Noradrenaline and Adrenaline (A*)

Question 1

Describe the synthesis pathway of noradrenaline.

Which enzyme is a phenotypic marker for noradrenergic neurones?

Answer 1

1 - Tyrosine hydroxylase converts L-tyrosine into L-DOPA.

2 - DOPA-decarboxylase converts L-DOPA into dopamine.

3 - Dopamine is transported into vesicles by a VMAT.

4 - In the vesicle, dopamine-beta-hydroxylase converts dopamine into noradrenaline.

• This is the enzyme that is a phenotypic marker for noradrenergic neurones.

For adrenaline:

5 - Phenylethanolamine N-methyltransferase (PNMT) converts noradrenaline into adrenaline.

Question 2

What is a heteroreceptor?

Answer 2

A heteroreceptor is a presynaptic release-modulating receptor, which is not activated by the neurotransmitters that are synthesized by the neuron on which it is located.

Question 3

List the adrenergic receptor subtypes.

To which GPCR alpha subunit are these subtypes bound?

Are these receptors excitatory or inhibitory?

How does the potency of adrenaline compare to that of noradrenaline for each of these subtypes?

Answer 3

Adrenergic receptor subtypes:

1 - Alpha 1 - Gq (excitatory).

2 - Alpha 2 - Gi (inhibitory).

• Adrenaline and noradrenaline have an equal potency for alpha 1 and 2 receptors.

3 - Beta 1 - Gs (excitatory).

4 - Beta 2 - Gs (excitatory).

5 - Beta 3 - Gs (excitatory - not mentioned in this lecture).

• Adrenaline and noradrenaline have an equal potency for beta 1 receptors.
• Adrenaline has a greater potency than noradrenaline for beta 2 and 3 receptors.

Question 4

List an agonist and antagonist for alpha 1 receptors.

What is the function of alpha 1 receptors?

Are alpha 1 receptors presynaptic or postsynaptic?

Answer 4

• Alpha 1 agonist: phenylephrine.
• Alpha 1 antagonist: prazosin.
• Alpha 1 receptors are involved in smooth muscle contraction.
• Alpha 1 receptors are postsynaptic.

Question 5

List an agonist and antagonist for alpha 2 receptors.

What is the function of alpha 2 receptors?

Are alpha 2 receptors presynaptic or postsynaptic?

Answer 5

• Alpha 2 agonist: clonidine.
• Alpha 2 antagonist: yohimbine.
• Alpha 2 receptors inhibit the release of noradrenaline and therefore act as a feedback mechanism.
• Alpha 2 receptors are both presynaptic (as autoreceptor and heteroreceptors) and postsynaptic.

Question 6

List an agonist and antagonist for beta 1 receptors.

What is the function of beta 1 receptors?

Are beta 1 receptors presynaptic or postsynaptic?

Answer 6

• Beta 1 agonist: dobutamine.
• Beta 1 antagonist: atenolol.
• Beta 1 receptors have positive inotropic and chronotropic effects.
• Beta 1 receptors are postsynaptic.

Question 7

List an agonist and antagonist for beta 2 receptors.

What is the function of beta 2 receptors?

Are beta 2 receptors presynaptic or postsynaptic?

Answer 7

• Beta 2 agonist: salbutamol.
• Beta 2 antagonist: butoxamine.
• Beta 2 receptors are involved in smooth muscle relaxation.
• Beta 2 receptors are postsynaptic.

Question 8

How do Gs alpha subunits cause a receptor potential?

Is the receptor potential excitatory or inhibitory?

Answer 8

Gs GPCRs causes a receptor potential by:

• Activating adenylyl cyclase.
• This increases cAMP, which activates PKA.
• PKA decreases K+ efflux and increases Na+ influx by a Ca2+-mediated mechanism.
• The overall effect of Gs is excitation.

Question 9

How do Gq GPCRs cause a receptor potential?

Is the receptor potential excitatory or inhibitory?

Answer 9

Gq GPCRs cause a receptor potential by:

1 - Upregulating PLC, which increases production of IP3 and DAG.

• IP3 opens ligand-gated Ca2+ channels.
• DAG activates PKC, which triggers various downstream pathways, e.g. leading to an increase in Ca2+ influx and a decrease in K+ efflux by closing K+ leak channels.
• The overall effect of Gq is excitation.

