Differential sensitivity of the On and Off visual responses to retinal ischemia

Retinal ischemia is a common condition that may lead into vision impairment and blindness. In this study, we evaluated changes separately in On and Off visual responses induced by retinal ischemia. To do this, reversible retinal ischemia was induced in anaesthetized rats by increasing the intraocular pressure until the eye fundus became whitish for either 30 or 60 min. Both electroretinogram (ERG) and multiunit neuronal activity in the superior colliculus (SC) were recorded simultaneously for at least 20 min before, during, and after ischemia. In addition, in normal eyes, intravitreal glycine (Gly) injections were performed to further investigate the mechanisms involved in this process. We found that collicular Off responses were more sensitive to ischemia than On responses. The Off response was the first one to decay at the time ischemia was induced and the last to recover after blood reperfusion. The duration of ischemia also differentially affected both responses. After 30 min of ischemia, 14% of SC recordings failed to recover Off responses. After 1 h of ischemia, the percentage of recordings that failed to recover Off responses increased to 50%. Post-ischemic ERGs remained unaltered in all cases. Intravitreal Gly injections caused suppression of Off responses in the SC. Higher doses caused suppression of both On and Off responses in the SC but with no effect on the ERG at the doses tested. In summary, Off responses were more sensitive than On responses to ischemia suggesting that different mechanisms drive the two types of responses. The recovery of transitory ischemia was not complete in the SC responses whereas the ERG remained unaltered, suggesting that retinal damage produced by ischemia is more prominent in ganglion cells. Our results provide critical information for understanding ischemia repercussions and visual processing in the early visual system.