U-89843A IS A NOVEL ALLOSTERIC MODULATOR OF GAMMA-AMINOBUTYRIC ACID(A) RECEPTORS

A group of pyrrolopyrimidine derivatives were examined for their inter action with rat recombinant gamma-aminobutyric acid (GABA), receptors using the whole cell patch damp and equilibrium binding techniques. In the alpha 1 beta 2 gamma 2 subtype of GABA(A) receptors expressed in human embryonic kidney cells, a prototype pyrrolopyrimidine, U-89843A (7H-pyrrolo[2,3-d]pyrimidine,6,7-methyl-2,4-di-1 -pyrrolidinyl,hydroch loride), dose-dependently enhanced 5 mu M GABA-induced Cl- currents wi th a maximal enhancement of 362 +/- 91%, a half-maximal concentration of 2 +/- 0.4 mu M and a slope factor of 1.1 +/- 0.4. The drug also inh ibited [S-35]t-butylbicyclophosphorothionate binding in rat cerebrocor tical membranes with a similar half-maximal inhibitory concentration. The enhancement of Cl- currents by U-89843A was insensitive to Ro 15-1 788 (a benzodiazepine antagonist), was also observed in the alpha 3 be ta 2 gamma 2 and alpha 6 beta 2 gamma 2 subtypes (no selectivity to di fferent alpha-isoforms unlike many benzodiazepines), but was absent in the receptor subtypes consisting of two subunits (alpha 1 beta 2, alp ha 1 gamma 2 and beta 2 gamma 2). it has been known that neurosteroids and barbiturates are uniformly active in both the two subunit recepto rs, substituted pyrazinones are only active in the alpha 1 beta 2 subt ype and loreclezole is active in the subtypes containing beta 2. We pr opose that U-89843A interacts with an allosteric site on GABA(A) recep tors distinct from the sites for benzodiazepines, barbiturates, neuros teroids, substituted pyrazinones or loreclezole. In the mouse, several analogs of U-89843A induced sedation with a rank order of potency ide ntical to that observed for their actions on [S-35]t-butylbicyclophosp horothionate binding and GABA-induced Cl- currents. Moreover, sedation induced by the analogs was not accompanied with a loss of righting re flex, unlike diazepam, barbiturates and neurosteroids. U-89843A appear s to represent a novel class of allosteric modulators of GABA(A) recep tors, pyrrolopyrimidines and may possess a unique therapeutic profile, arising from its interaction with GABA(A) receptors via a novel site which is formed from quaternary associations of the alpha, beta and ga mma subunits.