Subcellular topography of visually driven dendritic activity in the vertebrate visual system

Abstract

Neural pathways projecting from sensory organs to higher brain centers form topographic maps in which neighbor relationships are preserved from a sending to a receiving neural population. Sensory input can generate compartmentalized electrical and biochemical activity in the dendrites of a receiving neuron. Here, we show that in the developing retinotectal projection of young Xenopus tadpoles, visually driven Ca2+ signals are topographically organized at the subcellular, dendritic scale. Functional in vivo two-photon Ca2+ imaging revealed that the sensitivity of dendritic Ca2+ signals to stimulus location in visual space is correlated with their anatomical position within the dendritic tree of individual neurons. This topographic distribution was dependent on NMDAR activation, whereas global Ca2+ signals were mediated by Ca2+ influx through dendritic, voltage-dependent Ca2+ channels. These findings suggest a framework for plasticity models that invoke local dendritic Ca2+ signaling in the elaboration of neural connectivity and dendrite-specific information storage.

Figure 1. In Vivo Imaging of Visually Evoked Ca²⁺ Signals in Proximal and Distal Tectal Cell Dendrites

(A) Schematic of the retinotectal projection and visual stimulation. Flashing spots are presented in the visual field (left). Retinal ganglion cells (RGCs) project contralaterally to the tectal neuropil. Dorsal (D) and ventral (V) RGCs target lateral (L) and medial (M) tectal neuropil, respectively. The left half is a lateral view of the eye; the right half is a dorsal view of the left tectal lobe. (B) Experimental set-up. A two-photon laser scanning microscope is used to measure dendritic Ca²⁺ signals, while the retina is stimulated with flashing spots by using a projector and an image conduit. Infrared laser light detected by a photodiode in trans-illumination mode generates contrast images used for patch-clamp and single-cell electroporation. (C and D) Simultaneously acquired (C) negative-stain fluorescence image and (D) infrared contrast image of a patched tectal neuron in vivo. (E) Z-projection of a filled tectal neuron, overlaid with the IR contrast image of the tectum. Note different granularity in the periventricular cell body layer (upper left) and in the tectal neuropil (lower right). (F) ΔF/F transients recorded in proximal and distal dendritic regions and in the soma in response to 12 dimming spots (same as in [A]). The timing of stimuli is indicated by tick marks above traces. The location of dendritic regions is indicated by numbers in (E). Brackets and colors indicate groups of traces acquired in the same frame scan.

Figure 2. Visually Evoked Dendritic Ca²⁺ Signals Exhibit Topographic Bias

(A) Three-dimensional reconstruction of a tectal neuron (red) filled with OGB-1. Same orientation as in Figure 1E (the x axis is medial-to-lateral, the y axis is caudal-to-rostral, and the z axis is dorsal-to-ventral of the animal). The blue arrow indicates the dorsomedial-to-ventrolateral axis of the tectum. Ca²⁺ signals were measured in dendrites distal to the first branch point (black rectangles). Regions of interest (ROIs) are located in different z-planes. (B) Horizontal bar stimulation in five vertical positions to test dorsoventral topography in visual space. (C) ΔF/F transients in dendritic regions (1–4 in [A]) in response to horizontal bar stimulation. The stimulus consisted of five bars shown for 0.5 s every 5 s in pseudorandomized order at the positions indicated above traces. ΔF/F transients (black traces) averaged from five individual sweeps (gray traces) for each ROI. Individual sweeps were peak scaled before averaging. Approximate tuning curves for the average ΔF/F transient of dendritic regions 1–4 are indicated by dashed lines. The timing of flashing bars is shown above traces. (D) Normalized ΔF/F tuning curves of dendritic regions 1–4 calculated from average ΔF/F transients shown in (C) (mean ± SEM). Center-of-mass values (arrows) were calculated from these tuning curves by summing over stimulus positions, which were weighted by the amplitude of corresponding ΔF/F transients. (E) Mean-subtracted center of mass of tuning curves in different dendritic regions versus their position, Δu′, along the dorsomedial-to-ventrolateral axis in the tectum (blue arrow in [A]). Data are pooled from eight cells. The solid line is a straight line fit.

Figure 3. Line Scan Analysis of Dorsoventral Topographic Bias of Dendritic Ca²⁺ Signals

(A) Z-projection of a filled tectal neuron. (B) Reconstruction of boxed dendritic region in (A) with scan line (blue). Circles mark intersections of scan line with dendrites. Blue dashed lines indicate coordinates of dendritic locations (1–4) projected onto the medial-to-lateral tectal direction (u, arrow). (C) Individual ΔF/F transients acquired simultaneously in dendritic regions (1–4) in response to one sequence of horizontal bar stimulation. The sequence consisted of five bars shown for 0.5 s every 5 s in pseudorandomized order at the positions indicated above traces. (D) Normalized ΔF/F tuning curves from dendritic regions 1–4 measured in a single trial (same as [C]). Center-of-mass values (arrows) were calculated from these tuning curves by summing over stimulus positions, which were weighted by the amplitude of corresponding ΔF/F transients. (E) Mean-subtracted center of mass (ΔR) of tuning curves in different dendritic regions versus their Δu coordinate measured in a single cell. Data are pooled from 12 line scans in one cell (same cell as in [A]–[D]). The solid line is a straight line fit (r = −0.48, p = 0.0005). (F) Correlation coefficients determined as in (E) from line scan measurements in 15 cells. The mean correlation coefficient is r = −0.37 ± 0.05 (mean ± SEM).

