Toxicity of the specific antimuscarinic agent methoctramine and other non-specific anticholinergic drugs in human neuroblastoma cell lines in vitro

The highly selective cardiac-M(2) Muscarinic Acetylcholine Receptor (mAChR) antagonist methoctramine shows a number of concentration-dependent biochemical responses. At micromolar concentrations it interacts allosterically with the mAChR and has 'agonist-like' effects on the phosphoinositide and cyclic AMP second messenger systems. Direct stimulation or inhibition of second messenger systems has been reported to modulate cellular homoeostasis and differentiation. This study showed that methoctramine was toxic, in micromolar concentrations, to the human neuroblastoma cell lines SK-N-SH, LAN-5 and SH-EP1, the last being a clone that does not contain muscarinic receptors. The selective M2 mAChR antagonists 11-{2-[(diethylamino)methyl]-1-piperidinyl}-5,11-dihydro-6H-pyrido(2,3-6)(1-4)benzodiazepine-6-on (AF-DX 116) and gallamine, as well as the selective M1 and M3 antagonists pirenzepine and 4-diphenylacetoxy-n-methylpiperidine (4-DAMP), had no toxic effects. Lithium provided significant protection against methoctramine toxicity, whereas carbamylcholine, pertussis toxin and forskolin had no influence on its toxicity. At micromolar concentrations, the clinically used, non-selective mAChR antagonists ethopropazine, benztropine, trihexyphenidyl and orphenadrine displayed toxicity similar to that of methoctramine. Methoctramine, ethopropazine, benztropine and trihexyphenidyl enhanced significantly [(3)H]thymidine uptake at subtoxic concentrations. These results demonstrate that (a) the toxicity of methoctramine is by way of non-muscarinic mechanism, (b) some anticholinergic drugs commonly used in clinical medicine have toxic properties similar to those of methoctramine and (c) at subtoxic micromolar concentrations anti-muscarinic drugs have some trophic properties.