Dopamine (PD)

Question 1

how is dopamine synthesised?

Answer 1

Tyrosine to L-DOPA via Tyrosine hydroxylase then L-DOPA to Dopamine via DOPA-Decarboxylase
in the pre-synaptic terminal.
Dopamine is metabolised by MAO-B to ensure correct levels in the pre-synaptic terminal. Dopamine is then packaged into synaptic vesicles vis VMAT (vesicular mono-amine transporter)

Question 2

How is dopamine removed from the synapse?

Answer 2

• Dopamine is re-uptaken into the pre-syanptic terminal via dopamine transporter (DAT- dopamine-Na+ transporter) symporter driven by EC Na+ : as Na+ moved in down its gradient, dopamine is taken up against its gradient
• can also be taken into non-neuronal cells e.g. glial cells via EMT (extra-neuronal mono-amine transporter) - here it is metabolised by MAO-A, MAO-B or COMT

Question 3

What are the 2 classes of dopamine receptors?

Answer 3

D1-like and D2-like
D1-Like:
D1
D5

D2-Like
D2
D3
D4

• ALL are GPCRs but have different downstream signalling Mechanisms

Question 4

How do D-1 receptors work?

Answer 4

Act via G𝛼s- stimulates adenylate cyclase
Causes increase in cAMP production
Activates protein kinase A
This mediated downstream effects bt phosphorylating various proteins in the cell e.g. DARPP-32
DARPP-32 inhibits protein-phosphatase-1- usually dephosphorylates proteins so if is it inhibited, more proteins are being phosphorylated by PKA = Enhanced downstream effects

D1 pathways are excitatory
D1- receptor are found mainly on post-synaptic cells- dopamine acting at D1 act on post-synaptic membrane and stimul;ates excitatory responses.

Question 5

How do D2 receptors work?

Answer 5

These mediate inhibitory responses and are found pre or post synaptically.

They activate via G𝛼i/o = inhibit adenylate cyclase = decreased cAMP production

Pre-synaptic:
Gβ𝜸 Subunit can also inhibit voltage-gated calcium ion channels to decrease conc of calcium ions in cell = pre-synaptic inhibition = less calcium ions going into the cell as a result of action potential = decrease neurotransmitter release

Post-synaptic:
Also Gβ𝜸 Subunit activates K+ channels = K+ channels open and K+ moves out of neurone = hyperpolarisation = decreased excitatory response in post-synaptic neurones

Question 6

Is dopamine inhibitory or excitatory?

Answer 6

It can be both depending on which receptor it is acting on

Question 7

Give some examples of agonists at dopamine receptors and their selectivities.

Answer 7

Bromocriptine- non-selective
Pergolide- D2 selevtive over d1
Pramiprezole and ropinorole- d2-selective
Aripiprazole- partial agonist at D2

Question 8

Give some examples of antagonists at dopamine receptors and their selectivities.

Answer 8

• Haloperidol- non-selective
• Sulpiride- D2-like
• Domperidone- d2-like
• metoclopramide- d2-like

Question 9

Describe the 4 major dopamine pathways.

Answer 9

NIGROSTRIATAL PATHWAY:
- 75% (Most) of the dopamine in the brain is in this pathway
- Cell bodies within the substantial niagra of the midbrain project to the striatum
- important in the control of movement
- These neurones are lost in Parkinsons disease

MESOLIMBIC PATHWAY:
- Midbrain to the limbic areas
- Cell bodies are in the ventral tegmental area of the midbrain and project into parts of the limbic system e.g. amygdala, hippocampus, nucleus accumbens
- Involved in behaviour e.g. reward pathways
- Involved in the mediating the positive symptoms of schizophrenia
- importaNt in the MOA of drugs that are liable to abuse e.g. cocaine and amphetamine- these increase dopamine levels and so increase reward pathways

MESOCORTICAL PATHWAY:
- Cell bodies in VTA (above) of midbrain and project to the frontal cprtex
- involved in thought and cognition
- involved in mediating the negative symptoms of schizophrenia

TUBEROHYPOPHYSEAL PATHWAY:
- short path from hypothalamus to pituitary
- Regulate secretions of hormones from pituitary e.g. dopamine inhibits prolactin secretion

Question 10

Discuss the expression of the D1 and D2 receptors in the different dopamine pathways

Answer 10

D1: most abundant! - high density in nigrostriatal, mesolimbic and mesocortiyal pathways
D5: More restricted expression than D1- within limbic system e.g. hippocampus

D2: Highest density in nigrostriatal, mesolimbic and tuberohypophyseal paths and in the CTZ
D3: More restricted, weak expression
D4: weaker expression, found in various places

Question 11

What is the pathophysiology of Parkinsons in terms of dopaminergic neurones?

Answer 11

There is a loss of dopaminergic neurones in the nigrostriatal pathway (runs from the substantial viagra to the striatum).
60-80% of these neurones are lost before symptoms occur!

Question 12

What are the 2 pathways involved in control of movement in the nigrostriatal pathway?

Answer 12

Direct: enables movement
Goes to the striatum via the substantial niagra (then to the thalamus and motor cortex via an excitatory neurone = contraction)

Indirect: inhibits movement
Striatum to the globus palidus via the sub-thalamic nuclei to the pars reticulate in the substantial niagra (then to the thalamus and motor cortex via an excitatory neurone = contraction)

NEED THE DIAGRAM

Question 13

What do the indirect and direct pathway normally do?

