We show surgical implantation and Recording procedures to measure visual electrophysiological signals from the eye (electroretinogram) and brain (visual evoked potential) in conscious rats, which is more analogous to the human condition where recordings are conducted without anesthesia confounds.
The full-field electroretinogram (ERG) and visual evoked potential (VEP) are useful tools to assess retinal and visual pathway integrity in both laboratory and clinical settings. Currently, preclinical ERG and VEP measurements are performed with anesthesia to ensure stable electrode placements. However, the very presence of anesthesia has been shown to contaminate normal physiological responses. To overcome these anesthesia confounds, we develop a novel platform to assay ERG and VEP in conscious rats. Electrodes are surgically implanted sub-conjunctivally on the eye to assay the ERG and epidurally over the visual cortex to measure the VEP. A range of amplitude and sensitivity/timing parameters are assayed for both the ERG and VEP at increasing luminous energies. The ERG and VEP signals are shown to be stable and repeatable for at least 4 weeks post surgical implantation. This ability to record ERG and VEP signals without anesthesia confounds in the preclinical setting should provide superior translation to clinical data.
The ERG and VEP are minimally invasive in vivo tools to assess the integrity of retinal and visual pathways respectively in both the laboratory and clinic. The full-field ERG yields a characteristic waveform which can be broken down into different components, with each element representing different cell classes of the retinal pathway1,2. The classic full-field ERG waveform consists of an initial negative slope (a-wave), which has been shown to represent photoreceptor activity post light exposure2-4. The a-wave is followed by a substantial positive waveform (b-wave) which reflects electrical activity of middle retina, predominantly the ON-bipolar cells5-7. Furthermore, one can vary luminous energy and inter-stimulus-interval to isolate cone from rod responses8.
The flash VEP represents electrical potentials of the visual cortex and brain stem in response to retinal light stimulation9,10. This waveform can be broken down into early and late components, with the early component reflecting activity of neurons of the retino-geniculo-striate pathway11-13 and the late component representing cortical processing performed in various V1 laminae in rats11,13. Therefore simultaneous measurement of the ERG and VEP returns comprehensive assessment of the structures involved in the visual pathway.
Currently, in order to record electrophysiology in animals, anesthesia is employed to enable stable placement of electrodes. There have been attempts to measure ERG and VEP in conscious rats14-16 but these studies employed a wired setup, which can be cumbersome and may lead to animal stress by restricting animal movement and natural behavior17. With recent advances in wireless technology including improved miniaturization and battery life, it is now possible to implement a telemetry approach for ERG and VEP recording, decreasing the stress associated with wired recordings and improving long term viability. Fully internalized stable implantations of telemetry probes have proven to be successful for chronic monitoring of temperature, blood pressure18, activity19 as well as electroencephalography20. Such advances in technology will also assist with repeatability and stability of conscious recordings, increasing the platform's utility for chronic studies.
Ethics statement: Animal experiments were conducted in accordance with the Australian Code for the Care and Use of Animals for Scientific Purposes (2013). Animal ethics approval was obtained from the Animal Ethics Committee, University of Melbourne. The materials herein are for laboratory experiments only, and not intended for medical or veterinary use.
1. Preparing Electrodes
Note: A three channel transmitter is used for surgical implantation which enables 2 ERG and 1 VEP recording to be conducted simultaneously. The three active and three inactive electrodes need to be pre-fashioned into a ring shape before implantation in order to attach to the eye. For identification purposes, the manufacturer has enclosed active electrodes in half white, half colored plastic sheaths while inactive electrodes are covered in full colored sheaths. The ground electrode (clear plastic sheath) is left unaltered. For all active and inactive electrodes conduct steps 1.1, 1.2, 1.3 and 1.7.
2. Transmitter Implantation
3. Conduct ERG and VEP Recordings in Conscious Rats
The photoreceptor response is analyzed by fitting a delayed Gaussian to the leading edge of the initial descending limb of the ERG response at the top 2 luminous energies (1.20, 1.52 log c.s.m-2) for each animal, based on the model of Lamb and Pugh22, formulated by Hood and Birch23. This formula returns an amplitude and a sensitivity parameter, (Figure 1C and 1D, respectively). A hyperbolic function was fitted to the luminous energy response of rod bipolar cells for each animal, which also returned an amplitude and a sensitivity parameter, (Figure 1E and 1F respectively). Cone bipolar cell amplitude was analyzed as peak response of the waveform (top waveform of Figure 1A and 1B), with implicit time taken as the time it took to reach peak response. For further details please see Charng et al. 24.
