Dopamine D2 receptors mapped in rat brain with [3H](+)PHNO.

Previous work with membrane preparations had demonstrated that the agonist (+)-4-propyl-9-hydroxynaphthoxazine (PHNO) labels the high-affinity state of dopamine D2 receptors with 67-fold selectivity over D1 sites. In this study, quantitative autoradiography was used to examine the binding of 3HPHNO to rat brain sections. Highest binding densities were found in caudate-putamen, accumbens, and olfactory tubercles, as expected, and also in specific layers of the olfactory bulb. In addition, a second group of brain regions, including lateral septum, entorhinal cortex, molecular layer of hippocampus, and several brainstem structures showed low but consistent levels of binding. In all brain regions 3HPHNO binding (2 nM) was completely displaced by 10 microM sulpiride (> 99%). Addition of 150 microM guanilylimidodiphosphate, which normally converts D2 receptors from high to low affinity states, abolished 3HPHNO binding in all brain regions (> 99%), except for the islands of Callejas. This is likely to reflect binding to D3 sites in this area. Omission of preincubation in binding assays decreased 3HPHNO binding in a regionally dependent manner, with strongest effects (22%) seen in high-density areas. These preincubation results confirm that (+)PHNO may have limitations for in vivo imaging studies. On the other hand, 3HPHNO's negligible levels of non-specific binding compared to other agonists and overall selectivity would make it an excellent tool for in vitro autoradiographic monitoring of the high affinity state of D2 receptors.