The specific binding of [(11)C]doxepin, which has been used as a radioligand for mapping histamine H(1) receptors in human brain by positron emission tomography, was evaluated in five animal species. In mice the [(11)C]doxepin uptake was reduced by treatment with cold doxepin and two H(1) receptor antagonists, but not with H(2)/H(3) antagonists. The specific binding evaluated with treatment with (+)-chlorpheniramine (H(1) antagonist) was in the range of 10-30% in mouse, rat, rabbit, and monkey, but was not detected in guinea pig.
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