Optimal Electrode Placement for Conscious Wireless Telemetric Electroretinography

Abstract

Purpose: : Conventional rodent electroretinography (ERG) is conducted in anaesthetised animals. However, as anaesthesia alters the ERG response, conscious ERG recordings would be more translatable to the clinical setting. Long term implantation of electrodes for conscious wireless telemetric recording requires comfortable stable placement of electrodes whilst maintaining robust signals. Thus we aim to evaluate the optimal electrode positioning for conscious wireless telemetric ERG recordings.

Methods: : ERGs were conducted on anaesthetised Long-Evans rats (ket:xyl 60:5mg/kg) using implantable transmitters (CA-F40, Data Sciences International) at 0.58log.cd.s.m^-2. Whilst maintaining the active electrode on the central cornea the inactive was moved across the forehead (4 locations along midline; n=6) and compared to the conventional location on the sclera. Subsequently, the inactive electrode was stabilised at 5mm rostral to bregma and the active electrode was systematically moved from anterior to posterior of the globe (7 locations from central cornea to posterior sclera) at superior and nasal locations (n=6).

Results: : Physiotel transmitters provide better signal:noise ratios than conventional silver chloride electrodes (104±24 vs 20±7, p<0.05). Positioning the inactive electrode at the forehead produces larger ERG amplitudes and faster implicit times than at the sclera (F_1,68=19.1, p<0.001). However, different forehead locations did not alter the ERG response (F_3,15=2.01, p=0.15). Moving the active electrode from the corneal apex to the posterior sclera results in progressively smaller ERG amplitudes (F_6,68=10.6, p<0.001). Moving the active along the superior meridian gave larger ERG responses than the nasal meridian (F_1,65=6.69, p<0.05). In summary, an active electrode located superiorly 1mm behind the limbus combined with a forehead inactive electrode resulted in an ERG signal similar to the conventional ERG electrode location of active central cornea and inactive sclera (225±48μV vs 259±40μV, p=0.43).

Conclusions: : Long term implantation of ERG telemetry electrodes requires comfort and stability whilst maintaining signal quality. This can be achieved by placing the active electrode on the sclera thus avoiding the sensitive cornea. The inactive electrode can be placed on the forehead. This combination improves surgical outcomes and maintains robust ERG signals.

Keywords: electroretinography: non-clinical • transplantation • retina