Ex vivo Binding of the Agonist PET Radiotracer [11C]-(+)-PHNO to Dopamine D2/D3 Receptors in Rat Brain: Lack of Correspondence to the D2 Recepor Two-affinity-state Model

• Main Author: McCormick, Patrick N.
• Other Authors: Wilson, Alan A.
• Language: en_ca
• Published: 2010
• Subjects: positron emission tomography; dopamine D2 receptor; dopamine D3 receptor; agonist D2/D3 radiotracer; antagonist D2/D3 radiotracer; rat; antipsychotic drugs; autoradiography; ex vivo; 0347; 0419
• Online Access: http://hdl.handle.net/1807/26296

The dopamine D2 receptor exists in vitro in two states of agonist affinity: a high-affinity state mediating dopamine’s physiological effects, and a physiologically-inert low-affinity state. Our primary goal was to determine the in vivo relevance of this two-affinity-state model for the agonist PET radiotracer [11C]-(+)-PHNO, developed for measurement of the D2 high-affinity state. Our second goal was to characterize the regional D2 versus D3 pharmacology of [3H]-(+)-PHNO binding and assess its utility for measuring drug occupancy at both receptor subtypes. Using ex vivo dual-radiotracer experiments in conscious rats, we showed that, contrary to the two-affinity-state model, the binding of [11C]-(+)-PHNO and the antagonist [3H]-raclopride were indistinguishably inhibited by D2 partial agonist (aripiprazole), indirect agonist (amphetamine) and full agonist ((-)-NPA) pretreatment. Furthermore, ex vivo [11C]-(+)-PHNO binding was unaffected by treatments that increase in vitro high-affinity state density (chronic amphetamine, ethanol-withdrawal), whereas unilateral 6-OHDA lesion, which increases total D2 receptor expression, similarly increased the ex vivo binding of [11C]-(+)-PHNO and [3H]-raclopride. These results do not support the in vivo validity of the two-affinity-state model, suggesting instead a single receptor state for [11C]-(+)-PHNO and [3H]-raclopride in conscious rat. Importantly, we also demonstrated that the increased amphetamine-sensitivity of the agonist radiotracers [11C]-(+)-PHNO and [11C]-(-)-NPA, commonly seen in isoflurane-anaesthetized animals and cited as evidence for the two-affinity-state model, is due to the confounding effects of anaesthesia. Using in vitro and ex vivo autoradiography in rat and the D3 receptor-selective drug SB277011, we found that [3H]-(+)-PHNO binding in striatum and cerebellum lobes 9 and 10 was due exclusively to D2 and D3 receptor binding, respectively, but in other extra-striatal regions to a mix of the two receptor subtypes. Surprisingly, the D3 contribution to [3H]-(+)-PHNO binding was greater ex vivo than in vitro. Also surprising, several antipsychotic drugs, at doses producing 80% D2 occupancy, produced insignificant (olanzapine, risperidone, haloperidol) or small (clozapine, ~35%) D3 occupancy, despite similarly occupying both receptor subtypes in vitro. These data reveal a significant discrepancy between in vitro and ex vivo measures of dopamine receptor binding and suggest that the D3 occupancy is not necessary for the therapeutic effect of antispychotic drugs.