Protection of rabbit retina from ischemic injury by flupirtine

Purpose. The aim of this study was to determine whether flupirtine can slow down the changes seen in the rabbit retina after ischemia-reperfusion. Methods. A suction-cup procedure, which raises intraocular pressure, was used to give an ischemic insult to the rabbit retina. Electroretinograms were recorded before ischemia and at different periods after ischemia. In some instances, flupirtine was injected into the eye before ischemia. Immunohistochemistry was used to study the effect of ischemia-reperfusion on the gamma-aminobutyric acid (GABA) immunoreactivity and uptake of serotonin by the retina. The effect of flupirtine and ischemia on retinal adenosine triphosphate (ATP) levels were determined in in vivo and in vitro experiments. Results. Ischemia for 75 minutes causes a change in the nature of normal GABA immunoreactivity and a reduction in the b-wave of the electroretinogram. When flupirtine is injected into the vitreous humor at the onset of ischemic insult, the changes in GABA immunoreactivity are reduced and the recovery of the reduced b-wave of the electroretinogram after defined reperfusion times is enhanced significantly. Rat retinas incubated in vitro in physiological solution containing flupirtine caused a significant rise in the tissues' ATP content compared with control samples. However, incubation of the tissue in physiological solution saturated with nitrogen caused a drop in retinal ATP levels. Addition of flupirtine prevented this decrease from taking place. Serotonin injected into the vitreous humor of the rabbit eye is taken up by certain amacrine cells. The amount of serotonin taken up is reduced greatly in retinas, as judged by immunohistochemistry when tissues are subjected to ischemia. Because the ischemia was shown to cause a drop in the tissue ATP level, it is concluded that this is the cause of the reduced uptake of exogenous serotonin. Injection of flupirtine into the vitreous humor during ischemia enhanced the uptake of serotonin. Conclusions. Combined data show that flupirtine is a neuroprotective agent in retinal ischemia and that one mode of its mechanism of action is to influence ATP levels. Flupirtine may lower the activity of NMDA receptors, thus causing ATP levels to be less affected in the presence of the drug as a secondary effect.