Regional differences in mu 1-binding of [3H][D-Ala2,D-Leu5]-enkephalin: comparisons of thalamus and cortex in the rat.

Typically, mu 1-sites represent approximately 25-35% of binding in rat brain homogenates. Competition studies indicated that approximately 60% of [3H][D-Ala2,D-Leu5]-enkephalin ([3H]DADLE) binding in the thalamus was inhibited by low concentrations of morphine (2-5 nM). This high proportion of mu 1-binding was anticipated based upon the low levels of delta-sites and the high levels of mu 1-sites in this region observed in autoradiography studies. In contrast, morphine lowered [3H]DADLE binding by only approximately 5-15% in the cortex, a region known to possess large amounts of delta- and few mu 1-receptors. These results support previous autoradiography studies and illustrate the advantages of using tissue regions in the characterization of opiate receptor subtypes.