Spatial-temporal patterns of retinal waves underlying activity-dependent refinement of retinofugal projections

Abstract

During development, retinal axons project coarsely within their visual targets before refining to form organized synaptic connections. Spontaneous retinal activity, in the form of acetylcholine-driven retinal waves, is proposed to be necessary for establishing these projection patterns. In particular, both axonal terminations of retinal ganglion cells (RGCs) and the size of receptive fields of target neurons are larger in mice that lack the beta2 subunit of the nicotinic acetylcholine receptor (beta2KO). Here, using a large-scale, high-density multielectrode array to record activity from hundreds of RGCs simultaneously, we present analysis of early postnatal retinal activity from both wild-type (WT) and beta2KO retinas. We find that beta2KO retinas have correlated patterns of activity, but many aspects of these patterns differ from those of WT retina. Quantitative analysis suggests that wave directionality, coupled with short-range correlated bursting patterns of RGCs, work together to refine retinofugal projections.

Figure 1

MEA recordings of spontaneous activity reveal that retinal waves are present at 37°C but not 30°C in β2KO. (A) Examples of waves recorded from the same P6 WT retina at 30°C (top) and 37°C (bottom). (B) Examples of waves recorded from the same P6 β2KO retina at 30°C (top) and 37°C (bottom). Each dot represents multiunit activity recorded on an electrode at that site. The size of the dot is proportional to the amplitude of the spikes detected on the electrode and the color represents their frequency. Electrode spacing is 60 μm. (C) In WT retinas, waves travel faster at 37°C (black) than they do at 30°C (light gray) while, in β2KO retinas at 37°C (gray), waves travel even faster than in WT retinas at either temperature (p < 0.0001; one-way ANOVA with Bonferroni adjustment). (D) Quantification of the width of the wave front demonstrates that at 37°C, β2KO (gray) wave fronts are wider than WT wave fronts at either 30°C (light gray) or 37°C (black; p < 0.0001; one-way ANOVA with Bonferroni adjustment). Data in (C) and (D) are from WT 37°C: 2993 waves from 17 retinas; WT 30°C: 607 waves from 7 retinas; β2KO 37°C: 750 waves from 12 retinas. (E) Correlation index computed for spike trains from pairs of neurons and plotted as a function of the distance between electrodes on which the neurons were recorded. Plot summarizes data from multiple WT (black; n = 17) and β2KO (gray; n = 12) preparations and shows that CI decreases as a function of distance in both WT and β2KO retina. Error bars represent SEM. *** p < 0.001.

Figure 2

Temperature induces changes in spontaneous activity patterns of RGCs in WT and β2KO. (A) Peristimulus time histograms (PSTHs) of the mean spike rate of all neurons (bottom) at 30°C and 37°C (denoted by gray bar) from a P6 WT retina as well as single unit activity of 30 RGCs from a ten minute segment of the recording at 30°C (top left; denoted by open bar in bottom) and 37°C (top right; denoted by black bar in bottom). (B) PSTHs of the mean spike rate of all neurons (bottom) at 30°C and 37°C (denoted by gray bar) from a P6 β2KO retina as well as single unit activity of 30 RGCs from a ten min segment of the recording at 30°C (top left; denoted by open bar in bottom) and 37°C (top right; denoted by black bar in bottom). Correlated activity is seen at 37°C but not 30°C in the β2KO and the pattern of this activity is considerably different from WT patterns at either 30°C or 37°C. (C) High resolution spike rasters from WT (top) and β2KO (bottom) demonstrate that β2KO bursts appear to contain fewer spikes than WT. (D) RGCs spike more at 37°C (black) than at 30°C (gray) in both WT (left) and β2KO (right) retinas. (E) Waves occur more frequently at 37°C (black) than at 30°C (gray) in both WT (left) and β2KO (right) retinas. Data in (D) and (E) are from WT 37°C: 4378 neurons from 17 retinas; WT 30°C: 1331 neurons from 7 retinas, β2KO 37°C: 4078 neurons from 12 retinas, and β2KO 30°C: 2127 neurons from 6 retinas. Error bars represent SEM. ** p < 0.01; *** p < 0.001.

