The present results, based on the preparation and DA receptor affinity testing of a series of novel 2-substituted N-n-propylnorapomorphine (NPA) derivatives, indicate that affinity at D1 sites was reduced, but only moderately and without a clear relationship on the type of substituent, except that a 2-NH2 substituent markedly reduced D1 affinity. More importantly, however, D2 affinity usually was enhanced by 2-substitution of NPAs, and this effect was particularly striking with a 2-F substituent. Comparison of these results with N-n-propylaporphines (NPAs) to previous results with 2-N-methylaporphines (apomorphines) indicated, further, that the enhancement of D2 affinity was influenced appreciably, though somewhat inconsistently, by the N-alkyl side chain. A particularly important conclusion is that R-(-)-2-F-NPA had the highest D2 binding affinity (IC50 = 71 pM; Ki = 12 pM) and D2 selectivity (nearly 60000 by D1/D2 ratio of Ki values) of any ligand yet described (Table I), including a series of aminotetralines, ergolines, and phenethylamines which were evaluated in another report.12 The high affinity of 2-F-NPA led to the prediction that it would have high potency in a behavioral test of central DA agonist activity (induction of stereotyped gnawing in the rat), and it was found to be about ten-times more potent than NPA.12 In addition to the potential experimental or medicinal interest in such a potent and selective, centrally neuropharmacologically active D2 agonist as R-(-)-2-F-NPA, it should also be pointed out that this congener, R-(-)-2-NH2-NPA, could serve as a precursor for the preparation of 18F-labeled R-(-)-2-F-NPA, a potential imaging agent for positron emission tomography (PET) studies of agonist-labeled DA receptors in vivo. In this communication, we report an expedient synthesis of the radioiodinated isostere of glyburide, N-[2-[4-[[ [(cyclohexylamino)carbonyl]amino]sulfonyl]phenyl]ethyl]-5-[125I]iodo-2-methoxybenzamide (LY285110, compound 1), and its use to specifically label ATP-inhibited K channels in rat brain. Commercially available 5-iodosalicylic acid was converted to 5-iodo-2-methoxybenzoic acid by permethylation with dimethyl sulfate and potassium carbonate, followed by sodium hydroxide hydrolysis of the resulting ester. The acid chloride was generated with thionyl chloride and then reacted with 4-(2-aminoethyl)benzenesulfonate in 2 N sodium hydroxide to provide intermediate 5. Reaction of 5 with potassium carbonate and cyclohexyl isocyanate in acetone provided unlabeled 1. Compound 1 served as the starting material for the radionuclide-bearing version: reaction of 1 with hexamethylditin and tetrakis(triphenylphosphine)palladium(0) in refluxing dioxane for 3 h yielded arylstannane 6. Exposure of this arylstannane derivative to chloramine-T and sodium [125I]iodide in methanol, followed by HPLC purification produced [125I]-1. The product was pure by HPLC and comigrated on TLC plates with authentic unlabeled material; the radiochemical yield based on inorganic iodide was 78%. Importantly, the tin derivative 6 is stable, and [125I]-1 can be readily generated and purified from this storable precursor. Binding of [125I]-1 was evaluated by using rat brain homogenate preparations and a rapid filtration assay. The binding of [125I]-1 to rat brain membranes was saturable, and the nonspecific binding was low. Scatchard analysis of the saturation isotherm demonstrated a single, homogeneous population of binding sites with a Kd of 195 pM, and a Bmax of 340 fmol/mg protein. These data are in reasonable agreement with values reported for labeling of K channels in pig cortex microsomes by [3H]glyburide (Kd = 800 pM; Bmax = 400 fmol/mg protein).15 Glyburide and unlabeled 1 were potent inhibitors of specific [125I]-1 binding with Ki values of 608 pM and 2.6 nM, respectively. These data indicate that replacement of the chlorine atom in glyburide with iodine results in an approximately 4-fold decrease in affinity for the ATP-inhibited K channel; nevertheless, 1 still binds with high affinity to this site. Glipizide, another therapeutically useful sulfonylurea antidiabetic drug, was somewhat less potent as an antagonist of [125I]-1 binding, with a Ki value of 8.2 nM. The K channel opener pinacidil inhibited the specific binding of [125I]-1 but at concentrations in excess of 10^-7 M. In this communication we have described an efficient synthesis of an [125I]-labeled analogue of glyburide that binds avidly and specifically to sulfonylurea receptors in the brain. The specific binding of [125I]-1 to rat brain membranes could be inhibited in a concentration-dependent manner by sulfonylurea antidiabetic drugs which function as antagonists of ATP-dependent K channels. Because of its ease of preparation, high specific binding, and high specific activity, this compound appears to be a useful biochemical probe of ATP-inhibited K channels. These characteristics make this radiolabeled antagonist a particularly useful tool to study ATP-inhibited K channels where they are present in low density (e.g. vascular smooth muscle and the CNS) and for autoradiographic studies; in preliminary experiments17 we have obtained quantitative rat brain autoradiograms with 24-48-h exposure of brain slices labeled with [125I]-1. In addition to the advantage of rapid exposure time, the low-energy γ emissions from [125I]-1 allow autoradiographic quantification of K channels in white matter where lower energy β-emissions from tritium are quenched.