Open review: WN Grimes et al., Rod signaling in primate retina – range, routing and kinetics

This preprint presents new insights on visual processing in the retina, specifically how signals from rod photoreceptors are handled. Our visual system must operate over a huge range of light intensities, about 9 log units in the course of a day. In adaptation to this challenge the retina uses two kinds of photoreceptors: In the dimmest conditions only the sensitive rods are active, in the brightest conditions only the cones. In between the retina gradually switches from one input neuron to the other. However, even before the cones take over, the rod pathway undergoes substantial changes with increasing light level: the gain decreases and the speed of processing increases. This article challenges the prevailing notion of how those changes are accomplished.

When one inspects the neural circuits of the retina, the rods seem like an evolutionary afterthought added to a backbone circuit that was devoted to cones. The rod has no direct communication line to the output neurons, the retinal ganglion cells. Instead, its signals get spliced into the circuitry at various locations. The authors distinguish three routes: (1) an interneuron pathway through specialized rod bipolar and amacrine cells, which allows for high spatial convergence and high gain; (2) direct transmission of rod signals to cones via electrical junctions; and (3) transmission from rods to Off bipolar cells that predominantly handle cone signals. Each of these pathways imposes different gain and kinetics on the rod signal because different synaptic pathways are engaged.

With this background it has been proposed that the changes in human perception of rod stimuli that occur with increasing light level reflect changes in the neural routing of signals through the different synaptic pathways. In the mouse retina there is reasonable evidence for this, but the idea has received less scrutiny in primate retina, which ultimately matters for human vision. Here the authors come to the surprising conclusion that the primate retina uses pathway 1 for rod signals almost exclusively. Furthermore they claim that the changes in kinetics of rod signal processing observed in human psychophysics are largely explained by changes occurring within the rod photoreceptor cell itself. Parallel experiments in mouse retina using the same strategy gave very different results: a large contribution from the pathways 2 and 3 over the functional rod range.

The significance of this study lies first in revisiting a central problem of human vision, namely light adaptation. Second, it illuminates the relation between structure and function of neural circuits, in a case where both are exquisitely accessible by experiment; third it contributes to the question whether rodents offer the best model for the human nervous system.

The study looks very well designed and the experiments are executed with impressive skill. The main method was to record neural signals from diverse neurons in the retina that play different roles in the three rod pathways, and therefore sample signals from the 3 pathways in different proportions. For example the primate H1 horizontal cell soma should receive rod signals only through pathway 2, whereas the AII amacrine cell lies on both pathways 1 and 2. Also various pharmacological agonists and antagonists were used to block one pathway or another. The cross-species comparison with the mouse provides a positive control for the effects that seemed to be missing in the primate retina.

Regarding the conclusions drawn from the experiments, a few questions remain:

1. Psychophysics of parallel rod pathways. The authors suggest that the human perceptual effects can be explained by a single pathway for rod signals whose gain and kinetics vary as a function of light level. However, some psychophysical experiments show that multiple pathways for rod signals are in operation simultaneously, see for example the phenomenon of “rod self-cancellation” (Stockman and Sharpe, 2006). Can that be reconciled with the single-pathway hypothesis?
2. Health of rod-cone junctions. In terms of neural mechanisms, the authors suggest that in primate retina the rod-cone junctions are too weak to initiate a substantial pathway 2. Is it possible that these junctions are altered by the procedure of isolating the retina in vitro? The junctions seem to be sensitive to modulation, for example by circadian time, through the actions of dopamine (Ribelayga et al., 2008). That could be perturbed by isolating the retina, which would directly affect the main conclusion. How could one check whether these junctions are in the same state as in the intact human eye?
3. Strength of the third pathway. The single pathway conclusion seems less compelling for the Off retinal ganglion cells. Suppressing the On bipolars (which blocks pathway 1) eliminates 80% of the rod signals at a light level of 20R*/s, but only 20% at 200 R*/s (line 288). In between there is a large range and it all falls within the 300R*/s range that the authors consider for rod signaling. Thus it seems that there could be a substantial contribution from pathways 2 or 3 over a substantial part of the rod signaling range. This should be elaborated further.
4. A fourth pathway. Another potential pathway for rod signals was not considered here: rods excite H1 horizontal cells which inhibit cone terminals. There is reasonable evidence for this route in the mouse retina, where it seems to provide the entire receptive field surround of certain ganglion cells (Joesch and Meister, 2016; Szikra et al., 2014; Trümpler et al., 2008). This may affect the interpretation of some of the mouse experiments here. In the primate retina the H1 cell seems to be much less sensitive to direct rod input (Verweij et al., 1999), as stated in the manuscript (line 181).