Figure 4. NMDAR Activation Is Required for Topographic Bias of Dendritic Ca²⁺ Signals

(A) Individual ΔF/F transients acquired simultaneously in three dendritic regions (1–3) in response to horizontal bar stimulation during inhibition of NMDAR by APV (100 µM). (B) Mean-subtracted center of mass of dendritic tuning curves is not correlated with dendritic position Δu in the presence of APV (r = 0.04, p = 0.81). Data are from ten line scans in a single dendritic tree (same as in [A]). (C) Histogramand cumulative distribution of correlation coefficients for control cells (blue, n = 15) and cells when NMDARs were blocked by APV (red, n = 8). Medians of the distributions are significantly different (−0.406 [control] and +0.047 [APV], p = 0.0004, Wilcoxon’s rank sum test)

Figure 5. Voltage-Dependent Ca²⁺ Channels Mediate a Global Dendritic Ca²⁺ Signal during Tectal Cell Spiking

(A) Simultaneous recording of proximal dendritic ΔF/Ftransients and somatic cell spiking (I_{cell-attached}) in the cell-attached configuration. Lower traces: two examples of tectal spike bursts triggered by a dimming flash. (B) Relationship between ΔF/F transients in primary dendrite and somatic spike number (n = 6 cells). For each cell, the ΔF/F amplitude was normalized to the ΔF/F amplitude during a single spike (gray lines). Dots, individual ΔF/F amplitudes; diamonds, average ± SEM. The dashed curve is an exponential fit to average data. (C) Z-projection of a reconstructed tectal neuron. Dendritic regions in which ΔF/F transients were measured are indicated by rectangles 1–6. (D) Upper trace: action potentials evoked by somatic current injection (1, 2, 4, 8, and 16 action potentials). Lower traces 1–6: ΔF/F transients measured during somatic current injection in the rectangular regions shown in (C). Synaptic transmission was blocked by APV, CNQX, SR95531, and strychnine in the bath solution. (E) ΔF/F transient amplitudes measured in proximal and distal dendritic regions, plotted versus their intracellular distance from soma (n = 5 cells). Individual ΔF/F amplitudes from the same cell are indicated by identical symbols. ΔF/F amplitudes were normalized to the maximal amplitude measured within that cell, which could occur either distally or proximally. After normalization, data points from all cells were binned and averaged (diamonds, average ± SEM).

Figure 6. Dorsoventral Topographic Mapping of Presynaptic Axons

(A) Overlay of an IR image of a tectal lobe with fluorescence images of RGC axonal termination zones in the same lobe. Dashed curves demarcate the medial and lateral half of tectal neuropil (lower left and upper right, respectively). A pipette (upper left) was used to stabilize tectal preparation during the experiment. Inset: schematic of retinal injection sites. (B) Fractional intensity in tectal neuropil of axons from dorsal retina (red) and ventral retina (green). Individual experiments (circles) and average (thick line, error bars: SEM, n = 4 for each injection site)

Figure 7. Dorsoventral Topographic Mapping of Postsynaptic Population Activity

(A) Single dye-filled neuron (yellow), overlaid with a simultaneously acquired IR image of the entire tectal lobe. Cell position is measured from the medial pole as the fractional distance (blue curve, arrow) relative to total PVL length (red curve). The image plane is ~100 µm from the dorsal edge of the tectum. (M, C, L, R represent approximate medial, caudal, lateral, and rostral, respectively, in the tectum.) (B) Schematic diagram of dimming square positions, flashed for 0.5 s every 5 s. The sequence of spots is indicated by numbers (1–12). (C) Color-coded ΔF/F amplitudes (from average trace in [D]) mapped onto stimulus position. Center of mass (cross) of the receptive field was calculated as the weighted vector average of the four stimulus positions that evoked the largest responses in the primary dendrite. (D) ΔF/F transients measured in the primary dendrite of the neuron in (A) in response to dimming spots, as indicated in (B). Six individual traces (gray) and the average trace (black) are shown. (E and F) Coordinates of receptive field centers versus tectal cell position (0 is the most medial, 1 is the most lateral along the red line in [A]). The horizontal coordinate is not correlated with cell position, as indicated by a linear fit (E); correlation coefficient, r = −0.04; p = 0.83). (F) The vertical RF position is correlated with cell position along the PVL boundary. The solid line is a linear fit (r = −0.60; p = 0.0004). Data are from 31 cells.