Upregulation of HCN2 in ventral tegmental area is involved in morphine-induced conditioned place preference in rats

Morphine is an opioid commonly used to treat pain in clinic, but it also has the potential to be highly addictive, which can lead to abuse. Despite these known risks, the cellular and molecular mechanism of morphine conditioned place preference (CPP) is still unclear. In this study, using a rat model of chronic morphine administration, we found that compared with the control group, the mRNA and protein expression of HCN2 channel in the ventral tegmental area (VTA) were upregulated. Further immunofluorescence analysis showed that the fluorescence intensity of HCN2 channel of VTA dopaminergic neurons in morphine group was significantly enhanced, while the patch clamp recording of brain slices showed that both the magnitude and the density of I-h (HCN channel current) of VTA neurons were significantly increased. Moreover, intra-VTA infusion of ZD7288, a selective inhibitor of HCN channel, into rats of the morphine group decreased morphine CPP. Taken together, our results show that chronic morphine administration induces an upregulation of HCN2 in VTA dopamine neurons, while HCN inhibition reduces morphine CPP, suggesting that HCN channel may be a potential target for the treatment of morphine addiction.