Biochemistry 1.3 Neurotransmitters

July 5, 2018 | Author: lovelots1234 | Category: Serotonin, Acetylcholine, Chemical Synapse, Dopamine, Neurotransmitter
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BIOCHEMISTRY 1.3 Neurotransmi Neurotransmitters tters

Subject: Title: Lecturer: Batch/section:

UERMMMCI College of Medicine Biochemistry June 19, 2014 Date: (1.3) Biochemistry of Neurotransmitters Neurotransmitters Dr. Catherine L. Co-Reportoso st 2018A Sem/ A.Y.: 1 /A.Y. 2014-2015

Transcribers: Arce, J., Arquiza, A., Arriba, H., Avenir, M., Azarraga, C., Balberia, J. J. Trans Subject head: Evangelista, A. (9369390879/[email protected]) ( 9369390879/[email protected])

OUTLINE I. Neurotransmission II. Acetylcholine  A. Synthesis and Storage Storage B. Release C. Classes of Acetylcholine Acetylcholine Receptors D. Degradation E. Biologic Effects F. Clinical Application III. Catecholamines  A. Synthesis B. Storage C. Release D. Biologic Effects E. Reuptake F. Inactivation and Degradation G. Degradation of NE/E H. Clinical Application IV. Amino Acids and Derivatives Derivatives  A. Serotonin B. Histamine C. Glutamate D. Glycine E. Aspartate F. GABA G. Nitric oxide V. Summary tables


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OBJECTIVES Define neurotransmission and its basic components Identify the different groups of neurotransmitters Discuss the metabolism of each neurotransmitter as to precursor o  biochemical pathway o o synthesis and synthesis and storage release o  degradation o Briefly discuss the physiologic functions/effects of each neurotransmitter Explain the biochemical basis of some neurotransmitters o Myasthenia Gravis

Parkinson’s Disease   Depression o Carcinoid Syndrome o Allergic Reaction o o

I. NEUROTRANSMISSION essential for the process of communication between two neurons Presynaptic neuron

Chemicals are packaged into synaptic vesicle (S)

(+)stimulated/  depolarized

Released into the synaptic cleft (R)

Postsynaptic neuron

Bind to specific receptors   biologic effects (B)

*Reuptake (all NT except Ach) *Enzymatic inactivation (I)


Steps involved in synaptic transmission (S-R-B-I) 1. S-synthesis and storage 2. R-release 3. B-binding 4. I-inactivation Synapse – Synapse – the  the junction between 2 cells where the impulse is transmitted from one cell to another 2 types of synapses o  – less common; gap junctions   Electrical  – less link 2 cells and directly transmit transmit changes in membrane potential between cells




Chemical  –   –  most common type of synapse; NTs are released from a presynaptic neuron, and dock with receptor proteins on the postsynaptic neuron Neurotransmitter - endogenous chemicals which relay, amplify, and modulate signals between a neuron and another cell Different groups of NTs Cholinergic agent (Ach) o Catecholamines (dopamine, (dopamine, norepinephrine, o epinephrine) Amino acids and its derivatives derivatives (serotonin, o histamine, glutamate, aspartate, glycine, GABA, nitric oxide)

Resting level  – synaptotagmin1  synaptotagmin1   (a calcium protein) is bound to Q SNARE syntaxin, syntaxin , preventing the vesicle fusion If there is an influx of of extracellular extracellular Calcium, Calcium, o synaptotagmin1 is bound to calcium and syntaxin  syntaxin  is released, thus vesicle fusion is permitted. (Binding) Nicotinic AchR the endocytic vesicle loses their clathrin coat o + and are filled with Ach by the H -Ach antiport it is then translocated back to the active zone o the docked vesicles are held in the active o zone by synapsin1 o

C. Classes of Acetylcholine Receptors Nicotinic Acetylcholine Receptor (AchR) Ligand-gated ion channel (ionotropic o receptor) o Ach binds to each of the 2 alpha subunits of the receptor o Conformational change will then occur + o There is an influx of Na   ions resulting to depolarization of postsynaptic membrane 


