(-)-Baclofen and γ-aminobutyric acid inhibit calcium currents in isolated retinal ganglion cells

Of the various synaptic inputs known to converge upon retinal ganglion cells, the major inhibitory inputs are thought to be GABAergic. Although γ-aminobutyric add (GABA) is known to activate anion-selective ion channels in retinal ganglion cells, we have tested the possibility that GABA can also modulate cationic conductances in these cells, as seen in other central and peripheral neurons. Specifically, we have made whole-cell patch-clamp recordings to test whether voltage-gated calcium currents in isolated goldfish retinal ganglion cells are sensitive to GABA_B receptor ligands. (-)-Baclofen and GABA inhibited calcium currents activated by moderately long depolarizations and, during large depolarisations (e.g., to 0 mV), also appeared to accelerate the rate of current decay. The calcium current inhibition induced by (-)-baclofen and GABA was not prevented by 2-hydroxyasclofen, phaclofen, or bicuculline, even though bicuculline suppressed a GABA-activated conductance in these cells. These results demonstrate the presence of baclofen- and GABA-sentive calcium currents in vertebrate retinal ganglion cells as well as the coexistence of GABA_A and GABA_B receptors in individual retinal ganglion cells.