Figure 1A and B shows ERG waveform ± SEMs (n = 8) in conscious rats at day 7 and 28 post-surgery. The waveforms appear to be slightly larger at day 28 compared to day 7, but linear mixed model analysis revealed no significant time effect (p = 0.14 to 0.67) for photoreceptor (dark-adapted PIII) amplitude (Figure 1C) and sensitivity (Figure 1D); rod bipolar cell (dark-adapted PII) amplitude (Figure 1E) and sensitivity (Figure 1F); cone bipolar cell (light-adapted PII) amplitude (Figure 1G) and implicit time (Figure 1H). Similarly, VEP waveform SEMs (n = 8, Figure 2A) appear comparable at 7 and 28 days post-surgery, with amplitude (Figure 2B and 2C) and timing (Figure 2D – 2F) parameters showing no significant time effect (p = 0.20 to 0.93). These results indicate robust ERG and VEP signal stability.
Average signal-to-noise (SNR, n = 8) ratio of both ERG (Figure 3A) and VEP (Figure 3B) returned good stability over the five conscious recording sessions. In this scenario, ERG signal is defined as the as the amplitude of the ERG P2 response while noise is the maximal peak to trough amplitude calculated from a 10 msec pre-stimulus interval. In the VEP, P2-N1 amplitude is deemed as the signal while noise is also returned by the peak to trough of the 10 msec pre-stimulus interval. There was no significant time effect across the SNR of both ERG and VEP (p = 0.49 and 0.62 respectively).
Figure 1: Conscious Electroretinograms Exhibit Characteristic Waveforms and Repeatable Measurements. (A–B) ERG waveforms ± SEMs (n = 8) across a wide range of luminous energies at day 7 (A) and 28 (B) post-surgery. (C-F) Rod and cone ERG parameters are plotted against time after implantation. Rod (dark-adapted PIII) photoreceptoral amplitude (C) and sensitivity (D), rod bipolar cell (dark-adapted PII) amplitude (E) and sensitivity (F), and cone bipolar cell (light-adapted PII) amplitude (G) and implicit time (H) all showed stable recordings over the 5 sessions. All symbols indicate average value (± SEM). This figure has been modified from Charng et al.24 Figure 4. Please click here to view a larger version of this figure.
Figure 2: Conscious Visual Evoked Potentials Exhibit Characteristic Waveforms and Repeatable Measurements. (A) VEP waveforms ± SEMs (n = 8) are plotted at day 7 and 28 post surgery. (B–F) VEP amplitude and timing parameters are assessed over 1 month after implantation. P1-N1 (B) and P2-N1 (C) amplitude as well as P1 (D), N1 (E) and P2 (F) implicit time parameters were all stable over the 5 recording sessions. All symbols indicate average value (± SEM). This figure has been modified from Charng et al.24 Figure 6. Please click here to view a larger version of this figure.
Figure 3: The Telemetry System Demonstrates Stable Signal-to-Noise Ratio Over Time. The signal-to-noise ratio of the (A) ERG and (B) VEP were not significantly altered over time (n = 8). All symbols indicate average value (± SEM). This figure has been modified from Charng et al.24 Figure S1. Please click here to view a larger version of this figure.
Due to the minimally invasive nature of visual electrophysiology, ERG and VEP recordings in human patients are conducted under conscious conditions and only require the use of topical anesthetics for electrode placement. In contrast, visual electrophysiology in animal models is conventionally conducted under general anesthesia to enable stable electrode placement by eliminating voluntary eye and body movements. However, commonly used general anesthetics alter the ERG and VEP responses as shown by our previous publication24 and others25-27. As such development of a conscious ERG and VEP platform in a rodent model provides superior representation of physiological responses in animal models, which may in turn afford better translatability from preclinical to clinical findings. Another disadvantage of using anesthesia is that it limits the duration of an experiment. More specifically, the use of prolonged anesthesia as well as repeated administration of anesthetics can increase chance of adverse effects such as drug build up and associated respiratory problems28.