Question 10

How do Gi/o GPCRs cause a receptor potential?

Is the receptor potential excitatory or inhibitory?

Answer 10

Gi/o GPCRs cause a receptor potential by:

1 - Decreasing cAMP, therefore decreasing intracellular Ca2+.

2 - Increasing K+ inward rectifier channel activity.

• These are different to voltage-gated K+ channels in that rectifier channels are voltage independent and have 4 transmembrane domains instead of 2.
• The overall effect of Gi/o is inhibition.

Question 11

List the subtypes of alpha 1 and 2 receptors.

Answer 11

Alpha 1:

1 - Alpha 1A.

2 - Alpha 1B.

3 - Alpha 1D.

Alpha 2:

1 - Alpha 2A.

2 - Alpha 2B.

3 - Alpha 2C.

Question 12

What is the role of adrenaline in the CNS?

Answer 12

• There are very few adrenergic systems in the CNS (there are many more noradrenergic systems).
• The adrenergic systems that do exist have a role in control of blood pressure.

Question 13

Describe the anatomy of the noradrenergic system in the CNS.

Describe the functions of each component of the system.

Answer 13

• Noradrenergic cell bodies are found exclusively in the brainstem.
• These cell bodies have two main projections:

1 - From the ventral tegmental area (in the midbrain).

• Noradrenaline from these projections has a role in sexual and feeding behaviours.

2 - From the locus coeruleus (in the pons - the principal site for NAd synthesis), consisting of both ascending and descending projections, and a projection to the cerebellum.

• From the ascending projections, noradrenaline has a role in cognitive functions.
• From the descending projections, noradrenaline has a role in modulation of blood pressure and spinal nociceptive signalling.
• Locus coeruleus neurones have diffuse innervation, where individual neurons often branch to many different sites.
• Locus coeruleus neurones exert a neuromodulatory action through volume transmission. This is typical of noradrenaline throughout the body.

Question 14

Describe in detail the anatomy of the noradrenergic projections of the locus coeruleus (excluding the projection to the cerebellum).

Answer 14

Ascending projection:

1 - The ascending projection splits into ventral and dorsal bundles.

2 - As the bundles ascend, they reach the hypothalamus, where they recombine.

3 - As the recombined bundle ascends, it gives rise to a loop known as the stria terminalis.

4 - As it ascends further, it makes terminating projections to the cortex and hippocampus.

Descending projection:

1 - The descending projection gives rise to a projection at the nucleus tractus solitarius.

2 - It continues to descend to the spinal cord.

Question 15

What is the main function of the nucleus tractus solitarius?

Answer 15

The main function of the nucleus tractus solitarius is control of blood pressure.

Question 16

What are non-junctional varicosities?

Which neurones commonly have non-junctional varicosities?

Answer 16

• Non-junctional varicosities are swellings of axons (‘beads on a string’) that serve as points of release for neurotransmitters.
• This allows an individual neurone to have multiple ‘postsynaptic’ targets.
• Monoaminergic neurones often have non-junctional varicosities.

Question 17

Describe the neuromodulatory role of noradrenaline.

Answer 17

• As a neuromodulator, noradrenaline has concentration-dependent effects:
• Lower doses enhance effect of afferent input of the primary neurotransmitter.
• Higher doses inhibit effect of afferent input of the primary neurotransmitter.
• This is true regardless of whether the afferent input is excitatory or inhibitory.
• The function of this neuromodulatory role is to increase the ‘signal to noise ratio’, i.e. telling the postsynaptic neurone to pay attention to the most important input.
• This is a gating function.

Question 18

List 7 effects of noradrenergic signalling in the CNS.

At which regions of the brain does noradrenergic signalling mediate these effects?

Answer 18

1 - Central control of blood pressure (nucleus tractus solitarius).

2 - Control of hypothalamic hormone secretion (hypothalamus).

3 - Arousal and vigilance (thalamus).

4 - Memory (hippocampus).

5 - Endogenous analgesia (thalamus and spinal cord).

6 - Concentration (prefrontal cortex).

7 - Influencing sexual and feeding behaviour (ventral tegmental area).

8 - Motor control (see A* card 32)

• These effects make the locus coeruleus a good mediator of stress responses to urgent stimuli.

Question 19

Which adrenergic receptor has an influence on arousal?

Where is this receptor located?

How does this receptor work to increase arousal?

