W4 Noradrenergic Neurotransmission (Ben) Flashcards by Leiel Lalo

What is the general structure of a catecholamine?

A benzene diol (2 OH grps) with an amine-containing side chain

(img = benzene diol)

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From which AA are catecholamines synthesized?

Tyrosine

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What is the first step in the synthesis of catecholamines?

Substrate?

Enzyme (and its action)?

Product?

Hydroxylation of benzene ring…

Substrate: Tyrosine + O2

Enzyme: Tyrosine Hydroxylase

Product: DOPA + H20

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What is the cofactor for tyrosine hydroxylase?

And what does it become after the reaction?

Tetrahydrobiopterin

becomes dihydrobiopterin

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What is the inhibitor for the first enzyme in the synthesis of catecholamines?

What kind of inhibition is it?

What can it be used to treat?

α-methyl-p-tyrosine inhibits tyrosine hydroxylase

competitive inhibition
used to treat inoperable pheochromocytoma, an NE-producing tumor of the adrenal medulla

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What is the 2nd step in catecholamine synthesis, after hydroxylation?

Reactants?

Enzyme?

Products?

Decarboxylation of the tyrosine-derived carboxyl group…

Reactants: DOPA

Enzyme: Aromatic AA Decarboxylase

Products: Dopamine + CO2

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What is the cofactor for the 2nd enzyme in catecholamine synthesis?

Pyridoxal Phosphate (PLP)

active form of vitamin B6

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In what tissues is aromatic AA decarboxylase found?

In catecholaminergic AND serotonergic neurons, as well as blood vessel + kidney tissues

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What is the 3rd step in catecholamine synthesis, after decarboxylation?

Substrate?

Enzyme?

Products?

Hydroxylation of dopamine’s amine side chain (B position)…

Reactants: Dopamine + O2

Enzyme: Dopamine β-Hydroxylase

Products: Noradrenaline + H2O

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What is the co-factor for the B-hydroxylation of dopamine?

Ascorbic Acid

becomes dehydroascorbate

(via Dopamine B-hydroxylase)

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What is the last step in catecholamine synthesis, after the 2nd hydroxylation?

Reactants?

Enzymes?

Products?

Methylation of the amine group…

Reactants: Noradrenaline

Enzyme: Phenyl-ethanolamine N-methyl Transferase

Products: Adrenaline

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What is the cofactor for the enzyme which produces adrenaline?

S-Adenosyl Methionine

methylates noradrenaline and becomes S-Adenosyl Homocysteine

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What is the rate-limiting step in catecholamine synthesis?

The first step catalyzed by Tyrosine Hydroxylase…

hydroxylation of the benzene ring of Tyr

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Phenyl-ethanolamine N-methyl transferase is present only in which cells?

A cells of the adrenal medulla

and small groups of neurons in the brain stem

(only these cells produce adrenaline)

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Name 3 mechanisms for regulation of the enzyme which produces DOPA.

Tyrosine hydroxylase is regulated by:

Noradrenaline - negative feed-back inhibition
Ca⁺⁺ - activates the enzyme
Kinases - PKA/PKC/Ca-Calmod.-dep. Kinase all phosphorylate + stimulate the enzyme

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How does phosphorylation effect tyrosine hydroxylase?

It stimulates it by increasing affinity for its tetrahydrobiopterin co-factor.

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How does sympathetic activity affect noradrenaline synthesis?

In the short term?

And long term?

Include enzymes affected + mechanisms.

Short term: It increases tyrosine hydroxylase activity (despite the usual negative feed-back inhibition of the enzyme via noradrenaline).

Ca⁺⁺enters the nerve terminals via VDCCs in sympathetic activity, thus activating the enzyme.

Long term: Prolonged sympathetic activity leads to upregulation of de novo synthesis of Tyr hydroxylase and Dopamine B-hydroxylase.

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How do steroid hormones affect the synthesis of catecholamines?

by increasing activity of the N-methyl Transferase…

they increase the synthesis of adrenaline

(according to the physio lecture, it is specifically cortisol which does this, when it reaches the adrenal medulla in high concentration as the microcirculation of the zona fasciculata flows towards the medulla)

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What altered catecholamine substrate can be used to treat a cardiovascular disorder?

What disorder + how?

α-methyl DOPA

results in production of α-methyl noradrenaline which has about 1/10 the vasopressor effect normal NA, so is an effective treatment for hypertension

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Describe the uptake of catecholamines into synaptic vesicles.

2 important components

It is a secondary active transport, driven by an H⁺-ATPase pump.

The pump creates a proton gradient which drives VMTA2, a non-specific transporter of biogenic amines.

