The Brain's Chemical Messengers
Different neurotransmitters specialize in different jobs. Acetylcholine (ACh) activates skeletal muscle at the neuromuscular junction and supports memory in the brain; its loss is strongly linked to Alzheimer's disease. Dopamine drives reward, motivation, and motor control — too little in the substantia nigra produces the motor symptoms of Parkinson's disease, while dopamine excess is implicated in the positive symptoms of schizophrenia. Serotonin regulates mood, sleep, and appetite and is the target of SSRIs (selective serotonin reuptake inhibitors) used to treat depression. Norepinephrine promotes arousal and alertness and is central to the fight-or-flight response.
Two neurotransmitters dominate ordinary synaptic traffic by sheer volume: glutamate, the brain's main excitatory transmitter and essential for learning and long-term potentiation, and GABA (gamma-aminobutyric acid), the main inhibitory transmitter, which dampens neural excitability and is boosted by anti-anxiety drugs like benzodiazepines. Endorphins are the brain's own opioid-like painkillers, released during stress or exertion.
| Term | Effect on the receptor | Example |
|---|---|---|
| Agonist | Mimics or enhances the neurotransmitter's effect | Nicotine mimics ACh at its receptors |
| Antagonist | Blocks or reduces the neurotransmitter's effect | Curare blocks ACh receptors, causing paralysis |
An agonist can act directly (binding and activating the receptor itself) or indirectly (increasing how much natural neurotransmitter is available, e.g. by blocking reuptake). An antagonist typically works by occupying the receptor site without activating it, physically preventing the natural neurotransmitter from binding.
Common pitfall: assuming "antagonist" always means "produces no effect." An antagonist has a very real effect — it is just the effect of removing the neurotransmitter's normal action, which can itself be dramatic, as when naloxone rapidly reverses an opioid overdose.