003 biokimia saraf

Neuro biochemistry

Dr. SarYOno, SKp.,MKes. Medical biochemistry unit

Neuron Communication by neurons is based on

changes in the membrane’s permeability to ions.

A typical neuron has a dendritic region and an axonal region.

The dendritic region is specialized to receive information, typically neurotransmitters; it then undergoes graded potentials.

The axonal region is specialized to deliver information: after undergoing action potentials, neurotransmitters are released from the axon terminal.

Synapses An interneuronal junctions Two kinds of synapses

Chemical synapses Electric synapses

Chemical Synapses Vesicles contain neurotransmitters that can

alter the ionic conductivity of the postsynaptic membrane

The postsynaptic membrane is separated from the presynaptic membrane by a synaptic cleft having a width of 20 nm

An increase in sodium ion/Na+, permeability tends to depolarize the postsynaptic membrane (excitatory)

An increase in potassium ion, permeability hyperpolarizes the membrane (inhibitory)

Chemical vs. Electric SynapseElectrical synapse• Impulses can be regenerated without interruption in adjacent cells• Gap junctions

– Adjacent cells electrically coupled through a channel– Each gap junction is composed of 12 connexin proteins

• Examples– Smooth and cardiac muscles, brain, and glial cells

Chemical synapse• Terminal bouton is separated from postsynaptic cell by synaptic cleft• NTs are released from synaptic vesicles• Vesicles fuse with axon membrane and NT released by exocytosis• Amount of NTs released depends upon frequency of APExamples: otot lurik/rangka

Chemical vs. Electric Synapse

Chemical Synapse• Unidirectional• Current Flow Limited By Neurotransmitter Diffusion• Nonlinear

Electric Synapse• Bidirectional• Very Fast Current Flow• Linear

Ion gating in axon Changes in membrane potential caused by ion flow

through ion channels Voltage gated (VG) channels open in response to change

in membrane potential Gated channels are part of proteins that comprise the

channel Can be open or closed in response to change

2 types of channels for K+ 1 always open 1 closed in resting cell

Channel for Na+ Always closed in resting cells

• Some Na+ does leak into the cells

How A Nerve Cell Fires Nerve cell membrane is a lipid bilayer with

embedded proteins. ATP-powered ion pumps keep outside of

membrane + charged, inside – charged. Channels in membrane can let + ions pass

through. Channels normally closed. Neurotransmitter gated channels collapse

(‘depolarize’) voltage gradient. Voltage gated channels propagate

depolarization in a wave down axon.

Action potentials Stimulus causes depolarization to threshold Voltage gated (VG) Na+ channels open

Electrochemical gradient inward + feedback loop

Rapid reversal in membrane potential from –70 to + 30 mV VG Na+ channels become inactivated

VG K+ channels open Electrochemical gradient outward -feedback loop

Action potentials Depolarization and repolarization occur via diffusion, do not

require active transport Once AP completed, Na+/K+ ATPase pump extrudes Na+,

and recovers K+ All or none

When threshold reached, maximum potential change occurs Amplitude does not normally become more positive than + 30

mV because VG Na+ channels close quickly and VG K+ channels open

Duration is the same, only open for a fixed period of time Coding for Stimulus Intensity

Increased frequency of AP indicates greater stimulus strength Recruitment

Stronger stimuli can activate more axons with a higher threshold

(Potensial berjenjang)

Synaptic transmission NT release is rapid because many vesicles form fusion

complexes at docking site AP travels down axon to bouton VG Ca2+ channels open

– Ca2+ enters bouton down concentration gradient– Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs

Ca2+ activates calmodulin, which activates protein kinase Protein kinase phosphorylates synapsins

– Synapsins aid in the fusion of synaptic vesicles

Inhibit transmiter release

Inhibit transmiter uptake

organofosfat

The Dopamine The Dopamine HypothesisHypothesis

F. I. Carroll et al, Journal of Medical Chemistry 42, 2721-36 (1999)

Synapses are major targets of neuroactive drugs

CocaineX

Adenosine

CaffeineX

Nicotine

•Caffeine: inhibits adenosine receptors•Nicotine: activates acetylcholine receptors•Cocaine: inhibits uptake of DA, NE, 5HT•Ethanol?