Answer 13

Normally, these pathways from the basal ganglia inhibit the inhibitory neurone that runs from the substantial niagra to the thalamus- this provides negative input and so prevents unwanted movement.
When inhibition is switched off = signal is sent to the motor cortex allowing for purposeful movement.

NEED THE DIAGRAM

Question 14

How do the dopaminergic neurones in the nigrostriatal pathway affect the indirect and direct pathways?

Answer 14

NEED THE DIAGRAM

Dopaminergic neurone has cell body in the substantial niagra, in the pars compact and project into the striatum.
- These neurones make synapses with the direct and indirect pathway:
Excitatory input to direct pathway
Inhibitory input to indirect pathway
Dopamine is the NT in both cases, but in the direct pathway the neurones synapse to a D1 receptor (excitatory) but when it synapses with neurones in the indirect pathway, this is by D2 receptors (inhibitory).

DIRECT:
- Dopamine acts on D1 receptor = stimulates downstream neurone the runs from striatum to the substantial viagra to pars reticular. This neurone is inhibitory = stimulating an inhibitory neurone, so it will release its inhibitory neurotransmitter (GABA) = inhibits the next neurone in the chain (goes from the pars reticular up to the thalamus).
- Inhibiting an inhibitory neurone means the next neurone in the chain is stimulated - this is the neurone from the thalamus to the motor cortex = this promotes movement

INDIRECT:
- Dopamine released from nigrostriatal pathway and acts at D2 receptors
- This switches off the next neurone in the chain- neurone is going to the globus pallidus. As this neurone is inhibitory, the next neurone in the chain is going to be stimulated- an inhibitory neurone.
- This inhibits the next neurone in the chain- an excitatory neurone = switched off
- this then switches off the neurone to the thalamus
- next neurone is switched on- the neurone that goes from the thalamus to the motor cortex = promotes movement

Therfore, input from direct or indirect result in movement = nigrostriatal pathways always involve in movement.

Question 15

What happens to these indirect and direct pathways in PD?

Answer 15

We lose the neurones in the nigrostriatal pathway.
Normally, input from both the direct and indirect pathway cause movement. But if these neurones are lost, this input is lost which promotes movement = inhibited movement which explains the bradykinesia of PD

Question 16

What is the pathophysiology behind the tremor symptom of PD?

Answer 16

There are an abundance of cholinergic interneurones in the striatum.
Dopamine, normally inhibits acetyl choline release from striata neurones but in PD were there is a decrease in dopamine, there is an increase in Ach = hyperactivity of cholinergic synapse which causes tremor- a hallmark of PD.

Question 17

What are lewy bodies?

Answer 17

These are intracellular aggregations of a protein called 𝛼-synuclein found in neuronal cells.
- 𝛼-synuclein is a highly abundant (1% of protein within cytosol) neuronal protein found at the synapse that is involved in exocytosis of vesicles and vesicle recycling
- In PD, there is a mis-folding of this protein, leading to aggregation and plaque formation.
Additionally, in PD, removal of 𝛼-synuclein is impaired- usually there is a turnover of this protein by proteosmal pathway that breaks the protein down = doesn’t work properly in PD = accumulation and plaque formation
- A mutation in 𝛼-synuclein can cause early-onset PD and increase the number of lewy bodies. Alos mutation means neurones can’t release NTs properly

Question 18

What is the role of mitochondrial damage in development of PD?

Answer 18

Dopaminergic neurones need a lot of energy for function so are more susceptible to mitochondrial damage.

evidence:
- various chemicals have been found to induce PD e.g. Rotenone (pesticide) and MPTP (synthetic opioid byproduct). Have found that these both cause mitochondrial stress and dysfunction = death of dopaminergic neurones
- PARKIN & PINK1 gene mutations can lead to early-onset PD- these genes are both involved in mitochondrial turnover and so If they are mutated, damaged mitochondria accumulate as turnover is blocked leading to dysfunction. this occurs in other cells too but as dopaminergic neurones are particularly sensitive to mitochondrial damage - death.
Also, damage mitochondria generate reactive oxidative stress = ROS produced = these damage lipids/proteins/nucleic acids within the cell. Also, once cells start to die = neuroinflammation e.g. microglia come in to tidy up but chronic inflammation = cell death

Question 19

Does Levodopa cross the BBB?

Answer 19

Yes it can cross the BBB
it is synthesised to dopamine then by dopa decarboxylase as it is a precursor

Question 20

What is often given in combination with levodopa?

Answer 20

A dopamine decarboxylase inhibitor e.g. Carbidopa or Benserazide
These act peripharly (don’t cross BBB) to prevent the conversion of levodopa to dopamine in the periphery so that more levodopa can cross the BBB (also dopamine synthesised in the periphery can cause a lot of side effects)

Question 21

What are examples of COMT inhibitors and how do they work?

Answer 21

The COMT inhibitors, tolcapone and entacapone, inhibit the enzyme catechol-o-methyl-transferase (COMT), they prevent peripheral degradation of levodopa, allowing a higher concentration to cross the blood-brain barrier

Question 22

What are examples of and how do MAO-B inhibitors work?

Answer 22

MAO-B (monoamine oxidase) causes the breakdown of dopamine and so by inhibiting this enzyme, there is less metabolism of dopamine so it can have increased action.
examples e.g. selegiline and rasagiline

Question 23

What are examples of dopamine agonists used PD?

Answer 23

• Pramiprexole, ropinorole, rotigotine = selective for D2
• Bromocriptine, apomorphine = less selective so are used less

These drugs directly stimulate dopamine receptors