Effects of betahistine at histamine H3 receptors: mixed inverse agonism/agonism in vitro and partial inverse agonism in vivo

We previously suggested that therapeutic effects of betahistine in vestibular disorders result from its antagonist properties at histamine H(3) receptors (H(3)Rs). However, H(3)Rs exhibit constitutive activity, and most H(3)R antagonists act as inverse agonists. Here, we have investigated the effects of betahistine at recombinant H(3)R isoforms. On inhibition of cAMP formation and [(3)H]arachidonic acid release, betahistine behaved as a nanomolar inverse agonist and a micromolar agonist. Both effects were suppressed by pertussis toxin, were found at all isoforms tested, and were not detected in mock cells, confirming interactions at H(3)Rs. The inverse agonist potency of betahistine and its affinity on [(125)I]iodoproxyfan binding were similar in rat and human. We then investigated the effects of betahistine on histamine neuron activity by measuring tele-methylhistamine (t-MeHA) levels in the brains of mice. Its acute intraperitoneal administration increased t-MeHA levels with an ED(50) of 0.4 mg/kg, indicating inverse agonism. At higher doses, t-MeHA levels gradually returned to basal levels, a profile probably resulting from agonism. After acute oral administration, betahistine increased t-MeHA levels with an ED(50) of 2 mg/kg, a rightward shift probably caused by almost complete first-pass metabolism. In each case, the maximal effect of betahistine was lower than that of ciproxifan, indicating partial inverse agonism. After an oral 8-day treatment, the only effective dose of betahistine was 30 mg/kg, indicating that a tolerance had developed. These data strongly suggest that therapeutic effects of betahistine result from an enhancement of histamine neuron activity induced by inverse agonism at H(3) autoreceptors.