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What is the transporter responsible for neurotransmitter uptake into vesicles?

What is its substrate specificity?

VMTA2 - Vesicular Membrane Transporter 2

12 TM domain transporter with broad substrate specificity for biogenic amines

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What drug disrupts the uptake of catecholamines into synaptic vesicles?

How?

What can it treat?

Reserpine

irreversibly inhibits uptake via inhibition of VMTA2 + thus depletion of synaptic vesicles (b/c constant leakage is not balanced by uptake)
can be used to treat hypertension

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What psychiatric disorder is associated with disrupted VMTA2 activity?

How is it treated?

Bipolar Disorder

treated with lithium

(check notes for the mechanism)

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What two enzymes are involved in metabolism of noradrenaline?

Monoamine Oxidase
COMT - Catechol O-methyl Transferase

Where is **monoamine oxidase** located within a cell?

On the **outer membrane** of the **mitochondria**. (Although _some_ MAO B can be found in the synapse, according to the lecture.)

What are the two isoenzymes of MAO and how do their substrate specificities differ?

1. **MAO A** - specific for _noradrenaline_ + _serotonin_ 2. **MAO B** - specific for _dopamine_ + _synthetic substrates_

What is an inhibitor for **MAO A**?

**Clorgilin** (check notes for anything signif about this to add)

Name two **MAO B** inhibitors. One is a treatment for a neurological disorder. Which one, what disorder and how?

1. **Selegylin** 2. **Deprenyl** * used for _Parkinson's disease_ * decreases dopamine breakdown via MAO B, thus increasing DA levels

Name an important action of _extra-neuronal_ _MAO_. What happens to this action due to MAO inhibitors and what is the effect?

Gastrointestinal + hepatic MAO breaks down ingested **biogenic amines** (tyramine, phenylethylamine), preventing them from entering the circulation. MAO inhibition can allow these amines into the circulation, resulting in **hypertension**, etc.

What co-factor does COMT require?

**SAMe** - methylates the product of MAO + reductase enzyme using methyl from SAMe (resulting in SAHcy)

What important metabolite of _CNS_ _noradrenaline_ can be measured in bodily fluids? Which fluids?

**MHPG** **(3-Methoxy-4-Hydroxy-phenylglycol)** - found in _urine_ and _CSF_ - indicative of CNS NA activity ( picture below is wrong... its 4-hydroxy, not 1 )

What metabolite of _PNS noradrenaline_ can be found in urine? What can elevated levels be indicative of?

**VMA (Vanillylmandelic Acid****)** - high levels can indicate **pheochromocytoma**

How are catecholamines re-uptaken into _presynaptic neurons_?

**Secondary active transport** via an **Na-Catecholamine Symporter**

What happens to presynaptic catecholamine re-uptake in the case of _sustained depolarization_ of the presynaptic neuron?

High levels of **Na⁺** at the synaptic terminal **reverse** the direction of the Na-Catecholamine symporter. (Because it relies on the sodium gradient set up by the Na-K-ATPase for normal re-uptake function + the gradient is reversed in depolarization.)

Describe the structure of the Na-Catecholamine Symporter. Its specificity? Its efficiency?

It is a **12 TM domain** protein. It is **specific** for individual catecholamines (NA, DA), but diff. transporters have structural similarity. It **does not capture all** the synaptic catecholamine. Some diffuses away.

What are 2 inhibitors of the Na-Catecholamine Symporter?

**Cocaine** **Tricyclic Antidepressants**

Where are _all but one_ of the enzymes of catecholamine synthesis found in the cell? Which one is not found there and where is it found?

Most enzymes are in the **cytosol**... ...but **dopamine B-hydroxylase** is found **in synaptic vesicles**.

What (sort of strange) thing happens to noradrenaline + dopamine in synaptic vesicles? (HINT: In the case of NA in adrenal medulla cells, it allows the synthesis of adrenaline to take place once the NA has been made by dopamine B-hydroxylase.)

NA + DA **constantly leak** from synaptic vesicles into the cytosol and are returned to them by VMAT2. In adrenal medulla cells, this allows the _cytosolic_ methyl transferase enzyme to convert NA --\> A.

How can the G proteins coupled to the alpha-1 through beta-3 noradrenergic receptors be remembered?

**QISSS**

Compare the NA/A affinities of the B1 and B2 adrenergic receptors.

