and cerebellar cortex. However, in a semiquantitative visual examination of more than 200 brain regions, added exogenous GABA increased BZ binding in all regions. In a quantitative analysis of 19 regions, exogenous GABA uniformly stimulated [3H]flunitrazepam binding, the effect being proportional to the regional density of BZ receptors. No relationship was seen between the magnitude of the stimulation and the distribution of high affinity GABA receptors. In the mounted tissue sections, BZ binding appeared influenced by endogenous GABA since it was reduced by preincubation or by the addition of bicuculline. Taken together, these data suggest that most or all BZ receptors can be influenced by GABA and are coupled to a type of GABA receptor. However, the BZ-linked GABA receptor could represent either a subpopulation of GABA binding sites or a distinct receptor not labeled under the conditions used in these and other experiments.