Anatomical distribution of sodium-dependent [3H]naloxone binding sites in rat brain

✍ Scribed by Laura J. Sim-Selley; Ruoyu Xiao; Steven R. Childers

• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: English
• Weight: 236 KB
• Volume: 35
• Category: Article
• ISSN: 0887-4476
• DOI: 10.1002/(sici)1098-2396(20000315)35:4<256::aid-syn3>3.0.co;2-7

✦ Synopsis

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 µM 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.