Primate retinas contain two major
ganglion cell types. Midget (or P type) 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 further subdivided into a and b subtypes, according to the level of their 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 whole-cell, patch-clamp recordings from identified
ganglion cells in slice preparations of macaque and baboon retinas. We found that midget and parasol cells of both a and b types had similar responses to
excitatory amino acids, including
kainate, alpha-amino-3-hydroxy-5-methylisoxalzole-4-propionic
acid, and
N-methyl-D-aspartate, with reversal potentials 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 types of
ganglion cells also had similar responses to bath-applied inhibitory
amino acids. All cells had both
gamma-aminobutyric acid and
glycine receptors with reversal potentials near the equilibrium potential for Cl-, and the relative amplitudes of the responses to excitatory and inhibitory
amino acids were similar among the various cell types. These results suggest that the differences in response properties of the different classes of
ganglion cells in primate retina may be determined, to a significant degree, by the properties of the amacrine and bipolar cells that provide their input rather than by the nature of their postsynaptic receptors.