Abstract
Purpose :
The narrow-field amacrine cells that contain vesicular glutamate transporter 3 (vGluT3) make a large proportion of their synapses onto ganglion cell dendrites in primate retinas, and they are expected to play important roles in visual information processing. The specific types of neurons that interact with vGluT3 cells had not been identified, and that was the goal of this study.
Methods :
Central retinal tissue from an adult macaque was processed for serial blockface scanning EM, and a volume of images of the inner retina located 2 mm temporal to the center of the fovea was analyzed using Viking software. Neurons were visualized using an open-source Matlab program (SBFSEM-tools) and identified morphologically.
Results :
One vGluT3 cell was reconstructed and identified by its two dense dendritic arbors in the inner plexiform layer (IPL), one centered at 30% of the IPL depth and the other at 50%. The vGluT3 cell made synapses onto midget and parasol of ganglion cells, including both ON and OFF subtypes, and onto a broad thorny ganglion cell. In many instances, wide-field amacrine cells that made synapses onto the vGluT3 cell also made synapses directly onto the postsynaptic ganglion cells, a finding suggesting that the synapses from vGluT3 amacrine cells onto ganglion cells are excitatory. There were synaptic interactions between vGluT3 cells and every subtype of bipolar cell whose axonal arbors overlapped with its dendrites. There were interactions between vGluT3 cells and several other amacrine cell types, including: stellate wavy, wispy, AII and A13.
Conclusions :
Taken together, these findings suggest that vGluT3 cells in primates would respond to both increments and decrements in light intensity in their receptive field centers, and they would have strong inhibitory surrounds, as in other mammalian retinas. The interactions with amacrine cells suggest that the light responses are transient and receive input from the primary rod pathway. Our results may also reconcile two apparently contradictory observations in primate retinas. Parafoveal midget ganglion cells receive numerous synapses from amacrine cells, but little or no inhibitory input is detected in recordings from those cells. We propose that the amacrine cell synapses onto parafoveal midget ganglion cells in the earlier studies may have included excitatory synapses from vGluT3 cells.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.