This study showed that the telemetry system in conscious rats returned robust ERG and VEP signal stability for at least 28 days post-surgery. Our group is the first to conduct conscious wireless ERG and VEP responses simultaneously24 and this manuscript details the surgical and recording procedures involved. Comparison to other surgical procedures conducted with wired conscious ERG and VEP recordings illustrate superior stability in the ERG and equivalent repeatability in VEP recordings over a 1 month period15.
The surgical techniques and subsequent conscious recordings have the potential to be applied to various animal models. The platform has potential utility in multiple applications where it is beneficial to avoid confounds associated with anesthesia29. These include drug discovery, improved translation to human studies, and chronic or longitudinal experiments.
Possible modifications to the technique include altering the number of biopotential channels implanted and simultaneously recorded. This can vary from 1 to 4 biopotential leads and thus could measure visual evoked electrophysiology from between 1 eye to 2 eyes and 2 visual cortices. Note that the alteration in the number of biopotentials channels also leads to modification of the band-width recorded which will have implications for high frequency electrophysiological signals. For example the 3 channel biopotential transmitter used in this study (F50-EEE) was chosen to show that it is possible to simultaneously record visually evoked responses from the retina and visual cortex of a conscious rat. However, these 3 channel transmitters have a band-width of 1 – 100 Hz, which can faithfully record ERG a- and b-waves but will alter oscillatory potentials due to their higher frequency24. In contrast, if it was of interest to the study to record oscillatory potentials then a transmitter with less recording channels (i.e., broader band-width) could be employed. It is also possible for the light stimulus to be altered, for example instead of conducting full-field ERG and VEP, visual physiology in response to flicker stimuli may also be utilized.
One major limitation in translating this technique to other animal models is the size of the animal's eye. One should have no problem implanting the ocular electrodes to animals bigger than rats. However, it would be challenging to implant the ERG electrode onto a mouse eye due to the smaller working area. The cortical implantation, on the other hand, should be relatively straightforward to perform in most laboratory animals.
There are several aspects of the surgery that need to be closely observed to ensure successful implantation. It is imperative that the ERG electrode ring is formed into a smooth ring due to irritation that may be induced by any rough edges on the loop. The implantation of ERG active electrodes is facilitated by two concurrent experimenters, one to stabilize the eye while the other attaches the electrode to the sclera. Particular care has to be taken to ensure the scleral suture (2.2.19) is only half-thickness, as a full thickness scleral suture will puncture the eyeball and cause vitreous leakage. The implantation of electrodes onto the skull (VEP active and ERG/VEP inactive electrodes) is less technically demanding than that of ERG electrodes. Nevertheless, it is imperative that once the electrodes are anchored to the skull, the wires are allowed to uncurl naturally to reduce any unnecessary tension. Acclimatization to the recording restrainer prior to surgical implantation is advantageous to reduce excessive movements during ERG and VEP recordings.
The authors have nothing to disclose.
JC would like to acknowledge the David Hay Memorial Fund, The University of Melbourne for financial support in writing this manuscript. Funding for this project was provided by an ARC Linkage grant 100200129 (BVB, AJV, CTON).
