ANTAGONISM BY 8-HYDROXY-2(DI-N-PROPYLAMINO)TETRALINE AND OTHER SEROTONIN AGONISTS OF MUSCARINIC M1-TYPE RECEPTORS COUPLED TO INOSITOL PHOSPHOLIPID BREAKDOWN IN HUMAN IMR-32 AND SK-N-MC NEUROBLASTOMA-CELLS

IMR-32 and SK-N-MC cells were found to contain [H-3]quinuclidinyl benzilate specific binding sites inhibited by pirenzepine in a manner suggesting the presence of both M1-type and M2-type muscarinic receptor recognition sites. Neither cell had detectable [H-3]8-OH-DPAT binding sites. Carbachol stimulated the rate of inositol phospholipid breakdown in IMR-32 and SK-N-MC human neuroblastoma cells with an EC50 value of about 50-mu-M in both cases. Pirenzepine inhibited the carbachol (100-mu-M)-stimulated inositol phospholipid breakdown in both cells with Hill slopes of unity and IC50 values of 15 nM (IMR-32) and 12 nM (SK-N-MC). The 5-HT1A receptor agonist 8-OH-DPAT competitively inhibited carbachol-stimulated inositol phospholipid breakdown with pA2 values of 5.78 (IMR-32) and 5.61 (SK-N-MC). These values are consistent with the inhibitory potency of 8-OH-DPAT towards [H-3]quinuclidinyl benzilate binding in these cells. The 5-HT agonists 5-MeODMT and buspirone at micromolar concentrations inhibited carbachol-stimulated breakdown in IMR-32 cells. The inhibition by 8-OH-DPAT and 5-MeODMT was not affected by preincubation with (-)alprenolol. 5-HT (10-100-mu-M) was without effect on either basal or carbachol-stimulated breakdown. It is concluded that IMR-32 and SK-N-MC neuroblastoma cells express muscarinic M1-type but not serotoninergic receptors coupled to phosphoinositide-specific phospholipase C. 8-OH-DPAT acts as a weak antagonist at these muscarinic receptors.