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Central dopaminergic and
serotonergic function studied
with positron emission tomography
Per Hartvig,
Uppsala University PET Centre
Uppsala University PET Centre
´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
Uppsala University PET Centre
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.
0.016
0.015
0.014
0.013
0.012
0.011
0.01
0.009
0.008
0.007
BASELINE
OSU6162
0.006
Uppsala University PET Centre
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
0.017
Baseline
0.016
OSU
0.015
OSU+R-BH4
0.014
OSU+RBH4+TYR
0.013
0.012
0.011
Baseline
R-BH4
0.01
R-BH4+TYR
Figure 2. Attenuation of R-BH4-induced upregulation
of striatal L-[-11C]DOPA influx by OSU6162
Uppsala University PET Centre
Clinical studies using OSU6162
•
•
•
•
Parkinson disease
Huntington chorea
Schizophrenia
(Alcoholism, smoking cessation)
Uppsala University PET Centre
Apomorphine effect on dopamine
synthesis rate, k3
% Change in dopamine
synthesis rate
0
0.1
-5
0.1 mg/kg/h
-10
0.5 mg/kg/h
1.0 mg/kg/h
-15
-20
-25
Uppsala University PET Centre
”Tune” dependent change of
dopamine synthesis rate
%Change after apomorphine infusion
5
0
0.0110
0.0120
0.0130
0.0140
0.0150
-5
-10
-15
0.0160
Apomorphine 0.1 mg/kg
induced decrease of dopamine change is dependent
on baseline dopamine
tuning in the Rhesus
monkey
-20
Baseline dopamine synthesis rate
Uppsala University PET Centre
Effect of L-DOPA in early and
advanced Parkinsons disease
C hange%
L-DOPA infusion saturates
dopamine synthesis in early
Parkinson´s disease
100
80
Advanced PD
E a r ly P D
60
40
In advanced disease a loss of
presynaptic dopamine receptors
explains the induction of rate
20
0
-2 0
-4 0
C a u d a te
N u c le u s
P u ta m e n
V e n tra l s lic e
P u ta m e n
D o rs a l s lic e
Uppsala University PET Centre
Is L-DOPA an endogenous
neurotransmitter ? (Miwa, Goishima 1993)
0.018
L-DOPA infusion 3 or 15
mg/kg/h induces an increase
in dopamine synthesis rate.
0.015
0.012
0.009
0.006
1
2
Uppsala University PET Centre
Effect of 5R-erythro-tetrahydrobiopterin on dopamine synthesis
6R-erythro-5,5,7,8 tetrahydrobiopterin, the endogenous cofactor for the
hydroxylases induces an
increased dopamine
synthesis rate
L-[-11C]DOPA
Baseline
Uppsala University PET Centre
6R-BH4 infusion
6R-erythro-5,6,7,8-tetrahydrobiopterin
• Pharmacological effects
– Release of monoamines and serotonin
– Receptor effects
– Enhances synthesis of monoamines
–
Hydroxylase
AADC
– Tyrosine
–

Biopterin,BH4
L-DOPA

Pyridoxin
Uppsala University PET Centre
Dopamine
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)
– Parkinson´s disease
– Alzheimer´s disease
Uppsala University PET Centre
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
6-fluoro-L-[-11C]DOPA
Baseline
Uppsala University PET Centre
6R-BH4 infusion
Multitracer protocol on dopamine
function in toxicology
• Toxic
Dopamine Presynaptic Postsynaptic
• reaction synthesis terminals
terminals
• MPTP

• Manganese 
• Wilson

• disease



Uppsala University PET Centre
()


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
Uppsala University PET Centre
Radiotracers used with PET for
studies on presynaptic serotonin
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[-11C]L-tryptophan
Carboxy - [11C]L-tryptophan
5-hydroxy- [-11C]L-tryptophan
[11C] --methyl-L-tryptophan
Uppsala University PET Centre
Brain serotonin synthesis
CH3 COOH
HO
COOH
AADC
TH
NH2
N
N
5-Hydroxyt rypt ophan (5-HT P )
T ryptophan
HO
HO
CHO
MAO
N
NH2
N
5-Hydroxyt rypt amine (5-HT )
HO
NH2
COOH
N
5-Hydroxyindoleacet ic acid (5-HIAA)
Uppsala University PET Centre
AD
Positron emission tomography
11C-Tryptophan
5-hydroxy-11C-tryptophan
Uppsala University PET Centre
Brain utilization rate
5-Hydroxi (-11C)tryptophan
SUV
0.90
Time to peak
15 min
Rate
Striatum
7.0 x 10-3
Frontal ctx
3.3
Temp ctx
1.2
11C-tryptophan
1.1
15 min
- 2.0 x 10-3
- 1.5
- 0.7
______________________________________________________________________________
11C-TRP give
insignificant 11C-HT during PET, but might
show some specific uptake of the tracer
Uppsala University PET Centre
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
Uppsala University PET Centre
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
Uppsala University PET Centre
Effect of pyridoxine on the decarboxylation rate
of 5-hydroxytryptophan
0.014
0.012
1
2
3
4
5
6
7
Rate, k3
0.01
0.008
0.006
0.004
0.002
0
Baseline
Pyridoxine
Uppsala University PET Centre
Selectivity of aromatic amino acid
decarboxylase
Treatment
Decarboxylation rate, K3 of
L-DOPA 5-hydroxitryptophan
______________________________________________
Pyridoxine
10 mg bolus
0
+
Tetrahydrobiopterin
1 –15 mg/kg/h
+
0
Uppsala University PET Centre
Effect of bolus doses of amino acid on
decarboxylation rate of L-DOPA and 5-HTP
120
k3,5 of baseline
100
80
5-HTP
L-DOPA
60
40
20
0
0
0.5
3
5
10
15
25
Dose of amino acid, mg/kg
Uppsala University PET Centre
30
Factors regulating uptake of amino acids to
the brain and neurotransmitter synthesis
•
•
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•
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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
Uppsala University PET Centre
Effect of glucose infusion on uptake of 5hydroxytryptophan derived radioactivity
Norm alized uptake
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
base
300mg/kg/h
500mg/kg/h
Uppsala University PET Centre
Cerebral presynaptic synthesis in depression
Normalized radioactivity
1
0.9
0.8
0.7
0.6
Healthy
0.5
Depressed
0.4
0.3
0.2
0.1
0
0
5 10 15 20 25 30 35 40 45 50
Time, minutes
Uppsala University PET Centre
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
Putamen
0.0072
0.0081
________________________________________________
Uppsala University PET Centre
Brain disposition of precursor
amino acids
•
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Disease
SUV
Sex
F>M
Depression

Schizophrenia

Tourette

OCD

ECT

Synthesis rate
Med pre frt ctx
Do, ganglia

ganglia

ganglia

Uppsala University PET Centre
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.
Uppsala University PET Centre
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. tetrahydrobiopterin 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
Uppsala University PET Centre
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
Uppsala University PET Centre
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)
Uppsala University PET Centre
Limitations in studies with 5-hydroxy
11
[ C]tryptophan
• 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
Uppsala University PET Centre
Theses at UUPC
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Peter Bjurling
Lars ReibringJoakim Tedroff
Karl Johan Lindner
Anna Ekesbo
Richard Torstenson
Ina Marteinsdottir
Pinelopi Merachtsaki
Radiosynthesis
Depression
L-Dopa in PD
Validation of 5 HTP
DA degeneration, OSU
Regulation of DA
SSRI responsive diseases
Regulation of serotonin
Uppsala University PET Centre