The Competition Between Endogenous Dopamine and Radioligands for Specific Binding to Dopamine Receptors

PDFlib PLOP: PDF Linearization, Optimization, Protection

Page inserted by evaluation versionwww.pdflib.com – [email protected]

440

Ann. N.Y. Acad. Sci. 965: 440–450 (2002). © 2002 New York Academy of Sciences.

The Competition Between Endogenous Dopamine and Radioligands for Specific Binding to Dopamine Receptors

PAUL CUMMING,a DEAN F. WONG,b ROBERT F. DANNALS,b NIC GILLINGS,a JOHN HILTON,b URSULA SCHEFFEL,b AND ALBERT GJEDDEa

aPET Center, Århus University Hospitals, Århus, DenmarkbRadiology Department, Johns Hopkins Medical Institutions, Baltimore, Maryland, USA

ABSTRACT: The ternary complex model of G-protein–linkage to receptorsholds that agonists increase the affinity of the receptors for the G protein. Con-sequently, an agonist can exert the greatest inhibition of the binding of radioli-gands which are also agonists. We hypothesized that competition fromendogenous dopamine in striatum of living mice should thus have a greater ef-fect on the binding of the D2,3 agonist N-[3H]propylnorapomorphine([3H]NPA), than on the binding of the D2,3 antagonist [11C]raclopride in livingbrain. The binding potential (pB(0)), defined as the ratio of bound-to-unboundligand after reserpine treatment, was measured in mouse striatum for[11C]raclopride ( C = 8.5), and for [3H]NPA ( = 5.3). Relative tothese baseline values after dopamine depletion, saline-treatment decreased thepB of [3H]NPA by one-half, while the pB of [11C]raclopride declined by onlyone-third. Amphetamine decreased the pB of [3H]NPA to a greater extent thanthat of [11C]raclopride. The apparent inhibition constant of endogenousdopamine depended on the dopamine occupancy and declined to a value 1.66times greater for [3H]NPA than for [11C]raclopride at its highest occupancies.Thus, the agonist binding was more sensitive than antagonist binding to com-petition from endogenous dopamine. Dopamine agonist ligands may be espe-cially useful for PET studies of dopamine receptor occupancy by endogenoussynaptic dopamine. Analysis of the effect of dopamine occupancy on the inhi-bition of agonist indicated a limited supply of G protein, with a maximum ter-nary complex fraction of 40% of maximum antagonist binding capacity.

KEYWORDS: dopamine; receptors; D2; affinity; agonist; [11C]raclopride; N-[3H]propylnorapomorphine; amphetamine; reserpine

INTRODUCTION

Dopamine D2,3 antagonist binding sites can be detected with [11C]raclopride andother benzamide radioligands for positron emission tomography (PET) or singlephoton emission computed tomography (SPECT) studies of living brain. The bind-

Address for correspondence: Paul Cumming, Ph.D., PET Center, Århus University Hospitals,Nørrebrogade 44, Århus, Denmark.

[email protected]

pB 0( )RAC pB 0( )

NPA

441CUMMING et al.: COMPETITIVE BINDING TO DOPAMINE RECEPTORS

ing of exogenous radioligands to dopamine receptors in vivo is normally reduced bycompetition from endogenous dopamine. Consequently, pharmacological depletionof dopamine increases the availability of dopamine receptors in brain of rodent,1,2

non-human primate,3 and human.4 Conversely, stimulation of dopamine release withamphetamine, or blockade of dopamine transport with cocaine, causes further dis-placement of benzamide radioligands from specific D2 binding sites in striatum ofliving rat,1 baboon,5,6 and human.7,8 Thus, altered synaptic dopamine concentra-tions can be measured in vivo.