AChReceptor

Synapses are the targets of therapeutic drugs Synapses are also the sites of actions of many classes of

therapeutic drugs which act upon the brain. 1)     Antidepressant drugs generally act by inhibiting the

uptake of serotonin and norepinephrine 2)     An important class of analgesic drugs, the opiate

analgesics, activate receptors for neurotransmitters known as the endorphins and enkephalins

3)     Antipsychotic drugs block or inhibit receptors for dopamine

4)     Some types of anticonvulsant drugs potentiate the effects of the neurotransmitter GABA

•Antidepressant drugs: serotonin uptake inhibitors• Analgesics (morphine): opiate receptor agonists• Antipsychotic drugs: DA receptor antagonists•Anticonvulsant drugs; GABA, modulators•Antianxiety agents: GABA, modulators

Zigmond et al

(1999) Fig 8.3

Stages 1 & 2 Accumulation of a precursor amino acid into the neuron which is metabolized to yield the mature transmitter (ZZ)

Stage 3 Transmitter is then accumulated into vesicles by the vesicular transporter for storage and release. Stages 4 & 5 Transmitter is released into synaptic cleft to interact with post-synaptic receptors or autoreceptors that

regulate transmitter release, synthesis or firing rate.Stage 6 – 9 Inactivation and termination of the action of the released transmitter by reuptake through neuronal

transporter proteins, enzymatic degradation, uptake by glial cells or passive diffusion

LIFE CYCLE OF A TRANSMITTER

Neurotransmiter (bekerja cepat)

Klas I ; asetilkolin Klas II (amina); norepinefrin, epinefrin,

dopamin, serotonin, histamin Klas III (asam amino); GABA, glisin,

glutamat, aspartat Klas IV; NO

Neuropeptida (bekerja lambat)

Releasing hormon Peptida hipofise; LH, GH, endorfin, prolaktin,

oksitosin dll Peptida usus dan otak; enkefalin, substansi P,

gastrin, insulin, glukagon dll Dari jaringan lain; angiotensin, bradikinin,

kalsitonin dll

Neurotransmiter vs neuropeptida

Bekerja cepat Molekul kecil Disintesis di

sitosol Tempat kerja

di membran pasca sinap

Bekerja lambat

Molekul besar

Disintesis di ribosom, retikulum endoplasma, aparatus golgi

Tempat non sinaps, di sel prasinaps maupun pasca sinaps

Letak neurotransmiter penting

Asetilkolin-disekresi di sebagian besar otak, otot rangka dll

Norepinefrin- disekresi di neuron yang badan selnya di batang otak dan hipotalamus

Dopamin – di substansia nigra GABA- di medula spinalis, serebelum, ganglia

basalis, sebagian korteks Serotonin- di rafe medial batang otak Glutamat – di presinap sensorik korteks

Neurotransmitter in the CNS -There is a broad set of different neurotransmitters in the CNS-Neurotransmitters can be activating or inhibitory-Neurotransmitter bind to specific receptors and cause excitatory post synaptic potential (EPSP) or inhibitory post synaptic potentials (IPSP) -Activating transmitter act by opening kation-channels (Na+,K+, Ca2+)-Inhibitory transmitter act by opening anion (mainly Cl- -Channels)-Important neurotransmitter:

-Acetylcholine (Transmitter of Neuro-muscular junctions)-Dopamine-Serotonin-Glycin (inhibitory)-Aspartate-Glutamate-GABA (-amino-butyric acid, inhibitory)-Adrenaline/Noradrenaline (Autonomous nervous system)

Neurotransmitters

DopamineNorepinephrineEpinephrine

Glutamate -Aminobutyrate

Serotonin

Tyrosine

Tryptophan

PLP (vit B6)

deCO2ase

PLP (vit B6)

deCO2ase

PLP (vit B6)

deCO2ase

Amino AcidPrecursors

PLP:piridoksal fosfat

Pathway

Dopamine

Norepinephrine

Serotonin(5-HT)

