Localization of opioid receptor antagonist [3H]-LY255582 binding sites in mouse brain: Comparison with the distribution of mu, delta and kappa binding sites

Agonist stimulation of opioid receptors increases feeding in rodents, while opioid antagonists inhibit food intake. The pan-opioid antagonist, LY255582, produces a sustained reduction in food intake and body weight in rodent models of obesity. However, the specific receptor subtype(s) responsible for this activity is unknown. To better characterize the pharmacology of LY255582, we examined the binding of a radiolabeled version of the molecule, [3H]-LY255582, in mouse brain using autoradiography. In mouse brain homogenates, the Kd and Bmax for [3H]-LY255582 were 0.156 ± 0.07 nM and 249 ± 14 fmol/mg protein, respectively. [3H]-LY255582 bound to slide mounted sections of mouse brain with high affinity and low non-specific binding. High levels of binding were seen in areas consistent with the known localization of opioid receptors. These areas included the caudate putamen, nucleus accumbens, claustrum, medial habenula, dorsal endopiriform nucleus, basolateral nucleus of the amygdala, hypothalamus, thalamus and ventral tegmental area. We compared the binding distribution of [3H]-LY255582 to the opioid receptor antagonist radioligands [3H]-naloxone (mu), [3H]-naltrindole (delta) and [3H]-norBNI (kappa). The overall distribution of [3H]-LY255582 binding sites was similar to that of the other ligands. No specific [3H]-LY255582 binding was noted in sections of mu-, delta- and kappa-receptor combinatorial knockout mice. Therefore, it is likely that LY255582 produces its effects on feeding and body weight gain through a combination of mu-, delta- and kappa-receptor activity.