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Receptors sit everywhere, so drugs misfire
Quieting and Stimulating
Alcohol, barbiturates, and sedatives quiet the brain by increasing inhibitory release, which raises the threshold for activation. Adderall, caffeine, nicotine, speed, and modafinil lower it, generally by increasing neuromodulators that make neurons more likely to fire. That creates higher RPM across the brain and body, with some shaking and agitation, and it is hard to shut off the mind under stimulants.
Opening Plasticity
Psilocybin raises serotonin massively and unmasks lateral connections in the brain, which allows new associations to form in a condensed timeframe. MDMA increases serotonin and dopamine, and both open a heightened window for plasticity in a specific context. ADHD stimulants ramp up activity in the prefrontal cortex, which makes them a nonspecific focus enhancer.
Side Effects
Because these receptors are expressed all over the brain, certain antidepressants cause sexual side effects, weight gain, or excessive weight loss, and certain drugs cause hypersexuality. Electrical stimulation has the same problem. What each of these tools lacks is spatial and temporal specificity.
How does caffeine keep the brain alert?
Adenosine builds up in the brain the longer a person is awake, and it binds to receptors that dampen neural activity and promote sleepiness. Caffeine has a similar shape and sits in those receptors without activating them. That blocks adenosine's signal while the adenosine itself keeps accumulating. The caffeine in a cup of coffee reaches its peak level in the blood in roughly 30 to 60 minutes.
Why does psilocybin change how the brain connects?
The body converts psilocybin to psilocin, which binds strongly to serotonin 2A receptors. Those receptors are dense on large pyramidal neurons in the cortex. Brain imaging studies led by Robin Carhart-Harris at Imperial College London found that under psilocybin the usually tightly coupled default mode network loses some of its coherence, while communication between other brain regions increases.
Why do serotonin drugs affect so many body systems?
Serotonin acts through at least 14 receptor types, and they are spread across the brain, blood vessels, and gut. About 90 percent of the body's serotonin is in the intestines, where it helps regulate movement of the gut. A drug that changes serotonin signaling therefore reaches far beyond mood circuits. That breadth is why selective drugs and targeted stimulation are active research areas.
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