PPA Module 2b Lecture 2.9

Introduction to Biogenic Amines and Serotonin

  • Overview of Module 2.9: Focus on serotonin (5-hydroxytryptamine)

  • Learning objectives:

    • Understand synthesis, transport, and signaling of serotonin

    • Describe distribution of serotonin in the brain and its receptors

    • Explain main serotonergic pathways

    • Understand the actions of serotonin and drug interactions with the serotonergic system

Definition of Serotonin

  • Chemical name: 5-hydroxytryptamine (5-HT)

  • Commonly heard as the neurotransmitter serotonin

  • Widely found in nature, specifically in venoms and biological systems

Biological Significance

  • Historical context: Discovery in the 1930s in enterochromaffin-like cells in the GI tract

  • Major production site: 90% of serotonin is synthesized in the GI tract

    • Role in gut motility and digestion

    • Function in platelets: not produced there, but stored for clotting (known as serum tonic)

    • Influence on vasculature

Role in Central Nervous System (CNS)

  • Focus of this module centers on serotonin's role as a neurotransmitter in the CNS

  • Additional roles:

    • Vasoconstriction

    • Mediation of inflammation

    • Gastrointestinal contraction and peristalsis

Serotonin Synthesis

  • Cannot cross the blood-brain barrier; requires local synthesis

  • Derived from the amino acid tryptophan via a two-step process:

    1. Tryptophan hydroxylation:

    • Enzyme: tryptophan hydroxylase

    • Converts tryptophan to 5-hydroxytryptophan (5-HTP)

    1. Decarboxylation:

    • Enzyme: aromatic amino acid decarboxylase (AADC)

    • Converts 5-HTP to serotonin

  • Rate-limiting enzyme: tryptophan hydroxylase

  • PCPA (parachlorophenylalanine): Inhibitor of tryptophan hydroxylase, leading to decreased serotonin synthesis

Transport and Storage

  • Serotonin is packaged into vesicles by VMAT2 (vesicular monoamine transporter 2)

    • VMAT2 function: Ion cotransport, exchanging protons for serotonin

    • Risperidone: Drug that blocks VMAT2, leading to serotonin depletion and dysphoria

Serotonin Metabolism

  • Primary breakdown pathway through monoamine oxidase (MAO):

    1. First Step:

    • Conversion of serotonin to 5-hydroxyindoleacetaldehyde

    1. Second Step:

    • Further degradation to 5-hydroxyindoleacetic acid (5-HIAA) by aldehyde dehydrogenase (ALDH)

  • In the pineal gland:

    • Serotonin is a precursor for melatonin synthesis, important for circadian rhythms and sleep regulation

  • Synthesis pathway from serotonin to melatonin:

    • Enzymes involved:

    • AANAT (N-acetyltransferase) converts serotonin to N-acetylserotonin

    • ASMT (acetylserotonin methyltransferase) further converts N-acetylserotonin to melatonin

Serotonin Receptors

  • Overview of serotonin receptors:

    • Types:

    • At least 14 subtypes of serotonin receptors

    • Most are metabotropic; only 5-HT3 is ionotropic

    • 5-HT3 Receptors:

    • Ligand-gated ion channels, excitatory, fast action via depolarizing EPSP

  • Key receptors and their pathways:

    • 5-HT1: Inhibitory, controls adenylate cyclase activity

    • 5-HT2: Activates PLC, involved in excitatory signaling

    • 5-HT4: GS protein coupled, activates adenylate cyclase

    • 5-HT5: Inhibitory, affects adenylate cyclase

    • 5-HT6 and 5-HT7: GS protein couple and activate adenylate cyclase

Drug Interactions with Serotonin Receptors

  • Buspirone:

    • Agonist at 5-HT1 receptors, used as anxiolytic and antipsychotic

  • LSD:

    • Potent hallucinogen, acts on 5-HT2A receptors causing hallucinations and euphoria

  • Selective serotonin reuptake inhibitors (SSRIs):

    • Example: Fluoxetine (Prozac)

    • Mechanism: Blocks serotonin transporter, allowing more serotonin in the synapse

    • Key point: SSRIs work by allow serotonin to linger longer in the synapse, but immediate antidepressant effect is not observed, indicating complex intracellular mechanisms involved in their action

Serotonin Pathways

  • Major serotonergic projections originate from the midbrain raphe nuclei:

    • Dorsal (rostral) and caudal raphe nuclei

    • Projections widely distributed throughout the brain, influencing multiple systems

Specific Functions of Serotonin

  • Neuromodulation: Inhibitory effects by 5-HT1 receptors

  • Regulation of sleep and sedation: Activation leads to calmness and drowsiness

  • Anxiety and depression management: Postsynaptic 5-HT1A receptors in the hippocampus and amygdala associated with mood regulation

  • Cognition: Mechanisms through 5-HT2 receptors in the prefrontal cortex

  • Psychotic symptoms: Overstimulation leads to hallucinations as seen with LSD

  • Regulation of sexual behavior: Increased serotonin associated with decreased sexual activity

  • Caudal raphe projections:

    • Primarily to cerebellum, brainstem, and spinal cord

    • Function: Regulation of nausea, vomiting (via 5-HT3 receptors in the area postrema) and pain modulation (via projections to spinal cord lamina II)

Summary of Important Serotonin Receptor Drugs

  • Buspirone: Acts on 5-HT1A, reduces anxiety

  • LSD: Agonist primarily at 5-HT2A, causes hallucinations

  • Fluoxetine (Prozac): SSRI, used as an antidepressant by inhibiting serotonin reuptake

  • Ondansetron and Granisetron: 5-HT3 receptor antagonists, anti-nausea drugs used in chemotherapy

  • Sumatriptan: Agonist at 5-HT1B/1D, used for migraine treatment

Conclusion

  • Serotonin is a multifaceted neurotransmitter that affects mood, cognition, digestion, and other vital physiological processes. Understanding its pathways and interactions with various receptors is crucial for developing therapeutic strategies for anxiety, depression, and other related disorders.