N-ethylmaleimide-induced changes in agonist affinity for histamine H1-receptors in the guinea pig brain.

The effect of the thiol-alkylating agent, N-ethylmaleimide (NEM), on histamine (HA) H1-receptors from guinea pig cerebellum, labeled with [3H]mepyramine, was investigated. The properties of [3H]mepyramine binding (apparent dissociation constant and maximal number of sites) were not modified by prior treatment of the membranes with 2 or 5 mM NEM. This treatment did not change either the inhibition curves of d-chlorpheniramine or mianserin, two H1-receptor antagonists. In contrast, treatments of membranes with NEM significantly decreased the IC50 values of HA and the slope indexes (pseudo Hill coefficients) of HA inhibition curves, which became inferior to unity. These effects were irreversible, and their extent related to the NEM treatment duration and the NEM concentration. A computer analysis of the data indicated that part of the H1-receptors were converted from a state of low affinity for the amine (IC50 value of 75 microM) into a high agonist affinity state (IC50 value of 2 microM). The change was less marked for partial agonists than for HA. The NEM-induced change was observed in the presence and in the absence of Na+ ions, known to decrease the affinity of HA for H1-receptors. Agonists or antagonists did not protect against the modification of HA affinity induced by NEM. The digitonin-solubilized receptors retained their sensitivity to NEM. Among other thiol reagents, iodoacetamide and iodoacetic acid were ineffective, and organic mercurial agents strongly reduced the number of [3H]mepyramine-binding sites. NEM treatment might alkylate a critical thiol group located outside the ligand-binding domain of the H1 receptor and thereby stabilize the latter in a conformation distinct from that of the activated state.