Autoradiographic analysis of mu 1, mu 2, and delta opioid binding in the central nervous system of C57BL/6BY and CXBK (opioid receptor-deficient) mice

Abstract

The recent development of in vitro autoradiography techniques has enabled investigators to determine the distribution and relative levels of multiple ligand binding sites in discrete anatomical areas. In this study we used semi-quantitative in vitro autoradiography to compare the levels of binding to central mu 1, mu 2, and delta opioids sites in two strains of mice, C57BL/6BY and CXBK. The CXBK strain is known to be deficient in whole brain opioid binding sites and to be less sensitive than the C57 strain to the analgesic and locomotor stimulatory effects of opiates and opioids. Delta sites were visualized using [ 3H][ d-Ala 2- d-Leu 5]-enkephalin (DADL) plus a low concentration of morphine, total mu sites (mu 1 and mu 2) were visualized using [ 3H]dihydromorphine (DHM), and mu 2 sites were visualized using [ 3H]DHM plus a low concentration of DADL. Binding to mu 1 sites was determined by subtracting mu 2 binding from total mu binding. We found that the two strains did not consistently differ in the levels of delta sites; in some areas the CXBKs had lower levels but in many areas they had levels equal to or greater than those for the C57s. The CXBK strain, however, either had less or the same amount of mu binding as the C57 strain in all areas studied. The CXBK strain was especially deficient in mu 1 binding, particularly in areas involved in pain processing.