Dopamine D2 receptor radiotracers [11C](+)-PHNO and [3H]raclopride are indistinguishably inhibited by D2 agonists and antagonists ex vivo

IntroductionIn 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 [11C](+)-PHNO ([11C](+)-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 [3H]raclopride, which binds to both affinity states.MethodsWe 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 [11C](+)-PHNO and [3H]raclopride. Rats were sacrificed 60 min after radiotracer injection. Striatum, cerebellum and plasma samples were counted for 11C and 3H. The specific binding ratio {SBR, i.e., [%ID/g (striatum)−%ID/g (cerebellum)]/(%ID/g (cerebellum)} was used as the outcome measure.ResultsIn response to D2 antagonists, partial agonist or full agonist, [11C](+)-PHNO and [3H]raclopride SBRs responded indistinguishably in terms of both ED50 and Hill slope (e.g., (−)-NPA ED50 values are 0.027 and 0.023 mg/kg for [11C](+)-PHNO and [3H]raclopride, respectively). In response to AMPH challenge, [11C](+)-PHNO and [3H]raclopride SBRs were inhibited to the same degree.ConclusionsWe have shown that the SBRs of [11C](+)-PHNO- and [3H]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.