Heterogeneity of opioid receptor binding in brain slices

A methodological approach was established for the study of ligand binding to multiple opioid receptors in slices from rat brain striatum. Specific binding of radiolabeled opiates was resolved from total binding with enantiomers or excess unlabeled ligand. Equilibrium binding of triated etorphine, dihydromorphine, and ethylketocyclazocine, and competitive displacement of [3H]etorphine and [3H]dihydromorphine by the unlabeled opiates were used to assess both high and low affinity receptor sites. The high-affinity binding components of the radiolabeled opiates were characterized by linear Scatchard plots, Kd values of 2.8-3.7 nM, and binding site densities of 180-297 fmol/mg protein. The displacement of [3H]etorphine by morphine and ethylketocyclazocine displayed Hill coefficients of 0.62 and 0.47, respectively, and revealed receptor sites with much lower affinities than those described by the direct binding of these opiates. On the other hand, both morphine and ethylketocyclazocine displaced [3H]dihydromorphine with similar high potencies (apparent Kd's, 3-4 nM). The results support the feasibility of using brain slices as a cellular preparation to study opioid receptor mechanisms.