Evidence for sigma opioid receptor: binding of [3H]SKF-10047 to etorphine-inaccessible sites in guinea-pig brain.

A portion of the specific binding of tritiated SKF-10047 to the guinea-pig brain suspension of the particulate fraction is not inhibited by the strong narcotic analgesic l-etorphine. The binding properties of these etorphine-inaccessible (EI) sites were examined. The specific binding of [3H]SKF-10047 to the EI sites is saturable. Scatchard analysis of the saturation curve revealed a single class of binding sites with apparent Kd of 252 nM and an estimated Bmax of 663 fmol/mg of protein. The EI binding was reduced by heat treatment, trypsin digestion and phospholipase C digestion. The presence of sodium ions slightly increased specific EI binding. Lithium ion increased the EI binding by about 38% at the optimal concentration of 1 mM. Divalent cations such as Mg++, Ca++ and Mn++ reduced EI binding. Morphine-like drugs such as morphine, levorphanol and naltrexone were poor inhibitors for the EI binding, whereas opioid derivatives such as pentazocine, dextrallorphan, cyclazocine, SKF-10047 and dextrorphan were potent inhibitors. Nonopioid drugs such as haloperidol, imipramine, pimozide and propranolol were also potent inhibitors of the EI binding. Distribution of the EI sites in brain was different from that of the mu receptor: highest concentration of EI sites was found in brainstem, midbrain and cerebellum, whereas lower concentrations were found in striatum and cortex. It is suggested that EI sites are not mu receptors but may represent sigma receptors in the central nervous system, mediating psychotomimetic effects of several opioids and other drugs.