PREGNENOLONE SULFATE MODULATES NMDA RECEPTORS, INDUCING AND POTENTIATING ACUTE EXCITOTOXICITY IN ISOLATED RETINA

Pregnenolone sulfate (PS) acts as a positive allosteric modulator of N -methyl-D-aspartate (NMDA) receptor-mediated responses, In the retina, we previously observed that the synthesis of pregnenolone and PS incr eases after stimulation of NMDA receptors and blockade of the synthesi s reduces retinal cell death. This study was carried out to explore in the isolated and intact retina the possible role of PS in NMDA-induce d excitotoxicity, Lactate dehydrogenase (LDH) measurements and morphol ogical analysis revealed that a 90-min exogenous application of PS at 0.1-500 mu M concentrations potentiated NMDA-induced cell death and at 50-500 mu M concentrations caused cytotoxicity. After 45 min, either NMDA or PS caused no significant LDH release; but their co-application resulted in a high degree of toxicity. In addition, we found that a m ild NMDA insult developed into serious damage when even low PS concent rations (0.1-10 mu M) were used. Toxicity-inducing and -potentiating e ffects were specific to PS modulatory action on NMDA receptors, in tha t they were blocked by 4-(3-phosphonopropyl)2-piperazinecarboxylic aci d (CPP) and MK-801 but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CN QX), and neither dehydroepiandrosterone sulfate nor pregnenolone cause d LDH release. Prevention of degenerative signs was seen in retinae pr etreated with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS ), a Cl- channel blocker, thus indicating a Na+/Cl--dependent acute mo de of excitotoxic cell death responsible for PS toxicity, The positive interaction between the neurosteroid and NMDA receptors was further p roved by a PS dose-dependent increase in NMDA-induced stimulation of [ H-3] MK-801 binding to retinal membranes, The results suggest a crucia l role of PS in retinal vulnerability and propose the toxicity-potenti ating effects as an important key in linking NMDA-induced endogenous s ynthesis to acute excitotoxicity. J. Neurosci. Res. 54:787-797, 1998. (C) 1998 Wiley-Liss, Inc.