Interaction of the preferential D3 agonist (+)PHNO with dopamine D3-D2 receptor heterodimers and diverse classes of monoamine receptor: relevance for PET imaging

(+)-4-Propyl-9-hydroxynaphthoxazine ((+)PHNO) is a high affinity, preferential dopamine D-3 versus D-2 agonist employed in view of its high specificity and excellent signal-to-noise ratio as a radiotracer for positron emission tomography (PET) imaging. Surprisingly, its profile at other classes of monoamine receptor remains undocumented. In addition to hD(3) and hD(2L) receptors, (+)PHNO revealed high affinity at hD(4.4) but not hD(1) or hD(5) receptors. It also revealed significant affinity for several other G protein-coupled monoaminergic receptors, in particular h5-HT1A and h5-HT7. (+)PHNO behaved as a full agonist at hD(4.4) and h5-HT1A receptors with potencies comparable to its actions at hD(3) and hD(2L) receptors, and with less potency at 5-HT7 receptors. In binding assays with membranes derived from cells co-expressing hD(3) and hD(2L) receptors and labeled with [H-3]Nem-onapride or [H-3]Spiperone, the proportion of high affinity binding sites recognized by (+)PHNO was higher than an equivalent mixture of membranes from cells expressing hD(3) or hD(2L) receptors, suggesting that (+)PHNO promotes formation of hD(3)-hD(2L) heterodimers. Further, in cells co-expressing hD(3) and hD2L receptors, (+)PHNO showed higher efficacy for inhibiting forskolin stimulated adenylyl cyclase and inducing adenylyl cyclase super sensitization than in cells transfected with only hD(2L) receptors. In conclusion, (+)PHNO is a potent agonist at hD(4.4), h5-HT1A and h5-HT7 as well as hD(3) and hD(2L) receptors, and it potently activates dopamine hD(3)-hD(2L) heterodimers. These interactions should be considered when interpreting PET studies with [C-11](+)PHNO and may be relevant to its functional and potential clinical properties in Parkinson's disease and other disorders.