Abstract
The binding of [3H]-beta-endorphin to rat brain homogenates is complex. Although Scatchard analysis of saturation studies yields a straight line, detailed competition studies are multiphasic, suggesting that even at low concentrations of the compound, the 3H-ligand is binding to more than one class of site. A portion of [3H]-beta-endorphin binding is sensitive to low concentrations of morphine or D-Ala2-Leu5- enkephalin (less than 5 nM). The inhibition observed with each compound alone (5 nM) is the same as that seen with both together (each at 5 nM). Thus, the binding remaining in the presence of both morphine and the enkephalin does not correspond to either mu or delta sites. The portion of [3H]-beta-endorphin binding that is inhibited under these conditions appears to be equally sensitive to both morphine and the enkephalin and may correspond to mu1 sites. Treating membrane homogenates with naloxonazine, a mu1 selective antagonist, lowers [3H]- beta-endorphin binding to the same degree as morphine and D-Ala2-Leu5- enkephalin alone or together. This possible binding of [3H]-beta- endorphin to mu1 sites is consistent with the role of mu1 sites in beta- endorphin analgesia and catalepsy in vivo.