title: Dopamine D2 receptor radiotracers [(11)C](+)-PHNO and [(3)H]raclopride are indistinguishably inhibited by D2 agonists and antagonists ex vivo.
abstract: BACKGROUND
  In vitro, the dopamine D2 receptor exists in two states, with high and low affinity for agonists. The high-affinity state is the physiologically active state thought to be involved in dopaminergic illnesses such as schizophrenia. The positron emission tomography radiotracer [(11)C](+)-PHNO ([(11)C](+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol), being a D2 agonist, should selectively label the high-affinity state at tracer dose and therefore be more susceptible to competition by agonist as compared to the antagonist [(3)H]raclopride, which binds to both affinity states.
  
  
  METHODS
  We tested this prediction using ex vivo dual-radiotracer experiments in conscious rats. D2 antagonists (haloperidol or clozapine), a partial agonist (aripiprazole), a full agonist [(-)-NPA] or the dopamine-releasing drug amphetamine (AMPH) were administered to rats prior to an intravenous coinjection of [(11)C](+)-PHNO and [(3)H]raclopride. Rats were sacrificed 60 min after radiotracer injection. Striatum, cerebellum and plasma samples were counted for (11)C and (3)H. The specific binding ratio {SBR, i.e., [%ID/g (striatum)-%ID/g (cerebellum)]/(%ID/g (cerebellum)} was used as the outcome measure.
  
  
  RESULTS
  In response to D2 antagonists, partial agonist or full agonist, [(11)C](+)-PHNO and [(3)H]raclopride SBRs responded indistinguishably in terms of both ED(50) and Hill slope (e.g., (-)-NPA ED(50) values are 0.027 and 0.023 mg/kg for [(11)C](+)-PHNO and [(3)H]raclopride, respectively). In response to AMPH challenge, [(11)C](+)-PHNO and [(3)H]raclopride SBRs were inhibited to the same degree.
  
  
  CONCLUSIONS
  We have shown that the SBRs of [(11)C](+)-PHNO- and [(3)H]raclopride do not differ in their response to agonist challenge. These results do not support predictions of the in vivo binding behavior of a D2 agonist radiotracer and cast some doubt on the in vivo applicability of the D2 two-state model, as described by in vitro binding experiments.
authors.affiliation: Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada M5S 1A8. patrick.mccormick@camhpet.ca
authors.forename: Patrick N
authors.initials: PN
authors.lastname: McCormick
authors.forename: Shitij
authors.initials: S
authors.lastname: Kapur
authors.forename: Philip
authors.initials: P
authors.lastname: Seeman
authors.forename: Alan A
authors.initials: AA
authors.lastname: Wilson
mesh_terms: D000818:Animals; D002250:Carbon Radioisotopes; D018491:Dopamine Agonists / Q000494:pharmacology*; D018492:Dopamine Antagonists / Q000494:pharmacology*; D065127:Dopamine D2 Receptor Antagonists; D008297:Male; D010078:Oxazines / Q000378:metabolism*; D020891:Raclopride / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017448:Receptors, Dopamine D2 / Q000819:agonists / Q000378:metabolism*; D014316:Tritium
chemical_list: D002250:Carbon Radioisotopes; D018491:Dopamine Agonists; D018492:Dopamine Antagonists; D065127:Dopamine D2 Receptor Antagonists; D010078:Oxazines; D017448:Receptors, Dopamine D2; D014316:Tritium; C042631:naxagolide; D020891:Raclopride