Projections from lateral habenular to tail of ventral tegmental area contribute to inhibitory effect of stress on morphine-induced conditioned place preference.

Physical stress is one of the most important factors affecting morphine-induced conditioned place preference (CPP). Convincing evidences demonstrate that physical stress can activate lateral habenular (LHb) neurons. However, the mechanism by which physical stress regulates morphine-induced CPP through LHb remains unclear. In this study, we examined the impact of forced swimming stress (FSS) on morphine-induced CPP in rats. We found that FSS significantly decreased the CPP scores of rats compared with the normal morphine administration rats. Meanwhile, we detected the expression of DARPP-32 phosphorylation (p-DARPP-32) in the nucleus accumbens (NAc), and CaMKII in LHb. The results show that FSS enhanced the expression of CaMΚII in LHb, while it reduced the level of p-DARPP-32 expression in the NAc. Furthermore, by microinjecting AAV-CaMKII or AAV-RNAi into LHb, we demonstrated that an overexpression of CaMKII could reduce morphine-induced CPP scores of rats, while knock-down CaMΚII could restore morphine-induced CPP scores, which were interfered by FSS. In addition, by microinjecting DiI into the ventral tegmental area (VTA) and tail of VTA (tVTA) unilaterally, and an anterograde tracing virus (AAV-CaMKII-mCherry) into LHb unilaterally, we verified the neural projections from LHb to tVTA. Taken together, our findings suggest that FSS could activate LHb neurons through CaMΚII, and inhibit morphine-induced CPP through the LHb-tVTA pathway.

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