AMINO-ACID RECEPTORS OF MIDGET AND PARASOL GANGLION-CELLS IN PRIMATE RETINA

Primate retinas contain two major ganglion cell types. Midget (or P ty pe) cells have relatively sustained responses to light; the amplitude and polarity of these responses vary with stimulus wavelength. Parasol (or M type) cells are more sensitive to stimulus contrast and respond more transiently but are not selective for color. Both types can be f urther subdivided into a and b subtypes, according to the level of the ir dendritic stratification in the inner plexiform layer. To determine whether differences in receptors for amino acid transmitters are the basis for any differences in ganglion cell light responses, we made wh ole-cell, patch-clamp recordings from identified ganglion cells in sli ce preparations of macaque and baboon retinas. We found that midget an d parasol cells of both a and b types had similar responses to excitat ory amino acids, including kainate, pha-amino-3-hydroxy-5-methylisoxal zole-4-propionic acid, and N-methyl-D-aspartate, with reversal potenti als near the equilibrium potential for cations. Kainate responses were blocked by 6,7-dinitroquinoxaline, and N-methyl-D-aspartate responses were blocked by D-(-)-2-amino-7-phosphonoheptanoic acid. The four typ es of ganglion cells also had similar responses to bath-applied inhibi tory amino acids. All cells had both gamma-aminobutyric acid and glyci ne receptors with reversal potentials near the equilibrium potential f or Cl-, and the relative amplitudes of the responses to excitatory and inhibitory amino acids were similar among the various cell types. The se results suggest that the differences in response properties of the different classes of ganglion cells in primate retina may be determine d, to a significant degree, by the properties of the amacrine and bipo lar cells that provide their input rather than by the nature of their postsynaptic receptors.