Selective binding of 2-[125I]iodo-nisoxetine to norepinephrine transporters in the brain.
A radioiodinated ligand, (R)-N-methyl-(2-[(125)I]iodo-phenoxy)-3-phenylpropylamine, [(125)I]2-INXT, targeting norepinephrine transporters (NET), was successfully prepared. A no-carrier-added product, [(125)I]2-INXT, displayed a saturable binding with a high affinity (K(d)=0.06 nM) in the homogenates prepared from rat cortical tissues as well as from LLC-PK(1) cells expressing NET. A relatively low number of binding sties (B(max)=55 fmol/mg protein) measured with [(125)I]2-INXT in rat cortical homogenates is consistent with the value reported for a known NET ligand, [(3)H]nisoxetine. Competition studies with various compounds on [(125)I]2-INXT binding clearly confirmed the pharmacological specificity and selectivity for NET binding sites. Following a tail-vein injection of [(125)I]2-INXT in rats, a good initial brain uptake was observed (0.56% dose at 2 min) followed by a slow washout from the brain (0.2% remained at 3 hours post-injection). The hypothalamus (a NET-rich region) to striatum (a region devoid of NET) ratio was 1.5 at 3 hours post-i.v. injection. Pretreatment of rats with nisoxetine significantly inhibited the uptake of [(125)I]2-INXT (70-100% inhibition) in locus coeruleus, hypothalamus and raphe nuclei, regions known to have a high density of NET; whereas escitalopram, a serotonin transporter ligand, did not show a similar effect. Ex vivo autoradiography of rat brain sections of [(125)I]2-INXT (at 3 hours after an i.v. injection) displayed an excellent regional brain localization pattern corroborated to the specific NET distribution in the brain. The specific brain localization was significantly reduced by a dose of nisoxetine pretreatment. Taken together, the data suggest that [(123)I]2-INXT may be useful for mapping NET binding sites in the brain.
Kung
Mei-Ping
MP
Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
Choi
Seok-Rye
SR
Hou
Catherine
C
Zhuang
Zhi-Ping
ZP
Foulon
Catherine
C
Kung
Hank F
HF
D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D005473:Fluoxetine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D018374:LLC-PK1 Cells; D008297:Male; D008657:Metabolic Clearance Rate; D050484:Norepinephrine Plasma Membrane Transport Proteins; D009928:Organ Specificity; D011485:Protein Binding; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013552:Swine; D027981:Symporters / Q000378:metabolism*; D014018:Tissue Distribution
C493911:2-iodonisoxetine; D050484:Norepinephrine Plasma Membrane Transport Proteins; D019275:Radiopharmaceuticals; C493226:Slc6a2 protein, rat; D027981:Symporters; D005473:Fluoxetine