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Neurotransmitters are the molecules through which neurons communicate with each other at synapses and, by extension, the molecules ultimately responsible for practically every mental, emotional, cognitive, and motor state of the human body; the main ones are serotonin, dopamine, norepinephrine, GABA, glutamate, acetylcholine, and the endocannabinoids, each with its own functional profile, specific brain circuits, and characteristic receptors; nutritional, supplemental, and pharmacological modulation of these systems is the basis of practically all psychiatric treatments and of many nootropic or emotional well-being interventions, a modulation that should be understood with nuance because the popular simplification ("serotonin is the happiness hormone," "dopamine is the pleasure one") is misleading and creates mistaken expectations.
Serotonin (5-HT) is probably the most mentioned and least understood neurotransmitter in the popular world. Its functions are multiple: regulation of mood, sleep, appetite, pain sensitivity, sexual function, and gastrointestinal motility (where approximately 90% of the body's serotonin is found). The simple monoaminergic hypothesis of depression ("low serotonin causes depression") is greatly oversimplified, and modern data suggest that depression is a far more complex disorder in which serotonin is one of several systems involved.
Modulation: dietary tryptophan is the precursor; its brain availability depends on the transporter that competes with other neutral amino acids, which improves with carbohydrate intake (insulin lowers the plasma competitors). 5-HTP is the intermediate metabolite with modest support in some mood disorders. SSRIs (fluoxetine, sertraline, escitalopram) are the most widely used antidepressants and act by blocking synaptic serotonin reuptake.
Dopamine is not the "pleasure molecule" but the one for desire, motivation, and reward prediction. Central dopaminergic systems: mesolimbic pathway (motivation, reward, addiction), mesocortical pathway (executive cognition), nigrostriatal pathway (motor control; its degeneration causes Parkinson's disease), tuberoinfundibular pathway (prolactin inhibition).
Nutritional modulation: dietary tyrosine is a precursor; tyrosine supplementation (100-150 mg/kg) has been shown to improve cognitive performance under acute stress (sleep deprivation, extreme cold, sustained cognitive demand). Mucuna pruriens provides L-DOPA directly with significant pharmacological effects (relevant in Parkinson's disease, not as a casual supplement).
Glutamate is the main excitatory neurotransmitter of the CNS; GABA is the main inhibitory one. The balance between the two determines neuronal excitability; benzodiazepines (alprazolam, diazepam) potentiate GABA, and anticonvulsants modulate both systems.
Nutritional modulation: oral GABA has questionable brain bioavailability (it does not cross the blood-brain barrier well), so GABA supplements have modest effects. L-theanine from green tea is probably the nutritional GABAergic modulator with the best profile. Magnesium (a cofactor of glutamate NMDA receptors), glycine (a co-agonist of the NMDA receptor, also GABAergic), valerian, passionflower, and lavender have documented mild GABAergic effects.
Acetylcholine is the main neurotransmitter of the neuromuscular junction and of brain circuits involved in attention, memory, and learning; degeneration of cholinergic neurons in the nucleus basalis of Meynert is one of the central features of Alzheimer's disease.
Modulation: nutritional precursors (dietary choline, phosphatidylcholine/lecithin, alpha-GPC, citicoline) have modest clinical support in cognitive impairment. Huperzine A is an acetylcholinesterase inhibitor (a mechanism analogous to Alzheimer's drugs such as donepezil) with support in some studies. Bacopa monnieri has documented activity on the cholinergic system.
Norepinephrine (noradrenaline) modulates alertness, attention, the acute stress response, heart rate, and blood pressure. Its precursor is tyrosine (DOPA → dopamine → norepinephrine pathway). Modulation: rhodiola rosea, eleuthero, and schisandra have adaptogenic effects on the noradrenergic-cortisol system.
The endocannabinoid system (anandamide, 2-AG, CB1 and CB2 receptors) regulates appetite, pain, mood, immune function, and many other functions; exercise increases endocannabinoids and contributes to the "runner's high"; CBD (cannabidiol) has a complex clinical profile with support in refractory epilepsy, anxiety, and chronic pain.
Neurotransmitter systems are very complex and highly interconnected; nutritional interventions usually have modest magnitudes compared with established psychiatric pharmacology, but they can be useful as complements or in mild situations; serious modern psychiatry recognizes that depression, anxiety, and other disorders are not "simple chemical deficits" and that combined treatment (pharmacology when appropriate + psychotherapy + lifestyle + supplements as a complement) is usually more effective than any single intervention; simplistic claims about "boosting serotonin/dopamine" should be evaluated with skepticism.
Information, not medical advice.