Here, we present a protocol that simplifies the measurement of light evoked electroretinogram responses from larval zebrafish. A novel cone-shaped sponge-tip electrode can help to make the study of visual development in larval zebrafish using the electroretinogram ERG easier to achieve with reliable outcomes and lower cost.
The zebrafish (Danio rerio) is commonly used as a vertebrate model in developmental studies and is particularly suitable for visual neuroscience. For functional measurements of visual performance, electroretinography (ERG) is an ideal non-invasive method, which has been well established in higher vertebrate species. This approach is increasingly being used for examining the visual function in zebrafish, including during the early developmental larval stages. However, the most commonly used recording electrode for larval zebrafish ERG to date is the glass micropipette electrode, which requires specialized equipment for its manufacture, presenting a challenge for laboratories with limited resources. Here, we present a larval zebrafish ERG protocol using a cone-shaped sponge-tip electrode. The novel electrode is easier to manufacture and handle, more economical, and less likely to damage the larval eye than the glass micropipette. Like previously published ERG methods, the current protocol can assess outer retinal function through photoreceptor and bipolar cell responses, the a- and b-wave, respectively. The protocol can clearly illustrate the refinement of visual function throughout the early development of zebrafish larvae, supporting the utility, sensitivity, and reliability of the novel electrode. The simplified electrode is particularly useful when establishing a new ERG system or modifying existing small-animal ERG apparatus for zebrafish measurement, aiding researchers in the visual neurosciences to use the zebrafish model organism.
The zebrafish (Danio rerio) has become a widely used genetic vertebrate model, including studies of the visual neurosciences. The increasing popularity of this species can be attributed to advantages including ease of genetic manipulation, the highly conserved vertebrate visual system (neuron types, anatomical morphology and organization, and underlying genetics), high fecundity and lower cost of husbandry compared to mammalian models1. The non-invasive electroretinogram (ERG) has long been used clinically to assess human visual function, and in the laboratory setting to quantify vision in a range of large and small species including rodents and larval zebrafish2,3,4,5. The most commonly analyzed ERG components are the a-wave and b-wave, originating from the light-sensing photoreceptors and bipolar interneurons, respectively. In larval zebrafish, distinct layers in the retina are established by 3 days post-fertilization (dpf) and the morphology of the photoreceptor cone terminal synapses mature before 4 dpf6,7. Outer retinal function of larval zebrafish is thus established before 4 dpf, meaning that the ERG is measurable from this early age onwards. Because of the short experimental cycle and the high-throughput properties of the model, the ERG has been applied to larval zebrafish for functional assessment of disease models, analyzing color vision and retinal development, studying visual circadian rhythms and testing drugs8,9,10,11,12.
However, current approaches for larval zebrafish ERG has some complexities that may make it harder to adopt. Published larval zebrafish ERG protocols commonly use a glass micropipette filled with conductive liquid as the recording electrode3,4,5,13, which requires a high quality micropipette tip3. Specialized equipment, such as a micropipette puller and in some cases a microforge, are required for their manufacture. This can be a challenge for laboratories with limited resources and leads to extra costs even when adapting available small animal ERG systems for measurement of larval zebrafish visual function. Even when smoothed, the sharp micropipette tip can damage the surface of the larval eye. Additionally, commercial micropipette holders for electrophysiology are constructed with a fixed silver wire. These fixed wires become passivated after repetitive use, requiring the purchase of new holders leading to increased maintenance costs.
Here we describe an ERG method using a cone-shaped sponge-tip recording electrode, that is particularly useful for adapting established small-animal ERG setups for larval zebrafish ERG measurements. The electrode is easily made using common polyvinyl acetate (PVA) sponge and fine silver wire without any other specialized equipment. Our data show that this novel electrode is sensitive and reliable enough to demonstrate the functional development of retinal neural circuits in larval zebrafish between 4 and 7 dpf. This economical and practical sponge-tip electrode may be useful to researchers establishing new ERG systems or modifying existing small-animal systems, for zebrafish studies.
All electroretinogram (ERG) procedures were performed according to the provisions of the Australian National Health and Medical Research Council code of practice for the care and use of animals and were approved by the institutional animal ethics committee at the University of Melbourne.
