The displacement of 3H-diprenorphine from opiate receptors by mu-selective opiates was measured in the mouse striatum and thalamus in vivo. In addition, the regional distribution of opiate receptor binding using 3H-diprenorphine, 3H-naloxone and 3H-lofentanil was measured. The displacement of 3H-diprenorphine by naloxone and carfentanil in vivo showed no differences in the striatum and thalamus suggesting that 3H-diprenorphine binds only to one opiate receptor subtype in vivo. This finding is substantiated by the observation that the mu selective ligands 3H-naloxone and 3H-lofentanil have the same in vivo distribution of receptor binding as 3H-diprenorphine. The implication of these findings for PET imaging of opiate receptor subtypes is discussed.
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