Fentanyl and its derivatives are considered among the most potent opiate analgesicleuphoriants. The pharmacological literature generally supports a p 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, ['Hlcarfentanil 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 p opiate agonist radioligand ['HIDAMGO ([D-Ala2-MePhe4-Gly-o15]enkephalin) was also used. The working hypothesis was that if carfentanil was acting through another high-affinity site besides the p opiate receptor, the distribution pattern of the autoradiographic image produced by ['Hlcarfentanil should be significantly different than the autoradiographic pattern displayed by the well-characterized and selective p opiate ['HIDAMGO. Thirty-five brain regions were examined for specific ['Hlcarfentanil and ['HIDAMGO 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 fmoVmg T.E. for [3H]carfentanil; 162 ? 13 fmol/mg T.E. for ['HIDAMGO). 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 p opiate receptor as reported previously by other groups. Despite the nearly identical distribution patterns, an intriguing difference in the interaction of DAMGO and carfentanil with the p opiate receptor was observed. The biologically active nonhydrolyzable analog of GTP, GTPyS, was able to completely abolish or greatly diminish specific ['HIDAMGO binding depending on brain region; GTPyS had little or no effect on specific ['Hlcarfentanil binding. This latter difference in the molecular interaction of DAMGO and carfentanil with the p opiate receptor may indicate that some of the observed differences in the effects of fentanyl-like opiates may be due to a difference in the intrinsic activity of the fentanyl derivatives at the p opiate receptor. c) 1993 Wiley-Liss, Inc.