THE PHARMACOLOGY OF THE BENZODIAZEPINE SITE OF THE GABA-A RECEPTOR IS DEPENDENT ON THE TYPE OF GAMMA-SUBUNIT PRESENT

The pharmacology of native and recombinant GABA-A receptors containing either gamma 1,gamma 2 or gamma 3 subunits has been investigated. The pharmacology of native receptors has been investigated by immunoprecipitating receptors form solubilised preparations of rat brain with antisera for individual gamma-subunits and analysing their radioligand binding characteristics. Receptors containing a gamma 1-subunit do not bind benzodiazepine radioligands with high affinity. Those containing either a gamma 2 or gamma 3 subunit bind [H-3]flumazenil with high affinity. Some compounds compete for these binding sites with multiple affinities, reflecting the presence of populations of receptors containing several different types of alpha-subunit. Photoaffinity-labelling of GABA-A receptors from a cell line stably expressing GABA-A receptors of composition alpha 1 beta 3 gamma 2 followed by immunoprecipitation of individual subunits revealed that the alpha and gamma but not the beta-subunit could be irreversibly labelled by [H-3]flunitrazepam. The properties of recombinant receptors have been investigated in oocytes expressing gamma 1,gamma 2, or gamma 3 subunits in combination with an alpha and a beta-subunit. Some compounds such as zolpidem, DMCM and flunitrazepam show selectivity for receptors containing different gamma-subunits. Others such as Cl 218,872 show no selectivity between receptors containing different alpha-subunits. These data taken together suggest that the benzodiazepine site of the GABA-A receptor is formed with contributions from both the alpha and gamma-subunits.