Lec 15- Dopamine

Question 1

DA (dopamine brief history)

Answer 1

• Pre-1960 DA and DOPA intermediates in the biosynthesis of NA from tyrosine
• 1959 Horykiwicz reduction in DA in Parkinson’s disease. 1961- replenishment of stores with DL-DOPA
• 1972 Kebabian and Calne- Biochemical studies second messenger assays
• 1976 Snyder- DA hypothesis of schizophrenia
• 1979- D1 and D2 receptor subtypes
• The late 1980s- five receptor subtypes

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Question 2

Anatomy of central dopaminergic pathways

Answer 2

• Originate in the midbrain
• A8-A15 cell groups
• Nigro-striatal system
• Mesolimbic/Mesocortical pathways
• Tubero-hypophyseal system

Question 3

Nigro-striatal system

Answer 3

• Substantia nigra pars compacta- where cell bodies sit
• sends projections through medial forebrain bundle to striatum

Question 4

Mesolimbinc/ Mesocortical Pathways

Answer 4

• Mesolimbic pathways
  
  • Cells sit in VTA project through medial forebrain bundle into the nucleus accumbens
• Mesocorticalpathway
  
  • VTA => Hippocampus, pre-frontal cortex, motor cortex

Question 5

Tubero-hypophyseal system

Answer 5

• Pituitary => hypothalamic nucleus

Question 6

Consequences of DA release

Answer 6

• DA has 3 functional consequences in the brain
  
  • Motor control (A9, nigrostriatal system 75%)
  • Behavioural effects (A10, mesolimbic and mesocortical system to accumben, amygdala and cortex)
  • Endocrine control (A12, tubero-hypophyseal system arcuate nucleus of hypothalamus to medial eminence and pituitary gland)

Question 7

Nigro-striatal system

Answer 7

• A9, Substantia nigra pars compacta to corpus striatum
• Motor control, motor planning
  
  • Loss of DA results in Akinesia- parkinson like symptoms
  • Unilateral 6-OH DA lesions- circling towards the lesioned side. DA agonist injection induce turning toward injection site
  • Parkinson’s disease is associated with a deficiency of nigrostriatal dopaminergic neurones

Question 8

Mesolimbic/Mesocortical pathways

Answer 8

• A10- Ventral tegmental area to nucleus accumbens, amygdala and cortex
• Motivation, behavioural reward, cognition
• Implicated in drug self-administration as shown by animal models using amphetamine (DA releasing agent) or apomorphine (DA agonist)
  
  • Rewarding properties blocked by DA antagonist
• Large doses give rise to stereotyped repetitive inappropriate behaviour patterns such as licking, rearing, gnawing etc- psychosis response
• Side effect of L-DOPA= psychosis and shizophrenia (excess DA)

Question 9

Tubero-hypophyseal system

Answer 9

• A12, arcuate nucleus of the hypothalamus to medial eminence and pituitary gland
• Hormone release from the anterior pituitary is regulated by DA. DA inhibits prolactin release and increases growth hormone release.
• Anti-psychotic D2 antagonists cause breast development and lactation

Question 10

Other sites of action

Answer 10

• Chemo-receptor trigger zone, DA agonist all cause nausea and vomitting by action at
• Vascular smooth muscle- dilation of blood vessels

Question 11

Major function divisions of DAergic pathways

Answer 11

• Nigrostriatal
  
  • Motor
  • Internal modulation of basal ganglia
  • Parkinson’s disease
• Mesolimbic
  
  • Motivation, emotion
  • Reward system
  • Addiction
• Mesocortical
  
  • Cognition, arousal
  • Schizophrenia

Question 12

DA neurones

Answer 12

• All contain Tyrosine hydroxylase
• Identified easily electrophysiologically- Tonic/burst firing- 6Hz
• controlled by inputs from the cortex (GLU), SNr (GABA), Raphe (5-HT) Locus coeruleus (NADR)
• Release at classical synapses by calcium dependent exocytosis. Also a non-exocytotic release from dendrites and fibre varicosities
• Neuromodulatory- Regulate and enable function

Question 13

Biosynthesis of DA

Answer 13

• DA cells dont have dopamine-B-hydroxylase (so we dont make NA)

Question 14

Signal termination

1. Re-uptake in synaptic terminals by DA transport

Answer 14

• The main mechanism of termination- blocked by cocaine (also 5-HT and NADR)
• Mice have been produced in which the DA transporter gene has been disrupted
• It has been shown that in these animals the DA released remains in the synaptic cleft for about 100 times longer than in normal animals
• The animals display the hyper-locomotion that one normally sees in animals that are treated with cocaine

Question 15

Signal termination

2. Enzymic degradation

Answer 15

• To dihydroxyphenyl acetic acid (DOPAC) by MAO and aldehyde dehydrogenase
• To Homovanillic acid (HVA) by COMT, MAO and (aldehyde dehydrogenase)
• DOPAC and HVA excreted in urine, provide an index of DA release. HVA used as the index of DA turnover

Question 16

Dopamine receptors

Answer 16

• G-protein coupled receptor
• D1-like and D2-like families
• Major subdivision through positive or negative coupling to adenylate cyclase
  
  • D1 = Increase adenylate cyclase and cAMP

Question 17

Dopamine receptors

Answer 17

• D1 like receptors activate adenylate cyclase to increase cAMP levels
  
  • Consist of two subtypes D1 and D5
• D2 like receptors inhibit adenylate cyclase to decrease cAMP levels
  
  • Consist of 3 subtypes: D2, D3 and D4

Question 18

A G-protein Coupled DA receptor

Answer 18

• 7 membrane spanning helices
• Bundling forms ligand binding site
• D1 short 3rd intracell Loop
• D2 Long 3rd intracell Loop

Question 19

DA receptors

Answer 19

• D1 family= D1 +D5
  
  • Gs (+) adenylate cyclase
• D2 family= D2 (short and long); D3; D4
  
  • Gi/o (-) adenylate cyclase
  • Increase K+ (inward rectifier)- more negative membrane potential
  • Decrease Ca2+ (VDCC)- Ca dependent exocytosis

Question 20

Agonist pharmacology of DA receptor

Answer 20

Question 21

Antagonist pharmacology of DA receptors

Answer 21

• Because clozapine only acts on D4 we reduce side effect profile of the drug

Question 22

General characteristics - D1 family

Answer 22

• D1,5
• No discriminating agents
• Low affinity for DA
• Post-synaptic transmission

Question 23

General characteristics- D2 family

Answer 23

• D2,3,4
• Selective agents becoming available
• High affinity
• Post-synaptic transmission
• Pre-synaptic heteroreceptor
• Autoreceptor role

Question 24

Autoreceptor functions

Answer 24

• If DA is released on post-synaptic D2 like receptors
  
  • K+ increase = reduced firing = reduced AP = reduced Ca dependent exocytosis = reduced DA release
  • NB- Receptors can give pre-synaptically, different location gives the same result

Question 25

Tissue distribution-highlights

Answer 25

• mRNA used for distribution studies
• Distinct but overlapping distributions
• D1 and D2 (most abundant)- found in all terminal fields (often co-localised)
• D3 and D4 more selectively found in limbic areas (emotional, hypothalamus, amygdala) (i.e. input from VTA, cognition/emotion)
• D5 only in hippocampus, hypothalamus and thalamus

Question 26

Location and function of DA receptor

Answer 26

• Clozapine = can cause agranulocytosis

Question 27

Targets for pharmacological therapy

Answer 27

• Schizophrenia
• Attention deficit disorder
• Drug dependence
• Endocrine functions
• Movement disorders
  
  • Parkinson’s disease