Tyrosine

Tryptophan

PLP (vit B6)

AAA deCO2ase

PLP (vit B6)

AAA deCO2ase

Catecholamines

DOPA

DOHase Vit C, O2

Tyr OHase

THBP, O2

5HTPTrp OHase

O2THBP,

EpinephrinePNMT

SAMSAHC

THBP: tetrahydrobiopterin

Acetylcholine+ cholineAcetyl-CoAPyruvate

PDH complex(FAD, lipoamide, TPP)

Choline acetyltransferase

Acetylcholine

Acetylcholinesterase

Acetate + choline

Reuptake or diet

Catecholamine Biosynthesis

CH2CHCO2-

NH3+

HO

CH2CHCO2-

NH3+

HO

HO

CH2CH2NH2

HO

HO

CHCH2NH2

HO

HO

OH

CHCH2NHCH3

HO

HO

OH

Tyr hydroxylase

O2

Tyrosine Dihydroxyphenylalanine (DOPA)

Dopamine

DOPAdecarboxylase CO2

Dopaminehydroxylase

Norepinephrine

Catechol

Epinephrine(Adrenaline)

SAM

S-Adenosyl-homocysteine

Methyl transferase

DOPA, dopamine, norepinephrine,and epinephrine are all neurotransmitters

L-DOPA in Parkinsonism

Blood Brain

Blood Brain Barrier

L-DOPA L-DOPA Dopamine

Dopamine

HO

HO CH2-C-CO2H

CH3

NHNH2Carbidopa

Blocks

Parkinsonism associated with dopamine in brain through loss ofneurons in basal ganglia.Carbidopa + L-DOPA

Monoamine Oxidase (MAO)MAO

(in mitochondria)

R R’OH H NorepiOH CH3 EpiH H Dopamine

CHCH2NHR'

HO

HO

RCHCHO

HO

HO

R

CHCO2H

HO

HO

RUrinary metaboliteMAO inhibitors (e.g., tranylcypromine) are useful

in the treatment of depression Brain levels of dopamine and norepi.; also serotonin

Aldehydedehydrogenase

R=OH Vanillylmandelic acid (VMA)R=H Homovanillic acid (HVA)

Catechol-O-Methyl Transferase (COMT)

CHCH2NHR'

HO

HO

RCHCH2NHR'

HO

CH3O

R

COMT

Inactive metabolite

SAM S-Adenosyl-homocysteine

• COMT found in cytoplasm• Terminates activity of catecholamines• Catecholamine excretion products result from combined actions of MAO and COMT• Inhibitors of COMT (e.g., tolcapone) useful in Parkinson’s disease

Active catecholamine

Tryptophan Metabolism: Serotonin Formation

NH

CH2CHCO2-

NH3+

NH

CH2CHCO2-

NH3

HO

+

NH

CH2CH2NH2

HO

Tryptophan(Trp)

Indole ring

Trphydroxylase

O2

5-Hydroxy-tryptophan

Decarboxylase

CO2 5-Hydroxy-tryptamine (5-HT);Serotonin

Serotonin• Serotonin formed in:

• Brain (neurotransmitter; regulation of sleep, mood, appetite) • Platelets (platelet aggregation, vasoconstriction)• Smooth muscle (contraction) • Gastrointestinal tract (enterochromaffin cells - major storage site)

• Drugs affecting serotonin actions used to treat: • Depression

•Serotonin-selective reuptake inhibitors (SSRI) • Migraine• Schizophrenia• Obsessive-compulsive disorders • Chemotherapy-induced emesis

• Some hallucinogens (e.g., LSD) act as serotonin agonists

Serotonin Metabolism: 5-HIAA

NH

CH2CH2NH2

HO

NH

CH2CHOHO

NH

CH2CO2HHO

Serotonin

MAO

Dehydrogenase

5-Hydroxyindole acetic acid (5-HIAA) (Urine)

Carcinoid tumors: • Malignant GI tumor type• Excretion of large amounts of 5-HIAA

