In vivo binding of [125I]NH2-carfentanil to mu opioid receptors in mouse brain.

A functionalized derivative of the mu opioid agonist carfentanil was synthesized (NH2-carfentanil) and showed high specific activity when radiolabeled with iodine. [127I]NH2-carfentanil displayed high affinity and pronounced mu-binding selectivity with a delta/mu selectivity ratio of over 1200. The ability of [125I]NH2-carfentanil to interact in vivo with opioid receptors was determined in mouse brain using ex vivo binding techniques. Twenty minutes after intraperitoneal injection, 0.1% of the [125I]NH2-carfentanil injected into the mouse was present in the brain. [125I]NH2-carfentanil specific binding was inhibited by co-injection of naloxone or morphine while naltrindole, a delta-selective antagonist, was unable to displace the bound radioligand. Autoradiographic experiments revealed a heterogeneous distribution of [125I]NH2-carfentanil specific binding sites, maximal binding occurred in areas with high densities of mu receptors. Peripherally administered iodo-NH2-carfentanil selectively labelled central mu opioid receptors in mouse indicating great potential for single photon emission computed tomography studies.