VARIOUS SIGMA-LIGANDS EXERT LONG-TERM PROTECTION AGAINST GLUTAMATE TOXICITY IN PRIMARY HIPPOCAMPAL-NEURONS - APPARENT NON INVOLVEMENT OF IDENTIFIED SIGMA-2 SITES

Sigma-2 binding sites were identified in membranes of primary rat hipp ocampal neuronal cultures using [H-3]-(1,3-ortho-di-tolylguanidine) (D TG) (K-D = 29 nM and B-max = 7.4 pmoles/mg protein). Inhibition of bin ding by the pentazocine isomers (K-i values 1600 nM and 45 nM for the (+)- and the (-)-isomer respectively) and by six other compounds with various selectivity for sigma-1 and sigma-2 sites was performed. The s igma binding sites showed the features of sigma-a sites. Chronic treat ment of primary rat hippocampal neuronal cultures from day 1 to day 7 in vitro with the sigma site binding compounds carbetapentane, fenprop imorph, haloperidol, opipramole and sabeluzole resulted in protection of the neurons against glutamate-induced toxicity. IC50-values for neu roprotection after chronic treatment and IC50-values for binding to si gma-2 sites were similar. In contrast, the purported sigma agonist DTG , when applied up to 1 mu M, could not elicit such protective action. The neuroprotective activity of the former drugs was not affected by c o-treatment with an excess of DTG. We conclude that sigma-2 sites, occ upied by DTG, are probably not involved in the neuroprotective action following chronic application of the above sigma ligands against gluta mate-induced toxicity in rat hippocampal neurons. Both the functional role of sigma sites and the mechanism of the neuroprotective action of the sigma ligands are still moot.