Abstract

Human SK-N-SH neuroblastoma cells expressed sigma-1 and sigma-2 receptors with similar pharmacological profiles to those of rodent-derived tissues, although sigma-2 receptors exhibited some affinity differences that might suggest heterogeneity or species differences. Structurally diverse sigma ligands produced two types of increases in intracellular (cytosolic) Ca²⁺ concentration ([Ca²⁺]_i) in these cells. CB-64D, CB-64L, JL-II-147, BD737, LR172, BD1008, haloperidol, reduced haloperidol, and ibogaine all produced an immediate, dose-dependent, and transient rise in [Ca²⁺]_i. Sigma-inactive compounds structurally similar to the most active sigma ligands and ligands for several neurotransmitter receptors produced little or no effect. The high activity of CB-64D and ibogaine (sigma-2-selective ligands) compared with the low activity of (+)-pentazocine and other (+)-benzomorphans (sigma-1-selective ligands), in addition to enantioselectivity for CB-64D over CB-64L, strongly indicated mediation by sigma-2 receptors. The effect of CB-64D and BD737 was blocked by the sigma antagonists BD1047 and BD1063, further confirming specificity as a receptor-mediated event. The transient rise in [Ca²⁺]_i occurred in the absence of extracellular Ca²⁺ and was completely eliminated by pretreatment of cells with thapsigargin. Thus, sigma-2 receptors stimulate a transient release of Ca²⁺ from the endoplasmic reticulum. Prolonged exposure of cells to sigma-receptor ligands resulted in a latent and sustained rise in [Ca²⁺]_i, with a pharmacological profile identical to that of the transient rise. This sustained rise in [Ca²⁺]_i was affected by neither the removal of extracellular Ca²⁺ nor thapsigargin pretreatment, suggesting latent sigma-2 receptor-induced release from thapsigargin-insensitive intracellular Ca²⁺ stores. Sigma-2 receptors may use Ca²⁺ signals in producing cellular effects.