Effects of the histaminergic system on the morphine-induced conditioned place preference in mice

The effects of an H 2 receptor antagonist, a histidine decarboxylase inhibitor and a histamine precursor on the morphine-induced place preference in mice were examined. Morphine (1–7 mg/kg) produced a place preference in a dose-dependent manner. This morphine-induced place preference was significantly antagonized by the dopamine (DA) D 1 receptor antagonist SCH 23390. The histamine precursor, L-histidine, attenuated the morphine (7 mg/kg)-induced place preference. On the other hand, the histidine decarboxylase inhibitor, α-fluoromethylhistidine (α-FMH), significantly potentiated the morphine (1 mg/kg)-induced place preference. This potentiation was antagonized by SCH 23390. The H 2 receptor antagonist zolantidine (0.3 mg/kg) significantly potentiated the morphine-induced place preference. Surprisingly, zolantidine (1 mg/kg) alone also produced a significant place preference. The zolantidine-induced place preference was antagonized by SCH 23390. In addition, zolantidine (1, 3 and 10 mg/kg) significantly increased DA turnover (DA ratio) in the limbic forebrain (nucleus accumbens and olfactory tubercle), implying that zolantidine may activate the mesolimbic DA system. Moreover, co-administration of zolantidine dose-dependently increased morphine (10 mg/kg)-induced DA turnover in the limbic forebrain. These results suggest that the activation of histaminergic neurons may attenuate the rewarding effect of morphine, while the inhibition of histaminergic neurons may potentiate the rewarding effect of morphine. Furthermore, potentiation of the morphine-induced rewarding effect by inhibition of histaminergic neurons may be mediated by D 1 receptors. We also demonstrated that the H 2 receptor antagonist zolantidine may activate the mesolimbic DA system, and as a result, zolantidine itself produces a rewarding effect and potentiates the morphine-induced rewarding effect.