Modulation of the discriminative stimulus effects of cocaine and methamphetamine by the histaminergic system.
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The role of the histaminergic system in the discriminative stimulus effects of cocaine and methamphetamine was examined in rats trained to discriminate between saline and cocaine (10 mg/kg) or methamphetamine (1.0 mg/kg). L-histidine (400 mg/kg), a precursor of histamine, significantly enhanced the discriminative stimulus effects of cocaine and methamphetamine. Previous studies have revealed the existence of several histamine receptor types, H1-, H2-, and H3-receptors. These enhancing effects of L-histidine on the discriminative stimulus effects of cocaine and methamphetamine were attenuated by 5.0 mg/kg of pyrilamine (an H1-receptor antagonist), but not by 1.0 mg/kg of zolantidine (an H2-receptor antagonist), suggesting that these enhancing effects of L-histidine were mediated through the activation of H1-receptors. Thioperamide (7.5 mg/kg), an H3-receptor antagonist, also significantly enhanced the discriminative stimulus effects of cocaine and methamphetamine. However, neither pyrilamine nor zolantidine affected the enhancing effects of thioperamide, unlike the results attained with L-histidine. Therefore our findings suggest that the histaminergic system may modify the discriminative stimulus effects of cocaine and methamphetamine mediated through H1- and H3-receptors.