MUSCARINIC RECEPTOR-MEDIATED DUAL REGULATION OF ADP-RIBOSYL CYCLASE IN NG108-15 NEURONAL CELL-MEMBRANES

Cyclic ADP-ribose (cADP-ribose) is an endogenous modulator of ryanodin e-sensitive Ca2+ release channels. An unsolved question is whether or not cADP-ribose mediates intracellular signals from hormone or neurotr ansmitter receptors. The first step in this study was to develop a TLC method to measure ADP-ribosyl cyclase, by which conversion of [H-3]NA D(+) to [H-3]cADP-ribose was confirmed in COS-7 cells overexpressing h uman CD38. A membrane fraction of NG108-15 neuroblastoma x glioma hybr id cells possessed ADP-ribosyl cyclase activity measured by TLC. Carba mylcholine increased this activity by 2.6-fold in NG108-15 cells overe xpressing mi or m3 muscarinic acetylcholine receptors (mAChRs), but in hibited it by 30-52% in cells expressing m2 and/or m4 mAChRs. Both of these effects were mimicked by GTP, Pretreatment of cells with cholera toxin blocked the activation, whereas pertussis toxin blocked the inh ibition. Application of carbamylcholine caused significant decreases i n NAD(+) concentrations in untreated m1-transformed NG108-15 cells, bu t an increase in cholera toxin-treated cells. These results suggest th at mAChRs couple to ADP-ribosyl cyclase within cell membranes via trim eric G proteins and can thereby control cellular function by regulatin g cADP-ribose formation.