II. ACETYLCHOLINE (Ach) for more


A. Synthesis and Storage Acetylcholine is synthesized in the cytosol of nerve terminals An acetyl acetyl group group from acetyl CoA transferred to choline o Catalyst: Choline Acetyl Transferase Stored inside the synaptic vesicle via Vesicular acetylcholine transporter (VAChT) o With the use of VATPase Proton Pump + H   goes out of the synaptic vesicle in o exchange for the Ach 

Muscarinic Acetylcholine Receptor G-Protein gated (metabotropic receptor) Ach binds to the receptor mediated by the Go Protein Alpha subunit releases bound GDP and o binds itself to GTP Alpha subunit detaches from the G-protein o complex and will interact with the effector K+ channel opens   adenylyl cyclise is o inactivated  activation of some other enzymes o

B. Release Neuron is stimulated 2+ o Voltage gated ion channel of Ca  opens 2+ Extracellular Ca  go in o Exocytosis of synaptic vesicles is stimulated 




G. Clinical Application: Myasthenia Gravis

D. Degradation No reuptake occurs Acetylcholinesterase Located in the post synaptic membrane o Facilitates enzymatic inactivation of Ach o Hydrolyzes Ach to form acetate and choline o o Acetate diffuses away, while choline is transported back to the presynaptic neuron by a choline carrier

Myasthenia Myasthenia Gravis  – an autoimmune disorder

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E. Biologic effects Peripheral Nervous System Acetylcholine is found in the NMJ  – o Neuromuscular junction o The AchR that is found here are exclusively nicotinic Main Function: Stimulation of muscle fiber = o contraction Central Nervous System o Acetylcholine is found in the interneurons o There is a cholinergic projection from the nucleus basalis Myenert to the forebrain neocortex that is associated with limbic structures AchR in the CNS are nicotinic and muscarinic o The degradation of this pathway is often o associated with Alzheimer’s disease.

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Antibodies that circulate the body bind to AchR at the neuromuscular junction (NMJ) Result: Acetylcholine of post synaptic receptor in the NMJ is blocked   no depolarization and no muscle contraction will take place There is an inability to form a coupled reaction Symptom: Drooping of both eyelids, extensive muscle weakness Medications o Immunosuppressive drugs: prednisone, cyclosporine, mycophenolate mofetil and azathioprine  Controls antibodies Acetylcholinesterase inhibitors: neostigmine o and pyridostigmine  slows down acetylcholinesterase giving Ach a longer time to stimulate receptors

III. CATECHOLAMINES Examples of catecholamines are: epinephrine (adrenaline), norepinephrine (noradrenaline), dopamine Contains catechol, composed of a phenyl ring with two adjacent hydroxyl side groups (found in all catecholamines)

A. Synthesis of Synthesis of Catecholamines 1.

Phenylalanine  Phenylalanine  from the liver is converted into tyrosine by tyrosine by phenylalanine hydroxylase. Tyrosine can also be supplied by the diet. 2. Hydroxylation of tyrosine by tyrosine hydroxylase → dihydroxyphenylalanine (DOPA (DOPA). ). 3. The decarboxylation of DOPA by pyridoxal phosphate (DOPA decarboxylase) → dopamine 4. Dopamine is now stored stored in the synaptic vesicle where dopamine β-hydroxylase β-hydroxylase (DBH; this




enzyme is only present within these storage vesicles) → norepinephrine . 5. Norepinephrine is now methylated by phenylethanolamine N-methyltransferase → N-methyltransferase epinephrine B. Storage of Catecholamines Transported into the vesicles via VMAT2 (vesicle monoamine transporter 2). Vesicular ATPase (V ATPase) pumps protons into the vesicle to be exchanged for positively charged catecholamine. Serotonin and histamine can also be transported by VMAT2. 

C. Release of Catecholamines Catecholamines When an action potential hits the nerve terminal, 2+ 2+ Ca channels open → Ca  enters (the vesicles fuse with neuronal membrane) which then triggers the release of the content (neurotransmitter, ATP, chromogranins etc.) into the synaptic vesicle. Ready f or exocytosis. They now diffuse across the synaptic cleft → they they produce their specific response.

