Quantitative autoradiographic mapping of opioid receptors in the brain of δ-opioid receptor gene knockout mice

Using quantitative receptor autoradiography we have determined if deletion of the delta-opioid receptor gene (Oprd1) results in compensatory changes in the expression of other opioid receptors. Gene targeting was used to delete exon I of the mouse delta-opioid receptor gene and autoradiography was carried out on brains from wild-type, heterozygous and homozygous knockout mice. delta-Opioid receptors were labeled with [H-3]deltorphin 1 (7 nM), mu- with [H-3]DAMGO (4 nM), and kappa- with [H-3]CI-977 (2.5 nM) or [H-3]bremazocine (2 nM in the presence of DPDPE and DAMGO) and non-specific binding determined with naloxone. [H-3]Deltorphin I binding was reduced by approximately 50% in heterozygous animals. In homozygous animals specific binding could only be detected after long-term film exposure (12 weeks). Regions exhibiting this residual [H-3]deltorphin I binding correlated significantly with those demonstrating high levels of the mu-receptor and were abolished in the presence of the mu-agonist DAMGO. Autoradiographic mapping showed significant overall reductions in [H-3]DAMGO and [H-3]CI-977 binding throughout the brain following loss of both copies of the Oprd1 gene. In contrast, overall levels of [H-3]bremazocine binding were higher in brains from -/- than +/+ mice. Our findings suggest that residual [H-3]deltorphin I binding in the brain of delta-receptor gene knockout mice is the result of cross-reactivity with mu-sites and that there are no delta-receptor subtypes derived from a different gene. Changes in mu- and kappa-receptor labeling suggest compensatory changes in these subtypes in response to the absence of the delta-receptor. The differences in [H-3]CI-977 and [H-3]bremazocine binding indicate these ligands show differential recognition of the kappa-receptor. (C) 2002 Elsevier Science B.V. All rights reserved.