Spontaneously active networks in the outer and inner degenerated rd10 retina

Abstract

Interaction between spontaneously active networks is prominently present in the central nervous system (CNS). Network interaction at both local and global levels mediates communication across different regions and thus, plays a crucial role in the brain’s physiology (Andrews-Hanna et al., 2010; Oswald et al., 2017). Alterations in the network dynamics have been designated as a hallmark of neuronal dysfunction in various neurodegenerative diseases (Palop et al., 2007; Xu et al., 2015; Wang et al., 2017), signifying their relevance in clinical applications. However, these network activities that seem to be a suitable therapeutic target require a complete understanding towards it at a cellular level, which is still lacking. Similar to other neurological diseases, in a majority of retinal diseases, such as Retinitis Pigmentosa (RP), the degeneration and subsequent loss of photoreceptors generates enhanced spontaneous signals in several retinal neurons (Trenholm et al., 2012; Biswas et al., 2014; Haq et al., 2014). The degenerated retina is thought to form spontaneously active networks in the outer and inner retina (reviewed in Euler and Schubert, 2015). Because these networks were studied only in isolation, it is still unclear whether and how these interact. But since the networks are structurally connected via bipolar cells (BCs), they may modulate each other. Here, we used the primary rod degeneration rd10 mouse model, to investigate the interaction between spontaneously active networks at both somatic and synaptic levels. We established a method of dual recordings to record somatic calcium (Ca2+) signals from the neuronal population in the outer and inner retina using two-photon microscopy. Signal dynamics between the networks displayed spatio-temporal variability, however, the signals became similar in the later degeneration phase. We further assessed the interaction at the synaptic level and examined the role of BCs as potential linking neurons by implementing a vertical imaging technique. This allowed us to record near-simultaneous spontaneous glutamate release at the BC dendrites and axon terminals at the outer (OPL) and inner plexiform layers (IPL), respectively. Disconnecting outer and inner retinal networks by blocking glutamatergic input at the BC dendrites, drastically affected inner retinal signals suggesting that the inner retinal network is largely modulated by the outer retinal signals, most likely via BCs. Finally, our data also revealed the existence of multiple correlated active clusters which might reflect the presence of parallel pathways involved in the generation and propagation of spontaneously active networks in the degenerated retina.