BACLOFEN REDUCES GABA(A) RECEPTOR RESPONSES IN ACUTELY DISSOCIATED NEURONS OF BULLFROG DORSAL-ROOT GANGLIA

The effect of baclofen on the function of the gamma-aminobutyric acid( A) (GABA(A)) receptor was examined in acutely dissociated neurons of b ullfrog dorsal root ganglia (DRG) by using the whole-cell voltage-clam p method. Baclofen (0.1-100 mu M) depressed the inward currents produc ed by GABA (100 mu M) and muscimol (100 mu M) Baclofen shifted the con centration-response curve for GABA (1 mu M-1 mM) downward. Baclofen de creased the maximum response (V-max) to GABA without changing the appa rent dissociation constant (K-d), suggesting a noncompetitive antagoni sm. The effect of baclofen on the GABA current was blocked by antagoni sts for the GABA(B) receptor; the rank order of potency was P-[3-Amino propyl]-P-diethoxymethylphosphinic acid (CGP 55845A) >> -dichloropheny l)ethyl]amino-2-(S)-hydroxypropyl-P- benzylphosphinic acid (CGP 35348) > saclofen >> phaclofen. Baclofen produced an irreversible depression of the GABA current in neurons dialyzed with an internal solution con taining guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S, 100 mu M). I ntracellular guanosine 5'-O-(2-thiodiphosphate) (GDP beta S, 100 mu M) blocked the inhibitory effect of baclofen on the GABA current. Forsko lin (10 mu M) and dibutyryl N-6, 2'-O-dibutyryladenosine 3':5'-cyclic monophophate (db-cyclic AMP) (200 mu M) depressed the GABA current. N- (2-aminoethyl)-5-isoquinolinesulfonamide (H-9, 40 mu M) and N-(2-guani dinoethyl)-5-isoquinolinesulfonamide (HA-1004, 50 mu M), protein kinas e A (PKA) inhibitors, reduced the depressant effect of baclofen on the GABA current. The baclofen-induced depression of the GABA current was blocked by PKI(5-24), a specific PKA inhibitor, but not by PKC(19-36) , a specific protein kinase C (PKC) inhibitor. We suggest that GABA(B) receptors regulate the GABA(A) receptor function through a G-protein linked to the adenylyl cyclase-PKA. pathway in bullfrog DRG neurons. ( C) 1997 Wiley-Liss, Inc.