GABA B receptors expressed in Xenopus oocytes by guinea pig cerebral mRNA are functionally coupled with Ca 2+-dependent Cl − channels and with K + channels, through GTP-binding proteins

Abstract

Transmembrane currents induced by (−)-baclofen (BAC), a specific agonist of the γ-aminobutyric acid-B 9GABA B) receptor, in Xenopus oocytes injected with guinea pig cerebral mRNA were electrophysiologically and pharmacologically characterized under a voltage-clamp condition. The oocytes injected with mRNA acquired responsiveness to BAC and showed two types of currents at a holding potential of −50 mV. One was the slow and smooth inward current which had a short lateny and associated with a decrease in membrane conductance, and its amplitude was decreased by hyperpolarization and increased by depolarization. The other was the large fast oscillatory inward current with a long-latency, which was decreased in amplitude by depolarization and reversed at −26 mV. Both currents were not blocked by bicuculline but were depressed by 2-hydroxysaclofen (2-OH-SAC), though the smooth current was less sensitive to 2-OH-SAC; about 40% blockade at the 2-OH-SAC concentration capable of abolishing the oscillatory current. The smooth current was depressed by Ba 2+. The intracellular injection of EGTA into oocytes abolished the oscillatory current but did not affect the smooth current. The oscillatory current was time-dependently attenuated and almost abolished by intracellularly injected pertussis toxin (PTX), while the smooth current was not depressed by this toxin even when the oscillatory current was nearly abolished. The intracellular injection of GTP-γ-S into oocytes attenuated both oscillatory and smooth currents. These results suggest the possibility that GABA B receptors expressed in Xenopus oocytes by cerebral mRNA are functionally coupled with two signal transduciton systems, one is the opening of Ca 2+-dependent cl − channels mediated by PTX_sensitive GTP-binding protein(s) and the other is the closure of K + channels through PTX-insensitive GTP-binding protein(s).