Serotonin Metabolism: Melatonin

NH

CH2CH2NHCOCH3

H3CO

NH

CH2CH2NH2

HO2 Steps

Serotonin Melatonin

Melatonin:• Formed principally in pineal gland• Synthesis controlled by light, among other factors• Induces skin lightening• Suppresses ovarian function• Possible use in sleep disorders

Nitric Oxide• Cell messenger• Implicated in a wide range of physiological and pathophysiological events:• Vasodilation:

• Activates guanylyl cyclase cGMP• Nitroglycerin Glycerin + NO• Sildenafil (Viagra): in vascular smooth muscle:

NO cGMP GMPPhospho-diesterase-5

Blocks

Synthesis of Nitric Oxide

NH3+NH2

H2N=C-HNCH2CH2CH2CHCO 2-

+

NH3+

NH2CONH CH2CH2CH2CHCO 2- + NO

Nitric oxide synthase (NOS)

Arginine

Citrulline

Tryptophan Metabolism:Biosynthesis of Nicotinic Acid

NH

CH2CHCO2-

NH3+

TryptophanN

CO2H

Nicotinic acid (Niacin)

Several steps

Nicotinamide adenine dinucleotide (NAD)

GABA

GLUTAMATE GABA + CO2

GLU DECARBOXYLASE GABA IS THE MAJOR INHIBITORY NEURO-TRANSMITTER IN

BRAIN GLU IS THE MAJOR EXCITATORY NEURO-TRANSMITTER

STIMULATION OF NEURONS BY GABA PERMEABILITY TO CHLORIDE IONS

BENZODIAZEPINES (VALIUM) ENHANCE MEMBRANE PERMEABILITY OF Cl IONS BY GABA

GABAPENTIN PROTECTS AGAINST GLU EXCITOTOXICITY

HISTAMINE

HISTIDINE HISTAMINE + CO2

HIS DECARBOXYLASE HISTAMINES INVOLVED IN

ALLERGIC RESPONSE H1 RECEPTORS IN GUT, BRONCHI

• STIMULATION SMOOTH MUSCLE CONTRN’• H1 RECEPTOR ANTAGONISTS

– CLARITIN, ZYRTEC, ETC

HISTAMINE

HISTAMINES INVOLVED IN CONTROL OF ACID SECRETION IN STOMACH

H2 RECEPTORS • STIMULATION HCl SECRETION• H2 ANTAGONISTS

– CIMETIDINE– RANITIDINE

H2 RECEPTORS IN HEART STIMULATION HEART RATE

SEROTONIN TRP 5-HYDROXYTRYPTOPHAN

TRP HYDROXYLASE REQUIRES 5,6,7,8

TETRAHYDROBIOPTERIN 5-HT SEROTONIN + CO2

AROMATIC ACID DECARBOXYLASE SEROTONIN CAUSES

SMOOTH MUSCLE CONTRACTION BRAIN NEUROTRANSMITTER MELATONIN SYNTHESIZED IN PINEAL

GLAND

CATECHOLAMINES EPI, NOREPINEPHRINE, DOPAMINE

AMINE DERIVATIVES OF CATECHOL

REACTIONS:

TYR L-DOPA TYR HYDROXYLASE

L-DOPA DOPAMINE + CO2 AROMATIC ACID DECARBOXYLASE

DOPAMINE NOREPINEPHRINE DOPAMINE β-HYDROXYLASE

NOREPINEPHRINE EPINEPHRINE REQUIRES SAM

L-DOPA AND DOPAMINE IN SUBSTANTIA NIGRA, CATECHOLAMINE

PRODUCTION STOPS AT DOPAMINE PARKINSON’S DISEASE: DEGENERATION OF

SUBSTANTIA NIGRA DOPAMINE TREAT BY GIVING PRECURSOR, L-DOPA DOPAMINE CANNOT CROSS BLOOD/BRAIN

BARRIER TRANSPLANTATION OF ADR. MEDULLA CELLS TO

BRAIN L-DOPA A PRECURSOR OF MELANIN

PRODUCTION

NOREPINEPHRINE EPINEPHRINE

S-ADENOSYLMETHIONINE

ACTIONS OF NOREPINEPHRINE NOT NEARLY AS ACTIVE AS EPINEPHRINE

DURING EXTREME STRESS CIRCULATORY SYSTEM

CONSTRICTS GREAT VEINS (2) VASOCONSTRICTIVE TO SKIN (1) VASOCONSTRICTION (1) EFFECTS ON

GI TRACT SPLEEN PANCREAS KIDNEYS

NEUROTRANSMITTER IN THE BRAIN

ACTIONS OF EPINEPHRINE

AS AN INSULIN ANTAGONIST ACTIVATES MUSCLE GLYCOGEN PHOSPHORYLASE

GLUCOSE-6-P USED IN GLYCOLYSIS TRIGGERS PHOSPHORYLATION (ACTIVATION) OF

HORMONE-SENSITIVE LIPASE IN FAT CELLS MOBILIZES FAT BY HYDROLYZING TGs

GLYCOGEN BREAKDOWN IN LIVER ACTIVATES GLUCONEOGENESIS IN LIVER INHIBITS FATTY ACID SYNTHESIS

ACTIONS OF EPINEPHRINE

ON CARDIAC MUSCLE β1 -ADRENERGIC RECEPTOR STIMULATION

HEART RATE AND CARDIAC OUTPUT• β-BLOCKERS BLOOD PRESSURE

DILATES CORONARY ARTERIES (β2) ON SMOOTH MUSCLE (β2-ADRENERGIC)

IN BRONCHIOLES, FOR EXAMPLE MUSCLE RELAXATION

ACTIVATION OF G-PROTEINS• cAMP , ETC

ASTHMA MEDICATIONS

Phenylalanine and tyrosine are precursors of the

catecholamines, dopamine, norepinephrine, and epinephrine.

• Dopamine has a role in the pathogenic mechanisms

that cause Parkinson’s disease.

• Parkinson’s disease is characterized by a loss of

dopaminergic neurons that results in diminished

levels of dopamine in the striatum.

Dopamine is synthesized in the cytoplasm and (normally) immediately sequestered in intracellular storage vesicles through the agency of α-synuclein.

2. Biosynthesis of the catecholamines

phenylalanine

tyrosine

Tyrosine 3-monooxygenase*

dihydroxyphenylalanine (DOPA)

dopamine

norepinephrine

epinephrine

*Rate-determining step

•

Tryptophanprecursor of 5-hydroxy-tryptamine (serotonin).

The broad actions of the neurotransmitter 5-hydroxytryptamine involve effects upon emotion, mood, and reward.

suicide and depression have been associated with reduced serotonergic transmission.

One recent postmortem study examined the capacity to synthesize 5-hydroxytryptamine and the capacity to bind 5-hydroxytryptamine.

Their findings indicated that the brains of suicide victims had had reduced serotonergic function.

2. Biosynthesis of 5-hydroxytryptamine

In the conversion of tryptophan into 5-hydroxy-

tryptamine, tryptophan undergoes hydroxylation to

yield 5-hydroxytryptophan that is then decarboxylated

to yield 5-hydroxytryptamine.

Reactions that convert tyrosine or tryptophan into neurotransmitters.

It has been shown that that the intensity and

duration of catecholamine signaling at the synapse is

governed by the efficiency of the re-uptake of released

neurotransmitter. Re-uptake occurs by means of high-

affinity membrane transporters. The finding that a

dopamine “re-uptake” transporter can serve as a

cocaine “receptor” has provided new insight into

mechanisms of addiction.

Cartoon depicting the binding of cocaine (and other agents) by the membrane transporters that facilitate reuptake.

D. Arginine is the precursor of the second

messenger nitric oxide. This a second messenger

that appears to have distinctly different functions in

different cell types. For example, the nitric oxide

synthesized by platelets is seen to inhibit platelet

aggregation and adherence. In this way it

contributes to the anti-thrombogenic properties of

the endothelium.

In the kidney (in experimental animals), a diminution in

nitric oxide was associated with glucocorticoid-induced

hypertension. Such studies were carried out because it

had been suggested that decreased nitric oxide

contributes to the impaired endothelium-dependent

vasodilatation seen in essential hypertension.

