title: Dopamine D(2/3) receptor occupancy of apomorphine in the nonhuman primate brain--a comparative PET study with [11C]raclopride and [11C]MNPA.
abstract: Binding studies in vitro have demonstrated that the dopamine D2 receptor may exist in two affinity states for agonists. The high affinity state is thought to represent the functional state of the receptor and proportions might alter during disease. In vitro studies further indicate that agonists induce measurable D(2) receptor occupancy at clinically relevant concentrations but only when measured at the high affinity state. Recently developed PET-radioligands, such as [11C]MNPA, have now made it possible to directly study agonist binding in vivo. The aim of this study was to compare the inhibition by apomorphine of agonist and antagonist radioligand binding to D(2/3) receptors in vivo. A total of 36 PET measurements were performed with the D(2/3) antagonist [11C]raclopride or the D(2/3) agonist [11C]MNPA in two cynomolgus monkeys. On each study day, a baseline measurement was followed by two consecutive pretreatment studies with rising doses of apomorphine (0.01, 0.05, 0.15, 0.5, 1.0, and 3.0 mg/kg). Binding potential (BP(ND)) values were calculated for the striatum with cerebellum as reference region. Apomorphine inhibited [11C]raclopride and [11C]MNPA binding in a dose-dependent manner and to a similar extent. ID(50) and K(i) values were 0.26 mg/kg and 29 ng/ml for [11C]raclopride and 0.50 mg/kg and 31 ng/ml for [11C]MNPA. The present observations do not support the existence of two affinity states in vivo. It might thus be speculated that all D(2/3) receptors are in the high affinity state at in vivo conditions.
authors.affiliation: Karolinska Institutet, Department of Clinical Neuroscience, Psychiatry Section, Stockholm, Sweden. sjoerd.finnema@ki.se
authors.forename: Sjoerd J
authors.initials: SJ
authors.lastname: Finnema
authors.forename: Christer
authors.initials: C
authors.lastname: Halldin
authors.forename: Benny
authors.initials: B
authors.lastname: Bang-Andersen
authors.forename: Balázs
authors.initials: B
authors.lastname: Gulyás
authors.forename: Christoffer
authors.initials: C
authors.lastname: Bundgaard
authors.forename: Håkan V
authors.initials: HV
authors.lastname: Wikström
authors.forename: Lars
authors.initials: L
authors.lastname: Farde
mesh_terms: D000818:Animals; D001058:Apomorphine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D001921:Brain / Q000000981:diagnostic imaging / Q000187:drug effects / Q000378:metabolism; D002250:Carbon Radioisotopes / Q000494:pharmacology; D065127:Dopamine D2 Receptor Antagonists; D005260:Female; D008252:Macaca fascicularis; D049268:Positron-Emission Tomography / Q000379:methods*; D011485:Protein Binding / Q000187:drug effects; D020891:Raclopride / Q000494:pharmacology*; D017448:Receptors, Dopamine D2 / Q000819:agonists / Q000378:metabolism; D050637:Receptors, Dopamine D3 / Q000819:agonists / Q000037:antagonists & inhibitors / Q000378:metabolism; D014018:Tissue Distribution
chemical_list: D002250:Carbon Radioisotopes; D065127:Dopamine D2 Receptor Antagonists; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; D020891:Raclopride; C009074:N-n-propylnorapomorphine; D001058:Apomorphine