Per Hartvig, Uppsala University PET Centre

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Per Hartvig, Uppsala University PET Centre
Central dopaminergic and serotonergic function
studied with positron emission tomography
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Biosynthesis of dopamine and serotonin

• Precursor amino acid (tyrosine, tryptophan)
• Hydroxylase (tetrahydrobiopterin)
• ?
• L-dopa or 5-hydroxytryptophan
• Aromatic amino acid decarboxylase
• (pyridoxine, vitamin B6)
• ?
• Dopamine or serotonin

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Effect of the the dopamine D2 antagonist OSU6162
Dopamine synthesis rate, k3, min-1 in Rhesus
monkeys before and after 3 mg/kg of OSU 6162.
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Effect of tyrosine and R-tetrahydrobiopterin on
dopamine synthesis rate and stabilization with
OSU 6162
Tyrosine and biopterin increases dopamine
synthesis rate The increased rate is stabilized
to baseline by OSU6162
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Clinical studies using OSU6162

• Parkinson disease
• Huntington chorea
• Schizophrenia
• (Alcoholism, smoking cessation)

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Apomorphine effect on dopamine synthesis rate, k3
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Tune dependent change of dopamine synthesis rate
Apomorphine 0.1 mg/kg induced decrease of
dopa- mine change is dependent on baseline
dopamine tuning in the Rhesus monkey
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Effect of L-DOPA in early and advanced Parkinsons
disease
L-DOPA infusion saturates dopamine synthesis in
early Parkinsons disease In advanced disease a
loss of presynaptic dopamine receptors explains
the induction of rate
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Is L-DOPA an endogenous neurotransmitter ? (Miwa,
Goishima 1993)
L-DOPA infusion 3 or 15 mg/kg/h induces an
increase in dopamine synthesis rate.
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Effect of 5R-erythro-tetrahydro-biopterin on
dopamine synthesis
6R-erythro-5,5,7,8 tetra- hydrobiopterin, the
endo- genous cofactor for the hydroxylases
induces an increased dopamine synthesis rate
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6R-erythro-5,6,7,8-tetrahydrobiopterin

• Pharmacological effects
• Release of monoamines and serotonin
• Receptor effects
• Enhances synthesis of monoamines
• Hydroxylase
  AADC
• Tyrosine ? L-DOPA
  ? Dopamine
• Biopterin,BH4
  Pyridoxin

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6R-erythro-5,6,7,8-tetrahydrobiopterin

• Clinical studies
• Infantil autistic disorder
• (Double blind 3 mg/kg cross over, randomized
  study in children 4-8 y with PET, neurochemistry,
  immunology and clinical evaluation)
• Parkinsons disease
• Alzheimers disease

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The importance of radiolabelling position, 11C
Dopa vs 18F-fluoro-dopa
No effect of tetrahydrobiopterin due to increased
synthesis of 3-O-methyl dopa which is Passing to
the brain giving increased background
radioactivity in the reference
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Multitracer protocol on dopamine function in
toxicology

• Toxic Dopamine Presynaptic Postsynaptic
• reaction synthesis terminals terminals
• MPTP ? ?
  (?)
• Manganese   ? ? ?
  
• Wilson ? ?
  ?
• disease

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Regulation of presynaptic dopamine function

• Supply of tyrosine and L-DOPA
• L-DOPA catalysing effect on synthesis
• Tetrahydrobiopterin effects on synthesis and
  release
• Presynaptic control in Parkinson disease
• Tune dependent control
• Dopamine stabilisers

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Radiotracers used with PET for studies on
presynaptic serotonin

• ?-11CL-tryptophan
• Carboxy - 11CL-tryptophan
• 5-hydroxy- ?-11CL-tryptophan
• 11C -?-methyl-L-tryptophan

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Brain serotonin synthesis
CH3
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Positron emission tomography

• 11C-Tryptophan 5-hydroxy-11C-tryptophan

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Brain utilization rate

• 5-Hydroxi (?-11C)tryptophan
  11C-tryptophan
• SUV 0.90
  1.1
• Time to peak 15 min
  15 min
• Rate
• Striatum 7.0 x 10-3
  - 2.0 x 10-3
• Frontal ctx 3.3
  - 1.5
• Temp ctx 1.2
  - 0.7
• __________________________________________________
  ____________________________
• 11C-TRP give insignificant 11C-HT during PET, but
  might show some specific uptake of the tracer

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Endogenous tracer substrates

• Uptake into the target tissue, passing over the
  BBB
• Regional tissue accumulation of tracer
• Uptake into target cells
• Complex with enzymes in target cells
• Formation of active transmitter
• Uptake of active transmitter
• Release of transmitter to the synapse
• Binding to target receptors
• Metabolism of transmitter with cumulation

