Multiplexing Visual Signals in the Suprachiasmatic Nuclei Adam R. Stinchcombe, Joshua W. Mouland, Kwoon Y. Wong, Robert J. Lucas, Daniel B. Forger  Cell Reports  Volume 21, Issue 6, Pages 1418-1425 (November 2017) DOI: 10.1016/j.celrep.2017.10.045 Copyright © 2017 The Author(s) Terms and Conditions

Cell Reports 2017 21, 1418-1425DOI: (10.1016/j.celrep.2017.10.045) Copyright © 2017 The Author(s) Terms and Conditions

Figure 1 An Overview of the Mathematical Model Describing the GABA-Mediated Neuronal Network within the SCN (A) The visual filed is divided into squares representing the receptive fields of ipRGCs. A bar of light (yellow) falling on some squares results in some SCN neuron(s) experiencing a current via projections from the retinal ganglion cells in the retinohypothalamic tract (RHT) to the SCN (blue arrows). Within the SCN network, neurons can inhibit (flatheads) or excite (arrowheads) other SCN neurons depending on the reversal potential of GABA. The population consists of both neurons excited (green) and inhibited (red) by GABA. Approximately 20% of SCN neurons receive direct input (bright red and green), which is excitatory. Additional model details are described in the Experimental Procedures and Figures S1–S3. (B) Voltage traces (coloring from A; thick [thin] lines for spontaneously firing [quiescent] neurons) for a full-field light stimulus (yellow background) and an intact or removed (blue bar) GABA network. Inset: experimental voltage traces from Kononenko and Dudek (2004) with regular action potentials without GABA (left) and irregular action potentials with GABA (right) agreeing with the simulation. (C) A Raster plot for a small number of neurons (coloring from A) are shown for a stimulus of a vertical bar of light presented at different positions in the visual field. Bars are presented for 0.25 s with 0.25 s of background of low ambient light between presentations. The first three positions for the first 1.5 s are diagramed to the right. Neurons without direct input also have different firing rates, but increase or decrease their firing rates depending on how they are connected to the rest of the network. Cell Reports 2017 21, 1418-1425DOI: (10.1016/j.celrep.2017.10.045) Copyright © 2017 The Author(s) Terms and Conditions

Figure 2 Firing Rate Responses (A) Four voltage traces of select SCN neurons under a full-field light step (upper two are experimental, lower two are from the simulation). The left two traces show an increase in the firing rate while the right two show a decrease. (B) A scatterplot of firing rates during the on and off phases of the stimulus averaged over the 10 s before and after the step from the simulation (filled circles with coloring from Figure 1A) and experiments (black). (C) (i) Firing frequency distribution for various temporal inversion frequencies for a 14 × 14 checkerboard. Each column corresponds to a different inversion frequency and shows the firing rate histogram in color, as the fraction of the all SCN neurons. SCN neurons fire primarily around 4 Hz and resonate with that stimulus frequency. (ii) On a coarse spatial grid of 4 × 4 checkers and a 2-Hz inversion frequency, neurons with direct input (histogram in green) fire at 4 Hz while those without (histogram in black) fire between 2 Hz, the stimulus frequency, and 4 Hz. (D) For a 4 Hz inverting checkerboard stimulus and a range of irradiances, average population firing rate as the ratio of the firing rate for a given spatial contrast to that at the same irradiance and zero contrast. The GABA network is (i) intact and (ii) removed showing that the network reduces the SCN’s response to contrast. (E) The (horizontal bar) receptive fields (i and ii) for two SCN neurons (without direct input) and the timing of their spikes relative (iii and iv) to a 4-Hz inverting 14 × 14 checkerboard. Both full field neurons fire after each switch of the 4-Hz stimulus (every 125 ms). The delay is consistent with measured empirical data as the input is filtered through the GABA network. Cell Reports 2017 21, 1418-1425DOI: (10.1016/j.celrep.2017.10.045) Copyright © 2017 The Author(s) Terms and Conditions

Figure 3 Receptive Field Mapping Simulations (A) Changes in firing rates of selected neurons within the simulation as a function of bar position (spanning 200° of the visual field) exhibit various receptive fields. Only responses to vertically oriented bars are shown although both vertical and horizontal presentations are simulated. The receptive fields of two neurons from a poor fit simulation are shown. (B) The receptive fields from four neurons from the best-fit simulation. (C) Cumulative distribution of vertical bar receptive field sizes for data (red), the best-fit simulation (black), and a poor-fit simulation (blue). (D) The fraction of randomly generated networks that cannot be rejected are shown for two key network parameters: the fraction of the connections that are excitatory and the connection density. From this, we infer that the SCN GABA network is 20% excitatory with each neuron being directly connected to around 10 (1% of N = 1,024) other neurons. (E) The size of the largest connected component as a fraction of the size of the network as a function of the connection density for N = 10,000 (the size of the SCN). The first, second (median), and third quartiles of the distribution as shown, but the curves appear on top of one another. (F) A histogram with 40 bins of the visual field distance between connected neurons, separated into four types of connections: direct inhibitory (−1, bright red), effectively inhibitory through one intermediate neuron (−2, dark red), direct excitatory (+1, bright green), and effectively excitatory through one intermediate neuron (+2, dark green); total percentages of types are shown. The solid black curve is the distribution of pairwise distances that would result from uniformly random centers in a network of N = 1,024 neurons. Cell Reports 2017 21, 1418-1425DOI: (10.1016/j.celrep.2017.10.045) Copyright © 2017 The Author(s) Terms and Conditions

Figure 4 The GABA Network within the SCN Can Enhance Visual Tasks (A) Two Raster plots show that the GABA network encourages spiking in the ventral SCN (neurons with light input and are inhibited by GABA) for a 5 s ramp stimulus at circadian time (CT) 14.7. The spikes from 149, not sequentially numbered, neurons are shown. (B) Mean firing rate for all neurons (black) and for each network type (coloring from Figure 1A) for a slow ramp stimulus at CT 12 in four columns: GABA and RHT intact, GABA disabled, GABA permuted, and RHT permuted. The firing rates can increase or decrease with input in a direction opposite from that predicted by the sign of the GABA input. (C) Cumulative distribution of the in vivo experimentally measured ratio of firing rate for a full-field stimulus against that of background for cells with broad (blue) and confined (black) receptive field responses. (D) Receptive field centers (coloring from Figure 1A) of spiking neurons from ten not-rejected networks laid over the ipRGC receptive field grid. (E) Procedure to locate a bright spot: (1) the light input is perturbed with Gaussian noise with standard deviation σ, then (2a) directly passed directly or (2b) filtered by the SCN, and then (3) the controller moves the center of the visual field toward the largest signal. Movie S1 demonstrates the effect of tracking a bright spot with and without SCN filtering. (F) Gaze trajectory starting from 60° down and away from the bright spot on a 14 × 14 grid and σ = 0.3. (G) The mean displacement from the target once it is found as a function of σ. SCN filtering holds the gaze closer to the target. Cell Reports 2017 21, 1418-1425DOI: (10.1016/j.celrep.2017.10.045) Copyright © 2017 The Author(s) Terms and Conditions