Connexin36 is essential for transmission of rod-mediated visual signals in the mammalian retina

Abstract

To examine the functions of electrical synapses in the transmission of signals from rod photoreceptors to ganglion cells, we generated connexin36 knockout mice. Reporter expression indicated that connexin36 was present in multiple retinal neurons including rod photoreceptors, cone bipolar cells, and AII amacrine cells. Disruption of electrical synapses between adjacent AIIs and between AIIs and ON cone bipolars was demonstrated by intracellular injection of Neurobiotin. In addition, extracellular recording in the knockout revealed the complete elimination of rod-mediated, on-center responses at the ganglion cell level. These data represent direct proof that electrical synapses are critical for the propagation of rod signals across the mammalian retina, and they demonstrate the existence of multiple rod pathways, each of which is dependent on electrical synapses.

Figure 1. Rod Pathways in the Mouse Retina Utilize Cone Circuitry

(A) In the primary rod pathway, rods synapse onto a single class of rod bipolar cell, which synapses onto the AII amacrine cell. The AII generates parallel streams of ON and OFF by forming excitatory electrical synapses with ON CBs and inhibitory glycinergic synapses with OFF CBs, respectively. (B) In an alternative rod pathway, rods and cones are directly coupled via electrical synapses, allowing rod excitation to be communicated to ON and OFF CBs via synapses in the cone pedicle. (C) A third pathway may function in the transmission of OFF information. In this path, rods make flat synapses onto a specialized bipolar cell type that synapses directly onto Off-center ganglion cells.

Figure 2. β-Gal Reporter Is Present in the ONL, Amacrine, and Bipolar Cell Layers of the INL and Some Small Cells in the GCL

(A) β-gal immunofluorescence visualized by confocal microscopy can be detected throughout the ONL, and the reporter distinctively labels the photoreceptor inner segments (asterisk). Furthermore, in the INL, β-gal labels cells with characteristic bipolar cell morphology (examples indicated with arrows) and amacrine cells immediately adjacent to the IPL (examples indicated with arrowheads). (B) β-gal immunohistochemistry detects the reporter in a limited number of neurons with small cell bodies in the GCL that may be displaced amacrine cells (arrow), and also in processes from the IPL immediately adjacent to the GCL (arrowhead). (C) No reporter can be detected by immunohistochemistry in WT retina. Scale bar equals 10 μm.

Figure 3. PLAP Reporter Allows Identification of Individual Retinal Neurons

(A) Similar to the distribution of β-gal, PLAP histochemistry labels neurons in the ONL (asterisk), neurons in the bipolar cell layer (arrows), as well as narrow-field and bistratified amacrine cells (arrowheads). (B and C) PLAP-positive cells in the bipolar cell layer extend processes into the OPL and also into the On (C) and Off (D) layers of the IPL, indicating Cx36 expression by at least two classes of bipolar cell. (D) Individual PLAP-positive amacrine cells have the morphological characteristics of type AII amacrine cells. Scale bars equal 10 μm.

Figure 4. Identification of β-Gal-Positive Neurons in the INL

(A) Some β-gal-positive bipolar cells colocalize with glycine, indicating Cx36 expression in On CBs (examples indicated with arrows). However, β-gal-positive bipolar cells without glycine indicate Cx36 expression by another type of CB (examples indicated by arrowheads). (B and C) Muller glia and rod bipolar cells do not express β-gal. β-gal-positive neurons in the bipolar cell layer do not colocalize with the Muller glia marker CRALBP or the RBC marker PKCα. PKCα also labels a small number of amacrine cells (star) that do not overlap with the β-gal-positive amacrine cells. (D) In Cx36^+/– mice, β-gal-positive amacrine cells located at the most vitreal edge of the INL also colocalize with GLYT1 (examples indicated with arrows), confirming the identification of these cells as AII amacrines. (E and F) Glycine is readily detected in +/– (E) but not KO (F) bipolar cells, suggesting that the gap junctions between AII amacrines and On CBs require Cx36. Sections were counterstained for GLYT1 to mark glycinergic amacrines.. Scale bars equal 10 μm.

Figure 5. Gap Junctions between AIIs and AII and On CBs Are Disrupted

(A and B) In WT mouse retina, neurobiotin (mw = 284 Da) injected into individual AII amacrine cells is detectable both in adjacent AIIs (A) and in the overlying CBs (B), consistent with the notion that gap junctions couple these cell types. (D and E) In dramatic contrast, neurobiotin is completely restricted to the injected AII in the Cx36 KO. (C, F, and G) Vertical sections were used to confirm the identities of injected cells. Scale bars equal 10 μm.

Figure 6. Responses of On-Center Ganglion Cells in WT and Cx36 KO Retina

(A) Representative spike trains recorded extracellularly from on-center ganglion cells in WT (left) and Cx36 KO (right) retina to a 500 ms step of full-field illumination of different intensities. Stimulus onset and offset are indicated by the step functions beneath each row of recordings. Both ganglion cells have similar response components, but KO cell is about 100-fold less sensitive than the WT cell. (B) Normalized responses of WT on-center ganglion cells as a function of light intensity. Each data point shows the average and standard error for a number of cells. The data were fit by Michaelis-Menten equations as described in the Experimental Procedures. Responses fell into four groups: high sensitivity (squares, n = 18), intermediate sensitivity (circles, n = 30), low sensitivity (triangles, n = 9), and wide operating range (diamonds, n = 6). Symbols along the abscissa indicate the response thresholds for each class of cell using a 5% of maximum response criterion. (C) Normalized responses of KO on-center ganglion cells as a function of stimulus intensity. All KO on-center ganglion cell had response characteristics similar to WT low sensitivity cells. The open square along the abscissa indicates the threshold of cells in the KO. Symbols indicating the thresholds of WT cell classes are provided for comparison.

Figure 7. Characterization of the Cone Threshold Using Flickering and Paired-Pulse Light Stimuli

(A) Power spectra for the ganglion cell responses to the 10 Hz flickering light stimuli of increasing light intensity. The response to dim flickering light (4.7 Rh*/rod/s) showed no 10 Hz signal above background noise. The 10 Hz signals are first seen in responses to light at 31 Rh*/rod/s and increases at greater light intensities. Asterisks indicate 10 Hz frequency peaks. (B) Scatter plot comparison of the normalized 10 Hz signals in the power spectra of the responses to 10 Hz flickering light. As seen in (A), 10 Hz signals are not seen with stimuli less than 31 Rh*/rod/s (WT threshold, filled circle; KO threshold, open square). (C) Extracellular recordings from a WT on-center ganglion cell. Presentation of the paired-pulse light stimulus is indicated by the light trace beneath each panel of records. The saturating pulse was followed by a pulse of varying intensity as indicated under each trace. (D) Normalized response of the cell as a function of the intensity of the second flash. The threshold for WT cone photoreceptors was 31.2 Rh*/rod/s (filled circle; KO threshold, open square).