Purpose: To compare the effects of glutamate agonists and different types of ischemic insult on choline acetyltransferase (ChAT) immunoreactivity in the rat retina.
Methods: Rat retinas were exposed to different glutamate agonists in vivo or in vitro for specific periods of time, and the retinas were then fixed and processed for the localization of ChAT immunoreactivity. In other experiments, rats were administered an ischemic insult either by ligaturing the carotids (two-vessel occlusion [2-VO] procedure), cannulating the anterior chamber, and raising the intraocular pressure (high intraocular pressure [HIOP] procedure) or placing a ligature around the optic nerve sufficiently tightly to prevent blood flow through the central retinal artery (vascular ligation [VL] procedure). The electroretinogram was recorded, and, after a specific period of time, reperfusion was allowed to occur. Thirty to 36 hours after reperfusion, the retinas were dissected and processed for the localization of ChAT, as well as for parvalbumin, Thy-1, and αPKC immunoreactivities.
Results: Of the glutamate agonists tested, only kainate reduced ChAT immunoreactivity significantly in vivo and in vitro. This effect of kainate could be counteracted by the antagonist CNQX (6-cyano-2,3- dihydroxy-7-nitroquinoxaline-2,3,-dione). The ChAT immunoreactivity was unaffected in retinas in which ischemia was induced by the 2-VO procedure. In contrast, ChAT immunoreactivity was obliterated in retinas in which the HIOP was used and drastically reduced when the VL procedure was used. Interestingly, neither αPKC nor Thy-1 immunoreactivities were affected in retinas subjected to HIOP or VL methods. However, parvalbumin immunoreactivity was reduced in the HIOP model but only slightly altered in the VL model.
Conclusions: The current results suggest that kainate receptors are associated with the cholinergic retinal neurones in the rat retina. Activation of these receptors by kainate causes a reduction in the neurones' ChAT content. This effect can be mimicked by subjecting the retina to a sufficiently harsh ischemic insult, as occurs in the VL and HIOP procedures. When the ischemic insult is mild, as in the 2-VO procedure, no obvious change in ChAT immunoreactivity is apparent. The HIOP procedure for inducing an ischemic insult was found to be the most severe of the three procedures used, because ChAT immunoreactivity was obliterated and clear changes in the parvalbumin immunoreactivity also were recorded. Interestingly, neither the HIOP nor the VL procedures caused a change in the Thy-1 and αPKC immunoreactivities.