Figure 1.

Early studies on positron emission tomography (PET) measurements of brain histamine H₁ receptor occupancy. These images show radioactivity in horizontal brain sections at the striatal level (upper) and the cerebellar level (lower) after intravenous injection of [¹¹C]doxepin into healthy volunteers. Terfenadine (60 mg), epinastine (20 mg), or d-chlorpheniramine (2 mg) was orally administered 1 h before the doxepin injection. For example, d-chlorpheniramine clearly decreased the accumulation of the [¹¹C]ligand in the brain, resulting in a histamine H₁ receptor occupancy of 76.8%. The brain histamine H₁ receptor occupancy (%) was defined and calculated as described [18]. Modified based on [18].