The pharmacodynamic response to G-protein–mediated neurotransmission oc-curs in proportion to the quantity of ternary agonist-receptor-G-protein complex,9 asdefined by DeLean et al.10 and Samama et al.11 In this scenario, the affinity of G-protein–coupled receptors for the agonist is low when the receptor is dissociatedfrom its G protein and high on interaction with the GTP-free form of the protein(GTP-shift; Jiang et al.12 This property of dopamine binding sites affects the inter-pretation of PET studies of dopamine release.13 An extension of the ternary complexmodel of del Castillo and Katz14 expresses the quantity of bound agonist as

where the subscript a refers to the specific agonist, the term Ba to the quantity of thereceptor-agonist complex which is not coupled to G protein, Ka to the basal affinityto the agonist, and K′a to the receptor affinity as modified by G-protein binding,

where Cg is the G-protein concentration, χg is the concentration of G protein relativeto its affinity for the ternary complex, and Kg the receptors’ affinity to the G protein.

The formation of the ternary complex results in a biphasic displacement of antag-onist radioligands by dopamine, with components at nM (D2

High) and µM concen-trations (D2

Low).15,16 The characteristic interaction between receptor agonists and Gproteins predicts that the inhibitory constant of dopamine toward an agonist is lowerthan the inhibitory constant toward an antagonist. We tested this prediction by com-paring the dopamine inhibitory constant toward the agonist N-[3H]propylapomor-phine ([3H]NPA) and the antagonist [11C]raclopride in striatum of living mice.

METHODS

Male mice (n = 126) of the CD1 strain (Charles River Laboratories, Wilmington,MA) weighing 25–28 g were used in these experiments, which were approved by theresearch ethics committee of Johns Hopkins Medical Institutions. In experiment 1,groups of 18 mice were pretreated with intraperitoneal injections of 200 µL salinevehicle, quinagolide (CV 205,502, Novartis) at a dose of 1 or 10 mg/kg, i.p., or am-phetamine sulfate (Sigma Chemicals, St. Louis, MO) at a dose of 10 mg/kg, i.p., ad-

, (1)

, (2)

Ba Bmax

Ca

Ka′--------

1Ca

Ka′--------+

⁄=

Ka′ Ka 1Cg

Kg------+

⁄ Ka

1 χg+---------------= =

442 ANNALS NEW YORK ACADEMY OF SCIENCES

ministered 10 min before the radiotracer injections. In experiment 2, groups of 18mice were pretreated with saline, or reserpine (5 mg/kg, i.p.) 20 h before tracer in-jection supplemented with α-methyl-p-tyrosine methyl ester (AMPT, 100 mg/kg,i.p., Sigma) 1 h before tracer injections, or nicotine (100 µg/kg, s.c.) 3 min beforetracer injection.

Mice were immobilized for bolus injection to a tail vein of 200 µL saline contain-ing 0.1 MBq N-[3H]propylnorapomorphine ([3H]NPA, New England Nuclear, spe-cific activity 2 GBq/µmol) and 7 MBq [11C]raclopride (specific activity 200 GBq/µmol at time of delivery) synthesized by the method of Ehrin et al.,17 at the JohnsHopkins GE PET Trace cyclotron facility. Mice were killed at 5, 10, 20, 30, 45, and60 min after tracer injection, with triplicate determinations.

Brains were rapidly removed and the cerebellar hemispheres and corpora striatadissected. After weighing, left striatum and cerebellar hemispheres were transferredinto plastic vials for immediate measurement of gamma emissions using a γ-counter(LKB 1285). Right striata and cerebellar hemispheres were transferred to 20 mLglass liquid scintillation vials to which was added 1 mL of organic base (Solvable,Packard, Downers Grove, IL). After dissolving overnight at 40°C, 10 mL of liquidscintillation “cocktail” (Formula 989, Packard, Downers Grove, IL) was added toeach vial, and the tritium concentration measured using a β-counter (Tricarb, Pack-ard Instruments, Downers Grove, IL). Radioactivities were calculated as percentageinjected dose per gram of tissue. Using the cerebellum radioactivities as the inputs,the binding potentials of [11C]raclopride and [3H]NPA in mouse striatum were cal-culated in triplicate by the reference-region method of Lammertsma,18 in the base-line condition and in the drug-treatment conditions.

