High-affinity binding of [3H]doxepin to histamine H1-receptors in rat brain: Possible identification of a subclass of histamine H1-receptors

The binding of the radioactively labeled tricyclic antidepressant, [³H]doxepin, to rat brain tissie was examined. Scatchard plots of specific [³H]doxepin binding indicated the presence of two distinct binding sites. The equilibrium dissociation constant (K_D) of the high-affinity site was 0.020 nM with a maximal binding capacity (B_max) of 13.7 fmol/mg protein. The corresponding values for the low-affinity site were 3.6 nM and 740 fmol/mg protein, respectively. The high-affinity site was sensitive to competition by pharmacologically relevant concentrations of histamine H₁ antagonists such as pyrilamine (K_D = 1.0 nM), diphenhydramine (K_D = 20 nM), d-chlorpheniramine (K_D = 1.7 nM), and 1-chlorpheniramine (K_D = 97 nM). The B_max for [³H]doxepin binding in the high-affinity H₁-receptor, however, was approximately 10% of the B_max obtained using [³H]pyrilamine to label the H₁-receptor. Various tricyclic antidepressants were very potent inhibitors at the high-affinity [³H]doxepin site. Their potencies, however, did not correlated with their potencies previously reported for the H₁-receptor. The regional distribution of [³H]doxepin high-affinity sites correlated with the known distribution of H₁-receptors in the rat brain. These results suggest that [³H]doxepin is binding to a subclass of histamine H₁-receptors.