Catecholamine Biochemistry: Synthesis, Pathways, and Neuropharmacology

Catecholamines: Structure and Synthesis

Atomic Structure of Catecholamines

Catecholamines are a class of neurotransmitters and hormones characterized by a catechol nucleus (a benzene ring with two hydroxyl groups) attached to an amino group. Major catecholamines include dopamine, norepinephrine, and epinephrine.

• Catechol nucleus: Benzene ring with two adjacent hydroxyl groups.

• Amino group: NH2 functional group attached via an ethyl chain.

Catecholamine Biosynthesis Pathway

The synthesis of catecholamines involves several enzymatic steps, each producing a specific intermediate.

1. Rate-limiting step: Tyrosine Hydroxylase (TH) converts tyrosine to L-DOPA.

2. L-DOPA Decarboxylation: Aromatic Amino Acid Decarboxylase (AADC) converts L-DOPA to dopamine.

3. Dopamine β-Hydroxylation: Dopamine β-Hydroxylase (DBH) converts dopamine to norepinephrine.

Key enzymes:

• Tyrosine Hydroxylase (TH): Rate-limiting enzyme in catecholamine synthesis.

• AADC: Found in liver, kidneys, stomach, and brain; decarboxylates L-DOPA and other aromatic amino acids.

• Dopamine β-Hydroxylase (DBH): Glycoprotein containing Cu2+; converts dopamine to norepinephrine.

Formula:

• $\text{Tyrosine} \xrightarrow{TH} \text{L-DOPA} \xrightarrow{AADC} \text{Dopamine} \xrightarrow{DBH} \text{Norepinephrine}$

Enzyme Inhibition and Drug Effects

Various drugs can inhibit enzymes in catecholamine synthesis, affecting neurotransmitter levels.

• AADC inhibitors: Prevent production of dopamine and norepinephrine, leading to L-DOPA accumulation.

• DBH inhibitors: DDC (diethyldithiocarbamate), Disulfiram, and chelating agents block DBH by removing copper, reducing norepinephrine and increasing dopamine.

Catecholamine Storage, Release, and Reuptake

Vesicular Transport

VMAT2 (Vesicular Monoamine Transporter 2) packages catecholamines into synaptic vesicles for release. Blocking VMAT2 depletes neurotransmitter stores.

Reuptake Inhibition

Catecholamine reuptake is blocked by several drugs, increasing synaptic levels.

• Cocaine

• Amphetamine

• Ritalin

• Mazindol

• Tricyclic antidepressants

Catecholamine Metabolism

Monoamine Oxidase (MAO) and Catechol-O-Methyltransferase (COMT)

MAO degrades catecholamines in presynaptic neurons if not stored in vesicles. Found throughout the body, especially on the outer mitochondrial membrane. COMT is found only in the brain and also degrades catecholamines.

• MAO subtypes:

  • MAO-A: Degrades serotonin (5HT), norepinephrine (NE), epinephrine (E), and dopamine (DA).

  • MAO-B: Primarily degrades dopamine.

• MAO inhibitors:

  • Irreversible: Iproniazid, Paragyline, Deprenyl

  • Reversible: Depends on concentration

Dopaminergic Pathways in the Brain

Nigrostriatal Pathway

Originates in the substantia nigra (midbrain), innervates the globus pallidus and caudate putamen, and produces dopamine. Involved in motor control.

Mesolimbic Pathway

Originates in the ventral tegmental area (midbrain), projects to the nucleus accumbens and limbic structures. Produces dopamine and is involved in reward and reinforcement.

Mesocortical Pathway

Originates in the ventral tegmental area, innervates the hippocampus, anterior olfactory nucleus, and cerebral cortex. Produces dopamine and is involved in cognition.

Neurotoxins and Disease

Neurotoxins Affecting Catecholaminergic Neurons

• 6-OHDA: Does not cross the blood-brain barrier; must be administered intracerebroventricularly.

• DSP-4: Selective neurotoxin for locus coeruleus noradrenergic neurons in rodents and birds.

• MPTP: Induces Parkinson's disease phenotype by targeting dopaminergic neurons in the substantia nigra.

Parkinson's Disease: Associated with death of dopaminergic neurons in the substantia nigra.

Dopamine Receptors

D1 and D2 Receptors

Dopamine receptors are classified as metabotropic (G-protein coupled) and are distributed throughout the brain.

• D1 receptors: High levels in substantia nigra, nucleus accumbens, olfactory tubercle; activate adenylate cyclase (Gs protein), increasing cAMP.

• D2 receptors: Located in striatum, substantia nigra, pituitary gland, limbic system; inhibit or have no effect on adenylate cyclase (Gi protein), decreasing cAMP.

• D3 + D5: Mainly in hypothalamus.

• D1 + D2: Mainly in corpus striatum.

Metabotropic receptor mechanism:

• Ligand (neurotransmitter) binds to extracellular domain.

