strychnine and Retinal-Degeneration

Retinal ganglion cells (RGCs) spike randomly in the dark and carry information about visual stimuli to the brain via specific spike patterns. However, following photoreceptor loss, both ON and OFF type of RGCs exhibit spontaneous oscillatory spike activity, which reduces the quality of information they can carry. Furthermore, it is not clear how the oscillatory activity would interact with the experimental treatment approaches designed to produce artificial vision. The oscillatory activity is considered to originate in ON-cone bipolar cells, AII amacrine cells, and/or their synaptic interactions. However, it is unknown how the oscillatory activity is generated in OFF RGCs. We tested the hypothesis that oscillatory activity is transferred from the ON pathway to the OFF pathway via the glycinergic AII amacrine cells. Using extracellular loose-patch and whole cell patch recordings, we recorded oscillatory activity in ON and OFF RGCs and studied their response to strychnine, a specific glycine receptor blocker. The cells were labeled with a fluorescent dye, and their dendritic stratification in inner plexiform layer was studied using confocal microscopy. Application of strychnine resulted in abolition of the oscillatory burst activity in OFF RGCs but not in ON RGCs, implying that oscillatory activity is generated in ON pathway and is transferred to OFF pathway, likely via the glycinergic AII amacrine cells. We found oscillatory activity in RGCs as early as postnatal day 12 in rd1 mouse, when rod degeneration has started but cones are still intact. This suggests that the oscillatory activity in rd1 mouse retina originates in rod pathway.

Topics: Action Potentials; Animals; Dendrites; Female; Fluorescent Dyes; Glycine; Glycine Agents; Male; Mice, Inbred CBA; Mice, Mutant Strains; Microscopy, Confocal; Patch-Clamp Techniques; Periodicity; Receptors, Glycine; Retina; Retinal Degeneration; Strychnine

Retinal prosthetic devices are being developed to bypass degenerated retinal photoreceptors by directly activating retinal neurons with electrical stimulation. However, the retinal circuitry that is activated by epiretinal stimulation is not well characterized. Whole-cell patch clamp recordings were obtained from ganglion cells in normal and rd mice using flat-mount and retinal slice preparations. A stimulating electrode was positioned along the ganglion cell side of the preparation at different distances from the stimulated tissue. Pulses of cathodic current evoked action potentials in ganglion cells and less frequently evoked sustained inward currents that appeared synaptic in origin. Sustained currents reversed around E(Cl) and were inhibited by blockade of α-amino-3-hydroxyl-5-methyl-4-isoxazole-proprionate (AMPA)-type glutamate receptors with 2,3-dihydroxy-6-nitro-sulfamoyl-benzo(f)-quinoxaline-2,3-dione (NBQX), γ aminobutyric acid a/c (GABA(a/c)) receptors with picrotoxinin, or glycine receptors with strychnine. This suggests that epiretinal stimulation activates glutamate release from bipolar cell terminals, which in turn evokes release of GABA and glycine from amacrine cells. Synaptic current thresholds were lower in ON ganglion cells than OFF cells, but the modest difference did not attain statistical significance. Synaptic currents were rarely observed in rd mice lacking photoreceptors compared to normal retina. In addition, confocal calcium imaging experiments in normal mice retina slices revealed that epiretinal stimulation evoked calcium increases in the outer plexiform layer. These results imply a contribution from photoreceptor inputs to the synaptic currents observed in ganglion cells. The paucity of synaptic responses in rd mice retina slices suggests that it is better to target retinal ganglion cells directly rather than to attempt to engage the inner retinal circuitry.

Topics: Animals; Biophysics; Calcium; Disease Models, Animal; Electric Stimulation; Evoked Potentials; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA-A Receptor Antagonists; Glycine Agents; In Vitro Techniques; Mice; Mice, Inbred C57BL; Mice, Mutant Strains; Microscopy, Confocal; Patch-Clamp Techniques; Picrotoxin; Quinoxalines; Retina; Retinal Degeneration; Retinal Ganglion Cells; Sesterterpenes; Strychnine; Visual Pathways

Topics: Alkaloids; Brazil; Cardiovascular Agents; Muscle Relaxants, Central; Retinal Degeneration; Retinal Detachment; Strychnine; Strychnos