title: Intrinsic oscillatory activity arising within the electrically coupled AII amacrine-ON cone bipolar cell network is driven by voltage-gated Na+ channels.
abstract: In the rd1 mouse model for retinal degeneration, the loss of photoreceptors results in oscillatory activity (∼10–20 Hz) within the remnant electrically coupled network of retinal ON cone bipolar and AII amacrine cells. We tested the role of hyperpolarization-activated currents (I(h)), voltage-gated Na(+) channels and gap junctions in mediating such oscillatory activity. Blocking I(h) (1 mm Cs(+)) hyperpolarized the network and augmented activity, while antagonizing voltage-dependent Na(+) channels (1 μm TTX) abolished oscillatory activity in the AII amacrine-ON cone bipolar cell network. Voltage-gated Na(+) channels were only observed in AII amacrine cells, implicating these cells as major drivers of activity. Pharmacologically uncoupling the network (200 μm meclofenamic acid (MFA)) blocked oscillations in all cells indicating that Na(+) channels exert their influence over multiple cell types within the network. In wt retina, occluding photoreceptor inputs to bipolar cells (10 μm NBQX and 50 μm l-AP4) resulted in a mild (∼10 mV) hyperpolarization and the induction of oscillatory activity within the AII amacrine-ON cone bipolar cell network. These oscillations had similar properties to those observed in rd1 retina, suggesting that no major degeneration-induced network rewiring is required to trigger spontaneous oscillations. Finally, we constructed a simplified computational model that exhibited Na(+) channel-dependent network oscillations. In this model, mild heterogeneities in channel densities between individual neurons reproduced our experimental findings. These results indicate that TTX-sensitive Na(+) channels in AII amacrine cells trigger degeneration-induced network oscillations, which provide a persistent synaptic drive to downstream remnant neurons, thus appearing to replace photoreceptors as the principal drivers of retinal activity.
authors.affiliation: Department of Anatomy, Dalhousie University, Halifax, NS, Canada B3H 1X5.
authors.forename: Stuart
authors.initials: S
authors.lastname: Trenholm
authors.forename: Joanna
authors.initials: J
authors.lastname: Borowska
authors.forename: Jiawei
authors.initials: J
authors.lastname: Zhang
authors.forename: Alex
authors.initials: A
authors.lastname: Hoggarth
authors.forename: Kyle
authors.initials: K
authors.lastname: Johnson
authors.forename: Steven
authors.initials: S
authors.lastname: Barnes
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Lewis
authors.forename: Gautam B
authors.initials: GB
authors.lastname: Awatramani
mesh_terms: D025042:Amacrine Cells / Q000502:physiology*; D000818:Animals; D017629:Gap Junctions / Q000502:physiology; D008564:Membrane Potentials; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D008954:Models, Biological; D051245:Retinal Bipolar Cells / Q000502:physiology*; D012165:Retinal Ganglion Cells / Q000502:physiology; D015222:Sodium Channels / Q000502:physiology*
chemical_list: D015222:Sodium Channels