Bioamplifier | ADInstruments | ML 135 | Amplifies ERG and VEP signals |
Carboxymethylcellulose sodium 1.0% | Allergan | CAS 0009000-11-7 | Maintain corneal hydration during surgery |
Carprofen 0.5% | Pfizer Animal Health Group | CAS 53716-49-7 | Post-surgery analgesia, given with injectable saline for fluid replenishment |
Chlorhexadine 0.5% | Orion Laboratories | 27411, 80085 | Disinfection of surgical instrument |
Cyanoacrylate gel activator | RS components | 473-439 | Quickly dries cyanoacrylate gel |
Cyanocrylate gel | RS components | 473-423 | Fix stainless screws to skull |
Dental burr | Storz Instruments, Bausch and Lomb | E0824A | Miniature drill head of ~0.7mm diameter for making a small hole in the skull over each hemisphere to implant VEP screws |
Drill | Bosch | Dremel 300 series | Automatic drill for trepanning |
Enrofloxin | Troy Laboratories | Prophylactic antibiotic post surgey | |
Ganzfeld integrating sphere | Photometric Solutions International | Custom designed light stimulator: 36 mm diameter, 13 cm aperture size | |
Gauze swabs | Multigate Medical Products Pty Ltd | 57-100B | Dries surgical incision and exposed skull surface during surgery |
Isoflurane 99.9% | Abbott Australasia Pty Ltd | CAS 26675-46-7 | Proprietory Name: Isoflo(TM) Inhalation anaaesthetic. Pharmaceutical-grade inhalation anesthetic mixed with oxygen gas for VEP electrode implant surgery |
Kenacomb ointment | Aspen Pharma Pty Ltd | To reduce skin irritation and itching after surgery | |
Luxeon LEDs | Phillips Lighting Co. | For light stimulation, twenty 5 watt and one 1 watt LEDs, controlled by Scope software | |
Needle (macrosurgery) | World Precision Instruments | 501959 | for suturing abdominal and head surgery, used with 3-0 suture, eye needle, cutting edge 5/16 circle Size 1, 15mm |
Needle holder (macrosurgery) | World Precision Instruments | 500224 | To hold needle during abdominal and head surgery |
Needle holder (microsurgery) | World Precision Instruments | 555419NT | To hold needle during ocular surgery |
Optiva catheter | Smiths Medical International LTD | 16 or 21 G | Guide corneal active electrodes from skull to conjunctiva |
Povidone iodine 10% | Sanofi-Aventis | CAS 25655-41-8 | Proprietory name: Betadine, Antiseptic to prepare the shaved skin for surgery 10%, 500 mL |
Powerlab data acquisition system | ADInstruments | ML 785 | Acquire signal from telemetry transmitter, paired to telemetry data converter |
Proxymetacaine 0.5% | Alcon Laboratories | CAS 5875-06-9 | Topical ocular analgesia |
Restrainer | cutom made | Front of the restrainer is tapered to minimize head movement, length can be adjusted to accommodate different rat length, overall diameter is 60 mm. | |
Scapel blade | R.G. Medical Supplies | SNSM0206 | For surgical incision |
Scissors (macrosurgery) | World Precision Instruments | 501225 | for cutting tissue on the abodmen and forhead |
Scissors (microsurgery) | World Precision Instruments | 501232 | To dissect the conjunctiva for electrode attachment |
Scope Software | ADInstruments | version 3.7.6 | Simultaneously triggers the stimulus via the ADI Powerlab system and collects data |
Shaver | Oster | Golden A5 | Shave fur from surgical areas |
Stainless streel screws | MicroFasteners | L001.003CS304 | 0.7 mm shaft diameter, 3 mm in length |
Stereotaxic frame | David Kopf | Model 900 | A small animal stereotaxic instrument for locating the implantation landmarks on the skull |
Surgical drape | Vital Medical Supplies | GM29-612EE | Ensure sterile enviornment during surgery |
Suture (macrosurgery) | Ninbo medical needles | 3-0 | for suturing abdominal and head surgery, sterile silk braided, 60cm |
Suture needle (microsurgery) | Ninbo medical needles | 8-0 or 9-0 | for ocular surgery including, suturing electrode to sclera and closing conjunctival wound, nylon suture, 3/8 circle 1×5, 30cm |
Telemetry data converter | DataSciences International | R08 | allows telemetry signal to interface with data collection software |
Telemetry Data Exchange Matrix | DataSciences International | Gathers data from transmitters, pair with receiver | |
Telemetry data receiver | DataSciences International | RPC-1 | Receives telemetry data from transmitter |
Telemetry transmitter | DataSciences International | F50-EEE | 3 channel telemetry transmitter |
Tropicamide 0.5% | Alcon Laboratories | Iris dilation | |
Tweezers (macrosurgery) | World Precision Instruments | 500092 | Manipulate tissues during abdominal and head surgery |
Tweezers (microsurgery) | World Precision Instruments | 500342 | Manipulate tissues during ocular surgery |