Acetylcholine promotes directionally biased glutamatergic retinal waves
Spontaneous retinal waves are a critical driving force for the self-organization of the mouse visual system prior to eye-opening. Classically characterized as taking place in three distinct stages defined by their primary excitatory drive, Stage II waves during the first postnatal week are propagated through the volume transmission of acetylcholine while Stage III retinal waves during the second postnatal week depend on glutamatergic transmission from bipolar cells. However, both late Stage II and early Stage III retinal waves share a defining propagation bias toward the temporal-to-nasal direction despite developmental changes in the underlying cholinergic and glutamatergic retinal networks. Here, we leverage genetic and pharmacological manipulations to investigate the relationship between cholinergic and glutamatergic neurotransmission during the transition between Stage II and Stage III waves in vivo. We find that the cholinergic network continues to play a vital role in the propagation of waves during Stage III after the primary mode of neurotransmission changes to glutamate. In the absence of glutamatergic waves, compensatory cholinergic activity persists but lacks the propagation bias typically observed in Stage III waves. In the absence of cholinergic waves, gap junction-mediated activity typically associated with Stage I waves persists throughout the developmental window in which Stage III waves usually emerge and lacks the spatiotemporal profile of normal Stage III waves, including a temporal-to-nasal propagation bias. Finally, we show that cholinergic signaling through {beta}2 subunit-containing nicotinic acetylcholine receptors, essential for Stage II wave propagation, is also critical for Stage III wave directionality.