Intrasynaptosomal free calcium levels in rat forebrain synaptosomes: Modulation by sigma (sigma) receptor ligands

The sigma receptor ligands (+) and (-) pentazocine and BD 1008 (1-100 mu M) were added to rat forebrain synaptosomes. Their effects on intrasynaptosomal free calcium ([Ca2+](i)) levels under basal conditions and after depolarisation with high potassium buffer (45 mM KCl), veratridine (25 mu M) and 4-aminopyridine (4-AP, 1 mM) were determined. The sigma ligands elicited significant, concentration-dependent decreases in basal [Ca2+](i) levels with an order of potency (-) pentazocine > (+) pentazocine = BD 1008. The sigma ligands (at the maximum effective concentrations) also significantly inhibited the rise in [Ca2+](i) levels produced by depolarisation with KCl, veratridine and 4-AP. The effect of (+) and (-) pentazocine (100 mu M) to inhibit the depolarisation-dependent increase in [Ca2+](i) levels was greater when veratridine and 4-aminopyridine were used to depolarise the synaptosomes than with KCl, whereas the effect of BD 1008 (100 mu M) was approximately equipotent using all three depolarising agents. However, BD 1008 was more potent to inhibit the KCl-induced rise in [Ca2+](i) compared to (+) and (-) pentazocine. The data demonstrate for the first time that sigma ligands decrease [Ca2+](i) levels in rat forebrain synaptosomes and this suggests a possible mechanism for the changes to neuronal protein phosphorylation and neurotransmitter release previously observed with sigma ligands.