Proposed antagonists at GABA B receptors that inhibit adenylyl cyclase in cerebellar granule cell cultures of rat

Abstract

The effects of various proposed GABA B receptor antagonists on baclofen-mediated inhibition of adenylyl cyclase were studied in cultured cerebellar granule cells from rat. (±)-Baclofen maximally inhibited adenylyl cyclase by approximately 60% of the basal enzyme activity with an EC 50 value of 10 μM. 3-Aminopropane sulfonic acid (3-APS) and 5-aminovaleric acid (5-AVA) produced similar responsesto that seen with (±)-baclofen. Saclofen reversed the action of (±)-baclofen , 50 μM, with a half maximal inhibitory concentration (IC 50) of about 1.0 mM. The most effective antagonist in blocking the action of (±)-baclofen was 3-aminopropyl-diethoxy-methyl-phosphonic acid (CGP 35 348). In the presence of (±)-baclofen, 50 μM, the IC 50 for CGP 35 348 was 290 μM and its inhibitory constant (K A) was 180 μM. The agonist-like actions of 3-APS and 5-AVA were antagonized by CGP 35 348 suggesting that 3-APS and 5-AVA may act as weak agonists at the GABA B receptor that inhibits adenylyl cyclase. All antagonists tested, except the new compound CGP 35 348, have very low potencies at GABA B receptors that inhibit adenylyl cyclase, though these compounds have been quite effective at other GABA B receptor-mediated events. Thus, the GABA B receptor which inhibits adenylyl cyclase differs pharmacologically from other reported GABA B receptor/effector systems and supports the existence of multiple receptor subtypes.