1. Buffer Preparation
2. Electrode Preparation
3. Zebrafish Preparation
4. Sponge Platform Preparation
5. Animal and Electrode Positioning
6. Electroretinogram Recording
7. Analysis
This section provides representative results for ERG measurements taken daily from 4 to 7 dpf. From 4 dpf, ERG responses show robust a- and b-wave components, which arise from photoreceptors and bipolar cells, respectively. At each age tested, the amplitude of the b-wave increased with light intensity (Figure 2; Figure 3). Notably, the sensitivity of the larval zebrafish retina to dimmer flashes increased with age. The a- and b-wave were not recognizable at intensities lower than -1.61 log cd.s/m2 at 4 dpf, whereas clear signals were detectable at these intensities for older larvae (Figure 2). The b-wave response grew substantially between 4 and 5 dpf (P < 0.0001; Figure 2A-B; Figure 3B). Although the b-wave at lower intensities showed little change between 5 and 7 dpf, the signal at 2.48 log cd.s/m2 was greater at 7 dpf compared with 5 and 6 dpf (P < 0.0001; Figure 2; Figure 3B). A- and b-wave implicit times became significantly faster after 5 dpf (P < 0.0001; Figure 3C-D). Overall, these results demonstrate maturation of zebrafish retinal function between 4 to 7 dpf. Interestingly, the a-wave amplitude appeared to decrease from 5 to 7 dpf (Figure 3A). This may be because the maturation of synaptic connections in the outer retina shortens the latency of bipolar cells responses, resulting in faster b-wave onset that masks the a-wave. Those wishing to study the a-wave can employ pharmacological treatment to block post-photoreceptoral responses (i.e. the b-wave component).
Figure 1: Zebrafish Ganzfeld ERG set up with the cone-shaped sponge-tip electrode. (A) The cone-shaped sponge tip and the chlorinated silver electrode are air dried before constructing the sponge-tip electrode. (B-C) Subsequently, the chlorinated silver wire is inserted into the sponge cone through the base to form the complete electrode. (D) In the typical larval zebrafish Ganzfeld ERG setup, the reference electrode is inserted into the sponge platform and the zebrafish larva is covered by the Ganzfeld bowl. (E) The sponge-tip electrode gently touches the central corneal surface of the larval eye. Please click here to view a larger version of this figure.
Figure 2: Representative average ERG traces of wild-type larval zebrafish. Average ERG traces of wildtype zebrafish at (A) 4 dpf (n = 8), (B) 5 dpf (n = 8), (C) 6 dpf (n = 7), and (D) 7 dpf (n = 9). Responses were elicited using flashes from white LEDs. At each age, the traces show responses to flashes of (from bottom to top) -2.75, -2.11, -1.61, -0.81, 0.06, 0.72, 1.55, 1.89, 2.18, 2.48 log cd.s/m2. Scale bar = 50 µV. Please click here to view a larger version of this figure.
Figure 3: ERG a- and b-wave amplitudes and implicit times for 4 to 7 dpf zebrafish. (A) Group average (± standard error of the mean) a-wave amplitude increased with flash intensity but decreased with age in 4–7 dpf larvae. (B) Average b-wave amplitude in 4–7 dpf larvae increased with flash intensity; amplitude grew between 4 and 5 dpf. (C) Average a-wave implicit time and (D) average b-wave implicit time became faster between 5, 6 and 7 dpf. Lines of best fit are derived from non-linear regression. Please click here to view a larger version of this figure.
Stimulus light intensity (log cd.s.m-2) | Number of repeats | Inter-stimulus interval (s) |
-2.75 | 3 to 6 | 10 |
(30 s before next) | ||
-2.11 | 3 to 6 | 10 |
(30 s before next) | ||
-1.61 | 3 to 6 | 10 |
(30 s before next) | ||
-0.81 | 3 to 6 | 10 |
(60 s before next) | ||
0.06 | 3 to 6 | 10 |
(60 s before next) | ||
0.72 | 1 to 3 | 60 |
1.55 | 1 to 3 | 60 |
1.89 | 1 to 3 | 60 |
2.18 | 1 to 3 | 60 |
2.48 | 1 to 3 | 60 |
Table 1: Example protocol of ERG recordings. Stimulus presentations start from the dimmest (top) and progress to brighter (bottom) light levels, with progressively longer inter-stimulus intervals to ensure that dark adaption is maintained. The number of signals averaged at each intensity depends on the signal-to-noise level.
Functional readouts such as the ERG have become increasingly important in the suite of tools used to study larval zebrafish8,9,12,14. Due to the small size of the larval zebrafish eye, glass micropipettes have been adapted as recording electrodes in most published protocols3,4,5,8,9,12,13,14. Here we describe a larval zebrafish ERG protocol using a simpler cone-shaped sponge-tip electrode. The novel electrode can be used to modify standard small-animal ERG systems to measure larval zebrafish retinal function without any additional equipment. The materials for making the sponge-tip electrode are simply commercial PVA sponge and 0.3 mm silver wire, which makes this more economical than previous approaches. Another advantage is that, in contrast to the hard and sharp micropipette tip, the gentler electrode sponge tip is less likely to damage the larval eye. Finally, the PVA sponge helps to maintain moisture to the larval eye throughout the recording.
