Invivo electroretinographic studies of the role of GABA_C receptors in retinal signal processing

All three classes of receptors for the inhibitory neurotransmitter GABA (GABAR) are expressed in the retina. This study investigated roles of GABAR, especially GABA_CR (GABA(A)-ρ), in retinal signaling invivo by studying effects on the mouse electroretinogram (ERG) of genetic deletion of GABA_CR versus pharmacological blockade using receptor antagonists. Brief full-field flash ERGs were recorded from anesthetized GABA_CR^-/- mice, and WT C57BL/6 (B6) mice, before and after intravitreal injection of GABA_CR antagonists, TPMPA, 3-APMPA, or the more recently developed 2-AEMP; GABA_AR antagonist, SR95531; GABA_BR antagonist, CGP, and agonist, baclofen. Intravitreal injections of TPMPA and SR95531 were also made in Brown Norway rats. The effect of 2-AEMP on GABA-induced current was tested directly in isolated rat rod bipolar cells, and 2-AEMP was found to preferentially block GABA_CR in those cells. Maximum amplitudes of dark (DA) and light-adapted (LA) ERG b-waves were reduced in GABA_CR^-/- mice, compared to B6 mice, by 30-60%; a-waves were unaltered and oscillatory potential amplitudes were increased. In B6 mice, after injection of TPMPA (also in rats), 3-APMPA or 2-AEMP, ERGs became similar to ERGs of GABA_CR^-/- mice. Blockade of GABA_ARs and GABA_BRs, or agonism of GABA_BRs did not alter B6 DA b-wave amplitude. The negative scotopic threshold response (nSTR) was slightly less sensitive in GABA_CR^-/- than in B6 mice, and unaltered by 2-AEMP. However, amplitudes of nSTR and photopic negative response (PhNR), both of which originate from inner retina, were enhanced by TPMPA and 3-APMPA, each of which has GABA_B agonist properties, and further increased by baclofen. The finding that genetic deletion of GABA_CR, the GABA_CR antagonist 2-AEMP, and other antagonists all reduced ERG b-wave amplitude, supports a role for GABA_CR in determining the maximum response amplitude of bipolar cells contributing to the b-wave. GABA_CR antagonists differed in their effects on nSTR and PhNR; antagonists with GABA_B agonist properties enhanced light-driven responses whereas 2-AEMP did not.