D. Biologic Effects Dopamine (dopaminergic area   physiologi  physiologic c function) Substantia nigra in the basal ganglia → Voluntary motor control   Ventral tegmental area (limbic part of striatum) → Motivation Arcuate nucleus and periventricular nucleus of the hypothalamus (pituitary gland) → Regulates the release of certain hormones


Important for attention, emotion, sleeping, dreaming and learning Responsible for the “fight or flight” response which in turn activates the sympathetic nervous system by: o Increasing heart rate o Release stored energy from fat o Prepares muscles for action

E. Reuptake of Catecholamine Catecholamine

F. Inactivation and Degradation of Catecholamines Reuptake back into the presynaptic terminal and diffusion away from the synapse terminates the action of catecholamines. Degradative enzymes are present in both the presynaptic terminal and in adjust cells.   Monoamine oxidase (MAO; intracellular  – responsible for the degradation of dopamine in the presynaptic neuron). Catechol-o-methyltransferase Catechol-o-methyltran sferase (COMT; extracellular – extracellular  – responsible  responsible for the degradation of dopamine if it is not transported back into the presynaptic neuron) Homovanillic acid is formed when o  Dopamine is oxidized oxidized by MAO → dihydroxyphenylacetic acid which is when acted upon by COMT. 

Synthesis of epinephrine from tyrosine





Dopamine is acted upon by COMT and and then oxidized by MAO to form the same product




Dopamine Analogue Analogue (Levodopa)  –   –  The administration of L-Dopa (The form that can cross the blood brain barrier) temporarily diminishes the motor symptoms Dopamine Agonists  Apomorphine, pramipexole, ropinirole, roligotine MAO Inhibitors  Selegiline, rasagiline  MAO breaks down dopamine secreted by the dopaminergic neurons, thus the inhibitors will allow the level of dopamine in the basal ganglia to increase help increase L-dopa in the striatum  IV. AMINO ACIDS AND DERIVATIVES  A. Serotonin B. Histamine C. Glutamate D. Glycine E. Aspartate F. GABA (gamma-aminobutyric acid) G. Nitric oxide

G. Degradation of Norepinephrine / Epinephrine MAO (oxidative deanimation) and COMT (Omethylation) act on norepinephrine / epinephrine to form vanillyl mandelic acid 


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Vasoconstrictor Stimulates smooth muscle

H. Clinical Application: Parkinson’s Disease Degradation of dopaminergic neurons in the substantia nigra of the midbrain The patient exhibit loss of voluntary motor control and trembling-like movements from their arms and legs. Common medications given: 




Synthesis of Serotonin   Tryptophan – Tryptophan – precursor  precursor to serotonin 1. Tryptophan taken up by serotonergic neurons in restricted brain areas such as the raphe nucleus 2. Once it i t enters the neurons, tryptophan hydroxylase adds the hydroxyl group and produces 5-hydroxytryptophan (5-HT) 3. 5-HT is further decarboxylated by dopa decarboxylase to produce serotonin 4. Serotonin is then stored in synaptic synaptic vesicles vesicles and and docked at the nerve terminals, where it awaits an action potential 5. Release of serotonin into the synaptic cleft activates serotonergic receptors in the post synaptic neurons

Implicated in “jet lag”

Synthesis of Melatonin Serotonin (precursor) 1. Acetylation of the amine group by N-acetyl transferase leading to N-acetyl serotonin 2. Methylation of the the OH group by 5-hydroxyindoleO-methytransferase catalyzing the transfer of a methyl group by S-adenosylmethionin to obtain acetyl-5 methoxytryptamine or melatonin 

Inactivation and Degradation of Serotonin 1. Serotonin is inactivated inactivated by MAO (monoamine oxidase) 2. Oxidative deamination by MAO 5  hydroxyindole acetaldehyde   oxidized into 5 hydroxyindole acetic acid (found in uine) by aldehyde dehydrogenase Storage of Serotonin Vesicular ATPase proton pump pump via vesicular monoamine transporter 2 (VMAT 2)

Clinical Application Depression    - decreased level of serotonin   Symptoms: o Lack of energy and motivation o Less alert than usual; lack of sleep o Low memory retention o Loss of interest; low self-esteem   Medications o Selective Serotonin reuptake inhibitors (SSRI’s)   Prozac  Block the reuptake of serotonin into the presynaptic Neuron Tricyclic antidepressant o Decrease serotonin reuptake and  subsequent rise in serotonin levels in the synapse 

