AbstractThe goals of these experiments were to describe the morphology and synaptic
connections of amacrine cells in the baboon retina that contain immunoreactive
vesicular glutamate transporter 3 (vGluT3). These amacrine cells had the
morphology characteristic of knotty bistratified type 1 cells, and their
dendrites formed two plexuses on either side of the center of the inner
plexiform layer. The primary dendrites received large synapses from amacrine
cells, and the higher-order dendrites were both pre- and postsynaptic to other
amacrine cells. Based on light microscopic immunolabeling results, these include
AII cells and starburst cells, but not the polyaxonal amacrine cells
tracer-coupled to ON parasol ganglion cells. The vGluT3 cells received input
from ON bipolar cells at ribbon synapses and made synapses onto OFF bipolar
cells, including the diffuse DB3a type. Many synapses from vGluT3 cells onto
retinal ganglion cells were observed in both plexuses. At synapses where vGluT3
cells were presynaptic, two types of postsynaptic densities were observed; there
were relatively thin ones characteristic of inhibitory synapses and relatively
thick ones characteristic of excitatory synapses. In the light microscopic
experiments with Neurobiotin-injected ganglion cells, vGluT3 cells made contacts
with midget and parasol ganglion cells, including both ON and OFF types. Puncta
containing immunoreactive gephyrin, an inhibitory synapse marker, were found at
appositions between vGluT3 cells and each of the four types of labeled ganglion
cells. The vGluT3 cells did not have detectable levels of immunoreactive
γ-aminobutyric acid (GABA) or immunoreactive glycine transporter 1.
Thus, the vGluT3 cells would be expected to have ON responses to light and make
synapses onto neurons in both the ON and the OFF pathways. Taken with previous
results, these findings suggest that vGluT3 cells release glycine at some of
their output synapses and glutamate at others.
Functional Characterization of a Vesicular Glutamate Transporter in an Interneuron That Makes Excitatory and Inhibitory Synaptic Connections in a Molluscan Neural Circuit
