https://www.pnas.org/doi/10.1073/pnas.0914801107

"We demonstrate here that M3-muscarinic receptor knockout mice show a deficit in fear conditioning learning and memory."

It's interesting the Mucinex antagonizes the M3 receptor. Behavioral effects of Mucinex are very understudied.

M3 receptors also classically handle lung "learning" and response to allergens. https://pubmed.ncbi.nlm.nih.gov/8441331/ - "Muscarinic receptor subtypes in airways"

It’s interesting to me that this is a combination medicine because the atypical antipsychotics (say https://en.wikipedia.org/wiki/Quetiapine) are “polypharmacy in a single molecule” in that they bind to both dopamine and serotonin receptors which cancel out terrible side effects such as tardive dyskinesia caused by earlier antipsychotics that work on the dopamine receptors (though most of those bind to histamine receptors causing strong sedation as well.)

If TFA is to be believed, schizophrenia is quite common so the market is vast and the usual excuses of "price has to be high to recoup R&D because of low demand" doesn't seem like it would apply.

A mere 1M worldwide prescriptions (there's 6x that amount of people with schizophrenia in America alone) means 22 billions per year raw revenue.