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.

Starburst amacrine cells release GABA and ACh. This study explores the coordinated function of starburst-mediated cholinergic excitation and GABAergic inhibition to bistratified retinal ganglion cells, predominantly direction-selective ganglion cells (DSGCs). In rat retina, under our recording conditions, starbursts were found to provide the major excitatory drive to a sub-population of ganglion cells whose dendrites co-stratify with starburst dendrites (putative DSGCs). In mouse retina, recordings from genetically identified DSGCs at physiological temperatures reveal that ACh inputs dominate the response to small spot-high contrast light stimuli, with preferential addition of bipolar cell input shifting the balance towards glutamate for larger spot stimuli In addition, starbursts also appear to gate glutamatergic excitation to DSGCs by postsynaptic and possibly presynaptic inhibitory processes ABSTRACT: Starburst amacrine cells release both GABA and ACh, allowing them to simultaneously mediate inhibition and excitation. However, the precise pre- and postsynaptic targets for ACh and GABA remain under intense investigation. Most previous studies have focused on starburst-mediated postsynaptic GABAergic inhibition and its role in the formation of directional selectivity in ganglion cells. However, the significance of postsynaptic cholinergic excitation is only beginning to be appreciated. Here, we found that light-evoked responses measured in bi-stratified rat ganglion cells with dendrites that co-fasciculate with ON and OFF starburst dendrites (putative direction-selective ganglion cells, DSGCs) were abolished by the application of nicotinic receptor antagonists, suggesting ACh could act as the primary source of excitation. Recording from genetically labelled DSGCs in mouse retina at physiological temperatures revealed that cholinergic synaptic inputs dominated the excitation for high contrast stimuli only when the size of the stimulus was small. Canonical glutamatergic inputs mediated by bipolar cells were prominent when GABA/glycine receptors were blocked or when larger spot stimuli were utilized. In mouse DSGCs, bipolar cell excitation could also be unmasked through the activation of mGluR2,3 receptors, which we show suppresses starburst output, suggesting that GABA from starbursts serves to inhibit bipolar cell signals in DSGCs. Taken together, these results suggest that starbursts amplify excitatory signals traversing the retina, endowing DSGCs with the ability to encode fine spatial information without compromising their ability to encode direction.