In vivo labeling of delta opioid receptors in mouse brain by [3H]benzylidenenaltrexone, a ligand selective for the delta₁ subtype

(E)-7-Benzylidenenaltrexone (BNTX) is a selective ligand for the putative deltai (δ₁) opioid receptor. To explore the feasibility of labeling δ₁ sites in vivo, we determined the cerebral distribution of radioactivity after systemic administration of [3H]BNTX to CD1 mice. Uptake was highest in striatum and lowest in cerebellum throughout the 4 hr time course. Specific radioligand binding, approximated as the difference in radioactivity concentrations between striatum and cerebellum, peaked at 0.32 percent injected dose/g at 30 min and comprised a modest 23% of total striatal radioactivity. For seven brain regions, radioactivity concentrations correlated with δ site densities known from prior in vitro studies (rs = 0.79, p = 0.03), and also with the uptake of N1′-([11C]methyl)naltrindole in vivo (rs = 0.78, p = 0.04) in mice. Specific binding in striatum, olfactory tubercles and cortical regions was saturable by BNTX, and was inhibited stereoselectively by the optical isomers of naloxone. Naltrindole and naltriben (NTB), δ antagonists, blocked 65 -99% of [3H]BNTX specific binding at a dosage of 5.0 μmol/kg. Similar doses of the μ antagonist cyprodime, or the K agonist U50,488H, did not inhibit binding. Adjusted for the four-fold greater brain penetration of NTB relative to BNTX, dose-response studies suggested that δ₁ selective BNTX (ED50 = 1.51 μmol/kg) was 50% more potent than δ₂ selective NTB (ED50 = 0.56 μmol/kg) in blocking specific [3H]BNTX binding in striatum. In CXBK mice, a strain with functional δ₁ but not δ₂ receptors in antinociceptive assays, radioligand uptake and distribution proved similar to that in CD1 mice. In sum, [3H]BNTX labels murine δ opioid receptors in vivo with a low extent of specific binding. The data is consistent with, but not conclusive for, selective labeling of the δ₁ subtype.