Functional, molecular and morphological heterogeneity of superficial interneurons in the larval zebrafish tectum

Abstract

The superficial interneurons, SINs, of the zebrafish tectum, have been implicated in a range of visual functions, including size discrimination, directional selectivity, and looming‐evoked escape. This raises the question if SIN subpopulations, despite their morphological similarities and shared anatomical position in the retinotectal processing stream, carry out diverse, task‐specific functions in visual processing, or if they have simple tuning properties in common. Here we have further characterized the SINs through functional imaging, electrophysiological recordings, and neurotransmitter typing in two transgenic lines, the widely used Gal4s1156t and the recently reported LCRRH2‐RH2‐2:GFP. We found that about a third of the SINs strongly responded to changes in whole‐field light levels, with a strong preference for OFF over ON stimuli. Interestingly, individual SINs were selectively tuned to a diverse range of narrow luminance decrements. Overall responses to whole‐field luminance steps did not vary with the position of the SIN cell body along the depth of the tectal neuropil or with the orientation of its neurites. We ruled out the possibility that intrinsic photosensitivity of Gal4s1156t+ SINs contribute to the measured visual responses. We found that, while most SINs express GABAergic markers, a substantial minority express an excitatory neuronal marker, the vesicular glutamate transporter, expanding the possible roles of SIN function in the tectal circuitry. In conclusion, SINs represent a molecularly, morphologically, and functionally heterogeneous class of interneurons, with subpopulations that detect a range of specific visual features, to which we have now added narrow luminance decrements.

Single-Cell RNA Sequencing Characterizes the Molecular Heterogeneity of the Larval Zebrafish Optic Tectum

“…Three of those 13 lines were further surveyed, resulting in the identification of multiple neuronal morphologies closely resembling those previously found in the adult goldfish optic tectum 23 . Further tectal diversity studies have resulted in the characterization of previously unidentified cell types within the optic tectum, as well as cell types that corroborate previous studies in other teleosts [24][25][26][27] . These cell type diversity studies (which combine functional imaging, electrophysiological recordings, and neurotransmitter typing) together with the recent publication of a larval zebrafish brain atlas 28 constitute a wealth of knowledge surrounding the zebrafish optic tectum.…”

“…We found few cholinergic neurons; however, two developing populations without a known neurotransmitter identity show expression of id2b, thus we hypothesize these populations may have PyrN, TLPN, or TGPN morphologies. Superficial interneurons (SINs) have also been described as primarily GABAergic and receiving glutamatergic input directly from retinal ganglion cells 27 . Of the eight populations in our data that appear to contain mature neurons, four are GABAergic and synapse with glutamatergic neurons.…”

Blind cavefish retain functional connectivity in the tectum despite loss of retinal input

“…In teleost fish, the optic tectum is the main visual processing region, it is involved in the detection and processing of visual stimuli to generate goal-directed motor behaviors such as prey capture (62). Similar to our findings in A. mexicanus, numerous cellular response types have been previously identified in the zebrafish tectum, including those which respond selectively to changes in light level (63). We identified shared cellular response types across surface and both cave populations of A. mexicanus, though the number and sensitivity of responsive cells was severely reduced across cave populations.…”

“…Previous studies have identified a laminar organization of the zebrafish tectum, with different layers corresponding to distinct functional specializations (27,65,66). Numerous cellular response types have been previously identified in zebrafish, including those which respond selectively to changes in light level (63). Their identification in A. mexicanus provides an opportunity to investigate the genetic basis of tectal wiring through hybridization experiments.…”

“…light level (63). Their identification in A. mexicanus provides an opportunity to investigate the genetic basis of tectal wiring through hybridization experiments.…”