QUANTITATIVE AUTORADIOGRAPHIC ANALYSIS OF [H-3] CARFENTANIL BINDING TO MU-OPIATE RECEPTORS IN THE RAT-BRAIN

Fentanyl and its derivatives are considered among the most potent opia te analgesic/euphoriants. The pharmacological literature generally sup ports a mu opiate receptor site of action for the fentanyl derivatives , but some observations suggest that other sites of action may be invo lved in producing the extremely potent fentanyl effects. In order to i nvestigate the mechanism of action of fentanyl-like drugs further, [H- 3]carfentanil was used as a radioligand to image high-affinity carfent anil binding sites in slidemounted sections of the rat brain (receptor autoradiography). In parallel studies the prototypical mu opiate agon ist radioligand [H-3]DAMGO ([D-Ala2-MePhe4-Gly-ol5] enkephalin) was al so used. The working hypothesis was that if carfentanil was acting thr ough another high-affinity site besides the mu opiate receptor, the di stribution pattern of the autoradiographic image produced by [H-3]carf entanil should be significantly different than the autoradiographic pa ttern displayed by the well-characterized and selective mu opiate [H-3 ]DAMGO. Thirty-five brain regions were examined for specific [H-3]carf entanil and [H-3]DAMGO binding. The absolute and relative densities of the sites were essentially identical. The highest levels of binding w ere observed in the ''patch'' areas of the striatum (131 +/- 5 fmol/mg T.E. for [H-3]carfentanil; 162 +/- 13 fmol/mg T.E. for [H-3]DAMGO). T he lowest levels were observed in the cerebellum where no specific bin ding of either radioligand was observed. The overall distribution patt ern of the two radio-ligands produced a correlation coefficient of 0.9 5; the distribution pattern was prototypical for the mu opiate recepto r as reported previously by other groups. Despite the nearly identical distribution patterns, an intriguing difference in the interaction of DAMGO and carfentanil with the mu opiate receptor was observed. The b iologically active nonhydrolyzable analog of GTP, GTPgammaS, was able to completely abolish or greatly diminish specific [H-3]DAMGO binding depending on brain region; GTPgammaS had little or no effect on specif ic [H-3]carfentanil binding. This latter difference in the molecular i nteraction of DAMGO and carfentanil with the mu opiate receptor may in dicate that some of the observed differences in the effects of fentany l-like opiates may be due to a difference in the intrinsic activity of the fentanyl derivatives at the mu opiate receptor.