Ex vivo [¹¹C]‐(+)‐PHNO binding is unchanged in animal models displaying increased high‐affinity states of the D₂receptor in vitro

Abstract

Dopamine (DA) D2 receptor supersensitivity has been linked to an increase in the density of the D2 high-affinity state as measured in vitro. The two- affinity-state model of the D2 receptor predicts that the ex vivo specific binding of [11C]-(+)-PHNO, an agonist radiotracer thought to bind selectively to the high-affinity state in vivo, should be increased in animal models that display in vitro increases in the proportion of receptors in the D2 high-affinity state. Here, we test this hypotheses by comparing the ex vivo SBR of [11C]-(+)-PHNO with that of the antagonist radiotracer [3H]-raclopride in three dopaminergically supersensitive rat models-AMPH-sensitized rats, rats withdrawn from chronic ethanol, and unilaterally 6-OHDA-lesioned rats-using ex vivo dual-radiotracer biodistribution studies. We find that in AMPH-sensitized rats and rats withdrawn from chronic ethanol treatment, models that exhibited approximately 4-fold increases in the D2 high-affinity state in vitro, the SBRs of [11C]-(+)-PHNO and [3H]-raclopride are unchanged relative to control rats. In unilaterally 6-OHDA-lesioned rats, we find that the increase in [11C]-(+)-PHNO SBR is no different than that observed for the antagonist radiotracer [3H]-raclopride (54% +/- 16% and 52% +/- 14%, respectively). In addition, the effect of acute AMPH pretreatment (4 mg/kg, i.v.) on the SBRs of [11C]-(+)-PHNO and [3H]-raclopride is equivalent in AMPH-sensitized (-38% +/- 12% and -36% +/- 8%, respectively) and in control rats (-40% +/- 11% and -38% +/- 7%). These data emphasize a significant discrepancy between in vitro and in vivo measures of D2 agonist binding, indicating that the two-affinity-state model of the D2 receptor may not apply veridically to living systems. The potential implications of this discrepancy are discussed.

The Antipsychotics Olanzapine, Risperidone, Clozapine, and Haloperidol Are D2-Selective Ex Vivo but Not In Vitro

“…Within this dose range, our ex vivo SBR (a saturable signal) should be relatively insensitive to changes in injected mass, as opposed, for example, to a dose range surrounding the EC 50 in which small changes in injected dose would result in large reductions in the SBR. Our striatal SBR of 4.5 is identical to that in our earlier reports, which used 2-to 10-fold lower injected mass (McCormick et al, 2009;McCormick et al, 2008;and unpublished data), suggesting that the tracer dose range for the D2 receptor extends at least as high as the current injected dose of 5.7 nmol/kg. Second, exceeding the tracer dose range would result in an underestimate of antipsychotic D2 receptor occupancy.…”

Isoflurane Anaesthesia Differentially Affects the Amphetamine Sensitivity of Agonist and Antagonist D2/D3 Positron Emission Tomography Radiotracers: Implications for In Vivo Imaging of Dopamine Release

“…For the tritium-labelled radiotracers, [ 3 H]-(+)-PHNO and [ 3 H]-raclopride, the small injected masses (~0.3 nmol/kg), are undoubtedly within the tracer dose range for these radiotracers (vide infra). For [ 11 C]-(+)-PHNO, the SBR seen here (4.1±0.8) is similar to that in our previous reports (~4.5) which utilised similar injected mass [8,33], but also to more recent experiments (4.4±1.0) which used~10-fold higher injected mass (unpublished data), indicating no significant mass effects even at this relatively high dose. Moreover, the SBRs obtained here with 3 nmol/kg [ 11 C]-(+)-PHNO and with 0.3 nmol/kg [ 3 H]-(+)-PHNO are similar (i.e., 4.1±0.8 and 3.5±1.1, respectively), indicating no significant mass effect in our [ 11 C]-(+)-PHNO data.…”

“…Recent work by our group has demonstrated that [ 11 C]-(+)-PHNO and [ 3 H]-raclopride are indistinguishably inhibited by pretreatment with both the exogenous agonist (−)-NPA, and the partial agonist aripiprazole over a large range of doses [8]. Also, in AMPH-sensitised and ethanol-withdrawn rats, which have been shown to display greatly increased high-affinity state in vitro [29,30], [ 11 C]-(+)-PHNO (and [ 3 H]-raclopride) SBR, are unchanged relative to control animals [31].…”

“…The ex vivo dual-radiotracer methodology used here allows the examination of the SBR of two radiotracers simultaneously in the same animal, greatly reducing both the number of animals required and the variability associated with between radiotracer comparisons [32,33]. We have previously shown (using pretreatment with the~100-fold D3-selective drug SB277011), that [ 11 C]-(+)-PHNO and [ 3 H]-raclopride SBR in striatum is due solely to D2 receptor binding [8].…”

Double Dissociation of the Effects of Haloperidol and the Dopamine D3 Receptor Antagonist ABT-127 on Acquisition vs. Expression of Cocaine-Conditioned Activity in Rats

“…Further evidence for ABT-127 D3 receptor specificity at low doses was found in experiments 2 and 3. Doses of 10 mg/kg or above were necessary for significant inhibition of (Kapur et al, 2000;McCormick et al, 2009) and -propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b] [1,4]oxazin-9-ol that was recently used in humans as a D3-preferring tool (Boileau et al, 2009;Graff-Guerrero et al, 2009) are not selective enough; therefore, additional methods/approaches are necessary. One such approach is to evaluate radioligand binding in a brain area that is enriched in one receptor subtype and not the other.…”