The natural β-carbolines facilitate inositol phosphate accumulation by activating small G-proteins in human neuroblastoma cells (SH-SY5Y)

The naturally occurring β-carbolines exert psychotropic actions in humans and have numerous behavioral effects in animals. The known in vitro activities of these substances do not provide a satisfactory explanation for their in vivo effects. The present study was undertaken to explore the possibility of a specific signal transduction pathway. The human neuroblastoma cell line SH-SY5Y was used as a model system. High-affinity binding sites for [ 3H]norharman (synonymous: β-carboline) were detected. Pharmacological characterization revealed displacement of the ligand by β-carbolines, to a weaker extent by indoleamines, but not by opioids, muscarinic receptor agonists, metabotropic glutamate receptor agonists or several peptide neurotransmitters. Inositol phosphate accumulation was only slightly affected by the β-carbolines. However, the action of carbachol was clearly facilitated in a dose-dependent and pertussis toxin-insensitive manner. Pretreatment of the cells with Clostridium difficile toxin B blocked the facilitating effect of the β-carbolines by concentrations which did not affect the action of carbachol alone. This suggests that low molecular weight GTP-binding proteins are involved in the facilitating action of the β-carbolines. This mechanism was further supported by experiments measuring the concentrations of phosphatidylinositol phosphates after various activating compounds. In conclusion, the facilitating effect of β-carbolines on inositol phosphate accumulation could play a part in the actions of β-carbolines and may be produced by stimulating the generation of phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P 2), the key component in the activation of phosphoinositide-phospholipase C.