Acetylcholine action

Some cholinesterase inhibitors also inhibit butyrylcholinesterase (pseudocholinesterase). Flowever, inhibition of butyrylcholinesterase plays little role in the action of indirect-acting cholinomimetic drugs because this enzyme is not important in the physiologic termination of synaptic acetylcholine action. Some quaternary cholinesterase inhibitors also have a modest direct action as well, eg, neostigmine, which activates neuromuscular nicotinic cholinoceptors directly in addition to blocking cholinesterase. 

A Atropinization (blocking acetylcholine action at parasympathetic nerve endings in the CNS)... 

Fig. 1 Mechanism of acetylcholine action. Formed in the synapse, the compound is released by exocytosis into the synaptic cleft, where it is rapidly hydrolyzed by acetylcholinesterase...

The pharmacological properties of hydroxyzine are quite similar to those of chlorpromazine. It has antihistamine and anti-acetylcholine actions as have, indeed, most of the derivatives listed. Like chlorpromazine, hydroxyzine is anticonvulsive, it potentiates the action of hypnotics, it produces hypothermia and hypotension, it depresses the reticular system and it has some anti-adrenaline action. Extrapyramidal side effects do not, apparently, occur, perhaps because of the drug s rather powerful anti-acetylcholine action which probably also explains such side effects as dryness of the mouth and the occasional occurrence of central excitation. Epileptiform seizures have also been seen during hydroxyzine therapy. The fact that anti-acetylcholine and antihistamine activity is found with many of the diphenylmethane derivatives, whether depressant or excitatory, suggests that the tranquillizing effect of hydroxyzine cannot be due to its antagonizing the action of acetylcholine or histamine. 

Durell, J, Garland, JT and Friedel, RO (1969) Acetylcholine action biochemical aspects. Science, 165, 862-866. 

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