system
Given the following post, respond as if you're commenting on Reddit:
human
How are neurotransmitters in a brain measured?

I've heard things like "adenosine increases while awake and decreases when sleeping". How is adenosine (and other NTs) measured in a living human? My understanding is the only way to measure neurotransmitters is to cut open the brain, which you can't do on a living person.
gpt
You can measure circulating blood amounts, which sometimes, but generally isn't very accurate at reflecting brain amounts. You can also measure levels of metabolites. Generally though you just can't measure them which is why psychiatry is a rough adventure to find meds that work for each individual and is almost entirely based on self-reporting.