Abstract
Purpose :
Thyroid hormone receptor beta 2 (trβ2) is necessary for red cone development in vertebrates. We investigated how the rest of the retinal circuit adjusts functionally to this loss of red cones. We recorded ON-bipolar ERGs and ganglion cell spikes in a crispr mutant zebrafish that lacks functional trβ2.
Methods :
Larval eyes at ages 5, 6, and 7 days post fertilization were isolated from spawns of mutant or wild type adults. To isolate ON-bipolar responses, eyes were perfused with MEM and CNQX during ERG recordings. No drugs were added to MEM to record ganglion cell spikes. The eye was exposed to 9 wavelengths of light (330-650nm) at 7 different irradiances. ON-bipolar responses were fit to a 4 cone model. Ganglion cell responses were fit to a 7 cone input model.
Results :
The contribution of each cone to the overall signal is broken down into Vr, Vg, Vb, and Vu for ON-bipolar cells. Mutants had a significantly lower red cone contribution (0.017±0.010) than wild types (0.391±0.009, t-test p<0.0001). Mutants also had significantly increased contributions from the green (0.373±0.032) and UV (0.362±0.027) cones as compared to wild types (Vg: 0.183±0.026, Vu: 0.230±0.021, t-test p<0.0001). Single cell spike recordings showed that the mutant ganglion cells did not have an ON or OFF input originating from 575nm or 556nm opsin cones, in contrast to wild types. Mutant and wild type ganglion cells receive excitatory and inhibitory inputs, but mutants had a maximum of five opsin inputs while wild types reached seven.
Conclusions :
A loss of functional trβ2 leads to alterations in retinal circuitry. Without trβ2 there is a significant increase in green cone contribution to ON-bipolar cell responses. Amplification from cones to bipolars may contribute to the increase in mid-wavelength responses as compared to the PIII cone responses. The increase in UV response in the cone layer is maintained in the ON-bipolar layer suggesting an increase in either the number of UV cones or in sensitivity. In the ganglion cell layer the loss of red response is maintained, but there are still excitatory and inhibitory inputs from the five other opsin types. There is also likely a regulatory relationship between trβ2 or red cones and UV cones since the increase in UV cone contribution occurs in the mutant. Aside from the loss of red cones, the circuit is functional and forms the expected connections from all other cone types.
This is a 2020 ARVO Annual Meeting abstract.