This protocol demonstrates how to perform whole-cell patch clamp recording on retinal neurons from a flat-mount preparation.
The mammalian retina is a layered tissue composed of multiple neuronal types. To understand how visual signals are processed within its intricate synaptic network, electrophysiological recordings are frequently used to study connections among individual neurons. We have optimized a flat-mount preparation for patch clamp recording of genetically marked neurons in both GCL (ganglion cell layer) and INL (inner nuclear layer) of mouse retinas. Recording INL neurons in flat-mounts is favored over slices because both vertical and lateral connections are preserved in the former configuration, allowing retinal circuits with large lateral components to be studied. We have used this procedure to compare responses of mirror-partnered neurons in retinas such as the cholinergic starburst amacrine cells (SACs).
As an easily accessible part of the central nervous system, the retina has for decades been a useful model in neuroscience studies. Genetic marking of neurons has allowed detailed characterization of synaptic connections in the retina. With many methodologies available to examine function and morphology of retinal neurons, the patch clamp recording technique has been instrumental in our current understanding of vertically transmitted signals in the retina. These signals are originated from photon absorption in photoreceptors and sent to brain visual centers through spiking of retinal ganglion cells (RGCs). Despite a large body of knowledge accumulated thus far, neural diversity in vascularized mammalian retina remains unsolved and obstructs the full appreciation of retinal circuits that subserve normal vision. This is in part because most recordings were performed on retinal slices to trade lateral circuit integrity for access to more proximal retinal neurons1-3. To gain a comprehensive picture on how retina computes visual signals, it is thus desirable to record neurons in flat-mounts wherein lateral connections, large and small, may be better preserved.
When synaptic transmission from photoreceptors to bipolar cells is interrupted due to a defective metabotropic glutamate receptor 6 (mGluR6) signaling pathway in depolarizing bipolar cells4-6 or simply as the result of photoreceptor loss in degenerated retinas7-10, many RGCs exhibit oscillatory activities. These oscillations originate from multiple sources, however the one involving gap junction coupling between AII amacrine cells (AII-ACs) and depolarizing cone bipolar cells (DCBCs) has received the most attention and hence is best understood1,7,11. We have found another source, which persists under pharmacological blockade of the aforementioned AII-AC/DCBC network and drives oscillation of OFF-type SACs in RhoΔCTA and Nob mice with deafferentated retinas7,8,12. Here we detail our protocol of preparing retinal flat-mounts for INL neuron recording. This approach uses commercial mouse lines (Jax stock no. 006410 and 007905) to mark cholinergic retinal neurons by fluorescent protein (tdTomato) expression that is identifiable under a fluorescent microscope equipped with contrast enhancing optics. Some experimental results acquired through this approach have been previously reported4,5,7,13.
Ethical approval – procedures involving animal subjects were conducted in accordance with the rules and regulations of the National Institutes of Health guidelines for research animals, as approved by the institutional animal care and use committee of Baylor College of Medicine.
1. External and Internal Solutions
2. Preparation for the Day of Recording
3. Retina Dissection
4. Whole-cell Patch Clamp Recording from Flat-mount Retina
Representative recordings of ON- and OFF-type SACs from a deafferentated mouse retina are shown in Figure 1. Cholinergic cells in both GCL and INL can be reliably identified by tdTomato fluorescence and targeted for whole-cell patch clamp recording under DIC (Figure 1A) to reveal oscillation of their membrane potentials (top traces) and the synaptic currents that drive it (bottom traces, Figure 1B). Inhibitory and excitatory synaptic currents are revealed by holding the cells at 0 mV and -75 mV, respectively, under voltage clamp conditions (Figure 1B, bottom traces). Typical SAC dendritic arborization and stratification levels in the IPL (Figure 1C) can be visualized by post hoc streptavidin staining for the internally dialyzed biocytin. The rhythmicity and frequency of membrane potential fluctuation and postsynaptic current changes can be quantified by calculating the power spectral density. Pharmacological blockade, or the lack thereof, can be used to discern different synaptic mechanisms underlying membrane potential oscillation7.
Figure 1. Morphology and membrane potentials of ON- and OFF-starburst amacrine cell in the Nob mouse retina. (A) Both ON-SACs at GCL and OFF-SACs at INL were readily identified by Cre-driven tdTomato expression and targeted for recording under DIC. Scale bars equal 20 µm as indicated. (B) The top traces are membrane potential changes recorded from ON- and OFF-SACs as indicated. The bottom traces are representative rhythmic excitatory postsynaptic current (EPSC) that drive the oscillation of membrane potential and the arrhythmic inhibitory postsynaptic current (IPSC). (C) Post hoc histological characterization of recorded cells indicates the typical SAC dendritic morphology and stratification levels in the IPL. Scale bars equal 50 µm as indicated. Please click here to view a larger version of this figure.
Many labs have recorded from GCL neurons in the flat-mount preparation15-18, but our procedures allow recording from INL neurons. We hereby emphasize several steps that are critical for successful routine recordings.
