Abstract
Purpose :
An essential step in visual processing is the segregation of signals into ON and OFF pathways by retinal bipolar cells (BCs). Glutamate released from photoreceptors modulates the photoresponse of ON BCs via metabotropic glutamate receptor 6 (mGluR6) and a G protein (Go) that regulates a cation channel, TRPM1. We examined the ON and OFF responses of retinal ganglion cells (RGC) in TRPM1 and mGluR6 knockout（KO）mice based on the prediction that the deficits in RGC responses would be similar in both cases.
Methods :
TRPM1 KO and mGluR6 KO mouse lines were generated by disruption of the TRPM1 or mGluR6 gene by homologous recombination, respectively (Koike et al., 2010, Masu et al., 1995). Spike discharges of RGCs in the isolated mouse retina were recorded by a multielectrode array (60pMEA200-30iR-Ti, MEA-60 system). The retina was stimulated diffusely by a 490 nm light emitting diode (LED; 4.44μW/cm2), 2 s with the light on (9.77μW/cm2) and 2 s with the light off (0.567μW/cm2) in Ringer solution. Data were analyzed by Lab chart and further sorted by Efficient Technology of Spike sorting (EToS4). To analyze retinal circuits, cryosections were subjected to immunohistochemistry and imaged with LSM700 confocal microscope. A computer model was created and simulations were performed.
Results :
Significant spontaneous activity was observed in the TRPM1 KO retinas but not in the WT or mGluR6 KO retinas. Spontaneous activity in the TRPM1 KO retina was inhibited by a gap junction inhibitor (MFA). Rod BC terminals in the TRPM1 KO retina were significantly smaller than those in WT or mGluR6 KO retinas. We equated the smaller size of the rod bipolar cell terminals in the TRPM1 KO retina with a decrease in transmitter release. We then used a model to examine the consequences of this morphological changes on the AII-AC network and identified a condition under which oscillations were induced in the network.
Conclusions :
Although both TRPM1 and mGluR6 regulate ON bipolar visual transmission, we found a significant difference in spontaneous activity between TRPM1 and mGluR6 KO mouse retinas. The smaller size of the rod bipolar cell terminals in the TRPM1 KO retina, but not in mGluR6 KO retina, are consistent with a decrease in transmitter release altering the AII-AC network to produce oscillations.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.