Catecholamines

Atomic Structure and Synthesis

Catecholamines are a class of neurotransmitters derived from the amino acid tyrosine. They play crucial roles in the central and peripheral nervous systems, affecting mood, attention, and autonomic functions.

• Atomic Structure: Catecholamines consist of a catechol nucleus (benzene ring with two hydroxyl groups) attached to an amine group.

• Synthesis Steps: The biosynthesis of catecholamines involves several enzymatic steps:

  1. Tyrosine is converted to L-DOPA by tyrosine hydroxylase (rate-limiting enzyme).

  2. L-DOPA is converted to dopamine by aromatic amino acid decarboxylase (AADC).

  3. Dopamine is converted to norepinephrine by dopamine β-hydroxylase.

  4. Norepinephrine is converted to epinephrine by phenylethanolamine N-methyltransferase (PNMT).

• Key Enzymes: Tyrosine hydroxylase is the rate-limiting enzyme. AADC is found in the kidneys and other tissues.

• False Neurotransmitters: AADC can decarboxylate tyramine, producing octopamine, which may act as a false neurotransmitter.

• Drug Effects: Inhibition of AADC (e.g., by carbidopa) blocks dopamine synthesis, affecting neurotransmission.

• VMAT2: Vesicular monoamine transporter 2 (VMAT2) packages catecholamines into vesicles. Blockade (e.g., by reserpine) depletes neurotransmitter stores.

Metabolism and Inhibition

• MAO and COMT: Monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) degrade catecholamines. MAO exists in two subtypes: MAO-A and MAO-B.

• MAO Inhibitors: Examples include phenelzine (irreversible), moclobemide (reversible), selegiline (irreversible), and tranylcypromine (irreversible).

Catecholaminergic Pathways

• Nigrostriatal Pathway: Located in the substantia nigra, innervates the striatum, and is involved in motor control.

• Mesolimbic Pathway: Originates in the ventral tegmental area (VTA), projects to limbic structures, and is involved in reward.

• Mesocortical Pathway: Projects from the VTA to the prefrontal cortex, involved in cognition.

Neurotoxins and Disease

• 6-OHDA: Selective neurotoxin for catecholaminergic neurons; crosses the blood-brain barrier.

• MPTP: Neurotoxin that targets dopaminergic neurons, associated with Parkinson's disease.

• Parkinson's Disease: Characterized by death of dopaminergic neurons in the substantia nigra.

Dopamine Receptors and Knockout Models

• D1 Receptors: Mostly located in the striatum; stimulate adenylyl cyclase via Gs protein.

• D2 Receptors: Inhibit adenylyl cyclase via Gi protein.

• Knockout Mice: D1, D2, and D4 knockout mice show distinct behavioral phenotypes, such as altered locomotion and reward processing.

Drugs Affecting Dopaminergic System

Norepinephrine and Adrenergic System

Locus Coeruleus and Pathways

Norepinephrine (NE) is a neurotransmitter involved in arousal, attention, and stress responses. The locus coeruleus is the primary source of NE in the brain.

• Locus Coeruleus: Innervates nearly all parts of the telencephalon and diencephalon, including the amygdala, thalamus, and cortex.

• Sympathetic Neurons: Most contain norepinephrine.

• Neuropeptide Y: Modulates NE release and function.

Adrenergic Receptors

• Types: Alpha (α1, α2) and Beta (β1, β2, β3) receptors, all G protein-coupled.

• Mechanisms: α1 receptors use Gq proteins (activate phospholipase C), α2 use Gi (inhibit adenylyl cyclase), β receptors use Gs (stimulate adenylyl cyclase).

• Clinical Applications: α2 agonists (e.g., clonidine) for hypertension; β-antagonists (e.g., propranolol) for cardiac conditions.

Serotonin (5-HT)

Synthesis and Metabolism

Serotonin is a monoamine neurotransmitter involved in mood, appetite, and sleep regulation.

• Synthesis Pathway:

  1. Tryptophan is converted to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase (rate-limiting step).

  2. 5-HTP is converted to serotonin (5-HT) by AADC.

• Genes: Mutations in Tph2 or AADC genes prevent serotonin synthesis.

• Transport: Serotonin is transported into vesicles by VMAT2; reserpine blocks this process.

• Removal: Serotonin is removed from the synaptic cleft by the serotonin transporter (SERT); SSRIs, TCAs, and MDMA affect this pathway.

• Catabolism: Serotonin is degraded by MAO-A to 5-hydroxyindoleacetic acid (5-HIAA).

Receptors and Functions

• Receptor Types: 14 known serotonin receptors; most are metabotropic except 5-HT3 (ionotropic).

• 5-HT1A Receptor: High concentration in hippocampus, cortex, raphe nuclei, and amygdala; inhibits cAMP synthesis.

• Agonists: Buspirone, 8-OH-DPAT, mCPP, and flesinoxan.

• Antagonists: WAY-100635.

• Knockout Effects: Genetic deletion leads to increased anxiety and altered stress responses.

• 5-HT2A Receptor: Located in cortex; involved in perception and cognition.

Glutamate and GABA

Glutamate

• Function: Major excitatory neurotransmitter; involved in learning and memory.

• Receptors: AMPA, NMDA, and kainate (ionotropic); mGluRs (metabotropic).

• Long-Term Potentiation (LTP): Strengthening of synaptic transmission, crucial for memory formation.

• Excitotoxicity: Excessive glutamate causes neuronal death (necrosis), implicated in neurodegenerative diseases.

GABA

• Function: Major inhibitory neurotransmitter; regulates neuronal excitability.

• Synthesis: Glutamate is converted to GABA by glutamic acid decarboxylase (GAD).

• Transport: GABA is packaged into vesicles by VGAT; removed by GAT transporters.

• Metabolism: GABA is degraded to succinate by GABA transaminase.

• Receptors: GABAA (ionotropic, Cl- channel), GABAB (metabotropic, G protein-coupled).

• Drugs: Vigabatrin inhibits GABA transaminase, increasing GABA levels; used in epilepsy.

Acetylcholine (ACh)

Synthesis and Function

• Synthesis: Choline acetyltransferase (ChAT) synthesizes ACh from choline and acetyl-CoA.

• Transport: VAChT packages ACh into vesicles; hemicholinium blocks choline uptake.

• Degradation: Acetylcholinesterase breaks down ACh in the synaptic cleft.

• Receptors: Nicotinic (ionotropic) and muscarinic (metabotropic) receptors.

• Nicotine: Agonist at nicotinic receptors; increases alertness and addiction potential.

• Muscarinic Receptors: Five subtypes (M1-M5); involved in autonomic and CNS functions.

Drugs Affecting Cholinergic System

• Anticholinesterases: Inhibit acetylcholinesterase, increasing ACh levels (e.g., physostigmine, neostigmine).

• Clinical Use: Alzheimer's disease, myasthenia gravis, and early-stage Parkinson's disease.

Summary Table: Major Neurotransmitters and Key Features

Additional info: Some details, such as specific drug mechanisms and knockout mouse phenotypes, were inferred from standard neurobiochemistry and pharmacology knowledge to provide a complete and academically useful study guide.