Effects of the H₃ receptor inverse agonist thioperamide on cocaine-induced locomotion in mice: Role of the histaminergic system and potential pharmacokinetic interactions

Rationale: Previous studies have shown that intraperitoneal injections of thioperamide, an imidazole-based H₃ receptor inverse agonist that enhances histamine release in the brain, potentiate cocaine-induced hyperlocomotion. The present study examined the involvement of the histaminergic system in these effects of thioperamide in mice. Materials and methods: We investigated whether immepip, a selective H₃ agonist, could reverse the potentiating effects of thioperamide. Moreover, the non-imidazole H ₃ inverse agonist A-331440 was tested on the locomotor effects of cocaine. Using high-performance liquid chromatography with ultraviolet detection, cocaine plasma concentrations were measured to study potential drug-drug interactions between thioperamide and cocaine. Finally, thioperamide was tested on the locomotor effects of cocaine in histamine-deficient knockout mice in order to determine the contribution of histamine to the modulating effects of thioperamide. Results: Thioperamide potentiated cocaine-induced hyperlocomotion in normal mice, and to a higher extent, in histamine-deficient knockout mice. A-331440 only slightly affected the locomotor effects of cocaine. Immepip did not alter cocaine-induced hyperactivity but significantly reduced the potentiating actions of thioperamide on cocaine's effects. Finally, plasma cocaine concentrations were more elevated in mice treated with thioperamide than in mice that received cocaine alone. Conclusions: The present results indicate that histamine released by thioperamide through the blockade of H₃ autoreceptors is not involved in the ability of this compound to potentiate cocaine induced-hyperactivity. Our data suggest that thioperamide, at least at 10 mg/kg, increases cocaine-induced locomotion through the combination of pharmacokinetic effects and the blockade of H₃ receptors located on non-histaminergic neurons.