Spatial receptive fields in the retina and dorsal lateral geniculate nucleus of mice lacking rods and cones

In advanced retinal degeneration loss of rods and cones leaves melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) as the only source of visual information. ipRGCs drive non-image-forming responses (e.g., circadian photoentrainment) under such conditions but, despite projecting to the primary visual thalamus [dorsal lateral geniculate nucleus (dLGN)], do not support form vision. We wished to determine what precludes ipRGCs supporting spatial discrimination after photoreceptor loss, using a mouse model (rd/rd cl) lacking rods and cones. Using multielectrode arrays, we found that both RGCs and neurons in the dLGN of this animal have clearly delineated spatial receptive fields. In the retina, they are typically symmetrical, lack inhibitory surrounds, and have diameters in the range of 10–30° of visual space. Receptive fields in the dLGN were larger (diameters typically 30–70°) but matched the retinotopic map of the mouse dLGN. Injections of a neuroanatomical tracer (cholera toxin β-subunit) into the dLGN confirmed that retinotopic order of ganglion cell projections to the dLGN and thalamic projections to the cortex is at least superficially intact in rd/rd cl mice. However, as previously reported for deafferented ipRGCs, onset and offset of light responses have long latencies in the rd/rd cl retina and dLGN. Accordingly, dLGN neurons failed to track dynamic changes in light intensity in this animal. Our data reveal that ipRGCs can convey spatial information in advanced retinal degeneration and identify their poor temporal fidelity as the major limitation in their ability to provide information about spatial patterns under natural viewing conditions

Responses to melanopsin-isolating steps and gradual irradiance ramps in retina.

<p>A: Trial bin count examples of two cells responding to a 71% contrast step (blue bar at 0 to 20 seconds) presented during protocol 1 (background log 12.75 melanopsin photons/cm²/s to step log 13.5 photons/cm²/s) before (i) and during (ii) synaptic blockade. Before and after graphs are scaled to same axis to show changes in baseline activity upon synaptic blockade. B: Example raster plots and PSTH to a 50ms rod favouring yellow flash from dark (flash intensity log 11.45 rod photons/cm²/s) under normal conditions (aCSF) and under synaptic blockade (5 minutes after application of L-AP4 + NBQX) in a cell that was identified to respond to our melanopsin-isolating stimulus. C: Averaged plots for firing rate over time of consistent (n = 31) melanopsin-step responsive cells (mean±SEM) before (left) and during (right) synaptic blockade. D: Mean response onset and offset latencies for individual melanopsin-step responsive cells in the presence (purple symbols) and absence (green) of synaptic blockade. E: Under protocol 2, retinal cells responding to a melanopsin-step (n = 22/ 314 cellls; 71% contrast) do so over a range of background irradiances on both the upward and downward phases of the ramp (Repeated measures 2-way ANOVA, step vs baseline firing rate x irradiance; main effects of irradiance (p<0.001), step vs baseline (p<0.001) and interaction (p<0.05); Bonferroni post-hoc comparisons marked on figure as ** and *** p<0.01 for ND2 upward ramp and ND1.5 for downward; n = 22 cells). F: Retinal melanopsin-step responsive cells also tracked the gradual change in irradiance during protocol 2, revealed as a change in firing rate (mean± SEM) as a function of ramp progression.</p

Responses of cells sensitive to the melanopsin-step over a range of irradiances.

<p>A: PSTHs for firing rate across melanopsin-isolating steps (71% contrast) over a range of backgrounds (across both increasing and decreasing arms of the ramp; n = 24 responsive cells mean±SEM). See 5Cii for significance values. B: Scatter plot displaying mean onset vs offset latencies for each cell as a function of background irradiance. Ci: Mean change in firing rate associated with the melanopsin-isolating step (firing rate during step—firing rate over previous 10 s) with increasing irradiance during the upwards (dark blue) and downwards (light blue) phases of the ramping protocol. Ci and ii Significant responses were recorded for steps against backgrounds ≥ 12.1 log melanopsin photons/cm²/s (ND2) on the ramp up and 13.1 log melanopsin photons/cm²/s (ND1) on the ramp down (2-way RM ANOVA step vs baseline firing rage x irradiance; main effects of irradiance (p<0.05), step vs baseline (p<0.001) and interaction (p<0.001); Bonferroni post-hoc comparisons p<0.01 for ND2 and above for upward ramp and ND1 for downward). D: Firing rate (mean±SEM; n = 54) of dLGN stepping cells from protocol 1 to a bright blue 10 sec step from dark (log 14.1 melanopsin photons/cm²/s; purple bar). Note sustained activity after the termination of the step, considered to be a feature of the melanopsin light response. E: There was no significant change in time averaged firing rate of cells responsive to the melanopsin step as a function of ramp progression (mean±SEM n = 24).</p

Light Stimulation Protocols.

<p>A: LED spectral power densities and <i>in vivo</i> photoreceptor spectral sensitivity (normalised). The output of blue and yellow LEDs was adjusted to produce equivalent effects on rods (black line). By contrast, the blue LED, always appeared brighter for melanopsin (green line). B: Protocol 1. Stimuli (30 or 20s melanopsin-isolating steps in dLGN and retina, respectively) presentations of the blue LED were interleaved with 210 or 180 sec of the (dLGN and retina, respectively) yellow to produce a ‘step’ visible only to melanopsin. The magnitude of this melanopsin step could be varied by mixing blue and yellow in the step (increasing the yellow and decreasing the blue elicited decreasing levels of contrast). C: Protocol 2. Starting at ND4, irradiance slowly ramped up (0.5 ND per 200 seconds) before remaining at a steady state for 10 seconds. At each 0.5ND, a blue melanopsin-isolating step (71%) is given for 30 seconds (total time per 0.5ND cycle = 4 minutes). This process was repeated until reaching ND0, at which point light-levels instead slowly ramped down and the process was repeated. D: The effective change in photon flux for melanopsin (green) and rods (black) across a full repeat of Protocol 2. Settings of ND filter at the point of each melanopsin isolating step are provided above. Di and ii: The starting position of the ramp (ND4 or ND0) was varied across experiments.</p

Retinal responses in <i>rd/rd cl</i> mice.

<p>A: Trial bin count examples (5 second bins of time) of three cells responding to a 20s 71% contrast step (background log 12.8 to step log 13.56 melanopsin photons/cm2/s) in a <i>rd/rd cl</i> retina. Note the variety in the latency of response offset in examples 1 and 2, and the poor response reproducibility that is typical of <i>rd/rd cl</i> mice in example 3. Colour bar to the right of the plot (FR Hz) denotes the firing rate of cells in this and subsequent Trial bin count figures. B: Averaged plots for firing rate over time of melanopsin-step responsive cells (mean±SEM) to a 20s step (Bi n = 15) and a 60s step (Bii n = 17). Both step durations elicit a significant change in firing to the baseline rate (**** P<0.0001). C: A plot of mean onset and offset latencies for each individual responsive cell over multiple stimulus repeats reveals the extremely sluggish nature of light responses in <i>rd/rd cl</i> mice.</p