The occupancy of dopamine was calculated from the decline of the binding po-tential from the dopamine-depleted baseline obtained with reserpine plus AMPT, ac-cording to the equation,

where σ is the occupancy of dopamine, pB(I) the binding potential of the ligand inthe presence of dopamine, and pB(0) the binding potential in the absence of dopam-ine. This plot has an ordinate intercept of zero and a slope of 1 − σ. However, in thecase of the relatively increased affinity and binding potential of an agonist ligand,the binding potential of the ligand in the presence of the inhibiting agonist was fittedby the equation,

where α varies with the endogenous inhibitor concentration relative to its apparentinhibitory constant and the product α pB(0) is a normalization for any difference ofinherent binding affinity or density of available receptors between an antagonist andan agonist in the absence of endogenous competition. Equation 5 follows rearrange-ment of equations 3 and 4 such that

pB(I) = (1 − σ) pB(0), (3)

pB(I) = (1 − e−α pB(0)) pB(0), (4)

(5)σ∆pB

pB 0( )------------ e

αpB 0( )–= =


for any ligand because, by definition, σ → e−1/χ for χ → ∞, when the occupancy ofthe endogenous inhibitor is the following function of the normalized inhibitor con-centration,

where χa is the concentration of the endogenous agonist inhibitor relative to its ap-parent affinity (χa = Ca/Ka′) such that

Equation 7 was used to determine the normalized dopamine concentrations inhibit-ing the binding of the antagonist and agonist ligands. The change of affinity respon-sible for this difference was computed as

where Cg/Kg is the concentration of G protein relative to its affinity for the receptors,and and are the apparent normalized concentrations of the endogenousagonist inhibitor measured with an agonist and an antagonist radioligand,respectively.

RESULTS

Typical time-radioactivity curves for [3H]NPA and [11C]raclopride in striatum ofgroups of six mice treated with reserpine (FIG. 1A), saline (FIG. 1B), amphetamine(FIG. 1C), and the high dose of quinagolide (FIG. 1D) are illustrated. The continuouslines are the results of fitting of the cerebellar time-radioactivity input curves to theconcentrations measured in striatum for the estimation of binding potential (pB) instriatum. The mean pB under the several conditions are summarized in FIGURE 2A.In the dopamine-depleted baseline, pB(0), the mean ± SD values were 5.30 ± 0.77 for[3H]NPA and 8.42 ± 1.62 for [11C]raclopride. In two replications of the normal orsaline-treated condition, the pB of [3H]NPA were reduced by 52%, whereas the pBof [11C]raclopride was reduced by 30%. Amphetamine reduced the pB of [3H]NPAby 73%, and the pB of [11C]raclopride by 47%. The low dose of quinagolide de-creased the pB of [3H]NPA by 77%, and the pB of [11C]raclopride by 63%, but thehigh dose decreased both pB measurements by about 94%. Nicotine challenge in-creased the pB of [3H]NPA by 10%, but did not alter the pB of [11C]raclopride.

From these changes of pB, relative to pB(0), the receptor occupancies, σa, werecalculated according to equation 6 and plotted as a function of the concentration ofdopamine relative to its affinity, χa (FIG. 2C), calculated from equation 7. This rela-tionship obeyed the Michaelis-Menten formulation, indicating the presence of a sin-

, (6)

. (7)

, (8)

σa

χa

1 χa+---------------=

χa

pB 0( )pB I( )------------ 1– 1

eαpB 0( ) 1–

------------------------= =

rK 1Cg

Kg------+

Ka

Ka′-------- 1 χg+

χaNPA

χaRAC

-------------- eαpB 0( )

RAC

1–

eαpB 0( )

NPA

1–

------------------------= = = = =

χaNPA χa

RAC


gle class of binding sites. In each pharmacological condition, the occupancy washigher for [3H]NPA, than for [11C]raclopride. The resulting pairs of σa versus χa(FIG. 2C), show that values of were higher than values of . FIGURE 2Dand 2E show the ratio between the antagonist and agonist affinities fitted to a second-order polynomial, which approaches a slope of 1.66 ± 0.11 (SE) at infinity of χa

NPA,indicating that the affinity of dopamine receptors toward an agonist approaches avalue of 1.66-fold that of the receptors toward an antagonist. Another way of ex-pressing the same result is to state that the Cg/Kg ratio (χg) declines to a ratio closeto 0.66 for large values of .

