Serotonin Biochemistry

Serotonin Synthesis Pathway

Serotonin (5-hydroxytryptamine, 5-HT) is a key neurotransmitter synthesized from the amino acid tryptophan. The pathway involves several enzymatic steps, with distinct differences between animals and plants.

• Precursor: Tryptophan is the starting molecule for serotonin synthesis.

• Key Steps in Animals:

  1. Tryptophan → 5-Hydroxytryptophan (5-HTP) via tryptophan hydroxylase (rate-limiting step)

  2. 5-HTP → Serotonin (5-HT) via aromatic L-amino acid decarboxylase

  3. Serotonin can be further converted to N-acetylserotonin and then to melatonin

• Key Steps in Plants: Similar pathway but with plant-specific enzymes.

Rate-limiting step: Tryptophan → 5-HTP by tryptophan hydroxylase.

Additional info: The pathway is tightly regulated, and the rate-limiting step is a major target for pharmacological intervention.

Genetic Regulation of Serotonin Synthesis

Mutations in genes encoding tryptophan hydroxylase can disrupt serotonin synthesis. There are two main isoforms:

• TPH1: Affects peripheral serotonin expression.

• TPH2: Brain-specific; mutations here impair central serotonin synthesis.

Serotonin Transport and Storage

• Vesicular Transport: Serotonin is transported into synaptic vesicles by VMAT2 (vesicular monoamine transporter 2).

• Inhibition: Reserpine blocks VMAT2, preventing vesicular storage and depleting serotonin levels.

Serotonin Removal from Synaptic Cleft

Serotonin is primarily removed from the synaptic cleft by reuptake through the SERT (serotonin transporter, 5-HT transporter).

• Drugs affecting reuptake:

  • SSRIs (e.g., Prozac): Inhibit SERT, increasing synaptic serotonin.

  • Cocaine: Inhibits SERT and dopamine transporter.

  • MDMA: Inhibits SERT and increases serotonin release.

Genetic Variants of SERT and Stress Response

The SERT gene has several allelic variants (S and L). These influence stress management and risk for mood disorders.

• Allele combinations: S-S, S-L, L-L

• Public health relevance: S-S genotype + stressful life events → higher vulnerability to depression.

• Individualistic cultures: Higher prevalence of mood disorders with S alleles.

• Collectivist cultures: May buffer against genetic risk.

• Animal models: S/L genotype in rhesus macaques under stress produces more ACTH than L/L.

Drugs Increasing Serotonin Release

• Para-chloroamphetamine: Used experimentally.

• Fenfluramine: Used for appetite suppression in obesity.

• MDMA: Recreational drug; increases serotonin release.

MDMA Neurotoxicity

MDMA is neurotoxic to serotonergic neurons, especially in cortical regions, leading to long-term deficits in serotonin signaling.

Serotonin Metabolism

After release, serotonin is degraded by monoamine oxidase (MAO) to 5-HIAA (5-hydroxyindoleacetic acid), its main metabolite.

• Key steps:

  1. Serotonin → 5-Hydroxyindoleacetaldehyde (via MAO)

  2. 5-Hydroxyindoleacetaldehyde → 5-HIAA (via aldehyde dehydrogenase)

Serotonin Receptors

Overview and Classification

There are 14 known serotonin receptor subtypes, classified into several families (5-HT1 to 5-HT7). They can be metabotropic (G-protein coupled) or ionotropic.

• Metabotropic: Most 5-HT receptors (e.g., 5-HT1, 5-HT2, 5-HT4, 5-HT5, 5-HT6, 5-HT7)

• Ionotropic: 5-HT3 receptor

Major Receptor Families and Properties

Mechanistic Differences: 5-HT1A vs 5-HT2A

• 5-HT1A: Activates K+ channels, inhibits adenylyl cyclase, hyperpolarizes neurons, reduces cAMP synthesis.

• 5-HT2A: Activates phosphoinositide pathway, increases Ca2+ in postsynaptic cells, leads to neuronal excitation.

Distribution of 5-HT1A Receptors

• Mainly in prefrontal cortex

• Also in hippocampus, amygdala, raphe nuclei, septal area

5-HT1A Receptor Function and Pharmacology

• Inhibitory receptor: Stimulation reduces cAMP synthesis and protein kinase A (PKA) activity.

• Agonists: Buspirone, ipsapirone, 8-OHDPAT

• Antagonist: WAY 100635

• Side effects of agonists: Appetite reduction, decreased anxiety and alcohol intake, hypothermia

• Genetic deletion: Induces anxiety-like behavior in mice

5-HT2A Receptor Function and Pharmacology

• Location: Large numbers in cerebral cortex

• Agonist: DOI (hallucinogenic)

• Antagonists: Ritanserin, ketanserin (antipsychotic)

Drug Effects on the Serotonergic System

Serotonin and Behavior

Serotonin and Aggression

• Serotonin production is inversely correlated with aggression.

• Low serotonergic activity is associated with increased aggression.

Serotonin and Anxiety

• SSRIs are the most common pharmacotherapy for anxiety disorders; they increase synaptic serotonin by inhibiting reuptake.

• 5-HT1A receptor: Main receptor involved in anxiety regulation; knockout mice show high anxiety behavior.

• Partial agonist: Buspirone is commonly prescribed.

• 5-HT2A and 5-HT2C receptors: Also regulate anxiety-like behaviors; agonists can increase anxiety, while genetic deletion has anxiolytic effects.

• Developmental role: Early postnatal presence or absence of 5-HT1A receptors in forebrain areas can alter adult anxiety expression.

Serotonin and Pain

• Serotonin plays a role in processing pain signals and inflammation at the level of the brain.

Serotonin and Feeding

• 5-HT1A receptor agonists stimulate somatodendritic autoreceptors in serotonergic neurons, reducing serotonin release and leading to increased food intake (hyperphagia, but depends on context and region).

Serotonin and Thermoregulation

• Mice lacking serotonergic neurons show defects in thermoregulation and episodes of apnea.

Serotonergic Neurotoxins

• MDMA, para-chloroamphetamine, and 5,7-dihydroxytryptamine are neurotoxic to serotonergic neurons.

Serotonin, Learning, and Memory

• Stimulation or inhibition of serotonergic receptors can affect learning and memory, depending on receptor subtype and neuroanatomical location.