Gliotoxic effects of alpha-aminoadipic acid isomers on the carp retina: a long term observation

The glutamate analogue, alpha-aminoadipic acid was intravitreally administered in the D-, DL- and L-forms to carp (Cyprinus carpio) retina in vivo. To make a quantitative assessment of its gliotoxic action, the activity of glutamine synthetase, whose localization was confirmed in glial Müller cells by an immunohistochemical technique, was examined at various intervals over one month. Intravitreal injection of 8 mumol alpha-aminoadipic acids reduced the glutamine synthetase activity within 4 h and maximally by 24 h. The maximum reduction evoked by L-, DL- and D-forms was about 65, 45 and 28% in reduction, and their minimum effective dose was 0.8, 1.5 and 2.0 mumol, respectively. At three to four days after alpha-aminoadipic acids injection, sodium dodecyl sulphate gel electrophoresis suggested that some retinal proteins including glutamine synthetase were significantly reduced, whilst others were increased. These biochemical changes were fully reversed one to two weeks after administration of the D- or DL-forms, but not until one month with the L-form. The electroretinographic b-wave, reflecting glial activity, was completely blocked by 8 mumol alpha-aminoadipic acids within 4 h. The electroretinographic b-wave was recovered first in the case of D- and then of DL-form at two to three weeks after injection, but only 50% recovery was seen in the case of L-form even two months later. A high dose of DL-alpha-aminoadipic acid (16 mumol) induced as long lasting a suppression in the glutamine synthetase and electroretinographic b-wave activities as 8 mumol L-alpha-aminoadipic acid. Therefore, the gliotoxic efficacy of L-alpha-aminoadipic acid at micromol orders was two-fold higher than that of DL-alpha-aminoadipic acid. Differences in the time-course of recovery of the suppression of glutamate synthetase and electroretinographic b-wave activities induced by alpha-aminoadipic acids are discussed in terms of its gliotoxicity.