FIGURE 3A and 3B show the relationship between the concentration and occupan-cy of dopamine and the concentration and occupancy of the G protein, showing thedecline of the estimated GTP-free G-protein concentration of elevated concentra-tions and occupancies of the endogenous agonist competitor.

FIGURE 1. Radioactivity concentrations for [3H]NPA and [11C]raclopride in mousestriatum during 60 minutes after intravenous injection of a single bolus containing 0.1 MBq[3H]NPA and 7 MBq [11C]raclopride to groups of six mice that had been pretreated with (A)reserpine (5 mg /kg, i.p.) 20 h prior to tracer followed by AMPT (100 mg /kg) one hour priorto tracers, (B) saline vehicle (200 µL, i.p.), (C) amphetamine (10 mg / kg, i.p.) 10 min priorto tracers, and (D) quinagolide (10 mg / kg, i.p.) 10 min prior to the tracer administration.Radioactivity concentrations are calculated as the percentage of total injected dose per mgtissue. Lines are the results of fitting a two-compartment model to the radioactivities assum-ing reversible specific binding in striatum and zero specific binding in the reference tissue,cerebellum.

χaNPA χa

RAC

χaagonist


FIG

UR

E2.

See

fol

low

ing

page

for

leg

end.


DISCUSSION

In the present study, the pB of [11C]raclopride in striatum was close to the ratioof radioactivity in striatum to that in cerebellum (less 1) at 30 min after [3H]raclo-pride injection.19 Likewise, the estimates of pB for [3H]NPA in mouse striatum wereconsistent with earlier reports of the binding of [3H]NPA in rodent striatum duringone hour of tracer circulation.20,21 The apomorphines, like raclopride, do not distin-guish between D2 and D3 receptors.22 The differential sensitivity of NPA and raclo-pride to competition from dopamine is not evident from the earlier in vivo analysesperformed in separate groups of mice. In the present study, we used the dynamictime-radioactivity curves measured in brain during 60 min of dual tracer circulationto calculate both pB in striatum at equilibrium.

The 6-hydroxybenzoquinoline quinagolide is a dopamine agonist binding to D2-like receptors in rat brain with an apparent affinty of 0.6 nM.23 In the present study,the low dose of quinagolide was slightly more effective at displacing [3H]NPA than[11C]raclopride, but the two ligands were not discriminated by the high dose ofquinagolide. Quinagolide, like the apomorphine derivatives, may also be a partialagonist, accounting for the ability of a low dose of quinagolide to differentiate onlypartially between the two radioligands.

Partial depletion of dopamine increases the availability of binding sites for[11C]raclopride and other benzamide radioligands in human brain. The extent of thisincrease was greater in patients with schizophrenia than in healthy volunteers,4 a find-ing which was interpreted to reveal the presence of elevated basal occupancy inschizophrenia. However, the partial dopamine depletion increased the availability ofD2 antagonist binding sites by less than 20%, even in the schizophrenic subjects. Incontrast, the present study and earlier results in experimental animals3,19,24 indicatebasal occupancy of antagonist bindings sites of about 40%. Since complete dopaminedepletions are not obtained in human PET studies, differences in pB could be attrib-uted to differential lability of the extacellular dopamine in addition to differences inbasal occupancy. The present results show that a radiolabeled dopamine agonist ishighly sensitive to changes in extracellular dopamine concentrations, and may thusbe more suited than antagonists for detecting pathophysiological changes in occupan-

