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

The sulfhydryl alkylating reagent N-ethylmaleimide (NEM) blocks opioid rece ptor binding and receptor/G-protein coupling. Sodium partially restores [H- 3]naloxone binding after inhibition by NEM to reveal sodium-dependent [H-3] naloxone sites, defined as binding in the presence of 50-100 mM NaCl after treatment of membranes or sections with 750 mu m NEM. In the present study, receptor autoradiography of [H-3]nalaxone binding in control and NEM-treat ed tissue was used to examine the anatomical distribution of sodium-depende nt [H-3]naloxone sites in rat brain. In brain membranes, the pharmacology o f sodium-dependent [H-3] naloxone sites was consistent with that of mu opio id receptors. Relatively high IC50 values for agonists and lack of effect o f Gpp(NH)p on DAMGO displacement of [H-3]naloxone binding in NEM-treated me mbranes indicated that the sodium-dependent sites were low affinity sites, presumably uncoupled from G-proteins. Autoradiograms revealed that NEM trea tment dramatically reduced [H-3]naloxone binding in all brain regions. Howe ver, [H-3]naloxone binding was increased in specific regions in NEM-treated sections in the presence of sodium, including bed nucleus of the stria ter minalis, interpeduncular nucleus, periaqueductal gray, parabrachial nucleus , locus coeruleus, and commissural nucleus tractus solitarius. Sodium-depen dent [H-3]naloxone binding sites were not found in other areas that exhibit ed [H-3]naloxone binding in control tissue, including the striatum and thal amus. These studies revealed the presence of a subpopulation of [H-3]naloxo ne binding sites which are sodium-dependent and have a unique regional dist ribution in the rat brain. (C) 2000 Wiley-Liss, Inc.