Baclofen reduces GABAA receptor responses in acutely dissociated neurons of bullfrog dorsal root ganglia.

The effect of baclofen on the function of the gamma-aminobutyric acidA (GABAA) receptor was examined in acutely dissociated neurons of bullfrog dorsal root ganaglia (DRG) by using the whole-cell voltage-clamp method. Baclofen (0.1-100 microM) depressed the inward currents produced by GABA (100 microM) and muscimol (100 microM). Baclofen shifted the concentration-response curve for GABA (1 microM-1 mM) downward. Baclofen decreased the maximum response (Vmax) to GABA without changing the apparent dissociation constant (Kd), suggesting a noncompetitive antagonism. The effect of baclofen on the GABA current was blocked by antagonists for the GABAB receptor; the rank order of potency was P-[3-Aminopropyl]-P-diethoxymethylphosphinic acid (CGP 55845A) > > 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P- benzyl-phosphinic acid (CGP 35348) > saclofen > > phaclofen. Baclofen produced an irreversible depression of the GABA current in neurons dialyzed with an internal solution containing guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S, 100 microM). Intracellular guanosine 5'-O-(2-thiodiphosphate) (GDP beta S, 100 microM) blocked the inhibitory effect of baclofen on the GABA current. Forskolin (10 microM) and dibutyryl N6, 2'-O-dibutyryladenosine 3':5'-cyclic monophophate (db-cyclic AMP) (200 microM) depressed the GABA current. N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9, 40 microM) and N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004, 50 microM), protein kinase 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 GABAB receptors regulate the GABAA receptor function through a G-protein linked to the adenylyl cyclase-PKA pathway in bullfrog DRG neurons.