FIGURE 2. Binding potentials of [11C]raclopride and [3H]NPA in mouse striatum ofmice after several pharmacological treatments, and the estimation of occupancy by dopam-ine, the putative endogenous agonist inhibitor, under the same condition. Panel A: Bindingpotentials of (left) [11C]raclopride and (right) [3H]NPA in mouse striatum at 20 h after re-serpine treatment, and after acute treatements with nicotine (0.1 mg /kg, s.c.), saline(200 µL, i.p., two separate experiments), amphetamine (1 mg / kg, i.p.), or quinagolide (1 or10 mg/kg). Panel B: Binding potentials in presence of endogenous agonist inhibitor re-leased by treatments versus endogenous-agonist-inhibitor-depleted binding potentials mea-sured after reserpine and AMPT treatment. Curves represent regression by Eq. (4) withcorresponding estimates of α listed in legend. Panel C: Occupancies of endogenous agonistinhibitor determined with antagonist (RAC, open circles) and agonist (NPA, filled circles)radioligand according to Eqs. (6) and (7), indicating greater estimates of occupancy with ag-onist (NPA) ligand. Panel D: Antagonist ligand-determined occupancies of endogenous ag-onist inhibitor versus agonist ligand-determined endogenous agonist inhibitor, indicatingsecond-order polynomial relationship. Panel E: Lower occupancies plotted at highermagnification.


cy of receptors in human brain. The present findings with [3H]NPA in living mousesuggest that 11C-labeled NPA25,26 is a good candidate for PET investigations of basaloccupancy of dopamine receptors by endogenous dopamine in living human subjects.

Interaction with G Protein

The outcome of the study is consistent with the hypothesis that dopamine or anexogenous agonist competitor occupies a greater fraction of dopamine receptorswhen the occupancy is measured with an agonist radioligand than when measured

FIGURE 3. Binding potentials of G protein versus binding potentials of endogenousagonist inhibitor. Panel A: Normalized concentration of G protein (χg) versus normalizedconcentration of endogenous agonist inhibitor ( ) determined with agonist radioligand(NPA) according to Eq. (7). Panel B: Normalized concentration of G protein (χg) versusbinding potential of endogenous agonist inhibitor determined with agonist radioli-gand. Panel C: Binding potential of GTP-free G protein (σg) versus binding potential of en-dogenous agonist inhibitor ( ) determined with agonist radioligand. Curve representsregression by Eq. (15), yielding estimates of total normalized concentration (χG) and bind-ing potential (pB) of G protein. Panel D: Ternary complex fraction of total binding capacity(σgσa

agonist) versus occupancy of endogenous agonist inhibitor (σaagonist), indicating maxi-

mum achievable activation of receptor (40%).

χaNPA

σaNPA

σaNPA


with an antagonist radioligand. We based the hypothesis on the claim that an agonistradioligand undergoes the same GTP-shift of affinity enjoyed by dopamine or anexogenous agonist competitor. The theory of the GTP-shift of affinity yields the pre-diction that the ratio between the antagonist and agonist affinities depends on the ra-tio between the concentration of the G protein, which forms the ternary complextogether with the receptor and its bound agonist. In the present study, this ratio wasfound to decline with increasing agonist concentrations, suggesting that the G-protein concentration is not so large as to render the unbound G-protein concentra-tion essentially independent of the binding. Thus,

where Bg is the difference between the total mass of G protein and the mass of un-bound GTP-free G protein, and σg and σa are the occupancies of the GTP-free Gprotein and agonist, respectively. It follows that

where CG is the concentration of total GTP-free G protein such that

which rearranges to

where χg is the normalized concentration of GTP-free G protein available for bind-ing. The equation further rearranges to,

where χG is the normalized concentration (CG/Kg) of total GTP-free G protein, and is the baseline binding potential of the G protein. The equation was solved for

the agonist occupancy,

and subsequently fitted to the experimentally observed relationship between σa andσg, as shown in FIGURE 3C, yielding the estimates for χG and .

The inherent affinities of the receptors toward G protein and agonist are of thesame order of magnitude (4.0 versus 5.3). They also show that the mass of availableG protein is only half of the mass of receptors (2.1 versus 4.0).

