title: Anatomical distribution of sodium-dependent [(3)H]naloxone binding sites in rat brain.
abstract: The sulfhydryl alkylating reagent N-ethylmaleimide (NEM) blocks opioid receptor binding and receptor/G-protein coupling. Sodium partially restores [(3)H]naloxone binding after inhibition by NEM to reveal sodium-dependent [(3)H]naloxone sites, defined as binding in the presence of 50-100 mM NaCl after treatment of membranes or sections with 750 microM NEM. In the present study, receptor autoradiography of [(3)H]naloxone binding in control and NEM-treated tissue was used to examine the anatomical distribution of sodium-dependent [(3)H]naloxone sites in rat brain. In brain membranes, the pharmacology of sodium-dependent [(3)H]naloxone sites was consistent with that of mu opioid receptors. Relatively high IC(50) values for agonists and lack of effect of Gpp(NH)p on DAMGO displacement of [(3)H]naloxone binding in NEM-treated membranes indicated that the sodium-dependent sites were low affinity sites, presumably uncoupled from G-proteins. Autoradiograms revealed that NEM treatment dramatically reduced [(3)H]naloxone binding in all brain regions. However, [(3)H]naloxone binding was increased in specific regions in NEM-treated sections in the presence of sodium, including bed nucleus of the stria terminalis, interpeduncular nucleus, periaqueductal gray, parabrachial nucleus, locus coeruleus, and commissural nucleus tractus solitarius. Sodium-dependent [(3)H]naloxone binding sites were not found in other areas that exhibited [(3)H]naloxone binding in control tissue, including the striatum and thalamus. These studies revealed the presence of a subpopulation of [(3)H]naloxone binding sites which are sodium-dependent and have a unique regional distribution in the rat brain.
authors.affiliation: Department of Pharmacology and Toxicology and Institute for Drug and Alcohol Studies, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298, USA. ljsimsel@hsc.vcu.edu
authors.forename: L J
authors.initials: LJ
authors.lastname: Sim-Selley
authors.forename: R
authors.initials: R
authors.lastname: Xiao
authors.forename: S R
authors.initials: SR
authors.lastname: Childers
mesh_terms: D000818:Animals; D001345:Autoradiography; D001665:Binding Sites; D001921:Brain / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D005033:Ethylmaleimide / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D009270:Naloxone / Q000493:pharmacokinetics*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000187:drug effects / Q000378:metabolism*; D012964:Sodium / Q000494:pharmacology*; D014018:Tissue Distribution; D014316:Tritium
chemical_list: D017450:Receptors, Opioid, mu; D014316:Tritium; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D006165:Guanylyl Imidodiphosphate; D009270:Naloxone; D012964:Sodium; D005033:Ethylmaleimide