Comprehensive Guide to Neurotransmitter Regulation and Dietary Influence

General Overview of Neurotransmitters (NTs)

• Definition: Neurotransmitters (NTs) are biologically active chemicals that mediate electrochemical transmission between neurons (neurotransmission) to interact with each other and their target tissues.
• Primary Functions: NTs control crucial organic functions for life, including movement, emotional responses, and the physical ability to feel pleasure and pain. They are involved in processing sensory information and motor-behavioral control.
• Classification: While the precise number of human NTs is unidentified, over $200$ have been identified and classified. Major categories include:
  • Amino Acids: Glutamate ($Glu$), Aspartate ($Asp$), $D$-serine ($D-Ser$), Gamma-aminobutyric acid ($GABA$), and Glycine ($Gly$).
  • Monoamines: Dopamine ($DA$), Norepinephrine ($NE$/Noradrenaline $NA$), Epinephrine ($EPI$/Adrenaline), Histamine ($His$), and Serotonin ($5-HT$).
  • Peptides: Somatostatin ($SST$), substance $P$, and opioids.
  • Purines: Adenosine triphosphate ($ATP$) and Adenosine.
  • Gaseous Substances: Nitric oxide.
  • Trace Amines: Tryptamine and phenethylamines.
• Mechanism of Action:
  • Synthesis: Produced in nerve endings from simple, abundant precursors such as amino acids.
  • Release: Secreted by neurons via a process called "action potential," triggered by an unbalanced ionic charge.
  • Synaptic Cleft: NTs are released into the synaptic cleft and bind to receptor proteins on the cell membrane of the target tissue.
  • Activation: This binding activates, disrupts, or functionally modifies the target cell.
• Types of Effects:
  • Excitatory: Induces alterations and signaling in the target cell.
  • Inhibitory: Blocks any possible changes that can occur; attenuates neuron activity.
  • Dual Function: Some NTs can exhibit both functions depending on the situation.

Neurotransmitter Imbalance and Healthcare

• Related Disorders: NT dysfunction is linked to various neurological and psychiatric conditions:
  • Parkinson’s Disease: Characterized by reduced $DA$ levels due to the loss of dopaminergic neurons in the substantia nigra.
  • Alzheimer’s Disease: Associated with a deficiency of Acetylcholine ($ACh$) in certain brain regions.
  • Schizophrenia: Linked to dysfunction of $DA$, $Glu$, and $GABA$. High levels of $DA$ in the frontal lobes lead to psychotic episodes.
  • Depression: Observed correlation with declines in $NE$ and serotonin levels.
  • Myasthenia Gravis: An autoimmune disease where antibodies inhibit $ACh$ receptors at the neuromuscular junction, leading to muscle weakness.
• Nutritional Psychiatry: Nutritional approaches are increasingly used in clinical practice as complementary medicine. Foods containing NTs or their precursors help maintain internal balance and prevent disorders.
• Limitations of Standard Medicine: While drugs exist for mental illness, they may pose side effects ($Bahrani\,et\,al.,\,2016$).

Systematic Literature Selection (PRISMA)

• Search Strategy: Records were collected matching MESH terms "neurotransmitters AND diet" or "neurotransmitters AND nutrition" across databases like Pubmed, Scopus, Web of Science, and Google Scholar.
• Inclusion Criteria: Papers written in English, published in indexed journals, focused on diet/nutrition's role in NT chemistry or function, preferably with a clinical view.
• Statistical Concordance: Cohen’s kappa ($k$) for paper selection was $91.37\%$ ($k = 0.70941$).

