Progressive visual sensitivity loss in the Royal College of Surgeons rat: Perimetric study in the superior colliculus

The Royal College of Surgeons rat has a retinal pigment epithelial cell def ect which causes a progressive loss of rods occurring primarily over the fi rst few months of life. We have studied the consequences of this degenerati ve process on visual sensitivity across the visual field. Sensitivities wer e determined in the superior colliculus for unit responses recorded from 22 days up to one year of age from sites encompassing the whole visual field representation. Following visual sensitivity assessment, retinae were exami ned anatomically at the light and electron microscopic level. At 22 days of age, sensitivities in dystrophic rats were comparable to those of non-dyst rophies at any age (40 +/- 1 and 41 +/- 1 dB. respectively), despite the fa ct that signs of degenerative events were clear at the electron microscopic level, including presence of pyknotic photoreceptor nuclei, disorganised o uter segments and accumulation of debris. However, loss in sensitivity was first detected only at 28-36 days of age (27 +/- 4 dB). From then on, sensi tivities progressively decreased to reach a plateau by 180-240 days (4 +/- 2 dB). Starring around 90 days and onward, there was a positive gradient of sensitivities from temporal to nasal field. Drops in visual sensitivity we re parallelled by several changes in visual response properties, including prolonged latency, inconsistent responsiveness, appearance of bursting spon taneous activity and activation of units by stimuli presented outside their classical receptive fields. The measure of visual sensitivities by recording visual responses at specif ic sites in the superior colliculus provides a reliable point-to-point asse ssment of retinal function comparable to visual perimetry testing in humans . This experimental approach provides the background for answering question s arising during the development of potential experimental therapies for re tinal degeneration using animal models like the Royal College of Surgeons r at. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.