Characterisation of the binding of the histamine H3 receptor agonist [3H] (R)-alpha methyl histamine to homogenates of rat and guinea-pig cortex.

We have examined the specific binding of [3H] (R)-alpha-methyl histamine to homogenates of rat and guinea-pig cerebral cortex. Using rat cortex, specific binding represented 90% of total binding. This specific binding was saturable (Bmax 131 fmol/mg protein) of high affinity (Kd 0.40 nM) and reversible. H3 receptor agonists and antagonists potently competed for binding. Antagonist affinities correlated with those reported in functional assays. Affinities derived using guinea-pig cortex were similar to those from rat cortex. GTP inhibited binding (IC50 181 microM). Magnesium ions caused no increase in specific binding but inhibited binding at low mM concentrations. Sodium ions inhibited binding at physiological concentrations (IC50 44 mM).