SUMMARY In the retina, presynaptic inhibitory mechanisms that shape directionally selective (DS) responses in output ganglion cells are well established. However, the nature of inhibition-independent forms of directional selectivity remains poorly defined. Here, we describe a genetically specified set of ON-OFF DS ganglion cells (DSGCs) that code anterior motion. This entire population of DSGCs exhibits asymmetric dendritic arborizations that orientate toward the preferred direction. We demonstrate that morphological asymmetries along with nonlinear dendritic conductances generate a centrifugal (soma-to-dendrite) preference that does not critically depend upon, but works in parallel with the GABAergic circuitry. We also show that in symmetrical DSGCs, such dendritic DS mechanisms are aligned with, or are in opposition to, the inhibitory DS circuitry in distinct dendritic subfields where they differentially interact to promote or weaken directional preferences. Thus, pre- and postsynaptic DS mechanisms interact uniquely in distinct ganglion cell populations, enabling efficient DS coding under diverse conditions.

Key points• In mouse models for retinal degeneration, photoreceptor death leads to membrane oscillation in the remnant AII amacrine-ON cone bipolar cell network through an unknown mechanism.• We found such oscillations require voltage-gated Na + channels and gap junctions but not hyperpolarization-activated currents (I h ).• Na + channels are expressed predominantly in AII amacrine cells and I h in ON cone bipolar cells, and appear to interact via gap junctions to shape oscillations.• Similar intrinsic oscillations arose in the wild-type (wt) AII amacrine-ON cone bipolar cell network when photoreceptor inputs to bipolar cells were pharmacologically occluded.• Computational modelling captures experimental findings when a low level of cellular heterogeneity is introduced in the coupled network.• These unique insights into the cellular mechanisms underlying spontaneous activity in the degenerating retina might aid in designing the most effective strategies to restore vision using retinal prosthesis. AbstractIn 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.

Adult rat parasympathetic ciliary ganglion (CG) neurons were retrogradely labelled by intraocular injection of the carbocyanine fluorescent dye 1,1-dioleyl-3,3,3',3'-tetramethylindocarbocyanine methanesulfonate (DiI). Whole-cell and nystatin perforated patch recording techniques were then used to examine the electrophysiological properties of labelled CG neurons growing in primary culture. The resting membrane potential of CG neurons in dissociated cell culture was -50 +/- 8 mV, and isolated neurons fired overshooting action potentials in response to depolarizing current injection. Voltage-clamp recordings of membrane currents revealed a transient tetrodotoxin-sensitive Na+ inward current and both sustained and transient outward K+ currents. Sustained outward K+ current was reduced (55-77%) by 5 mM tetraethylammonium and to a lesser extent (42-46%) by superfusion with nominally Ca2+ free external solution. Transient outward current was blocked by 100 microns 4-aminopyridine and exhibited steady-state inactivation at potentials depolarized to -50 mV. These data demonstrate that identified adult mammalian CG neurons can be successfully maintained in culture. Cultured CG neurons retain electrical excitability, with voltage-sensitive Na+ and K+ currents giving rise to action potentials.