The coupling of muscarinic receptors to hydrolysis of inositol lipids in human neuroblastoma SH-SY5Y cells

Carbachol (CCh)-stimulated hydrolysis of inositol lipids in human neuroblastoma SH-SY5Y cells was systematically characterized in parallel with the carbachol effects on cAMP formation. Carbachol concentration-dependently induced the hydrolysis of inositol lipids and formation of [³H]IP₃, [³H]IP₂ and [³H]IP₁ in these cells labeled with [³H]inositol. The maximal amount of [³H]IP₁ accumulated in presence of 10 mM LiCl was about 50-fold above the basal level. The EC₅₀ value of CCh was 14 μM. The muscarinic antagonists atropine, pirenzepine and 11-[[2-(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido [2,3-b] (1,4)-benzodiazepine-6-one (AF-DX) 116) competitively inhibited CCh-induced [³H]IP₁ accumulation. The functional inhibition constants (converted from the pA₂ values) were 0.24, 8.1 and 470 nM, respectively. These values are in good agreement with the inhibition constants of these drugs from antagonist/[³H]pirenzepine studies using intact cells. Forskolin, adenosine and PGE₁ stimulated cAMP formation in this cell line. Morphiine decreased PGE₁-induced cAMP formation as well as the basal cAMP formation. However, CCh did not stimulate or inhibit the basal cAMP formation. Also, CCh did not have any effects on the adenosine and PGE₁-induced cAMP formation in these cells. These data suggest that muscarinic M₁ receptors are coupled to the hydrolysis of inositol lipids and not to the adenylate cyclase system in human neuroblastoma SH-SY5Y cells.