The product of the occupancies of G protein and agonist defines the fraction ofthe total number of receptors in the ternary configuration. FIGURE 3D shows that this

Bg = Bmax σg σa, (9)

Bg = (CG − Cg) Vd, (10)

(CG − Cg)/Vd = Bmax σg σa, (11)

, (12)

, (13)

, (14)

Cg

Kg------ χg–

Bmax

KgVd-------------σgσa=

χG

σg

1 σg–---------------– pB 0( )

G σgσa=

pB 0( )G

σa

χG

pB 0( )G σg

------------------- 1

pB 0( )G

1 σg–( )---------------------------------–=

pB 0( )G


fraction reaches only 40% of the receptors at total agonist occupancy, indicating thatno more than 40% of the receptors can be fully activated. We speculate that the re-maining 60% of binding sites constitute the receptor reserve. Furthermore, changesin the abundance of G protein (Gi /o) may be implicated in the phenomenon of sen-sitization of dopamine receptors following chronic reserpine treatment27 or 6-hydroxydopamine lesions of the dopamine pathway.28

ACKNOWLEDGMENTS

Our research was supported by MRC (Denmark) grants 9701888 and 9802563and by the USPHS (NIH) Research Grants RO1 MH42821, RO1 DA11080, RO1AA12839, RO1 NS38927, ROI SA 09482, and K24 DA00412. The authors thankBisma El-Humadi and Paige Rauseo for expert technical assistance, and acknowl-edge the generous gift of quinagolide from Novartis.

REFERENCES

1. YOUNG, L.T. et al. 1991. Effects of endogenous dopamine on kinetics of [3H]N-methyl-spiperone and [3H]raclopride binding in the rat brain. Synapse 9: 188–194.

2. INOUE, O. et al. 1991. Difference in in vivo receptor binding between [3H]N-methyl-spiperone and [3H]raclopride in reserpine-treated mouse brain. J. Neural Transm.85: 1–10.

3. GINOVART, N., L. FARDE, C. HALLDIN & C.G. SWAHN. 1997. Effect of reserpine-induced depletion of synaptic dopamine on [11C]raclopride binding to D2-dopaminereceptors in the monkey brain. Synapse 25: 321–325.

4. ABI-DARGHAM, A. et al. 2000. From the cover: increased basal occupancy of D2 recep-tors by dopamine in schizophrenia. Proc. Natl. Acad. Sci. USA 97: 8104–8109.

5. DEWEY, S.L. et al. 1993. Striatal binding of the PET ligand 11C-raclopride is altered bydrugs that modify synaptic dopamine levels. Synapse 13: 350–356.

6. CARSON, R.E. et al. 1997. Quantification of amphetamine-induced changes in[11C]raclopride binding with continous infusion. J. Cereb. Blood Flow Metab. 17:437–447.

7. SCHLAEPFER, T.E. et al. 1997. PET study of competition between intravenous cocaineand [11C]raclopride at dopamine receptors in human brain. Am. J. Psychiatry 154:1209–1213.

8. LARUELLE, M. et al. 1997. Microdialysis and SPECT measurement of amphetamine-induced dopamine release in non-human primates. Synapse 5: 1–14.

9. BURSTEIN, E.S., T.A. SPALDING & M.R. BRANN. 1997. Pharmacology of muscarinic recep-tor subtypes constitutively activated by G proteins. Mol. Pharmacol. 51: 312–319.

10. DE LEAN, A., J.M. STADEL & R.J. LEFKOWITZ. 1980. A ternary complex modelexplains the agonist-specific binding properties of the adenylate cyclase-coupledbeta-adrenergic receptor. J. Biol. Chem. 255: 7108–7117.

11. SAMAMA, P., S. COTECCHIA, T. COSTA & R.J. LEFKOWITZ. 1993. A mutation-inducedactivated state of the beta 2-adrenergic receptor. Extending the ternary complexmodel. J. Biol. Chem. 268: 4625–4636.