What causes this receptor to be stimulated?

Answer 19

• Alpha 2B receptors have an influence on arousal.
• These receptors are found in the thalamus.
• Activation of alpha 2B receptors by locus coeruleus neurones increases thalamocortical activity, increasing arousal and vigilance.
• This might occur due to stressful stimuli (e.g. a fall in blood pressure, which the locus coeruleus can detect via its connections with the nucleus tractus solitarius).
• Remember the thalamus is the site at which all sensory information passes to be sent to the cortex for processing, so is important for arousal.

Question 20

Why does administration of yohimbine to the locus coeruleus cause stress, fear and insomnia?

Answer 20

• Yohimbine is an alpha 2 antagonist.
• It binds to alpha 2 autoreceptors, which normally inhibit noradrenaline release.
• By antagonising this inhibition, noradrenaline release from the locus coeruleus increases.
• This potentiates stress signals from the locus coeruleus, promoting arousal and vigilance.

Question 21

List 5 symptoms of pathologically overactive locus coeruleus activity.

Answer 21

Symptoms of pathologically overactive locus coeruleus activity include:

1 - Anxiety.

2 - Stress.

3 - Fear.

4 - Hyperarousal.

5 - PTSD.

Question 22

What is guanfacine?

How does this drug work?

What else might this drug be useful for?

Answer 22

• Guanfacine is an antihypertensive drug that targets the noradrenergic system.
• It is selective agonist for alpha 2A receptors.
• It binds to alpha 2A receptors in the nucleus tractus solitarius, which causes a decrease in blood pressure (remember alpha 2 receptors are inhibitory).
• This drug is also useful for:

1 - Reducing distractibility (increasing concentration / vigilance) in ADHD patients.

2 - Treating cognitive impairment (increasing concentration / vigilance) in schizophrenia and Alzheimer’s patients.

3 - Treating PTSD.

Question 23

What is modafinil?

How does this drug work?

What else might this drug be useful for?

Answer 23

• Modafinil is a drug used to enhance cognition that targets the noradrenergic system.
• It is a noradrenaline and dopamine reuptake inhibitor.
• It therefore enhances locus coeruleus activity, increasing noradrenergic effects on cognition.
• This drug is useful for:

1 - Reducing distractibility (increasing concentration / vigilance) in ADHD patients.

2 - Treating narcolepsy.

Question 24

List 2 conditions characterised (at least in part) by pathologically underactive locus coeruleus activity.

Answer 24

Conditions characterised by pathologically underactive locus coeruleus activity include:

1 - Depression.

2 - ADHD.

Question 25

List 2 drugs used to treat depression that targets the noradrenergic system. How do these drugs work?

Answer 25

Antidepressant drugs that target the noradrenergic system: 1 - Mirtazapine. - Mirtazapine is an alpha 2 antagonist, part of the noradrenergic and specific serotonergic antidepressant (NaSSA) drug class, which blocks alpha 2 adrenergic auto- and heteroreceptors. - This relieves alpha 2-mediated inhibition of locus coeruleus stimulation of the hippocampus, which is thought to be underactive in depression. 2 - Reboxetine. - Reboxetine is a noradrenaline reuptake inhibitor.

Question 26

Describe the role of the locus coeruleus in analgesia. Give an example of an analgesia-enhancing drug that targets the noradrenergic system. How does this drug work? What else might this drug be useful for?

Answer 26

- Descending projections of the locus coeruleus neurones are activated by noxious sensory stimuli. - Alpha 2 receptors on these descending projections mediate a stress-induced analgesia. - Clonidine is an alpha 2 agonist that targets spinal alpha 2 receptors to enhance locus coeruleus-mediated analgesia. - Clonidine also inhibits TRPV1 channels (remember these are involved in the sensation of heat). * The primary use of clonidine is as an antihypertensive.

Question 27

A*: Describe the role of noradrenaline in addiction. List 3 pieces of evidence for this.

Answer 27

- Addiction and reward is commonly associated with the mesolimbic pathway, which is mostly under the influence of dopamine. - However, mesolimbic structures such as the ventral tegmental area, bed nucleus of the stria terminalis, nucleus accumbens and the amygdala also receive facilitary input from noradrenergic neurones. - Alpha 1 receptors are thought to mediate noradrenergic modulation of dopamine release at these structures. Evidence: 1 - Rewarding stimuli, including substances of abuse such as amphetamine and cocaine, increase noradrenaline release in the prefrontal cortex. 2 - Lesions to the locus coeruleus has been shown to cause a decrease in dopamine release in the nucleus accumbens. 3 - Activation of the locus coeruleus has been shown to cause an increase in dopamine release in the ventral tegmental area.