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Biosynthesis of dopamine and serotonin
Precursor amino acid (tyrosine,
tryptophan) Hydroxylase (tetrahydrobiopterin
) ? L-dopa or 5-hydroxytryptophan
Aromatic amino acid decarboxylase (pyridoxine,
vitamin B6) ? Dopamine or serotonin
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Effect of pyridoxine on the decarboxylation rate
of 5-hydroxytryptophan
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Selectivity of aromatic amino acid decarboxylase
Treatment Decarboxylation rate, K3 of
L-DOPA 5-hydroxitryptophan
______________________________________________Py
ridoxine 10 mg bolus 0
Tetrahydrobiopterin 1 15
mg/kg/h 0
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Effect of bolus doses of amino acid on
decarboxylation rate of L-DOPA and 5-HTP
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Factors regulating uptake of amino acids to the
brain and neurotransmitter synthesis

• Plasma amino acids
• Diurnal rythm
• Age
• Gender
• Food and drinking
• Proteins, carbohydrates and fat
• Caffeine, ethanol
• Co-factors and vitamins (Pyridoxin B6, biopterin)
• Drugs, SSRI

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Effect of glucose infusion on uptake of
5-hydroxytryptophan derived radioactivity
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Cerebral presynaptic synthesis in depression
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Decarboxylation rate of 5-HTP in different brain
areas
Area Controls Depressed _________________
_________________________________ Lateral frontal
cortex 0.0011 min-1 0.0022 Medial frontal
cortex high
0.0029 0.0060
low 0.0001 0.0042 Caudate 0.0098 0.0098 Put
amen 0.0072 0.0081 ______________________
__________________________
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Brain disposition of precursor amino acids

• Disease SUV Synthesis
  rate
• Sex F gt M
• Depression ? Med pre frt
  ctx??
• Schizophrenia ? Do, ganglia
  ?
• Tourette ?
  ganglia ?
• OCD ? ganglia ?
• ECT ?

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Presynaptic serotonin function in social
phobia(Ina Marteinsdottir et al 2001)
Method Statistical evaluation of PET with
5-hydroxy- tryptophan by a pixel wise blocked
analysis of variance contrasting differences
between patients and controls. Results A a
focal hyposerotonergic tonus in social phobics
as compared to controls was evident in temporal
cortex (periamygdala/rhinal, temporal pole and
gyrus) frontal cortex, anterior cingulatae,
right insula and left basal ganglia.
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Regulation of aromatic amino acid decarboxylase
activity for L-DOPA and 5-hydroxytryptophan

• Several mechanisms regulate amino acid transport
  to the brain and presynaptic synthesis serotonin
• Synthesis of serotonin may be regulated by a
  similar decarboxylase enzyme but with different
  selectivity
• Modulating effect of enzyme co-factors e.g.
  tetrahydrobiopte-rin and vitamin B6 varied for
  the two transmitters
• Capacity limitation in transport and enzyme
  activity for 5HTP
• Limited capacity of amino acid transport may
  influence serotonin function with special impact
  in affective disorders

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What is measured with 5-HTP and PET ?

• 11C-labelling in carboxy and ?-position of 5-HTP
• Blockade of central decarboxylase with NSD1015
• Bioanalysis of brain radioactivity using rat
  brain homogenate by HPLC shows radiolabelled
  5-HTP, serotonin and metabolites
• Calculated rates analysed of brain radioactivity
  in rat brain are similar to rates measured in
  monkey and man with 5-HTP and PET

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Calculation of decarboxylation rate

• Brain reference region after validation of
  accumulation (Patlack plot, Hartvig et al 1992)
• Simulation of a brain refrence region with
  negligable 5HT synthesis gives rates close to
  measured (Blomquist et al 2001)
• Plasma as reference with metabolite correction
  shows regional 5HT rates in accordance with AADC
  activity (Hagberg et al JBFM, 2002)

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Limitations in studies with 5-hydroxy
11Ctryptophan

• Rapid in vivo metabolism to radiolabelled
  products
• Low plasma concentration of radioactivity
• Serotonergic activity in most brain areas - no
  obvious reference area in the brain
• Steady state in the brain not established in
  15-20 min
• Limited capacity for transport over BBB and for
  synthesis
• Use of tracer may occur in non-serotonergic
  neurons

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Theses at UUPC

• Peter Bjurling Radiosynthesis
• Lars Reibring- Depression
• Joakim Tedroff L-Dopa in PD
• Karl Johan Lindner Validation of 5 HTP
• Anna Ekesbo DA degeneration, OSU
• Richard Torstenson Regulation of DA
• Ina Marteinsdottir SSRI responsive diseases
• Pinelopi Merachtsaki Regulation of
  serotonin