98th Annual Meeting DOG 2000

K 150

The use of the paired-flash ERG technique to investigate rod function

C. Friedburg, M. M. Thomas, T. D. Lamb

Introduction: The a-wave of the electroretinogram (ERG) reflects photoreceptor activity [1]. Later, this activity is confounded by electrical signals originating from post-receptoral cells, resulting in the rise of the b-wave. To dissect the two signals at later times, a paired-flash technique was recently proposed [2]. Here we report on the possibilities and limitations of this technique when investigating rod function.

Methods: Photoreceptors maintain a circulating membrane current that is reduced in response to light, giving rise to the a-wave of the ERG. In the paired-flash technique, changes in the circulating current due to a first, test flash are derived from measures of the remaining current at discrete times. The latter are obtained from the a-wave response to a second, bright flash termed the probe which causes rapid and nearly complete suppression of the circulating current. Ganzfeld ERGs were recorded in mydriasis from 3 normal subjects using DTL electrodes positioned loosely in the lower fornix; these caused no complications and provided stable ERGs for several hours.

Results: The main drawback of the paired-flash technique is the time required for rods to recover from the bright probe flash, here 20-30 s for our standard probe flash of 8650 scot Td s. A representative recording of the rod photoreceptor response, averaging 8 responses to the probe presented alone and at 7 delays after the test, therefore takes about 30 min. The cone contribution needed for subtraction was determined by three approaches. The best estimate was obtained using a 4-flash technique that exploits the different recovery times of rods and cones. Data analysis was improved by considering a range of post-probe times rather than using measurement at a single time as previously. Rod responses derived with this method peaked at about 100 ms in the dark. With different steady background intensities, the response showed classical signs of light adaptation, namely desensitisation and acceleration of the recovery.

Discussion: Paired-flash ERG experiments allow in vivo investigation of rod photoreceptor function unavailable with single-flash ERG techniques.

[1] Breton et al. (1994) IOVS 35: 295; [2] Birch et al. (1995) IOVS 36: 1603
Department of Physiology, University of Cambridge, Downing Street Cambridge CB2 3EG, UK