MODULATION BY SIGMA LIGANDS OF N-METHYL-D-ASPARTATE-INDUCED [H-3] NORADRENALINE RELEASE IN THE RAT HIPPOCAMPUS - G-PROTEIN DEPENDENCY

The effects of the high affinity sigma (sigma) ligands 1,3-di(2-tolyl)guanidine (DTG), (+)N-cyclopropyl-methyl-N-methyl-1,4-diphenyl-1-ethyl-but-3-en-1-yl-amine hydrochloride (JO- 1 784), (+)3-[3-hydroxyphenyl]-N-(1-propyl)piperidine hydrochloride [(+)3-PPP] and haloperidol were studied on N-methyl-D-aspartate (NMDA)-evoked release of [H-3]noradrenaline (NA) from preloaded hippocampal slices made from Sprague-Dawley rats. The [H-3]NA release was evoked once by a 4 min exposure to NMDA, 40 min after the beginning of superfusion with a Mg++-free Krebs' solution. In the absence of any drug, NMDA evoked a concentration-dependent [H-3]NA release. Mg++ and EGTA abolished the [H-3]NA release induced by NMDA. JO-1784 and (+)3-PPP potentiated in a concentration-dependent manner NMDA-induced [H-3]NA release, without affecting the basal outflow. DTG concentration-dependently inhibited the overflow of [H-3]NA evoked by NMDA, without affecting the basal efflux. Haloperidol, which did not modify NMDA-evoked [H-3]NA release by itself, completely prevented the effects of JO-1784, (+)3-PPP and DTG. In contrast, spiperone, also a potent dopamine receptor antagonist but with low affinity for sigma binding sites, failed to prevent the potentiation of NMDA-evoked release of [H-3]NA by JO-1784 and (+)3-PPP. The possible involvement of G(i/o) proteins in the modulation by sigma-ligands of NMDA-evoked [H-3]NA release in the rat hippocampus was also investigated. To this end, G(i/o) proteins were inactivated with pertussis toxin (PTX), injected locally 3 to 11 days prior to the experiment or with in vitro preincubation with N-ethylmaleimide (NEM) for 30 min prior the experiment. Both the in vivo PTX and the in vitro NEM pretreatments abolished the potentiating effects of JO-1784 and (+)3-PPP, and shifted the concentration-effect curve for DTG-induced suppression of NMDA-evoked release of [H-3]NA to the right. These results suggest that the sigma-receptors mediating the effects of JO-1784, (+)3-PPP and DTG on NMDA-evoked release of [H-3]NA are coupled to G(i/o) proteins.