The major inhibitory
neurotransmitters GABA and
glycine provide the bulk of input to large-field
ganglion cells in the retina. Whole-cell patch-clamp recordings were used to characterize the
glycine- and
GABA-activated currents for morphologically identified ON-alpha
ganglion cells in the rabbit retina. Cells identified as ON-alpha cells by light evoked currents were intracellularly stained and examined by light microscopy which revealed dendritic stratification in the vitreal half of the inner plexiform layer and confirmed their physiological identity. All Ca(2+)-mediated synaptic influences were abolished with Co(2+), revealing two types of ON-alpha cell characterized by their different inhibitory current profiles. One group exhibited larger
glycine- than
GABA-activated currents, while the other group had larger
GABA- than
glycine-activated currents. Both cell types demonstrated
strychnine-sensitive
glycine-activated currents and
bicuculline-sensitive GABAA-activated currents. Surprisingly, both cell types expressed functional
GABAC receptors demonstrated by their sensitivity to TPMPA. In addition, the cells with larger
glycine-activated currents also possessed GABAB receptors, whereas those with larger
GABA-activated currents did not. Immunocytochemical experiments confirmed the presence of
glycine, GABAA, and GABAC receptor subunits on all physiologically identified ON-alpha
ganglion cells in this study. In addition, the GABAB receptor immunolabeled puncta were present on the cells with larger
glycine-activated currents, but not on the cells with the larger
GABA-activated currents. In conclusion, the presence of different functional
GABA and
glycine receptors determined physiologically correlated well with the specific
GABA and
glycine receptor immunolabeling for two neuropharmacological types of rabbit ON-alpha
ganglion cells.