Question 28

A*: List 3 noradrenergic drugs used to treat addiction. How do they work?

Answer 28

1 - Disulfiram, an inhibitor of dopamine-beta-hydroxylase, is used to treat alcohol addiction and is also thought to have efficacy for cocaine addiction. - Disulfiram inhibits noradrenaline synthesis, reducing reward from addictive substances. 2 - Prazosin, an alpha 1 antagonist, was shown to reduce nicotine-craving behaviours in rats (Forget et al., 2010). - Prazosin is thought to inhibit dopamine release in the nucleus accumbens of the mesolimbic pathway, reducing reward from addictive substances. 3 - Lofexidine is an alpha 2 antagonist used to prevent craving and treat withdrawal symptoms of opioids. - Opioid receptors are GPCRs, and cause inhibition via Gi/o signalling. - Lofexidine mimics the inhibitory effect of opioid receptors by activating alpha 2 receptors, which are also Gi/o-associated GPCRs.

Question 29

A*: If the locus coeruleus has such widespread connections, how is it able to selectively activate one structure (e.g. increase arousal) without activating all other innervated structures (e.g. analgesia)? What is the significance of this?

Answer 29

- There are two views on the architecture of the locus coeruleus: 1 - The locus coeruleus is a single, discrete nucleus that influences noradrenaline release globally in the CNS. - This theory suggests that the locus coeruleus is able to selectively activate targets due to differences in the postsynaptic targets, e.g. differences in translational profiles and synaptic architecture / strength. 2 - The locus coeruleus is modular and consists of many subsets of cell bodies. - This theory suggests that the component nuclei of the locus coeruleus have separate targets, synchronous spike timing and are able to be activated independently to produce a 'semiglobal' noradrenaline response. - The significance of this distinction is that if subpopulations of neurones in the LC can be attributed to specific functions, these subpopulations can be targeted to: 1 - Investigate 'druggable' characteristics of neuronal subpopulations governing a specific function. - This will enable development of more specific drugs with fewer side effects. 2 - Improve models of noradrenaline transmission.

Question 30

A*: Describe the role of noradrenaline in the pathophysiology of Parkinson's disease.

Answer 30

- Parkinson's disease is characterised by formation of Lewy bodies and Lewy neurites, which contain ubiquitin and alpha-synuclein respectively. - In Parkinson's disease, noradrenergic neurones in the locus coeruleus undergo morphological changes and degeneration due to buildup of alpha-synuclein. - This results in a decrease in neuromodulation of various brain functions, which in turn underpins the non-motor symptoms of Parkinson's disease such as anxiety, depression and cognitive impairment (see card 18).

Question 31

A*: What is droxidopa? Give an example of a disease that can be treated with droxidopa.

Answer 31

- Droxidopa is a prodrug that is converted into noradrenaline by the enzyme AAAD. - It recently gained approval for the treatment of hypotension in Parkinson's disease. - It is also thought to have other benefits for Parkinson's disease (see card 18), particularly for motor symptoms (see A* card 32).

Question 32

A*: Describe the role of noradrenaline in motor control.

Answer 32

- Both the cerebellum and dopamine neurones of the basal ganglia receive modulatory input from noradrenergic neurones of the locus coeruleus. - Noradrenaline can influence motor control in via these structures.

Question 33

A*: Describe the effect of noradrenaline on fat metabolism.

Answer 33

- Adipose tissue is innervated by noradrenergic neurones. - These neurones corelease noradrenaline and neuropeptide Y, which have a synergistic effect on fat metabolism in adipocytes. - The effect of noradrenaline / neuropeptide Y corelease on fat metabolism depends on: 1 - The presence of different noradrenaline / neuropeptide Y receptor subtypes. 2 - The distribution of noradrenaline / neuropeptide Y receptors in the adipocyte. 3 - The relative quantities of noradrenaline / neuropeptide Y release. - This system is a potential target for anti-obesity drugs, however further research into the signalling mechanism is required before a drug can be developed.