Raclopride studies of dopamine release: dependence on presynaptic integrity

doi:10.1016/S0006-3223(03)00288-9

Biological Psychiatry

Volume 54, Issue 11, 1 December 2003, Pages 1193-1199

https://doi.org/10.1016/S0006-3223(03)00288-9 Get rights and content

Abstract

Background

Raclopride and dopamine (DA) compete for in vivo binding to the D₂ receptors. Thus, measurements of raclopride binding provide a method to evaluate endogenous release in a variety of conditions. Amphetamine elicits DA release, provoking a rapid increase in synaptic DA and leading to a reduction in raclopride binding, which outlasts the temporary increase in extracellular DA concentrations by several hours. The mechanism responsible for the decrease in raclopride binding is still unclear.

Methods

We used a multiple ligand concentration receptor assay method in normal monkeys and in monkeys with varying degrees of lesion of the DA nigrostriatal terminals to measure the density and affinity of D₂ receptors after methamphetamine challenge.

Results

The reduced raclopride binding can be accounted for by a decreased affinity of the ligand to the receptors. There is a direct, nonlinear relationship between the presynaptic storage capacity and the change in raclopride binding after methamphetamine.

Conclusions

This observation may bear important implications for the understanding of diseases such as schizophrenia in which the marked increase in amphetamine-induced displacement of raclopride compared with normal control subjects suggests increased release of DA from presynaptic stores and potential abnormalities in presynaptic DA function.

Introduction

In recent years, a number of studies have used positron emission tomography (PET) or single photon emission tomography to investigate the effects of various drugs on dopamine (DA) release and the levels of endogenous DA in the striatum. The ligands of choice are usually benzamides such as [¹¹C]raclopride or [¹²³I]IBZM, tracers selective but with moderate affinity for the DA D₂ and D₃ receptors. This property is believed to lead to competition between the endogenous ligand DA and the tracer and thus to permit changes in endogenous DA to be reflected by opposite changes in the tracer binding potential (BP; see Laruelle 2000a for review). Taking advantage of this technique, the effects of amphetamine have been extensively studied in vivo in human and nonhuman primates. Most of the studies, however, have measured only the tracer’s BP, an estimate of the density over the affinity (B_max/K_d), predictably found to be decreased after amphetamine Breier et al 1997, Laruelle et al 1996.

The increase in extracellular DA measured by microdialysis peaks early (within 15–30 min) but subsides within 2 hours of drug administration and thus cannot explain the sustained decrease in BP of up to 6 hours after a single dose of amphetamine Carson et al 2000, Laruelle et al 1997. Prolonged alterations in raclopride binding have also been reported after a behavioral manipulation (Koepp et al 1998). A number of explanations have been put forward to explain sustained decreases, including mismatch between extracellular and intrasynaptic concentrations, receptor internalization, or the existence of non-DA mechanisms (Laruelle 2000a). Receptor internalization, however, would be reflected by reduced B_max. In contrast, if raclopride is binding to one receptor site with a single affinity, K_d will increase with increasing occupancy of the receptors by DA without changes in B_max. Preliminary reports by ourselves (Doudet et al 2000b) and Carson et al (2002) have demonstrated increased K_d with unchanged B_max.

We investigated the relationship between the changes in raclopride BP after methamphetamine and the integrity of the DA presynaptic system at baseline. We scanned normal monkeys and monkeys with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced progressive lesions of the DA nigrostriatal terminals. Multiple ligand concentration receptor assay studies were obtained using previously published methods Farde et al 1986, Holden et al 2002 to obtain an estimate of the density and affinity of the receptors at baseline and after methamphetamine. We hypothesized that if methamphetamine’s effects on raclopride binding were mainly due to acute DA release and receptor occupancy, the BP would decrease and K_d^app would increase in normal animals but that this effect would disappear with increasing loss of DA terminals. We correlated the percentage of change of raclopride BP with two indices of DA integrity and storage capacity, the influx rate constant of 6-[¹⁸F]fluoro-L-dopa (FDOPA) and the BP of [¹¹C]dihydrotetrabenazine (DTBZ), a tracer of the vesicular monoamine transporter. These measures of presynaptic integrity were obtained in concurrent studies in the baseline condition Doudet 2000, Doudet et al 1998.

