Implantation and Recording of Wireless Electroretinogram and Visual Evoked Potential in Conscious Rats

Summary

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.

Abstract

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.

Introduction

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 pathway^1,2. The classic full-field ERG waveform consists of an initial negative slope (a-wave), which has been shown to represent photoreceptor activity post light exposure^2-4. The a-wave is followed by a substantial positive waveform (b-wave) which reflects electrical activity of middle retina, predominantly the O....

Protocol

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 ....

Results

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 lumin.......

Discussion

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 publication.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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 ~ 0.7 mm 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 W and one 1 W 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, 15 mm
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, 60 cm
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, 30 cm
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