Estimating endogenous dopamine levels at D2 and D3 receptors in humans using the agonist radiotracer [(11)C]-(+)-PHNO.

Using positron emission tomography (PET) and an acute dopamine depletion challenge it is possible to estimate endogenous dopamine levels occupying dopamine D2/3 receptors (D2/3R) in humans in vivo. Our group has developed [(11)C]-(+)-PHNO, the first agonist radiotracer with preferential in vivo affinity for D3R. Thus, the use of [(11)C]-(+)-PHNO offers the novel possibility of (i) estimating in vivo endogenous dopamine levels at D2/3R using an agonist radiotracer, and (ii) estimating endogenous dopamine levels at D3R in extrastriatal regions such as the substantia nigra, hypothalamus, and ventral pallidum. Ten healthy participants underwent a [(11)C]-(+)-PHNO PET scan under baseline conditions and another under acute endogenous dopamine depletion achieved via oral administration of alpha-methyl-para-tyrosine (64 mg/kg). [(11)C]-(+)-PHNO binding was sensitive to acute dopamine depletion, allowing in vivo estimates of endogenous dopamine in D2R-rich regions (caudate and putamen), mixed D2/3R-rich regions (ventral striatum and globus pallidus), and extrastriatal D3R-rich regions (hypothalamus and ventral pallidum). Dopamine depletion decreased self-reported vigor, which was correlated with the reduction in dopamine levels in the globus pallidus. [(11)C]-(+)-PHNO is a suitable radiotracer for use in estimating endogenous dopamine levels at D2R and D3R in neuropsychiatric populations.

Striatal dopamine D2/3 receptor binding following dopamine depletion in subjects at Ultra High Risk for psychosis.

Altered striatal dopaminergic neurotransmission is thought to be fundamental to schizophrenia. Increased presynaptic dopaminergic activity ([18F]-DOPA PET) may predate the onset of psychotic symptoms and correlates to clinical symptoms in subjects at Ultra High Risk (UHR) for developing psychosis. Postsynaptic dopaminergic neurotransmission has not been investigated yet in UHR patients. We hypothesized that synaptic dopamine concentration would be increased in UHR patients, and that synaptic dopamine concentration would be related to symptom severity. 14 UHR patients and 15 age and IQ matched controls completed an [123I]-IBZM SPECT scan at baseline and again after dopamine depletion with alpha-methyl-para-tyrosine (AMPT). We measured changes in radiotracer binding potential, compared these between UHR patients and controls, and correlated these to symptom severity. The UHR group as a whole did not differ significantly from controls. AMPT significantly reduced symptom severity in the UHR group (p=0.014). Higher synaptic dopamine concentration predicted larger reduction of positive symptoms following depletion in the UHR group (p=0.01). In UHR patients, positive symptoms responded to dopamine depletion, comparable to observations in schizophrenia, suggesting a similar mechanism. Higher synaptic dopamine concentration was associated with more severe positive symptoms and a greater reduction of these symptoms following depletion.

Dopaminergic modulation of the human reward system: a placebo-controlled dopamine depletion fMRI study.

Reward related behaviour is linked to dopaminergic neurotransmission. Our aim was to gain insight into dopaminergic involvement in the human reward system. Combining functional magnetic resonance imaging with dopaminergic depletion by α-methylparatyrosine we measured dopamine-related brain activity in 10 healthy volunteers. In addition to blood-oxygen-level-dependent (BOLD) contrast we assessed the effect of dopaminergic depletion on prolactin response, peripheral markers for dopamine and norepinephrine. In the placebo condition we found increased activation in the left caudate and left cingulate gyrus during anticipation of reward. In the α-methylparatyrosine condition there was no significant brain activation during anticipation of reward or loss. In α-methylparatyrosine, anticipation of reward vs. loss increased activation in the right insula, left frontal, right parietal cortices and right cingulate gyrus. Comparing placebo versus α-methylparatyrosine showed increased activation in the left cingulate gyrus during anticipation of reward and the left medial frontal gyrus during anticipation of loss. α-methylparatyrosine reduced levels of dopamine in urine and homovanillic acid in plasma and increased prolactin. No significant effect of α-methylparatyrosine was found on norepinephrine markers. Our findings implicate distinct patterns of BOLD underlying reward processing following dopamine depletion, suggesting a role of dopaminergic neurotransmission for anticipation of monetary reward.

Development and validation of a liquid chromatography-tandem mass spectrometric method for the determination of alpha-methyltyrosine in human plasma.

A sensitive and selective LC-MS-MS method for the isolation and quantification of alpha-methyltyrosine (AMT) from human plasma is described. The method employs a simple protein precipitation using zinc sulfate and sodium hydroxide. This precipitation procedure produced samples with high aqueous content that could be directly injected into a LC-MS-MS system without compromising reverse-phase chromatographic performance. Chromatographic separation was performed on a MetaChem MonoChrom C(18) column (2.0 mm x 50 mm; 5 microm) at a flow rate of 1 mL/min. Compounds were eluted using a gradient mixture of water-acetic acid (100:0.1, v/v) and acetonitrile-acetic acid (100:0.1, v/v). The structural analog alpha-hydroxymethyltyrosine was used as the internal standard. Mass spectrometric detection was carried out with a triple quadrupole mass spectrometer. The method was validated and used to determine human plasma AMT concentrations, and has been implemented to derive pharmacokinetic parameters.

Imaging extrastriatal dopamine D(2) receptor occupancy by endogenous dopamine in healthy humans.

The effect of endogenous dopamine on in vivo measurement of dopamine D(2) receptors in extrastriatal regions (thalamus and temporal cortex) was evaluated with single photon emission computed tomography and the high affinity ligand [123I]epidepride by comparing the binding potential before and after acute dopamine depletion. Dopamine depletion was achieved by per-oral administration of 5.5 g/70 kg body weight alpha-methyl-para-tyrosine given in 37 h. The alpha-methyl-para-tyrosine treatment increased the binding potential significantly in the temporal cortex (13+/-15%, P=0.036) but not in the thalamus (2+/-9%). The increase of the binding potential in the temporal cortex correlated strongly with the increase of dysphoric mood evaluated by the Positive and Negative Symptom Scale (PANSS) (rho=0.88, P=0.004). These results imply that [123I]epidepride, coupled with acute dopamine depletion might provide estimates of synaptic dopamine concentration.

Imaging D2 receptor occupancy by endogenous dopamine in humans.

The impact of endogenous dopamine on in vivo measurement of D2 receptors in humans was evaluated with single photon emission computerized tomography (SPECT) by comparing the binding potential (BP) of the selective D2 radiotracer [123I]IBZM before and after acute dopamine depletion. Dopamine depletion was achieved by administration of the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (AMPT), given orally at a dose of 1 g every six hours for two days. AMPT increased [123I]IBZM BP by 28 +/- 16% (+/- SD, n = 9). Experiments in rodents suggested that this effect was due to removal of endogenous dopamine rather than D2 receptor upregulation. Synaptic dopamine concentration was estimated as 45 +/- 25 nM, in agreement with values reported in rodents. The amplitude and the variability of the AMPT effect suggested that competition by endogenous dopamine introduces a significant error in measurement of D2 receptors in vivo with positron emission tomography (PET) or SPECT. However, these results also imply that D2 receptor imaging coupled with acute dopamine depletion might provide estimates of synaptic dopamine concentration in the living human brain.