Dopmaine pathways (finished)

Question 1

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Describe the structure of dopamine

Answer 1

catechol ring= 6 carbons

Question 2

What are catecholamines?

Answer 2

= group of neurotransmitters which includes dopamine, noradrenaline + adrenaline

Question 3

When was dopamine synthesised?

Answer 3

1910

Question 4

Who were the 1st people to synthesise dopamine?

Answer 4

George Barger and James Ewens

Question 5

What are catecholamines derived from?

Answer 5

tyrosine

Question 6

What was dopamine initally considered to be?

Answer 6

a precursor to noradrenaline (norepinephrine) and adrenaline (epinephrine) with no function of its own

Question 7

When was dopamine established as a neurotransmitter?

Answer 7

1958 but Nobel Peace prize won for this in 2000

Question 8

Who established dopamine as a neurotransmitter with a function?

Answer 8

Arvid Carlsson

Question 9

Describe the synthesis of dopamine

Answer 9

Synthesised in 2 steps:

1. tyrosine hydroxylase (TH) converts tyrosine into L-DOPA by adding hydroxyl group (OH) to backbone of tyrosine- L-DOPA has short lifetime so is further converted by…
2. DOPA decarboxylase- converts L-DOPA rapidly into dopamine by removing carboxyl group

Question 10

What is considered to be the rate limiting step in dopamine synthesis?

Answer 10

= tyrosine hydroxylase

Question 11

What is the difference between the enzymes involved in dopamine synthesis?

Answer 11

Tyrosine hydroxylase = substrate specific- only converts tyrosine

DOPA decarboxylase = non substrate specific- so can convert any L amino acid

Question 12

What is the rate of dopamine synthesis regulated by?

Answer 12

Catecholamines can inhibit tyrosine hydroxylase (TH) found in cytosol

Question 13

Describe 2 reasons why synthesis of dopamine is altered

Answer 13

• Presynaptic dopamine receptors (D2) inhibit synthesis and release of dopamine by activating signalling pathways- feedback inhibition which are negatively or positively coupled to cAMP signalling + Voltage gated calcium channels
• Neuronal activity alters synthesis = levels of activity of neurons can change amount of dopamine produced- more activity = more dopamine produced

Question 14

Name a co-factor of tyrosine hydroxylase

Answer 14

Tetrahydrobiopterin (BH4) = crucial for correct function of tyrosine hydroxylase

Question 15

Name 2 dopamine transporter proteins

Answer 15

DAT
VMAT

Question 16

What is the structure of DAT and where is it located?

Answer 16

12 transmembrane-spanning protein ~619 amino acids- located on- pre-synaptic membrane

Question 17

What is the main function of DAT?

Answer 17

Takes up excess dopamine from synaptic cleft- known as a transporter

Question 18

Name 3 drugs that bind to DAT + what does this result in?

Answer 18

= Binding site for cocaine, amphetamine and methylphenidate

FUNCTION = Block the transporter (= DAT) that would usually take up excess dopamine = inhibits reuptake so increase dopamine in the synaptic cleft = activates post-synaptic receptors = increased signalling

Question 19

What is the main deactivation method used by DAT?

Answer 19

uses ion gradient created by the Na+/K+ ATPase- by coupling influx of sodium ions (= symporting ion) into cytosol, it also imports dopamine i.e. energy gradient created to move dopamine back into pre-synaptic neuron

Question 20

What is the function of VMAT?

Answer 20

= packages vesicles with dopamine- transport dopamine from cytosol into vesicles

Question 21

Describe the 2 differences between VMAT and DAT

Answer 21

• VMAT has similar structure to DAT but small difference which determines if it is found in pre synaptic membrane (DAT) or the vesicular membrane (VMAT)
• VMAT does not use Na+ as a symporting ion- instead uses hydrogen ions in presynaptic vesicles, they are pushed down their concentration gradients at same time dopamine is moved into synaptic vesicles

Question 22

What does reserpine do?

Answer 22

blocks VMAT-2 and depletes DA as it is not reintroduced into vesicles = Depletes all dopamine = results in Parkinsons

Question 23

What happens when the DAT gene is knocked out in animals?

Answer 23

elevates the synaptic level of DA in all brain regions with DA terminals – enhancing DA neurotransmission in the striatum results in these rats being hyperactive

Question 24

What is 6-hydroxydopamine?

Answer 24

(like dopamine but has + OH) – neurotoxin that is only taken up by DAT

Question 25

What does 6-OHDA do?

Answer 25

Inhibits mitochondrial respiratory chain leads to neuronal death = causes oxidative stress = damages neurons: - Auto-oxidation produces hydrogen peroxide - = Causes oxidative stress to mitochondria releasing cytochrome c leading to death of dopamine and noradrenaline neurons

Question 26

What happens when 6-OHDA is injected into striatum of mice ?

Answer 26

1. Induces oxidative stress in dopaminergic neurons 2. = reduction in dopamine signalling 3. = neurodegeneration by behavioural changes- Parkinsons

Question 27

Name the 2 enzymes that are responsible for the metabolism of dopamine

Answer 27

Monoamine oxidase (MAO) COMT

Question 28

Where is MAO found?

