Characterization of postsynaptic glutamate transporter functionality in the zebrafish retinal first synapse across different wavelengths

In the zebrafish retina, incident light undergoes wavelength-dependent
processing encompassing mechanisms such as color opponency, contrast
enhancement, and motion detection prior to neural transmission to the
brain proper. In darkness, photoreceptors continuously release glutamate
into the synaptic cleft, a process that diminishes in response to
increased light intensity, thereby conveying visual signals to ON and OFF
bipolar cells. Specifically, in zebrafish, the ON pathway signal
transduction is mediated by metabotropic glutamate receptor 6b (mGluR6b)
and Excitatory Amino Acid Transporters (EAATs). Here we demonstrate that
knockout of eaat5b and eaat7 disrupts electroretinogram responses to short
and long-wavelength stimuli while preserving middle-wavelength responses,
suggesting wavelength-specific roles. We found differential expression of
EAAT5b and EAAT7 in the outer plexiform layer, particularly in the strike
zone, crucial for prey capture, supporting task specific involvement of
these signaling pathways. In order to investigate this, we developed a
virtual hunting assay using UV light stimuli. Such a behavioral assay
targeting short and long wavelengths indicate that EAAT5b and EAAT7
influence UV-dependent prey detection and motion sensing differently. Our
findings highlight the importance of EAAT5b and EAAT7 in modulating light
integration dynamics in the zebrafish retina.