The key to successful application of the sponge-tip electrode is to ensure full saturation of the sponge. This normally takes no less than 15 minutes of soaking in 1x goldfish Ringer’s buffer. Incomplete saturation of the sponge can increase the noise level owing to faster drying of the electrode. For better signal collection, making new electrodes for each experimental session (generally < 8 h) is highly recommended. Repeat use can lead to reduced ERG signals, making inter-session comparisons more difficult.
When positioning the larval zebrafish onto the sponge platform, care must be taken to ensure that the eye to be measured is not in contact with any surrounding solution or the paper towel underneath the fish. Such contact shorts the electrical circuit, as the reference electrode is embedded in the sponge platform and reduces the ERG.
Even with well saturated electrode sponge tips, gradual drying occurs, which is evident as increased noise in ERG signals. Should this occur, drip one drop of 1x goldfish Ringer’s onto the base of the cone using 1 mL syringe and a 30 G x ½" needle. If adding the solution to the sponge tip does not reduce the noise level, check that the eye is not in contact with a surrounding fluid and ensure that the electrode tip is centered on the corneal apex.
The recordings in the representative results reported here were made with a bandpass setting of 1–300 Hz, which does not allow sampling of oscillatory potentials (OP)—wavelets on the b-wave derived from the third-order retinal neurons including amacrine and ganglion cells 15,16,17. A higher lowpass setting (e.g., 500 or 1,000 Hz) may be better suited for OP recording.
In summary, the cone-shaped sponge-tip electrode helps to simplify larval zebrafish ERG recording with existing small-animal ERG systems, providing reliable results. Representative results demonstrate that ERG amplitude grows between 4 and 5 dpf, with further maturation between 5 and 7 dpf manifesting as faster implicit times. Our simple ERG protocol with the economical and practical cone-shaped sponge-tip electrode can benefit investigators studying zebrafish retinal function. The technique can also be adapted to assess adult zebrafish or other vertebrate models with small eyes.
The authors have nothing to disclose.
Funding for this project was provided by a grant from the Melbourne Neuroscience Institute (to PTG, PRJ & BVB).
0.22 µm filter | Millex GP | SLGP033RS | Filters the 10× goldfish ringer's buffer for sterilizatio |
1-mL syringe | Terumo | DVR-5175 | With a 30G × ½" needle to add drops of saline to the electrode sponge tip to prevent drying and increased noisein the ERG signals. |
30G × ½" needle | Terumo | NN*3013R | For adding saline toteh sopnge tip electrode. |
Bioamplifier | ADInstruments | ML135 | For amplifying ERG signals. |
Bleach solution | King White | 9333441000973 | For an alternative method of sliver electrode chlorination. Active ingredient: 42 g/L sodium hypochlorite. |
Circulation water bath | Lauda-Königshoffen | MGW Lauda | Used to make the water-heated platfrom. |
Electrode lead | Grass Telefactor | F-E2-30 | Platinum cables for connecting silver wire electrodes to the amplifier. |
Faraday Cage | Photometric Solution International | For maintianing dark adaptation and enclosing the Ganzfeld setup to improve signal-to-noise ratio. | |
Ganzfeld Bowl | Photometric Solution International | Custom designed light stimulator: 36 mm diameter, 13 cm aperture size. | |
Luxeon LEDs | Phillips Light Co. | For light stimulation twenty 5W and one 1W LEDs. | |
Micromanipulator | Harvard Apparatus | BS4 50-2625 | Holds the recording electrode during experiments. |
Microsoft Office Excel | Microsoft | version 2010 | Spreadsheet software for data analysis. |
Moisturizing eye gel | GenTeal Gel | 9319099315560 | Used to cover zebrafish larvae during recordings to avoiding dehydration. Active ingredient: 0.3 % Hypromellose and 0.22 % carbomer 980. |
Pasteur pipette | Copan | 200C | Used to caredully transfer larval zebrafish. |
Powerlab data acquisition system | ADInstruments | ML785 | Controls the LEDs to generate stimuli. |
PVA sponge | MeiCheLe | R-1675 | For the placement of larval zebrafish and making the cone-shaped electrode ti |
Saline solution | Aaxis Pacific | 13317002 | For electroplating silver wire electrode. |
Scope Software | ADInstruments | version 3.7.6 | Simultaneously triggers the stimulus through the Powerlab system and collects data |
Silver (fine round wire) | A&E metal | 0.3 mm | Used to make recording and reference ERG electrodes. |
Stereo microscope | Leica | M80 | Used to shape and measure the cone-shaped sponge apex (with scale bar on eyepiece). Positioned in the Faraday cage for electrode placement. |
Tricaine | Sigma-aldrich | E10521-50G | For anaethetizing larval zebrafish. |
Water-heated platform | custom-made | For maintianing the temperature of the sponge platform and the larval body during ERG recordings |