High affinity binding of tricyclic antidepressants to histamine H₁-receptors: Fact and artifact

Six tricyclic antidepressants were tested for their ability to inhibit the binding of the histamine H₁-receptor antagonist [³H]pyrilamine to membrane fractions from whole rat brain. Calculated inhibition constants (K_i) for the antidepressants were in the range of 2.6 × 10⁻¹¹ to 2.3 × 10⁻⁷M and correlated very well with their equilibrium dissociation constants derived from biological assays of the H₁-receptor. Increasing the concentration of receptors present in the binding assay resulted in an overestimation of the calculated K_i's for doxepin, amitriptyline, and nortriptyline, but not for the lower affinity compounds of the series, imipramine, protriptyline, and desipramine. These results indicate: (1) the importance of receptor concentration in determining the potency of compounds which competitively inhibit, with very high affinity, the binding of a radioactively labeled ligand; (2) the need to correlate binding data with biological data.