The mechanism of retinal oscillations in Trpm1 KO mouse

Abstract

Purpose : Retinal ganglion cells (RGCs) of Trpm1 knockout (KO) mouse retina exhibit abnormal periodic spontaneous firings (oscillations). We performed electrophysiological and morphological experiments, as well as computer simulations to elucidate the mechanism of oscillations.

Methods : Trpm1 KO mice (1-month old) were used for experiments. Patch-clamp techniques were applied to αRGCs (soma size > 15 µm) in the wholemount retina. Cs-based (E_Cl = –60 mV) and K-based internal solutions with Neurobiotin were used for voltage- and current-clamp recordings, respectively. The retina was continuously superfused with Ames’ medium bubbled with 95% O₂/5% CO₂ at 32°C. After recordings, the retina was immunostained with ChAT and SMI-32 antibodies, and the recorded αRGCs were morphologically classified into ON and OFF types based on the dendritic stratification in the inner plexiform layer. For histochemistry isolated retinas were fixed and embedded in low-melting agarose, and then sectioned by a linear slicer every 50 µm. After blocking, retinal slices were incubated with the primary and secondary antibodies. Retinal circuit models were constructed to estimate the parameters for oscillations. We constructed a normal retinal model and a pathological model, and finally explored the parameters that cause oscillations in RGCs.

Results : Oscillations in αRGCs were generated by synaptic inputs. The reversal potentials of oscillations in ON and OFF αRGCs were −2.3 ± 4.0 mV (n = 7) and −56.7 ± 3.3 mV (n = 20), respectively. Thus, oscillations in ON and OFF αRGCs may be driven by glutamatergic inputs and by glycinergic and/or GABAergic inputs, respectively. The spike timings of oscillations in ON/ON or OFF/OFF αRGC pair and ON/OFF αRGC pair were in phase and antiphase, respectively. The frequency of oscillations was similar (8~9 Hz) in both ON and OFF αRGCs. Rod bipolar cells (RBCs) in Trpm1 KO retinas have significantly shorter axons and smaller terminals than those in WT retinas. The pathological retinal circuit model could reproduce oscillations in both ON and OFF αRGCs.

Conclusions : Our results suggest that oscillator may send opposite signals to ON and OFF pathways and that glutamatergic inputs from RBCs to AII amacrine cells (AII ACs) may be reduced due to smaller terminals of RBCs. Model simulation supports the hypothesis that the reduced glutamatergic inputs to AII ACs may be important for generating oscillations.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.