Examination of the involvement of supraspinal and spinal mu and delta opioid receptors in analgesia using the mu receptor deficient CXBK mouse

The role of mu and delta opioid receptors in the spinal and supraspinal analgesic actions of morphine and [D-Pen2, L-Pen5] enkephalin were examined in the tail-flick test utilizing the mu opioid receptor deficient CXBK mouse and BALB/cBy and C57BL/6By, the progenitor strains of CXBK. The analgesic effects of i.c.v. administered morphine were equivalent in the CRS-CD1 (Swiss) standard laboratory mice and the progenitor strains of CXBK. Morphine did not, however, produce analgesia in the CXBK mice at doses greater than 10 times the ED50 dose in the progenitor strains. Similarly, the analgesic effect of i.c.v. [D-Ala2, NMePhe4, Gly-ol]enkephalin, a highly selective mu receptor peptide agonist, also was reduced greatly in the CXBK mice. These data are consistent with the deficiency in mu opioid receptors observed autoradiographically in this strain. In contrast, the highly selective delta opioid receptor peptide agonist [D-Pen2, L-Pen5]enkephalin was equipotent i.c.v. in the CXBK mice and in the progenitor strains of CXBK. In contrast to the effects produced by i.c.v. administration, the analgesic effects of intrathecally administered morphine were similar between CRS-CD1 and CXBX strains of mice. These results suggest that 1) both mu and delta opioid receptors can mediate supraspinal analgesia and 2) that the receptor(s) involved in spinally mediated analgesia is (are) quite distinct from those involved supraspinally.