Lack of genotype effect on D1, D2 receptors and dopamine transporter binding in triple MOP-, DOP-, and KOP-opioid receptor knockout mice of three different genetic backgrounds

Abstract

We investigated D1, D2 receptors and dopamine transporter (DAT) binding levels in mice lacking all three opioid receptors and wild-type (WT) mice on three different genetic backgrounds. Quantitative autoradiography was used to determine the level of radioligand binding to the D1 and D2 receptors and DAT labeled with [(3)H]SCH23390, [(3)H]raclopride, and [(3)H]mazindol, respectively in triple-opioid receptor knockout (KO) and WT maintained on C57BL/6 (B6) and 129/SvEvTac (129) as well as C57BL/6 x 129/SvPas (B6 x 129) strains. No significant genotype effect was observed in D1, D2 receptors and DAT binding in any regions analyzed in any of the strains studied, suggesting that a lack of all three opioid receptors does not influence D1, D2 receptors and DAT expression, irrespective of their genetic strain background. However, strain differences were observed in D1 binding between the three strains of mice studied. Lower levels of D1 binding were observed in the substantia nigra of B6 x 129 WT mice compared with the 129 WT mice and in the olfactory tubercle of B6 x 129 WT compared with B6 WT and 129 WT mice. Lower levels of D1 binding were observed in the caudate putamen of B6 x 129 KO mice compared with 129 KO mice. In contrast, no significant strain differences were observed in D2 and DAT binding between the three strains of mice in any regions analyzed. Overall, these results indicate a lack of modulation of the dopaminergic system by the deletion of all three opioid receptors regardless of different background strains.

Fig. 1

A: Representative autoradiograms of [³H]SCH23390 binding to D1 receptors in the brain sections of wild-type (WT) and triple-opioid receptor knockout (KO) mice from different genetic backgrounds; C57BL/6 (B6), 129 (129), and C57BL/6 × 129 (B6 × 129). The sections shown are from the level of the caudate putamen (bregma 1.10 mm) and substantia nigra (bregma −3.16 mm). The color bar represents pseudocolor interpretation of black and white film images in fmol/mg tissue. B: Quantitative autoradiography of [³H]SCH23390 binding in the brains of wild-type (WT) and tripleopioid receptor knockout (KO) mice from different genetic background; C57BL/6 (B6), 129 (129), and C57BL/6 × 129 (B6 × 129). Values are expressed as means ± SEM of five to six mice. Three-way ANOVA showed there was a significant effect of strain (F_(2,402) = 36.21, P < 0.001), region (F_(14,402) = 537.60, P < 0.001), strain × region interactions (F_(28,402) = 3.44, P < 0.001), and strain × genotype interactions (F_(2,402) = 3.60, P < 0.05). *P < 0.05 compared with 129 KO; ^###P < 0.001 compared with B6 WT; ^†P < 0.05 and ^†††P < 0.001 compared with 129 WT. CPu, caudate putamen; Tu, olfactory tubercle; SN, substantia nigra.