The pattern of [3H]cyclofoxy retention in rat brain after in vivo injection corresponds to the in vitro opiate receptor distribution.

Cyclofoxy, a fluorinated analog of naltrexone, has been designed specifically to permit in vivo labeling of opiate receptors in experimental animals and ultimately, humans. Recently, using positron emission tomography (PET), 3-[18F]acetylcyclofoxy was shown to accumulate in opiate receptor-rich brain regions of a live baboon and to be stereospecifically displaced by injections of (-)-naloxone but not (+)-naloxone. Autoradiographic evidence is presented here that the unacetylated compound, [3H]cyclofoxy, labels a population of opiate receptors in brain after in vivo injections that is virtually identical to that labeled by [3H]naloxone. The in vivo binding patterns of [3H]cyclofoxy in brain were similar to those obtained following incubation of slide-mounted brain sections in vitro. Intravenous injections of [3H]cyclofoxy to rats yielded high (greater than 4:1) striatal and thalamic to cerebellar binding ratios in brain homogenates, supernatants and in 24 micron-thick brain sections 60 min after 30 mu Ci per animal (spec. act. = 16.4 Ci/mmol). Autoradiographs revealed the typical opiate antagonist binding profile with marked retention of [3H]cyclofoxy in the striatal patches, subcallosal streak, medial habenula and central thalamus with little retention of label in cerebellum. [3H]Cyclofoxy binding was reversible since the radiolabeled drug disappeared from brain tissue within 2 h after injections to rats, or could be removed from brain slices in vitro by washing slide-mounted tissue sections for 45 min. In addition, after in vitro washing the same brain sections again bound [3H]cyclofoxy or [3H]naloxone in the same pattern. When pre-washed brain sections were incubated with [3H]cyclofoxy in the presence of unlabeled naloxone, [3H]cyclofoxy binding was reduced to background levels. These data show that [3H]cyclofoxy labels-sensitive opiate receptors in vivo and in vitro. The present results combined with evidence that cyclofoxy demonstrates a low level of toxicity in animals suggest that cyclofoxy is an excellent tool with which to study the physiological role of opiate receptors in living animals using in vivo autoradiography, and in humans using PET.