12. JIANG, M. et al. 2001. Most central nervous system D2 dopamine receptors are coupledto their effectors by Go. Proc. Natl. Acad. Sci. USA 98: 3577–3582.

13. GJEDDE, A. & D.F. WONG. 2001. Quantification of neuroreceptors in living humanbrain V: Endogenous neurotransmitter inhibition of haloperidol binding in psychosis.J. Cereb. Blood Flow Metab. 21: 982–994.

14. DEL CASTILLO, J. & B. KATZ. 1957. Interaction at the endplate receptors between dif-ferent choline derivatives. Proc. R. Soc. London B. 146: 369–381.


15. CRESSE, I. & D.R. SIBLEY. 1979. Radioligand binding studies: Evidence for multipledopamine receptors. Commun. Psychopharmacol. 3: 385.

16. BATTAGLIA, G. & M. TITELER. 1982. [3H]N-Propylapomorphine and [3H]spiperonebinding in brain indicate two states of the D2-dopamine receptor. Eur. J. Pharmacol.81: 493–498.

17. EHRIN, E. et al. 1987. Synthesis of [methoxy-3H]- and [methoxy-11C]-labelled raclo-pride, a specific dopamine-D2 receptor ligand. J. Labelled Comp. Radiopharm. 24:931–939.

18. LAMMERTSMA, A.A. et al. 1996. Comparison of methods for analysis of clinical[11C]raclopride studies. J. Cereb. Blood Flow Metab. 16: 42–52.

19. ROSS, S.B. & D.M. JACKSON. 1989. Kinetic properties of the accumulation of 3H-raclo-pride in the mouse brain in vivo. Naunyn-Schmiedeberg’s Arch. Pharmacol. 340: 6–12.

20. HALL, M.D., P. JENNER & C.D. MARSDEN. 1983. Differential labelling of dopaminereceptors in rat brain in vivo: comparison of [3H]piribedil, [3H]S 3608, and [3H]N,n-propylnorapomorphine. Eur. J. Pharmacol. 87: 85–94.

21. VAN DER WERF, J.F., J.B. SEBENS, W. VAALBURG & J. KORF. 1983. In vivo binding of N-n-propylnorapomorphine in the rat brain: regional localization, quantification in stri-atum and lack of correlation with dopamine metabolism. Eur. J. Pharmacol. 87: 259–270.

22. LEVANT, B. 1997. The dopamine D3 receptor: neurobiology and potential clinical rele-vance. Pharmacol. Rev. 49: 221–252.

23. CHARUCHINDA, C., P. SUPAVILAI, M. KAROBATH & J.M. PALACIOS. 1987. Dopamine D2receptors in the rat brain: autoradiographic visualization using a high affinity selec-tive agonist ligand. J. Neurosci. 7: 1352–1360.

24. ROSS, S.B. & D.M. JACKSON. 1989. Kinetic properties of the in vivo accumulation of3H-(-)-N-n-propylapomorphine in mouse brain. Naunyn-Schmiedeberg’s Arch. Phar-macol. 340: 13–20.

25. HWANG, D.R., L.S. KEGELES & M. LARUELLE. 2000. N-[11C]propyl-norapomorphine: apositron-labelled dopamine agonist for PET imaging of D2 receptors. Nucl. Med.Biol. 27: 533–539.

26. GILLINGS, N. & P. CUMMING. 2000. Synthesis of (R)-[1-11C]propylnorapomorphine.7th International Symposium on the Synthesis of Isotopes and Isotopically LabelledCompounds. Dresden, June 18–22.

27. BUTKERAIT, P. & E. FRIEDMAN. 1993. Repeated reserpine increases striatal dopaminereceptor and guanine nucleotide binding protein RNA. J. Neurochem. 60: 566–571.

28. TENN, C.C. & L.P. NILES. 1997. Sensitization of G protein-coupled benzodiazepinereceptors in the striatum of 6-hydroxydopamine-lesioned rats. J. Neurochem. 69:1920–1926.

Documents

The Competition Between Endogenous Dopamine and Radioligands for Specific Binding to Dopamine Receptors