Circuit 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 a broad range of
areas. A gap junction antagonist, meclofenamic acid, reduced the frequency
and size of Stage 1 waves, nearly abolishing them. The general nAChR
antagonist, hexamethonium similarly nearly abolished Stage 1 waves.
Application of the α4β2 nAChR antagonist dihydro-ß-erythroidine only
slightly reduced the frequency of waves but significantly reduced the
number of cells that participated in waves. Thus, Stage 1 waves are
mediated by a complex circuitry involving subtypes of nAChRs and gap
junctions. Stage 1 waves in mice lacking the β2 subunit of the nAChRs
(β2-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), which undergo a
significant amount of cell death, in WT and β2-nAChR-KO mice. We found
that the developmental decrease of ipRGC density is preserved between WT
and β2-nAChR-KO mice, indicating that processes regulating ipRGC
distribution are not influenced by spontaneous activity.