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

Abstract 

Introduction

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 [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.

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 [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.

Results

In 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.

Conclusions

We 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.

Keywords: Positron emission tomography, Dopamine D2 receptor, [¹¹C](+)-PHNO, [³H]raclopride, Brain biodistribution, Rats