Solubilization and Characterization of Histamine H1 receptors in brain

[3H]Doxepin, a tricyclic antidepressant, binds with high affinity to guinea pig brain membranes with a drug specificity indicating an association with H1 histamine receptors. The [3H]doxepin binding site has been solubilized, with digitonin being the only detergent able to maintain specific binding after solubilization. After solubilization, the kinetics and drug specificity of binding are virtually identical with those obtained in the intact membranes, indicating a conservation of the transmitter binding site after removal of the receptor from its lipid environment. The regulation of agonist affinity for the histamine H1 receptor by cations is maintained after solubilization. Sodium and to a similar extent lithium, but not potassium, rubidium, or cesium, decrease the affinity of agonists for the receptor. The divalent cations manganese and magnesium maintain their ability to increase agonist affinity after solubilization. Guanine nucleotides, however, lose their ability to decrease agonist affinity for the histamine H1 receptor after solubilization. Histamine receptors in rat brain differ from guinea pig brain receptors in the potency of several antihistamines. The difference is maintained in the solubilized receptors. Sucrose gradient and gel filtration experiments indicated Mr approximately 430,000 for the receptor-digitonin complex. The isoelectric point of the receptor is 4.8. None of these physical techniques distinguishes between guinea pig and rat brain receptors.