Abstract
Purpose :
To investigate the role of trβ2/L-opsin expression on cone synaptic transmission we studied ERG b-waves from transgenic larvae where the thyroid nuclear receptor trβ2, responsible for the expression of L-opsin in red cones, is expressed in all differentiated cone types, producing mixed opsin cones (Suzuki et al., 2013).
Methods :
Sibling gnat2:mYFP-2A-trβ2 and wild type (WT) were distinguished by transgenic mYFP fluorescence in the pupil and pineal gland. Dissected eyes were perfused with oxygenated MEM containing either 20mM Na Aspartate or 50µM CNQX. L-Aspartate blocks post-synaptic glutamatergic receptors and isolates cone PIII responses, while CNQX blocks AMPA/kainate receptors, isolating ON-bipolar-cell b2 responses. ERG responses were recorded at 9 wavelengths (330-650nm) with either infrared (IR) or red (627nm) backgrounds. Spectra were fit by a ‘sum of Hill functions’ model (Nelson & Singla, 2009). Spectral sensitivity from ERG b2 responses were divided by those for PIII responses as a measure of cone signal amplification by ON-bipolar cells.
Results :
At 5dpf, the IR-adapted b2 ERG of WT larvae was more sensitive across all wavelengths than gnat2:trβ2. The b2 spectral shapes were similar . Red backgrounds increased WT b2-sensitivity at short wavelengths, as previously reported (Balraj & Nelson, 2017), but decreased gnat2: trβ2, b2 sensitivity across the spectrum. In WT, b2 sensitivity decreased slightly between 5dpf and 12dpf, while gnat2:trβ2 b2 sensitivity increased. At 12dpf red-adapted WT spectra were more sensitive than IR-adapted, but gnat2:trβ2 spectra were less sensitive. At 5dpf dark-adapted b2/PIII amplification was low (0.3-1.0) in both genotypes (Fig. 8A). With red adaptation, amplification increased to 9 in WT at 530-650nm, but in gnat2:trb2, it increased only to 2. At 12dpf IR adapted b2/PIII amplification increased slightly (1.0-3.0) in both genotypes. With red adaptation, long-wavelength amplification increased, to 24 in WT, but only to 10 in gnat2:trb2.
Conclusions :
In gnat2:trb2, b-wave sensitivity is developmentally delayed, and the ability of the b-wave to adapt to red backgrounds is severely depressed. We theorize that native dark-adapted L-opsin depresses synaptic transmission across cone types, and that the excess L-opsin in the mixed opsin cones of gnat2:trβ2 further inhibits synaptic function.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.