On the other hand, cortical cells from patients with

Alzheimer’s disease showed increased levels of nitric

oxide mRNA and protein. Hence, nitric oxide has been

associated with the progression of Alzheimer’s disease.

From an overview however, nitric oxide is generally

considered to be a vasodilator.

As a second messenger, nitric oxide up regulates

the activity of the enzyme that synthesizes cyclic GMP,

i.e., guanylyl cyclase. The cyclic GMP then up

regulates a cyclic GMP-dependent protein kinase. The

protein kinase, in turn, phosphorylates specific

substrates in order to bring about its effects.

2. Biosynthesis of nitric oxide

Nitric oxide is synthesized by the enzyme nitric

oxide synthase (NOS). There are three types of

enzymes, the neuronal nitric oxide synthase (nNOS),

the endothelial nitric oxide synthase (eNOS), and the

inducible nitric oxide synthase (iNOS). They each

catalyze the complex oxidation/reduction reaction that

utilizes one of arginine’s nitrogen atoms in order to

make the “free radical” nitric oxide.

The NOS reaction requires NADPH, FAD, and FMN along with tetrahydrobiopterin (BH4) and heme (Fe).

An electron transport chain has been formulated for the reaction.

V. Glutamate and its Receptors and Signaling The amino acid glutamate is an endogenous excitatory

neurotransmitter, utilized by neurons in the human

central nervous system and the peripheral nervous system. Glutamate acts by binding glutamate receptors that allow it to play a pivotal role in synaptic mechanisms

involved in learning and memory.

Learning and memory involve synaptic plasticity, the

phenomenon in which the efficacy, or efficiency, of

synaptic transmission varies in an activity-dependent

manner. The effect can be transient (short-term) or

persistent (long-term). Signal transduction involving

glutamate receptors governs the reactions involved in

these processes..

Cartoon depicting signal transduction involving the glutamate receptor in the phenomenon of synaptic plasticity.

Glutamate binds two types of receptors, the

ionotropic glutamate receptors and the metabotropic

receptors. There are three of the former, the NMDA

(N-methyl-D-aspartate), the AMPA (α-amino-3-

hydroxy-5-methyl-4-isoxazolepropionic acid), and the

kainate receptors. They are referred to as ionotropic

because they associate to form cation channels that

facilitate the entry of Ca2+ (or Na+) into the cell.

Glutamate receptor molecules form dimers and then tetramers that function as ion channels.

These ion channels are ligand-gated channels that

assume an “open” configuration upon glutamate (or

agonist) binding. They are locked in a “closed”

configuration when an antagonist is bound. It should

be remembered that when a protein binds a ligand, the

protein generally undergoes a conformational change

that can alter its properties and its activity.

Effect of an antagonist, a partial agonist, and the full agonist, glutamate upon the conformation of the glutamate receptor.

Excessive glutamate binding to glutamate

receptors in the brain can bring about excitotoxic

neuronal cell death, however. Excessively activated

receptors lead to brain damage seen with cerebral

ischemia, traumatic brain injury, and the neuro-

degeneration associated with Huntington’s chorea,

epileptic disorders, and Alzheimer’s disease.

These excitotoxic effects are due, in part, to

signaling downstream of certain glutamate receptors that

leads to in increased intracellular Ca2+. The ionotropic

N-methyl-D-aspartate (NMDA) glutamate receptor is

one of the principal ones that facilitates the entry of

extracellular calcium (Ca2+) into the neuronal cell.

Calcium (Ca2+) that enters the neuronal cell by way of

NMDA channels can, among other actions, up regulate

the synthesis of nitric oxide by the nNOS.

As a result of increasing intracellular Ca2+ NMDA glutamate receptors have broad influence, modulating the activities of a variety of protein kinases and phosphoprotein phosphatases.

Recently, it has been suggested that the NMDA glutamate receptor and a (D1) dopamine receptor cooperate in the molecular mechanisms of compulsion and persistence as related to drug adiction.