Dansyl derivatives of the opiate antagonist naloxazone and the α-adrenergic blocking drug 2-[(2′,6′-dimethoxy)phenoxyethylamino]methyl-benzodioxan (WB-4101) were synthesized as potential in vivo fluorescent labels for opiate and α-adrenergic receptors, respectively. When assayed in vitro dansyl analogs display Ki-values for [3H]naloxone and [3H]WB-4101 binding of 8 nM and 80 nM, respectively. The same patterns of histofluorescence are observed in rat brain slices after the intravenous or intracerebroventricular injection of both dansyl drugs. The same patterns are also observed in dansyl-propranolol and 9-aminoacridine-propranolol (9-AAPN) treated animals as well as in untreated control rats. Accordingly, the observed fluorescence does not reflect the labeling of any receptor but is consistent with the distribution of lipofuscin, an endogenous fluorescent compound.
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