FUNCTIONAL COMPARISON OF THE ROLE OF GAMMA-SUBUNITS IN RECOMBINANT HUMAN GAMMA-AMINOBUTYRIC ACID(A) BENZODIAZEPINE RECEPTORS

The effect of benzodiazepines on the activity of gamma-aminobutyric ac id (GABA)A receptors has been shown to be influenced by different alph a subunits and can also be affected by the presence of different gamma subunits. Previous studies have shown that receptors without a gamma subunit or those containing gamma1 are modulated to a lesser degree by benzodiazepines. Using the Xenopus oocyte expression system to expres s different subunit combinations, a detailed analysis of the pharmacol ogical modulation of GABA(A) receptors by various benzodiazepine site ligands has been carried out. We analyzed 14 compounds, varying throug h full agonist, partial agonist, antagonist, and inverse agonist, with receptors consisting of alpha2beta1, alpha2beta1gamma2S, and alpha2be ta1gamma1 and we demonstrate differences in their extent of potentiati on by different benzodiazepine-type ligands. Most compounds showed neg ligible effects on alpha2beta1 and most agonists, particularly the imi dazopyridines zolpidem, alpidem, and AHR14,749, exhibited less potenti ation with alpha2beta1gamma1 than with alpha2beta1gamma2S. The inverse agonists dimethoxy-4-ethyl-beta-carboline-3-carboxylate and Ro154513 did not act as inverse agonists and produced slight potentiation of al pha2beta1gamma1 receptors. Concentration-response curves were construc ted for five selected agonists to evaluate both affinity and efficacy differences between alpha2beta1gamma2 and alpha2beta1gamma1 receptors. Most compounds showed lower efficacy and up to 10-fold lower affinity with alpha2beta1gamma1. Zolpidem showed slightly higher affinity but an extremely low efficacy; FG8205 also showed a markedly lower efficac y and was the most selective compound for alpha2beta1gamma2S versus al pha2beta1gamma1 receptors. CL218,872 showed high efficacy with alpha2b eta1gammal and affinity similar to that with alpha2beta1gamma2 (being the least selective compound), suggesting that some low efficacy parti al agonists with gamma2-containing receptors may be more efficacious w ith gamma1-containing receptors. The antagonists Ro15-1788 and CGS8216 , although they blocked flunitrazepam potentiation of alpha2beta1gamma 2, could not block potentiation of alpha2beta1gamma1. This study demon strates that unique pharmacological profiles can be conferred by recep tors containing different gamma subunits.