Some other comments and suggestions:

5. The circuit diagrams in Figs 1, 3A, 6D are confusing to anyone who doesn’t already know these circuits by heart. This could be much improved. (1) Show all the circuit components (cells and synapses) in the same diagram, so one can compare the different pathways. As it stands the same circuit element gets relabeled from On bipolar to Off bipolar in different subpanels and then new synapses appear (Fig 3B-C). A fix to this problem can be found in Fig 1 of (Rivlin-Etzion et al., 2018). (2) To make any sense of these circuits it is essential to distinguish sign-preserving and sign-inverting synapses, for example the primary pathway to Off bipolars involves two sign inversions. That should be indicated by different symbols in the circuit diagram. See for example Fig 1 of (Soucy et al., 1998).
6. Lines 208ff: The comparison between the mouse and macaque is a key result, but a bit difficult to appreciate because it extends over multiple figures. For example Figs 6F and H don’t seem all that different: there’s a regime where only one pathway is active, and another where the other dominates, and then something in between. This transition happens at lower light levels for the mouse than the monkey. The significance becomes apparent only when one compares to the graphs of rod saturation (Fig 2B and Fig 4A), which happens at higher light levels in the mouse (half-saturation at 120 R*/s) than the monkey (45 R*/s). So the mouse has multiple pathways active within the regime where the rods are still functioning just fine, but not so for the monkey. Maybe one could combine these two graphs, e.g. by plotting the relative contribution of different pathways on the y-axis vs fraction of rod saturation on the x–axis. One curve for each species should make the difference clear.
7. Line 555, “Rods were selectively stimulated using the 405 nm LED”: This isn’t quite right. That wavelength stimulates the rod pigment and the L and M cone pigments equally well (relative to their peak absorption wavelength), because it falls into the beta band of all those absorption spectra (Schnapf et al., 1988). The 620 nm light on the other hand is selective for cones. Of course the ganglion cells are much more sensitive to rods because of the larger number and higher gain of rods (under conditions of the present experiments). So one might say that “the 405 nm light stimulates RGCs selectively through rods rather than cones”.
8. Fig 3 and its supplements: What is the meaning of “flash contrast” measured in %? The background is measured in units of R*/rod/s but flash strength in units of R*/rod. How does one get a dimensionless ratio?
9. Fig 3 supplement 2: Panel B legend in figure is confusing, suggests recordings are from a rod and a cone.
10. Fig 4: Panels A and B should use same symbol color for same species, for easier comparison.
11. Typos: line 142 “dominant”, line 155 “complementary”.
12. Wording: line 314 “speed up” or “accelerate” instead of “speed”?

References

Joesch, M., and Meister, M. (2016). A neuronal circuit for colour vision based on rod-cone opponency. Nature 532, 236–239.

Ribelayga, C., Cao, Y., and Mangel, S.C. (2008). The circadian clock in the retina controls rod-cone coupling. NEURON 59, 790–801.

Rivlin-Etzion, M., Grimes, W.N., and Rieke, F. (2018). Flexible Neural Hardware Supports Dynamic Computations in Retina. Trends Neurosci. 41, 224–237.

Schnapf, J.L., Kraft, T.W., Nunn, B.J., and Baylor, D.A. (1988). Spectral sensitivity of primate photoreceptors. Vis Neurosci 1, 255–261.

Soucy, E., Wang, Y., Nirenberg, S., Nathans, J., and Meister, M. (1998). A novel signaling pathway from rod photoreceptors to ganglion cells in mammalian retina. Neuron 21, 481–493.

Stockman, A., and Sharpe, L.T. (2006). Into the twilight zone: the complexities of mesopic vision and luminous efficiency. Ophthalmic Physiol Opt 26, 225–239.

Szikra, T., Trenholm, S., Drinnenberg, A., Juttner, J., Raics, Z., Farrow, K., Biel, M., Awatramani, G., Clark, D.A., Sahel, J.A., et al. (2014). Rods in daylight act as relay cells for cone-driven horizontal cell-mediated surround inhibition. Nat Neurosci 17, 1728–1735.

Trümpler, J., Dedek, K., Schubert, T., de Sevilla Müller, L.P., Seeliger, M., Humphries, P., Biel, M., and Weiler, R. (2008). Rod and cone contributions to horizontal cell light responses in the mouse retina. J Neurosci 28, 6818–6825.

Verweij, J., Dacey, D.M., Peterson, B.B., and Buck, S.L. (1999). Sensitivity and dynamics of rod signals in H1 horizontal cells of the macaque monkey retina. Vis. Res 39, 3662–3672.