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

Abstract

The effect of baclofen on the function of the γ-aminobutyric acidA (GABAA) receptor was examined in acutely dissociated neurons of bullfrog dorsal root ganglia (DRG) by using the whole-cell coltage-clamp method. Baclofen (0.1–100 μM) depressed the inward currents produced by GABA (100 μM) and muscimol (100 μM). Baclofen shifted the concentration-response curve for GABA (1 μM-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-benzylphosphinic 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) (GTPγS, 100 μM). Intracellular guanosine 5′-O-(2-thiodiphosphate) (GDPβS, 100 μM) blocked the inhibitory effect of baclofen on the GABA current. Forskolin (10 μM) and dibutyryl N6, 2′-O-dibutyryladenosine 3′:5′-cyclic monophophate (db-cyclic AMP) (200 μM) depressed the GABA current. N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9, 40 μM) and N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004, 50 μM), 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. Synapse 26:165–174, 1997. © 1997 Wiley-Liss, Inc.