• Receptor undergoes conformational change, activating associated G-protein (α, β, γ subunits).

• Signal transduction via intracellular enzymes or ion channels.

Common pathways:

• $Gs: \text{stimulates adenylate cyclase} \rightarrow \text{increases cAMP} \rightarrow \text{activates PKA}$

• $Gi: \text{inhibits adenylate cyclase} \rightarrow \text{decreases cAMP} \rightarrow \text{reduces PKA}$

Drugs Affecting the Dopaminergic System

Drug Actions Table

The following table summarizes drugs and their effects on the dopaminergic system:

Drug	Action
DOPA	Converted to DA in brain by AADC
Phenelzine	Inhibits MAO, therefore inhibits catecholamine catabolism
a-Methyl-para-tyrosine (AMPT)	Inhibits TH, therefore depletes catecholamines
Reserpine	Inhibits VMAT2, depletes catecholamines
6-OHDA	Neurotoxin for catecholaminergic neurons
Amphetamine	Releases catecholamines
Cocaine and methylphenidate	Antagonist for reuptake
Apomorphine	Agonist of DA receptors
SKF 38393	Agonist of D1 receptors
Quinpirole	Agonist of D2 and D3 receptors
SCH 23390	Antagonist of D1 receptors
Haloperidol	Antagonist of D2 receptors

Dopamine Transporter Knockout and Receptor Knockout Studies

Knockout Mouse Models

• DA transporter knockout: Excess dopamine in synaptic cleft causes extreme hyperactivity (not slothfulness).

• D1 knockout mice: Deficit in cognitive tasks.

• D2 knockout mice: Impairment in spontaneous movement, coordination, and posture.

• Double knockout (D1 and D2): Fatality within second to third week of life.

Pleasure and Reward System

Dopamine and Reward

Dopamine is a key neurotransmitter in the pleasure and reward system, reinforcing behaviors by its release in the nucleus accumbens (mesolimbic pathway). The ventral tegmental area and prefrontal cortex are also involved in processing anticipation, decision-making, and emotional response to rewards.

ADHD and Dopamine Transporter

ADHD and Dopamine

Attention Deficit Hyperactivity Disorder (ADHD) is associated with defects in the dopamine transporter (DAT), leading to altered dopamine supply and affecting attention and behavior.

Norepinephrine System

Locus Coeruleus and Norepinephrine

The locus coeruleus innervates nearly all parts of the telencephalon and diencephalon, including cortex, hippocampus, amygdala, septum, thalamus, and hypothalamus. Most sympathetic neurons contain norepinephrine, produced by noradrenergic neurons in the locus coeruleus via DBH.

Neuropeptide and Norepinephrine Modulation

• Neuropeptides and norepinephrine modulate each other's release via alpha2 autoreceptors and NPY (neuropeptide Y) receptors.

• Regulate vascular tone, appetite, anxiety, and stress responses.

Adrenergic Receptors and Signal Transduction

Adrenergic Receptor Subtypes

• Alpha-1 (α1): Gq protein, increases Ca2+, contraction.

• Alpha-2 (α2): Gi protein, inhibits neurotransmitter release, contraction of smooth muscle.

• Beta (β): Gs protein, smooth muscle relaxation, heart contraction.

All adrenergic receptors are metabotropic (G-protein coupled).

Alpha-Adrenergic Receptor Mechanism

1. Activation of Gq coupling protein.

2. Alpha subunit activates phospholipase C.

3. Release of IP3 and DAG from phosphatidylinositol.

4. IP3 stimulates release of Ca2+.

5. Ca2+ activates protein kinases; DAG activates protein kinase C.

Clinical Applications of α2 Receptor Stimulation

• Used in treatment of hypertension, sedation, and pain management.

• Modulates mood, hormone release, emotional behavior, and sleepiness.

Locus Coeruleus and Vigilance

Role in Alertness and Sleep

• Activity in locus coeruleus increases alertness; inhibition reduces activity.

• Regulates melatonin levels via retinohypothalamic tract (RHT).

• Norepinephrine quantity regulates sleep: more NE leads to less sleep (excitatory), less NE leads to more sleep.

• Projects to prefrontal cortex, influencing attention and working memory.

Prefrontal Cortex NE Receptor Affinity

• NE has higher affinity for alpha-2 receptors than alpha-1 in prefrontal cortex.

• Excess NE stimulates alpha-1 receptors, leading to cognitive impairment.

Emotionally Arousing Events

• Locus coeruleus releases noradrenaline during emotional arousal.

Norepinephrine and Feeding Behavior

• Paraventricular nucleus (PVN) is sensitive to norepinephrine stimulation.

• NE binding to alpha-adrenergic receptors in PVN stimulates feeding behavior by disinhibiting appetite-suppressing neurons.

Additional info: Some context and definitions were expanded for clarity and completeness, including mechanisms of receptor action and clinical applications.