C a rc rc i n o i d s y n d r o m e   


Reuptake: Serotonin Transporter (SERT) Biologic effects of Serotonin GIT (enterochromaffin cells) 80%   regulates intestinal movements   Brain   appetite, sexual behavior, and mood control Low level  depression Extremely high level   mania, reduced appetite and sexual behavior Pineal gland  regulation of sleep Blood platelets  vasoconstrictor  v asoconstrictor 

Increase in serotonin (overproduction from the tumor) Occurs in 5% with with carcinoid carcinoid tumor tumor in GI tract (midgut) Carcinoid tumor  –   –  malignant neuroendocrine tumor of the SI, producing serotonin  increase amount of serotonin secretion Symptoms: Flushing o  Diarrhea o  o Heart failure: serotonin induced fibrosis of the valvular endocardium B. HISTAMINE Produced by mast cells and by certain neuronal fibers within the brain Potent local mediator of allergic reactions

Synthesis Histidine (precursor) Histidine is decarboxylized decarboxylized by the enzyme histidine decarboxylase (requires pyridoxal phosphate) to histamine  

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Biologic effects of Melatonin Produced in the pineal gland Controls sleep-wake cycle (circadian rhythm) Conveys information about light-dark cycles of the body   




Release Depolariztion of nerve terminals  activation of release of histamine by voltage -dependent and calcium-dependent mechanism   activate both post synaptic and presynaptic receptors

  Diphenhydramine   Chlorpheniramine

o o

C. GLUTAMATE Synthesis Synthesized via glutamine and glucose Deamination of glutamine o The intermediate alpha ketoglutarate ketoglutarate (α KG) o from glucose metabolism (via TCA pathway) can be transformed into glutamate by  –  transfer of free   Dehydrogenation  –  ammonia to α KG to form glutamate   Transamination – Transamination – ammonia/amino  ammonia/amino group from any amino amino acid to form glutamate 

Storage Vesicular ATPase proton pump pump via vesicular monoamine transporter 2 (VMAT 2) 

Biologic Effects Chemical messenger that mediates a wide range of cellular responses e.g. immunologic responses 


Physiologic effects


H1 Lung, Brain, vessels Contraction of smooth muscle, sleep-wake regulation Allergic reaction

H2 Heart, brain, stomach Gastric acid secretion

Gastric ulcer

H3 Neurons

H4 Mast cells, eosinophils

Sleep, food intake


Cognitive impairment

Inflammation, immune response

Storage and Release Glutamate is stored in the synaptic synaptic vesicle via Vesicular Glutamate Transporter/VGluT, and subsequently released by exocytosis. 

Receptors Release is Ca2+ dependent 2 kinds: Inotropic - ligand-gated nonselective cation channels which allow the flow of K+, Na+ and sometimes Ca2+ in response to glutamate binding. N-methyl-D-aspartate N-methyl-D-aspartate (NMDA) receptors o Non-NMDA receptors o  α-amino-3-hydroxy-5-methyl-4isoxazolepropionic acid (AMPA)  Kainate receptors Metabotropic - share a common molecular morphology with other G protein –linked  –linked metabotropic receptors (mGluR), located in glial cells 

Inactivation and degradation of Histamine Histamine, unlike other neurotransmitters, neurotransmitters, is not recycled into the presynaptic terminal. Astrocytes, however, have a specific high-affinity uptake system for histamine and may be the major sites of the inactivation and degradation of histamine. Patients with allergic reaction cannot degrade histamine because they have low activity of diamineoxidase. st 1. 1   step: methylation. Histamine methyltransferase transfers a methyl group from S-adenosylmethionine (SAM) to a nitrogen ring of histamine to form methylhistamine nd 2. 2   step: oxidation. Oxidation by MAO-B, followed by an additional oxidation step. In the peripheral tissues, histamine undergoes deamination by diamine oxidase followed by oxidation to a carboxylic acid. 

