X-ray action on isolated rat retina and on its rhodopsin

In order to explain the formation mechanism of phosphenes detected by astronauts during space flights, the X-ray stimulation of the eye photoreceptor cells is investigated. The albinos rat retina, maintained in survival, was used. The electrophysiological response (ERG) induced by X-rays is found to be identical to the one produced by a visible light stimulation. Under our experimental procedure only a direct interaction between incident radiation and retina can induce such an ERG. A comparative study indicates that, in order to obtain the same ERG amplitude, the incident energy on the retina must be about 5 x 106 times larger for X-rays (E = 40 keV) than for visible light (d = 489 nm). The analysis of these results leads us to assume that the X-rays act on the rod photosensitive molecule: the rhodopsin. We studied the modifications of the rhodopsin absorption spectrum after X-ray irradiation. For exposures smaller than 5 x 105 R, a large increase of the absorption in the ultraviolet part of the spectrum is observed, while for exposures of 1.25 x 106 R, a 20% decrease is measured at the level of the characteristic peak of the rhodopsin (500 nm). By comparison to what is known for visible light, this important bleaching of the rhodopsin by X-rays can account for the generation of an ERG on the isolated retina. By analogy with the enzymatic inactivation after X-irradiation, we believe that the X-ray rhodopsin bleaching could be due to an energy transfer from the opsin to attachment site of the chromophoric group (11-cis retinal). The disorganization of this critical site of the molecule could be at the origin of the origin of the rod excitation, then of the ERG