Answer 28

On inner mitochondrial membrane in cells- e.g. neurons

Question 29

Where is COMT found?

Answer 29

Cytosol but also membrane bound

Question 30

Describe the role of COMT and MAO

Answer 30

- Each produce an intermediate COMT produces 3-MT MAO produces DOPAC - Enzymes act in either order to produce common metabolite HVA = inactive molecule that is secreted out of the body

Question 31

What is the difference between dopaminergic, noradrenergic and adrenergic neurons?

Answer 31

- Dopaminergic neurons = has tyrosine hydroxylade & DOPA decarboxylase - Noradrenergic neurons = also have dopamine b-hydroxylase - Adrenergic neurons (e.g. chromaffin cells in the adrenal medulla) = have PNMT on top of this

Question 32

Name the 5 subtypes of dopamine receptors and what do they all have in common?

Answer 32

= G-protein coupled receptors: D1 D2 D3 D4 D5

Question 33

Name the D1 like dopamine receptors

Answer 33

D1 and D5

Question 34

Name the D2 like dopamine receptors

Answer 34

D2 D3 D4

Question 35

On which neuron is each dopamine receptor found?

Answer 35

- D1 = Post-synaptic - D5 = Post-synaptic - D2= Pre- and Post-synaptic: Striatum high levels - D3 = Post-synaptic: Nucleus accumbens - VTA pathway Presynaptic in cortex? - D4 - Pre- and Post-synaptic: Frontal cortex, amygdala, hippocampus

Question 36

What is the main difference between D1 like receptors and D2 like receptors?

Answer 36

D1 = positively coupled to AC/cAMP signalling D2 = negatively coupled to AC/cAMP signalling

Question 37

Compare the structure of D1 like receptors and D2 like receptors

Answer 37

D1 has long c terminal- D2 has short c terminal In D2 there is a very large loop compared to D1

Question 38

Describe some different cellular downstream reponses in D1 and D2 receptors

Answer 38

D1 = more PIP hydrolysis, calcium is increased and PKC activation = excitability increased following activation of D1 D2 = increased potassium which hyperpolarises membrane, reduces calcium = excitability reduced following activation of D2

Question 39

How were dopamine neuronal pathways 1st established?

Answer 39

Falck – Hillarp Fluorescence 1960’s- wanted to look at brains were not necessarily looking for dopamine- to do this: 1. Freeze dried brain tissue to remove all water 2. Then exposed to formaldehyde vapor to dry it But they observed that after exposure to formaldehyde = converted Dopamine to a fluorescent isoquinoline molecule that could be seen as a yellow green colour under the microscope

Question 40

What method was used to improve Falck-Hillarp fluoresence

Answer 40

1980s- tyrosine hydroxylase immunohistochemistry produced even better resolution of visualizing dopaminergic neurons e.g. generate antibody against tyrosine hydroxylase = can specifically stain neurons

Question 41

Name the 4 different dopamine pathways

Answer 41

Nigrostriatal Mesolimbic Mesocortical Tuberoinfundibular

Question 42

What % of neurons are dopaminergic?

Answer 42

1%

Question 43

Where do neurons originate and stretch to in the nigrostriatal pathway?

Answer 43

neurons originates in substantia nigra and stretches to striatum

Question 44

Where do neurons originate and stretch to in the mesolimbic pathway?

Answer 44

neurons originates in tegmentum / VTA and stretches to nucleus accumbens

Question 45

Where do neurons originate and stretch to in the mesocortical pathway?

Answer 45

neurons originate in tegmentum / VTA and stretches to frontal cortex

Question 46

Where do neurons originate and stretch to in the tuberinfundibular pathway?

Answer 46

small pathway- relates to hypothalamus -arcuate nucleus of the hypothalamus innervate the median eminence in the anterior pituitary gland.

Question 47

What does the nigrostriatal pathway control for and what does degeneration of the neurons result in?

Answer 47

controls planning and execution of voluntary skeletal muscle movement. - = Degeneration of these neurones occurs in Parkinson’s disease leading to motor impairment symptoms

Question 48

What does the mesolimbic pathway control for and what does overactivity of the neurons result in?

Answer 48

part of the limbic system controlling emotion, pleasure reward and goal-directed behaviour - Over-activity in this pathway contributes to psychosis, delusions and hallucinations the positive symptoms of schizophrenia

Question 49

What does the mesocortical pathway control for and what does underactivity of the neurons result in?

Answer 49

neuronal activity is associated with emotion and motivation. - Under-activity in this pathway may contribute to negative symptoms (social withdrawal/cognitive dysfunction) seen in schizophrenia. - Cognitive side-effects sometimes known as ‘neuroleptic-induced deficit syndrome’ may also be mediated by this pathway.

Question 50

What does the tuberoinfundibular pathway control for?

Answer 50

Neuroendocrine associated pathway: - DA release inhibits prolactin secretion; involved in maternal behaviour (neuroendocrine function).