Quantitative autoradiographic analysis of [3H]carfentanil binding to mu opiate receptors in the rat brain

Abstract

Fentanyl and its derivatives are considered among the most potent opiate analgesic/euphoriants. The pharmacological literature generally supports a mu opiate receptor site of action for the fentanyl derivatives, but some observations suggest that other sites of action may be involved in producing the extremely potent fentanyl effects. In order to investigate the mechanism of action of fentanyl-like drugs further, [3H]carfentanil was used as a radioligand to image high-affinity carfentanil binding sites in slidemounted sections of the rat brain (receptor autoradiography). In parallel studies the prototypical mu opiate agonist radioligand [3H]DAMGO ([D-Ala2-MePhe4-Gly-ol5]enkephalin) was also used. The working hypothesis was that if carfentanil was acting through another high-affinity site besides the mu opiate receptor, the distribution pattern of the autoradiographic image produced by [3H]carfentanil should be significantly different than the autoradiographic pattern displayed by the well-characterized and selective mu opiate [3H]DAMGO. Thirty-five brain regions were examined for specific [3H]carfentanil and [3H]DAMGO binding. The absolute and relative densities of the sites were essentially identical. The highest levels of binding were observed in the "patch" areas of the striatum (131 +/- 5 fmol/mg T.E. for [3H]carfentanil; 162 +/- 13 fmol/mg T.E. for [3H]DAMGO). The lowest levels were observed in the cerebellum where no specific binding of either radioligand was observed. The overall distribution pattern of the two radioligands produced a correlation coefficient of 0.95; the distribution pattern was prototypical for the mu opiate receptor as reported previously by other groups.(ABSTRACT TRUNCATED AT 250 WORDS)