DIFFERENTIAL COUPLING OF G-PROTEIN ALPHA-SUBUNITS TO 7-HELIX RECEPTORS EXPRESSED IN XENOPUS OOCYTES

Xenopus oocytes were used to examine the coupling of the serotonin 1c (5HT1c) and thyrotropin-releasing hormone (TRH) receptors to both endo genous and heterologously expressed G protein alpha subunits. Expressi on of either G protein-coupled receptor resulted in agonist-induced, C a2+-activated Cl- currents that were measured using a two-electrode vo ltage clamp. 5HT-induced Cl- currents were reduced 80% by incubating t he injected oocytes with pertussis toxin (PTX) and inhibited 50-65% by injection of antisense oligonucleotides to the PTX-sensitive Go alpha subunit. TRH-induced Cl- currents were reduced only 20% by PTX treatm ent but were inhibited 60% by injection of antisense oligonucleotides to the PTX-insensitive Gq alpha subunit. Injection of antisense oligon ucleotides to a novel Xenopus phospholipase C-beta inhibited the 5HT1c (and Go)-induced Cl- current with little effect on the TRH (and Gq)-i nduced current. These results suggest that receptor-activated Go and G q interact with different effecters, most likely different isoforms of phospholipase C-beta. Go-expression of each receptor with seven diffe rent mammalian G protein alpha subunit cRNAs (Goa, Gob, Gq, G11, Gs, G olf, and Gt) was also examined. Co-expression of either receptor with the first four of these G alpha subunits resulted in a maximum 4-6-fol d increase in Cl- currents; the increase depended on the amount of G a lpha subunit cRNA injected. This increase was blocked by PTX for G alp ha oa and G alpha ob co-expression but not for G alpha q or G alpha 11 co-expression. Co-expression of either receptor with Gs, Golf, or Gt had no effect on Ca2+-activated Cl- currents; furthermore, co-expressi on with Gs or Golf also failed to reveal 5HT- or TRH-induced changes i n adenylyl cyclase as assessed by activation of the cystic fibrosis tr ansmembrane conductance regulator Cl- channel. These results indicate that in oocytes, the 5HT1c and TRH receptors do the following: 1) pref erentially couple to PTX-sensitive (Go) and PTX-insensitive (Gq) G pro teins and that these G proteins act on different effectors, 2) couple within the same cell type to several different heterologously expresse d G protein alpha subunits to activate the oocyte's endogenous Cl- cur rent, and 3) fail to couple to G protein alpha subunits that activate cAMP or phosphodiesterase.