The renin-angiotensin system--a possible neuromodulator in the human retina?

Morphology studies imply a possible contribution of the renin-angiotensin system (RAS) to neurotransmission in the mammalian retina. To investigate further the functional role of the RAS in the visual system of humans, we tested the effects of the angiotensin-converting enzyme (ACE) inhibitor Captopril (Capt; 25 mg) on electroretinogram (ERG) recordings, the cone flicker threshold (CFT), the FM-28-HUE test, and the psychophysical rod threshold in healthy volunteers. The parameters of renin, angiotensin, and blood pressure were controlled. We obtained the following results. First, Capt induced a significant decrease in amplitudes of the scotopic b-wave, oscillatory potentials, and the a-wave. The latency of responses to 30-Hz flicker was increased. Second, the CFT was reduced to a lower intensity, whereas the final rod threshold remained unchanged. These results are in accordance with previous immunocytochemical and electrophysiological data. They cannot be explained exclusively by the observed changes in systemic blood pressure. We conclude that the RAS may possibly be involved in retinal neurotransmission in humans in addition to its vascular effects.