Figure 3

Retinal waves travel in a preferred direction in WT but not β2KO retina. (A) A comparison of the fraction of neurons in each preparation that display a directional bias indicates that WT (black) and β2KO (gray) retinas have equivalent amounts of RGCs that display a directional bias (see Methods; p = 0.1522, Kruskal-Wallis ANOVA; WT: 2354 neurons from 17 retinas; β2KO: 1888 neurons from 12 retinas). (B) A comparison of the magnitude of the preparation directional bias demonstrates that β2KO retinas (gray) have less directional bias than WT (black). A large preparation directional bias indicates that waves possess a directional bias in a given recording (Nasal to Nasal: p = 0.01772; Temporal to Temporal: p = 0.00768; Dorsal to Dorsal: p = 0.0292; Ventral to Ventral: p = 0.02933; t-tests). Black filled circle indicates a WT preparation lacking a directional bias and a gray filled circle indicates a β2KO preparation with a directional bias. (C, D) Polar plots measuring the preparation directional bias of individual preparations from nasal (black), temporal (light gray), dorsal (dark gray; open arrowheads), and ventral (gray; arrow points) retinas. Each arrow represents the mean direction (angle) and mean strength (magnitude) of the directional bias of all the RGCs in an individual preparation and shows that retinal waves have a preferred direction in WT but not β2KO retinas. (C) WT preparations from each retinal quadrant have a directional bias (see Table 2) and the angle of bias is the same in Dorsal, Ventral, and Temporal retina, but of opposite polarity in Nasal retina (p < 0.05; Circular ANOVA with Wheeler-Watson test with Bonferroni correction). (D) β2KO preparations lack a directional bias (see Table 2) and the angle is the same in all quadrants in β2KO retina (p = 0.289; Circular ANOVA). (E, F) Distributions of angle of neuron directional bias for individual RGCs indicate that WT preparations (E) possess a directional bias in each quadrant while β2KO preparations (F) do not (Nasal: black, Temporal: filled, light gray, Dorsal: dark gray, Ventral: gray). In (A) and (B), plot shows values (circles) along with means (horizontal lines) and SEMs. N: Nasal; T: Temporal; D: Dorsal; V: Ventral; ** p < 0.01; * p < 0.05.

Figure 4

The probability that spatially distant RGCs will fire temporally-offset bursts is increased in an axis-specific manner in WT, but not β2KO retinas. Offset Correlation Indices (OCIs) computed for bursts fired during waves from pairs of neurons and plotted as a function of the distance between electrodes on which the neurons were recorded. (A) OCIs are larger for WT RGCs separated by 800-900 μm along the NT axis (filled circles) than along the axis orthogonal to it for time offsets of 4-5 seconds (open circles; 17 retinas; 209,566 RGC pairs) (B) In β2KOs retinas, OCIs are equivalent along both axes at the same time offset as in (A) (12 retinas; 257,567 RGC pairs). Error bars represent SEM. * p < 0.05.

Figure 5

β2KO RGCs fire bursts that contain fewer spikes and are less correlated than those in WT retina. (A) The percent of all spikes that are included in bursts is significantly reduced in β2KO RGCs (gray). (B) Inter-spike intervals (ISI) of bursts are significantly increased in β2KO RGCs (gray). (C) The mean burst duration is unchanged in β2KOs (gray). (D) A comparison of the percentage of all bursts that occur during waves reveals that the bulk of RGC bursts occur during waves in WT retinas (black), but in between waves in β2KO retinas (gray; WT: 2993 waves; β2KO: 760 waves; same number of bursts as A and B). (E) The bulk of high-frequency spiking occurs during wave-bursts in WT RGCs (left, black), but during non-wave-bursts in β2KO RGCs (right, gray). Data in (A-E) are from WT: 234,154 bursts from 4378 RGCs; β2KO: 369,124 bursts from 4078 RGCs. (F) Burst correlation index computed for bursts from pairs of neurons and plotted as a function of the distance between electrodes on which the neurons were recorded. Plot summarizes data from multiple WT (209,566 RGC wave pairs) and β2KO (430,088 RGC wave pairs and 257,567 RGC non-wave pairs) preparations and shows that BCI decreases as a function of distance for wave-bursts from WT (black) and β2KO (light gray) retina, but is significantly reduced in non-wave-bursts (gray) from β2KO retina. Data in (A-E) are from WT: 17 retinas, β2KO: 12 retinas. Error bars represent SEM. *** p < 0.001. n.s. not significant.

Figure 6

Models of how retinal waves mediate the refinement of visual system projections. (A) Synapse elimination: In WT mice, the directional bias of retinal waves along the NT axis during the first post-natal week helps refine projections along this axis by ensuring that RGCs separated by 800+ μm are more likely to fire temporally-offset bursts (top left, wave direction designated by arrows in retina; see Figure 4). This helps eliminate topographically incorrect projections by the second week (bottom left). Because the directional bias of waves is lost in β2KOs, this decreases the likelihood that RGCs separated by 800+ μm will fire temporally-offset bursts (top right). Topographically incorrect synapses are not eliminated and axonal projections fail to refine along the NT axis (bottom right). (B) Synapse strengthening: In WT retinal waves, short ISIs and strong local correlations within each eye ensure that only neighboring RGCs (a and b) drive post-synaptic bursting in target neurons, stabilizing these synapses but not ones from distant RGCs (c, left). In β2KOs, longer correlations allow distant RGCs (a,b, and c) to drive coincident post-synaptic spiking, stabilizing misdirected axonal projections (right). (C) Eye-specific segregation: In WT retinas, the lack of RGC activity during inter-wave intervals ensures that retinotopically matched regions of the two eyes are not co-active. This helps strengthen projections from one eye, thereby segregating inputs from the same eye (left). In β2KOs, inter-wave activity is increased leading to an increased probability of co-active inputs. This strengthens inputs from both eyes and segregation fails to occur normally (right).