The freshness and flatness of the retina are important for penetrating it with a recording pipette. In this regard, the firm attachment of the retina to the punched nitrocellulose membrane is paramount and is achieved by transient absorption of solution followed by timely rehydration (Step 3.4-3.6). During this short period, usually less than 30 sec, the vitreous behaves like a loose jelly and can be peeled off. Our technique is more efficient when compared to several published procedures, where the vitreous is removed manually in solution17,19 or by enzymatic actions18,20. Another frequently used method is to tear the inner limiting membrane off using an empty patch pipette for each cell to be recorded21,22. We did not pursue the tearing method for fear of dismounting the retina. However, peeling the somewhat transparent and jelly-like vitreous off may at times dislodge the retina from the membrane. This is therefore a step which requires practice as the retention of retina on the nitrocellulose membrane is essential for recording INL neurons. A good practice is to use well-flattened and fully hydrated nitrocellulose membrane. A point worth noting here is the apparent trade-off between the recordable area (i.e., the size of a punched hole), the flatness of the retina, and the firmness of attachment to the membrane. A larger recording window is preferred but a larger punched hole means that there is less membrane area for the retina to attach to. Similarly, mounting the retina over a smaller hole ensures firm attachment but flatness may be reduced, and so is recordable area.
To insert an electrode through the vitreous into the GCL and INL (Step 4.4), we apply a positive pressure to prevent electrode jamming. The immediate reduction of this pressure upon penetration into the retina is also critical for reducing staining background and for preserving oscillation. An important check point is to shorten the time between penetration and obtaining the seal, preferably less than 30 sec for GCL neurons and within 60 sec for INL neurons. Another important point is the 5 min wait period after the formation of the giga-ohm seal and before the membrane ruptures. This wait period ensures the clearing of the spilled internal solution in the path of the electrode and is especially relevant when a potassium based internal solution is used because the high potassium content may temporarily depolarize neighboring neurons and disturb oscillation. Finally, a somewhat unconventional feature of our approach (Step 4.7) is that we approach a cell, form a giga-ohm seal, and then gain whole-cell access under the current clamp mode, as advised by Dr. Rory McQuiston of Virginia Commonwealth University, who helped us with our initial electrophysiological recordings. This method allows the quick resistance change upon break-in to be captured by voltage changes (visible through an oscilloscope) and protects the recorded cell from the sudden swing of membrane potential at the whole-cell level. Another useful feature of our method is the negative current applied to the electrode tip, which helps attract positively charged membrane phospholipids and facilitates seal formation.
With regard to preserving lateral retinal circuits, a horizontally sliced preparation of the retina20 has been suggested. While this method efficiently exposes the horizontally expanding dendrites and synaptic connections in the IPL for recording and imaging, the vertical pathway is unfortunately disrupted and hence this method can only be used for limited purposes. Finally, the refinements we implement in tissue preparation and recording procedures allow routine targeted recordings of INL neurons such as OFF-SACs7. By incorporating two-photon imaging and contrast enhancing microscopy, targeting the bipolar cells and horizontal cells near the outer plexiform layer for patch clamp recording under various lighting conditions is now considered feasible in a flat-mount preparation.
The authors have nothing to disclose.
We thank Joung Jang and Xin Guan for technical assistance. We thank Dr. Rory McQuiston of Virginia Commonwealth University for setting up our first patch clamp rig and advices on experimental procedures. We thank Dr. Samuel Wu for suggestions on voltage clamp recording. The work is supported by NIH grants EY013811, EY022228 and a vision core grant EY002520. C-KC is the Alice R. McPherson Retina Research Foundation Endowed Chair at the Baylor College of Medicine.
Fixed-stage fluorescent microscope with DIC | Olympus | BX51-WI | |
Micromanipulators | Sutter | MP-225 | |
Patch clamp amplifier | A-M System | AM2400 | |
AD converter | National Instrument | NI-USB-6221 | |
Heater controller | Warner Instrument | TC-324B | |
Inline heater | Warner Instrument | SC-20 | |
Peristaltic pump | Rainin | Dynamax | |
pipette puller | Sutter Instrument | P-1000 | |
Glass tube with filament | King Precision Glass | Customized | |
Stimulator | A.M.P.I. | Master-8 | |
Biocytin | Sigma | B4261 | |
NaCl | Sigma | S6191 | |
KCl | Sigma | P5405 | |
NaHCO3 | Fisher | BP328-1 | |
Na2HPO4 | Sigma | S0876 | |
NaH2PO4 | Sigma | S5011 | |
CaCl2 | Sigma | C5670 | |
MgSO4 | Sigma | M1880 | |
D-glucose | Sigma | G6152 | |
K-gluconate | Sigma | G4500 | |
ATP-Mg | Sigma | A9187 | |
Li-GTP | Sigma | G5884 | |
EGTA | Sigma | E0396 | |
HEPES | Sigma | H4034 | |
KOH | Sigma | P5958 | |
Cs-methanesulfonate | Sigma | C1426 | |
CsOH | Sigma | 232041 | |
Syringer filter | Nalgene | 171 | |
1 ml syring | Rainin | 17013002 | |
10 ul pipette tip | Genesee Scientific | 24-130RL | |
Streptavidin-488 | ThermoFisher | S-11223 | |
10X PBS | Lonza | 17-517Q |