Cellular Mechanisms Underlying Embryonic Retinal Waves
Spontaneous activity is a hallmark of developing neural systems. In the retina, spontaneous activity comes in the form of retinal waves, comprised of three stages persisting from embryonic day 16 (E16) to eye opening at postnatal day 14 (P14). Though postnatal retinal waves have been well characterized, little is known about the spatiotemporal properties or the mechanisms mediating embryonic retinal waves, designated Stage 1 waves. Using a custom-built macroscope to record spontaneous calcium transients from whole embryonic retinas, we show that Stage 1 waves are initiated at several locations across the retina and propagate across finite regions of a broad range of areas. A gap junction antagonist, meclofenamic acid, reduced the frequency and size of Stage 1 waves but did not abolish them. The general nAChR antagonist, hexamethonium blocked Stage 1 waves, while they persisted in the presence of 4{beta}2 nAChR antagonist dihydro-{beta}-erythroidine, indicating that the spatiotemporal properties of Stage 1 waves are mediated by a complex circuitry involving subtypes of nAChRs and gap junctions. Stage 1 waves in mice lacking the {beta}2 subunit of the nAChRs ({beta}2-nAChR-KO) were reduced, but in contrast to WT mice, they persisted in the hexamethonium and were completely blocked by meclofenamic acid. To assay the impact of Stage 1 waves on retinal development, we compared the spatial distribution of a subtype of retinal ganglion cells, intrinsically photosensitive retinal ganglion cells (ipRGCs) in WT and {beta}2-nAChR-KO mice. We found that the developmental decrease of ipRGC density is preserved between WT and {beta}2-nAChR-KO mice, indicating that processes regulating ipRGC distribution are not influenced by spontaneous activity.