**B1** - _approx. equal affinity_ for both + tends to be in organs with sympathetic innervation, so _NA dominates_ **B2** - _higher affinity for A_ than NA, tends to be in places with less/no sympathetic innervation

What 2 places are B1 receptors found? And what are their effects? (of course there are more, but which 2 from lecture)

**Heart muscle** - positive inotropic + chronotropic effect **GI smooth muscle** - relaxation

What 3 places are B2 receptors found? And their effects? (again... more than 3 exist, but which 3 from lecture)

1. **Uterine muscle** - relaxation 2. **Skeletal muscle** - glycogenolysis, vasodilation 3. **Bronchi smooth muscle** - dilatation

Where are B3 receptors found mainly? Their effect?

**Adipose Tissue** - activate lipolysis

Describe the cascade of effects which results in the _immediate action_ of B1 receptors on heart muscle.

1. B1 --\> G_s --\> adenylyl cyclase --\> increased [cAMP] --\> PKA active 2. PKA **phosphorylates + activates LVDCC** (AKA DHP receptor) allowing calcium influx 3. Increased [Ca⁺⁺]_ic leads to **Ca-induced Ca Release** thru the **RYR**_2_**** (heart isoform) on the sarcoplasmic reticulum 4. Further increased [Ca⁺⁺]_ic**increases contraction force**

How do the DHP-RYR1/RYR2 mechanisms differ?

**RYR1** - in skeletal muscle, DHP activation simply _induces a conformational change_ in RYR1 leading to Ca++ release from SR **RYR2** - in heart, DHP activation leads to Ca++ influx into cell, which leads to _calcium-induced calcium release_ via RYR2 from the SR

What is the "late action" of the B1 receptor in heart muscle? Describe the cascade that produces this effect.

The late action is a **positive chrontropic** (^ HR) effect. 1. PKA **phosphorylates/deactivates** **_phospholamban_** 2. Phosphorylate phospholamban can no longer inhibit **SERCA** 3. SERCA **pumps Ca into SR** to increase the rate of relaxation of heart muscle

What two heart conditions can B1-selective antagonists treat due to their blocking of chrono/inotropic effects?

**Arrhythmia** **Angina**

make some cards about alpha and beta receptors on adipose tissue skipping them for now bc her slides are confusing... talking abt glycogenolysis in adipose tissue, which is VERY minimal if it happens at all

How is glycogen metabolism regulated by _cholinergic (nAChR) stimulation_ of skeletal muscle?

Neural stimulation of muscle contraction via nAChRs leads to _increased intracellular calcium_. This calcium binds to the _delta subunit_ (very similar to calmodulin) of _phosphorylase kinase_, partially activating it. (It is fully activated when A/B subunits are P-ated by PKA)

How does _adrenergic stimulation_ of skeletal muscle result in glycogen metabolism regulation?

1. B2 receptors --\> --\> --\> PKA activation 2. PKA **phosphorylates α/β subunits** of **phosphorylase kinase**, fully activating it 3. PKA also P-ates **glycogen synthase kinase**, which then P-ates + _inactivates_ **glycogen synthase**

How does adrenergic stimulation _via B receptors_ (not A!) affect smooth muscle? Give the detailed mechanism, not just the effect.

B receptor stimulation **relaxes** smooth muscle... 1. Gs --\> --\> --\> PKA 2. PKA **phosphorylates _myosin light chain kinase_**, which _inactivates_ it

What are the 2 relevant effects of α1 receptor stimulation? (According to the slides.)

1. Increased glycogenolysis 2. Vascular SM contraction

What are the 3 relevant effects of α2 receptor stimulation? (According to the slides.)

1. GI SM relaxation 2. inhibition of lipolysis 3. platelet aggregation

What are the 2 relevant effects of β1 receptor stimulation? (According to the slides.)

1. Stimulation of lipolysis 2. Increased HR + contractility

What are the 3 relevant effects of β2 receptor stimulation? (According to the slides.)

1. Increased glycogenolysis 2. Increased gluconeogenesis 3. SM relaxation * bronchi * vessels * GI SM

How does adrenergic stimulation affect glycogen metabolism in the liver? Give receptor, mechanism + final effect.

**α-1 receptors** lead to... * Calcium increase --\> Ca-Calmodulin complex * Activation of **phosphorylase kinase** (via CAM-dep. kinase?) * **P-ation** + **activation** of **glycogen phosphorylase** * Increased glycogenolysis

How does adrenergic stimulation have a _two-fold_ effect on GI smooth muscle?

1. Direct stimulation via B1 receptor --\> relaxation 2. Indirect effect via A2 receptor on cholinergic axon terminals _inhibits_ ACh release to mAChRs --\> further relaxation (not entirely sure thats what this picture means since she hasn't gone over it yet in lecture, but it's the best i could figure from Wiki)

make a card abt this img... dont understand/remember how Ca-CAM can activate MLCK and lead to vasoconstriction