Clinical Application Allergic reaction 



Release of histamine histamine and other inflammatory agents Symptoms: Localized allergic reaction o Runny nose o Watery eyes o Constriction of bronchi o o Tissue swelling Medications:


Reuptake and Degradation of Glutamate Nerve terminals terminals and glial cells reuptake the glutamate released from the nerve terminals. In the glia, glutamate is converted into glutamine by glutamine synthetase. Glutamine is inactive in a sense that it cannot o activate glutamate receptors It is then released released from the glial cell cell into the o extracellular fluid where nerve terminals take up glutamine (glutamine   glutamate; glutamateglutamine cycle) o

Biologic Effect Functions Functi ons as major excitatory excitat ory NT within the CNS, leading to depolarization of neurons Glutamate plays a role in learning and memory processes, as well as motor m otor function 



 Amyotrophic lateral sclerosis (ALS)/Lou Gehrig’s disease - characterized by degeneration of the motor neurons in the anterior horn of the spinal cord, brainstem, and cerebral cortex.   Excitotoxicity  –   –  neuronal death by prolonged stimulation of neurons by excitatory amino acids Excess glutamate can overstimulate the brain and cause seizures


Synthesis transamination reaction catalyzed by aspartate aminotransferase, AST 

D. GLYCINE Synthesis & Storage Reaction is catalyzed by serine hydroxymethyltransferase (SHMT) Transfer of the hydroxymethyl group from serine→tetrahydrofolate serine→tetrahydrofolate (THF), producing glycine. Stored in neuronal synaptic vesicles by vesicular inhibitory amino acid transmitter (VIAAT). 

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can also be derived from asparagine through the action of asparaginase

Reuptake Released glycine is taken up by neurons by an active sodium-dependent mechanism i nvolving specific membrane transporters (glycine transporter 2). 

Degradation Three pathways of degradation for glycine Predominant: the glycine cleavage enzyme catabolizes glycine to carbon dioxide, ammonia and a one-carbon fragment in t he form of a derivative tetrahydrofolate (THF) called N5, N10methylene THF (glycine+THF+NAD+ (glycine+THF+NAD+  N5,N10methylene THF + NADH+CO2+NH4) Reversal of glycine biosynthesis biosynthesis from serine with serine hydroxymethyl transferase. The resulting serine is then converted to pyruvate by serine dehydratase (2 steps)   Glycine serine (serine OH-CH3 transferase)   Serinepyruvate (serine dehydratase) Glycine is converted to glyoxylate by D-amino acid oxidase   Glycineglyoxylate (aa oxidase)   Glyoxylateoxalate (lactate DH) 

Receptors Inotropic (same with glutamate): o N-methyl-d-aspartate (NMDA) o  Α-amino-3-hydroxy-5-methyl-4 Α-amino-3-hydroxy-5-methyl-4isoxazolepropionic acid (AMPA) Kainite o  Biologic effect Major excitatory NT in the spinal cord 

Degradation of Aspartate

Biologic Effect Most important inhibitory neurotransmitter in the spinal cord, lower brainstem, and retina Function as coagonist at the NMDA glutamate receptor glycine promotes the actions of glutamateglycine serves both inhibitory and excitatory functions within the CNS 

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F. GABA AKA gamma-aminobutyric gamma-aminobutyric acid major inhibitory neurotransmitter in the brain (central nervous system) prevents over-excitation



Synthesis synthesized by decarboxylation of glutamate by the enzyme glutamic acid decarboxylase


Storage Vesicular Inhibitory Amino Acid Transporter (VIAAT) Vesicular GABA transporter (VGAT) 

Release and Binding GABA is released into the presynaptic cleft after depolarization 2 receptors: GABA A - ligand-gated ion channels o GABA B - G protein-coupled receptors o 

Inactivation/Degradation GABA is converted back to glutamate via GABA shunt (Catalyzed by enzyme GABA-T, or ααoxoglutarate transaminase in the presence of ααketoglutarate to form 2 m olecules Glutamate & Succinic semialdehyde (SSA)) 

G. NITRIC OXIDE Relaxation of smooth muscles

Nitric oxide synthase




References th

Biochemistry, 4  ed by Garrett, pp 1046-1056 nd Mark’s Basic Medical Biochemistry 2  ed, pp 888-899 th Biochemistry, 4  ed by Voet and Voet, pp 779-785




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