Dopamine (DA)

• Origins: Synthesized in small clusters of nuclei within the midbrain: the ventral tegmental area and the substantia nigra ($Bjorklund\,\&\,Dunnett,\,2007$).
• Biosynthesis:
  1. Starts from the precursor amino acid $L$-Tyrosine ($L-Tyr$).
  2. Converted to $L$-Dopa by the enzyme Tyrosine hydroxylase.
  3. $L$-Dopa is converted to $DA$ by the enzyme aromatic $L$-amino acid decarboxylase (DOPA decarboxylase).
  4. Cofactors: Synthesis requires tetrahydrobiopterin, iron ($Fe$), copper ($Cu$), zinc ($Zn$), and Vitamin $B$ complex elements.
• The Blood-Brain Barrier (BBB): $DA$ absorbed from food cannot cross the BBB; it must be synthesized within brain tissue from precursors like $L$-Dopa, which can cross.
• Functions: Reward learning, prediction of future rewards (money, pleasure, food), motor control, and motivation.
• Dietary Sources for DA Support:
  • Highest Precursor ($L-Dopa$) Content: Mucuna pruriens (velvet beans), Vicia faba (fava/broad beans), Cassia and Bauhinia seeds.
  • Common Foods: Fish, eggs, spirulina, meat, poultry, chocolate, apples, avocados ($4\text{--}5\,\mu g/g$), bananas (pulp: $8\,\mu g/g$; peel: $700\,\mu g/g$), watermelon, tomato, spinach, pumpkin seeds, and turmeric.
  • Clinical Example: Cooking $250\,g$ of velvet beans was found to significantly elevate $DA$ levels and lesson Parkinson's symptoms if eaten $1\text{--}2$ hours after a meal.
• Dietary Warnings: Excess saturated fats (animal fats, butter, palm/coconut oil) can interfere with $DA$ uptake in the brain. Prolonged high-fat diets can induce alterations in $DA$ reuptake.

Acetylcholine (ACh)

• Definition: The primary NT of the parasympathetic nervous system; essential for attention, understanding, and memory.
• Synthesis: Produced by the enzyme choline acetyltransferase which adds a choline molecule to an acetyl portion from Acetyl-CoA.
• Clinical Relevance:
  • Muscle Function: Toxins like botulin and curare cause paralysis by blocking $ACh$ release or receptor sites.
  • Cognition: Reduced levels in the elderly are linked to delirium, fluctuations in consciousness, and disordered thinking.
• Dietary Sources:
  • Rich Precursor (Choline) Sources: Egg yolks, Brussels sprouts, dairy products, liver, salmon, soybeans, mung beans, and lentils.
  • Plant Sources: Urtica dioica L. and Urtica aureus L. (nettles) contain $0.5\,\mu mol/g$ dry weight in roots. Also found in citrus, strawberries, and radishes.
• Mistletoe: Contains $ACh$, $His$, and $GABA$, traditionally used for treating high blood pressure and hysteria.

Serotonin (5-HT)

• Nickname: "The Happy Molecule."
• Distribution: Synthesized in the brain (raphe nucleus) and the gut ($95\%$ of the body's serotonin comes from the gastrointestinal tract).
• Synthesis Chain: $L-Tryptophan\,\rightarrow\,5-Hydroxytryptophan\,\rightarrow\,Serotonin$. Enzymes involved: Tryptophan hydroxylase and amino acid decarboxylase.
• Functions: Regulation of mood, appetite, social behavior, sleep, circadian rhythms, and gastrointestinal motility.
• Dietary Sources:
  • Fruits/Veg: Kiwi, bananas, cherries, papayas, pineapples, plums, and tomatoes.
  • Nuts/Seeds: Walnuts, hazelnuts, coffee beans.
  • Other: Paprika, turkey, and bacon.
• Supplementation: $L-Tryptophan$ ($Trp$) supplements can increase brain serotonin, though cofactors and precautions against adverse drug reactions are necessary.

Gamma-Aminobutyric Acid (GABA)

• Role: The most important inhibitory NT in the human brain cortex. It ensures neural impulses are not transmitted "too quickly," aiding relaxation and mood stabilization.
• Synthesis: Produced from Glutamate ($Glu$) via the enzyme glutamic acid decarboxylase ($GAD$).
• Cofactor: Pyridoxal phosphate (Vitamin $B6$).
• Disorders: Deficiency is linked to anxiety (treated with benzodiazepines to increase signaling), epilepsy, and Huntington's disease.
• Dietary Sources:
  • High Concentrations: Raw spinach ($414\,nmol/g$), potatoes, sweet potatoes, cruciferous vegetables, and Shiitake mushrooms.
  • Teas: White tea has the highest concentration among Chinese varieties.
  • Fermentation: Produced in high levels in $GABA$ soy yogurt and black raspberry juice.
  • Functional Example: Consuming $10\,g$ of chocolate containing $28\,mg$ of $GABA$ was found to ameliorate stress reactions during tasks.

