Data from: Spectral inference reveals principal cone-integration rules of the zebrafish inner retina

In the vertebrate retina, bipolar cells integrate the signals from
different cone types at two main sites: directly, via dendritic inputs in
the outer retina, and indirectly, via axonal inputs in the inner retina.
Of these, the functional wiring of the indirect route, involving diverse
amacrine cell circuits, remains largely uncharted. However, because
cone-photoreceptor types differ in their spectral sensitivities, insights
into the total functional cone-integration logic of bipolar cell might be
gained by linking spectral responses across these two populations of
neurons. To explore the feasibility of such a “spectral-circuit-mapping”
approach, we here recorded in vivo responses of bipolar cell presynaptic
terminals in larval zebrafish to widefield but spectrally resolved flashes
of light. We then mapped the results onto the previously established
spectral sensitivity functions of the four cones. We find that this
approach could explain ∼95% of the spectral and temporal variance of
bipolar cell responses by way of a simple linear model that combined
weighted inputs from the cones with four stereotyped temporal components.
This in turn revealed several notable integration rules of the inner
retina. Overall, bipolar cells were dominated by red-cone inputs, often
alongside equal sign inputs from blue- and green-cones. In contrast,
UV-cone inputs were uncorrelated with those of the remaining cones. This
led to a new axis of spectral opponency which was mainly set-up by
red-/green-/blue-cone “Off” circuits connecting to “natively-On” UV-cone
circuits in the outermost fraction of the inner plexiform layer – much as
how key colour opponent circuits are established in mammals. Beyond this,
and despite substantial temporal diversity that was not present in the
cones, bipolar cell spectral tunings were surprisingly simple. They either
approximately resembled both opponent and non-opponent spectral motifs
already present in the cones or exhibited a stereotyped non-opponent
broadband response. In this way, bipolar cells not only preserved the
efficient spectral representations in the cones, but also diversified them
to set up a total of six dominant spectral motifs which included three
axes of spectral opponency. More generally, our results contribute to an
emerging understanding of how retinal circuits for colour vision in
ancestral cone-tetrachromats such as zebrafish may be linked to those
found in mammals.