L-glutamate: a neurotransmitter candidate for cone photoreceptors in the monkey retina

In order to examine whether L-aspartate and L-glutamate are photoreceptor transmitters in monkey retina, we have carried out two different studies: an autoradiographic localization of the high-affinity uptake sites for aspartate (Asp) and glutamate (Glu), and an immunocytochemical localization of the biosynthetic enzyme, L-aspartate aminotransferase (AAT). Our results show that L-Glu is taken up by a sodium-dependent, high-affinity uptake system with a Km = 8 +/- 3.5 X 10(-6) M and a Vmax = 48 +/- 14 X 10(-12) mol/min/mg protein; 3H-L-Glu or 3H-L- and D-Asp are taken up and accumulated by rod somata and inner segments in retinas incubated at 37 degrees C or at ambient temperature; cones accumulate 3H-L-Glu at ambient temperature but not at 37 degrees C. Neither 3H-L-Asp nor D-Asp is accumulated by cones at either temperature; rods and cones show the same labeling pattern irrespective of whether they are located in the central or peripheral retina; and antiserum to AAT preferentially stains the cone somata, inner segments, and synaptic pedicles, while the outer segments are negative. Both foveal and peripheral cones are strongly reactive, but rods show little reactivity. These findings point to the existence of major differences between rods and cones in the uptake and metabolism of L-Asp and L-Glu and are consistent with L-Glu being a cone transmitter in the monkey retina.