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

Previous work with membrane preparations had demonstrated that the agonist (+)‐4‐propyl‐9‐hydroxynaphthoxazine (PHNO) labels the high‐affinity state of dopamine D 2 receptors with 67‐fold selectivity over D 1 sites. In this study, quantitative autoradiography was used to examine the binding of [ 3 H](+)PHNO 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 [ 3 H](+)PHNO binding (2 nM) was completely displaced by 10 μM sulpiride (>99%). Addition of 150 μM guanilylimidodiphosphate, which normally converts D2 receptors from high to low affinity states, abolished [ 3 H](+)PHNO binding in all brain regions (>99%), except for the islands of Callejas. This is likely to reflect binding to D 3 sites in this area. Omission of preincubation in binding assays decreased [ 3 H](+)PHNO 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, [ 3 H](+)PHNOs 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 D 2 receptors. © 1994 Wiley‐Liss. Inc.