PHARMACOLOGY OF THE GABA(B) RECEPTOR IN AMPHIBIAN RETINA

Amacrine and ganglion cells in the amphibian retina contain GABA(B), a s well as GABA(A), receptors. Baclofen, a GABA(B) agonist, hyperpolari zes the dark membrane potential of these third order neurons and makes their light responses more transient. GABA(B) receptors in the retina have a similar agonist profile to GABA(B) receptors described at othe r sites in the brain. Namely, preferential activation by the R-enantio mer of baclofen, and agonist sensitivity in the order 3-aminopropylpho sphinic acid > baclofen >> 3 -aminopropylphosphonic acid. The GABA(B) receptor was not activated by 4-aminobutylphosphonic acid. Several ant agonists, such as phaclofen, saclofen, and 2-hydroxysaclofen, were ine ffective in the amphibian retina. However, CGP35348 blocked the action of applied baclofen and produced effects on the light response that w ere opposite to those of baclofen. Applied agonists and antagonists su pport the hypothesis that GABA(B) receptors serve to regulate the bala nce of sustained and transient signals to the inner retina.