Glutamate (Glu)

• Role: The most plentiful amino acid in the brain and the primary excitatory NT. Essential for synaptic plasticity (underpinning learning and memory).
• Toxicity: Overstimulation by $Glu$ can be poisonous to brain cells; hence, it is strictly regulated and stored within cells.
• Synthesis: Formed from glutamine by the enzyme glutaminase. Lysine is a significant precursor for de novo manufacture.
• Dietary Sources:
  • Natural Protein: Meats, seafood, stews, seaweeds, and cheeses.
  • Additives: Monosodium glutamate ($MSG$) and salts like sodium, potassium, calcium, and magnesium glutamate (labeled as $E620\text{--}E625$).
  • Free Glutamic Acid: Found in Parmesan cheese, miso, soy sauce, and tomato sauce.

Norepinephrine (NE) and Epinephrine (Epi)

• Norepinephrine ($NE$):
  • Function: Stress hormone and NT; triggers the "fight or flight" response and increases arousal, wakefulness, and mental clarity.
  • Synthesis: Produced from $DA$ using the enzyme dopamine beta-hydroxylase.
  • Dietary Sources: Bananas, beans, cheese, chicken, and oatmeal.
• Epinephrine ($Epi$):
  • Function: Produced by chromaffin cells in the adrenal gland; prepares the body for extreme situations by increasing heart rate and glucose production ($glycogenolysis$).
  • Dietary Sources: Sparse research exists, but coffee, tea, citrus, and vanilla are known to raise levels.

Histamine (His)

• Origins: Released from histaminergic neurons in the tubero-mammillary nucleus ($TMN$) of the posterior hypothalamus.
• Arousal State: Firing rate of $His$ neurons is high during wakefulness, slow during relaxation, and stops during $REM$ and $NREM$ sleep.
• Functions: Regulates sleep-wake cycles, inflammatory response, and appetite. Disruption is linked to Schizophrenia and alcohol abuse behavior.
• Metabolism: Broken down by the enzyme diamine oxidase ($DAO$).
• Dietary Sources:
  • High Histamine: Fermented foods (yogurt, kefir, sauerkraut, kimchi), aged cheeses, salami, sausages, kombucha, and soy sauce.
  • Histamine Releasers (trigger mast cell release): Shellfish, pork, egg whites, lemon, lime, chocolate, and strawberries.

Aspartate (Asp)

• Forms: Exist as $D$-aspartate ($D-asp$) and $L$-aspartate ($L-asp$).
• Bio-role: Found predominately in the neuroendocrine system; regulates hormones like glucocorticoids, prolactin, and oxytocin ($D\,Aniello,\,2007$).
• Precursor: $D$-aspartate racemase converts $L-asp$ to $D-asp$ in neuronal production.
• Dietary Sources: Oysters, wild game, sausage, oat flakes, asparagus, and molasses from sugar beets.

Discussion and Future Outlook

• Natural Sources: Psychobiotics, plant extracts, and vegetables provide a roadmap for avoiding medicine side effects.
• Uncertainties: There is a critical need to explore whether nutritional NTs can evade gut microbiota metabolism, bypass splanchnic metabolism, and traverse the BBB.
• Therapeutic Recommendations (Preliminary):
  • $ACh$-rich diet for Alzheimer's/Dementia.
  • Glutamate-free diet for Migraines/Epilepsy.
  • $GABA$-rich diet for Anxiety/Sleeplessness.
  • $DA$-rich diet for Parkinson's.
  • $5-HT$-rich diet for Depression.
  • Histamine-free diet for Vascular Headaches.
• Conclusion: Maintaining sufficient amounts of basic nutrients—protein (for amino acids), Vitamin $B$, Vitamin $C$, and oligo-elements—is essential for a healthy brain and balanced NT levels.