Journal of Neuroscience, Vol 6, 637-643, Copyright © 1986 by Society for Neuroscience
L-glutamate: a neurotransmitter candidate for cone photoreceptors in the monkey retina
PV Sarthy, AE Hendrickson and JY Wu
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.