Brain Research, 304 (1984) 1-7 1 Elsevier BRE 10114 Research Reports Histamine H 1 Receptors in Human Brain Labelled with [3H]Doxepin SHIGENOBU KANBA and ELLIOqT RICHELSON Departments of Psychiatry and Pharmacology, Mayo Foundation, Rochester, MN 55905 (U.S.A.) (Accepted November 22nd, 1983) Key words: tricyclic antidepressants -- histamine H 1receptors -- human brain-- mequitazine -- antihistamines-- [3H]doxepin-- mianserin Doxepin, a tricyclic antidepressant, is one of the most potent histamine H 1antagonists. Therefore, the binding of [3H]doxepin to human brain membranes was examined. Scatchard analysis revealed two distinct binding sites. The high-affinity binding site with a dissocia- tion constant (Ko + S.E.M.) of 3.1 + 0.3 x 10 q0 M was pharmacologicallyidentified as histamine H 1receptors. Dissociation curves at low concentrations of [3H]doxepin were biphasic, suggesting several possibilitiesabout the interaction between [3H]doxepin and hista- mine H 1receptors. Tetracyclic antidepressants, mianserin and maprotiline, were very potent, with Kos of 3.6 + 0.7 x 10 -10M and 7.9 + 0.5 x 10- l0 M, respectively. Mequitazine, a new antihistamine with a weak sedative effect, had a Ko of 5.8 + 0.8 x 10- 9, making it ten times as potent as the classic antihistamine diphenhydramine. The highest binding of [aH]doxepin to histamine H 1 receptors was found in cerebral neoeortex and the limbic system. The distribution of histamine H 1receptors in human central nervous system did not correlate with the previously reported distributions in rat brain and guinea pig brain determined by [3H]doxepin binding. INTRODUCTION of Pathology, Mayo Clinic a~d Foundation at the Recently, some tricyclic antidepressants have been shown to be potent competitive inhibitors of histamine Ht receptors. Of all the drugs studied, dox- epin is the most potent histamine HI antagonist known as determined in biological assays with mu- rine neuroblastoma cells ]3 and guinea pig ileum6; and in several radioligand binding studies of H1 receptors labeled with [3H]pyrilaminea,s, 19. [3H]Doxepin binds to rat brain membranes at two distinct sites~S, 20. The high-affinity binding site is associated with histamine H x receptors; the low affinity site, however, has not been identified. For the present report, we extended our studies on the characteristics of histamine H 1 re- ceptors by studying the binding of [3H]doxepin to hu- man brain. MATERIALS AND METHODS Brain tissue preparation Human brain was obtained from the Department time of autopsy. The time elapsed between death and the autopsy was approximately 8 h. Most of the re- ported results were based on studies with brain tissue from a 28-year-old female whose death was caused by aortic rupture without obvious brain damage. She had no reported mental illness nor had she received any prescribed medication. The brain was quickly dissected into individual regions by a neuropatholo- gist and stored at --180 °C in a liquid nitrogen refrig- erator. For the preparation of the homogenate, white matter was removed from the cortex as much as pos- sible. The tissue was then homogenized with a Poly- tron (on channel 8) in 50 vols. of 50 mM Na-K phos- phate buffer (pH 7.4) and the homogenate was cen- trifuged at 48,000 g for 10 min. The pellet was sus- pended in the same volume of buffer and the centri- fugation was repeated. The final pellet was sus- pended in the Na-K phosphate buffer to obtain a pro- tein concentration of approximately 2 mg/ml. Frontal cortex was used for saturation, kinetics, and competi- tion experiments. Correspondence: E. Richelson, Departments of Psychiatry and Pharmacology, Mayo Foundation, Rochester, MN 55905, U.S.A. 0006-8993/84/$03.00 © 1984 Elsevier Science Publishers B.V.