Origin of transient and sustained responses in mammalian ON retinal ganglion cells (RGCs)

Abstract

Purpose : RGCs generate sustained vs. transient photoresponses to encode low vs. high stimulus frequencies. For ON RGCs, two studies have examined whether this dichotomy originates at ON bipolar cells’ glutamate receptors. When Awatramani & Slaughter (J Neurosci 2000) applied the mGluR6 antagonist CPPG to mimic light, they found that it depolarized tonic ON bipolar and ganglion cells far more than phasic ones and proposed that mGluR6 transduction underlies the tonic/phasic dichotomy. By contrast, Kaur & Nawy (J Physiol 2012) challenged that conclusion by showing that responses to CPPG were uniform across ON bipolar cell types. Thus, the role of mGluR6 in shaping photoresponse kinetics remains controversial. Further, these papers studied only salamander, and mammalian retinas have not been tested. We reexamined this question using mice.

Methods : Whole-cell current-clamp recordings were made from randomly targeted ON bipolar cells in retinal slices, and GFP-labeled RGCs in flat-mount retinas. Three mouse lines were used for the RGC recordings: opn4^Cre/+;GFP labeling melanopsin RGCs (Ecker et al. Neuron 2010), TRHR-GFP labeling a type of ON-OFF direction-selective RGCs (Rivlin-Etzion et al. J Neurosci 2011), and Hoxd10-GFP labeling ON and ON-OFF RGCs innervating the accessory optic system (Dhande et al. J Neurosci 2013). Stimuli were full-field 10s 480nm light steps at 8.5 log – 11.5 log quanta cm^-2 s^-1. To quantify photoresponse sustainedness, the response amplitude near the end of the stimulus was divided by the peak amplitude to obtain a final-to-peak ratio. CPPG or Cd²⁺ was then applied to block endogenous mGluR6 activation.

Results : ON bipolar cells exhibited a wide range of photoresponse transience with final-to-peak ratios ranging from 0 to 0.9. Both CPPG and Cd²⁺ induced larger depolarizations in bipolars with more tonic photoresponses, showing a direct correlation between the amplitude of CPPG/Cd²⁺-induced depolarization and the final-to-peak photoresponse ratio. RGC photoresponse transience was similarly diverse, with melanopsin RGCs being the most tonic and TRHR+ RGCs being the most phasic. CPPG induced far greater depolarizations in the more tonic RGCs.

Conclusions : Our observations paralleled those by Awatramani & Slaughter, suggesting that in mouse, phasic and tonic RGC light responses result from selective bipolar input and that these two channels originate at ON bipolar cells’ dendrites.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.