Section snippets

Methods and materials

Rhesus monkeys (Macaca mulata) were studied at baseline and after acute administration of methamphetamine (mAMPH). Six monkeys were included as normal control animals (NORM; n = 6) and weight-, age-, and sex-matched with the Parkinsonian animals. Six monkeys had bilateral MPTP-induced lesions of the nigrostriatal DA terminals and four had a right unilateral lesion. Among the lesioned animals, three bilateral animals and three unilateral animals were classified as “asymptomatic,” that is, they

Results

The time–activity curves between 30 and 60 min pre- and post-mAMPH in both striatum and cerebellum were constant in time, suggesting little or no effect of blood flow on our results (data not shown). The data for the right striata are summarized in Table 1. Table 2 shows separately the data for the left and right striata of the unilaterally lesioned animals (three asymptomatic: Sc, Ke, Mi; one symptomatic: Ka), demonstrating the consistently attenuated response to mAMPH in the right-lesioned

Discussion

Two important findings arise from this study. First, we demonstrated in vivo the origin of the decrease in BP that has been consistently reported, in both human subjects and nonhuman primates, after acute exposure to amphetamine or methamphetamine. Second, we demonstrated that at least partial DA innervation is required for this effect to take place. In agreement with Carson et al (2000), early studies in two animals confirmed that administration of a single dose of amphetamine or

Acknowledgements

This work was supported by Canadian Institutes of Health Research (formerly Medical Research Council of Canada) Grant MPO 14535. We thank Astra Research Centre for their gift of the raclopride precursor. We also thank the staff of the UBC/TRIUMF PET program for their assistance and contribution to this work. TRIUMF is funded by a contribution from the National Research Council of Canada. We are grateful to Dr. T.J. Ruth (head, PET program) and J.A. Stoessl (director, Pacific Parkinson Research

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NeuroImage

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Equilibrium analysis to quantify dynamic positron emission tomography (PET) with bolus followed by continuous tracer infusion and acute amphetamine challenge assumes that all tissue kinetics attain steady states during pre- and post-challenge phases. Violations of this assumption may result in unreliable estimation of the amphetamine-induced percent change in the binding potential (ΔBP%).
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Original articleRaclopride studies of dopamine release: dependence on presynaptic integrity

Abstract

Background

Methods

Results

Conclusions

Introduction

Section snippets

Methods and materials

Results

Discussion

Acknowledgements

Brain Res Rev

Eur J Pharmacol

Brain Res

Brain Res

Schizophrenia is associated with elevated amphetamine-induced dopamine concentrationsEvidence from a novel positron emission tomography method

Proc Natl Acad Sci U S A

Amphetamine-induced dopamine release in the rat striatumAn vivo microdialysis study

J Neurochem

Scatchard analysis with bolus/infusion administration of [C-11]racloprideAmphetamine effects in anesthetized monkeys

Amphetamine-induced dopamine releaseDuration of action assessed with [11C]raclopride in anesthetized monkeys

In vivo [3H]spiperone bindingEvidence for accumulation in corpus striatum by agonist-mediated receptor internalization

J Cereb Blood Flow Metab

Striatal binding of the PET ligand 11C-raclopride is altered by drugs that modify synaptic dopamine levels

Synapse

PET studies in the MPTP model of Parkinson’s disease

6- [18F]Fluoro-L-DOPA PET studies of the turnover of dopamine in MPTP-induced parkinsonism in monkeys

Synapse

Multiple ligand concentration receptor assays (MLCRA)Sequential vs non-sequential measurements of density and affinity of dopamine D2 receptors with [11C] raclopride. Application to amphetamine effects

NeuroImage

In vivo PET studies of the dopamine D2 receptors in rhesus monkeys with long-term MPTP-induced parkinsonism

Synapse

Differential effects of amphetamine and a dopamine transporter blocker on the density and affinity of DA D2 receptors in monkeysPET studies with [11C[raclopride

NeuroImage

Original article
Raclopride studies of dopamine release: dependence on presynaptic integrity

Amphetamine-induced dopamine releaseDuration of action assessed with [¹¹C]raclopride in anesthetized monkeys

In vivo [³H]spiperone bindingEvidence for accumulation in corpus striatum by agonist-mediated receptor internalization

Striatal binding of the PET ligand ¹¹C-raclopride is altered by drugs that modify synaptic dopamine levels

6- [¹⁸F]Fluoro-L-DOPA PET studies of the turnover of dopamine in MPTP-induced parkinsonism in monkeys

Multiple ligand concentration receptor assays (MLCRA)Sequential vs non-sequential measurements of density and affinity of dopamine D2 receptors with [¹¹C] raclopride. Application to amphetamine effects

Differential effects of amphetamine and a dopamine transporter blocker on the density and affinity of DA D2 receptors in monkeysPET studies with [¹¹C[raclopride