It's an interesting hypothesis.

It's an interesting hypothesis. Some people respond to choline supplements for migraine or depression, or to racetams that modulate that brain system (ostensibly depleting brain choline). Too much or too little choline can cause headaches.

Amitriptyline is also one of the most effective antidepressants. There are two types of ACh receptors - muscarinic and nicotinic. I believe amitriptyline has inhibitory actions on both. I'd be wary of drawing too many conclusions from that med though - it binds to a lot of receptor subtypes. Anticholinergic medications are also associated with dementia, used in the long-term.

The brain is very complex, and it seems unlikely to me there is one root cause of depressive symptoms. For instance, the BDNF/TrKB pathway is also likely to be involved, as a hippocampal neurogenesis actuator (indirectly in the case of antidepressants). This is likely why the SERT-selective antidepressants take weeks to work - the effect is indirect. But some people respond to atypical dopaminergic or serotonergic antidepressants straight away (like mirtazapine, bupropion, or modafinil).

This isn’t medical advice, and I’m not a doctor.