Objectives: The drug seeking motivation is one of the most important internal reasons of relapse. During a certain period of withdrawal, drug seeking motivation increases over time. The ventral pallidum (VP) is responsible for the coding and transforming of the reward motivation. However, the mechanisms of how VP-mediated abnormal enhancement of drug seeking motivation after withdrawal and the functional specificity of its subregions are still unclear. After withdrawal, the level of the transmitter-dopamine (DA) in the nucleus accumbens (NAc) decreases, which results to the weakened projection of GABAergic pathway of NAc-VP, and the latter may lead to the hyperfunction of VP neurons, and cause the enhancement of drug-seeking motivation eventually. In this study, we established the cocaine self-administration(SA) model in rats, and used the break point (BP) test to evaluate drug seeking motivation. On the basis of this animal model, we then used immunohistochemical method and in-vivo fiber photometry to detect the roles of the three functional sub-regions of VP in the enhancement of drug seeking motivation after withdrawal, and employed retrograde tracing technology to further clarify the regulatory function of NAc projection to the key VP subregions. Then, we used chemogenetic technology to explore the effect of NAc-VP projection on drug seeking motivation after withdrawal. The implement of this study will help us understand the regulatory mechanism of abnormal drug-seeking motivation after withdrawal, and provide novel ideas to intervene relapse.
Methods: In this study, we used the cocaine self-administration model in rats, and we conducted the following experiments: 1) we tested the level of drug seeking behavior at different time points after withdrawal; 2) we used the immunohistochemical method to detect the expression level of c-Fos positive cells in the key subregions of VP, and adopted the fiber photometry recordings of the expression level of Ca2+ to further clarify the subregional specificity and its neuron activation level; 3) we then used the fiber photometry recordings and retrograde tracing technology to clarify the functional change of NAc-VP neural projection; 4 ) at last, we used chemogenetic technology to verify that the weakened NAc-VP neural projection participates in the enhancement of drug-seeking motivation.
Results: 1) On the 14th day after withdrawal, the drug seeking motivation in rats was significantly enhanced; 2) The expression level of c-Fos positive cells in ventromedial VP( VPvm) and ventral lateral VP(VPvl) subregions were significantly enhanced after withdrawal. Fiber photometry experiments showed that neurons in the VPvm subregion were significantly activated after withdrawal; 3) Fiber photometry and retrograde tracing experiments showed that NAc-VP neural projection was weakened after withdrawal; 4) The results of chemogenetic experiments showed that specifically inhibit NAc-VP neural projection increase the level of drug seeking motivation.
Conclusions: This study clarified that VP neurons participate in the enhancement of drug-seeking motivation after withdrawal with subregional specificity, and the weakened NAc-VP neuronal projection mediates the strengthening of drug-seeking motivation after withdrawal, and this regulation function may work in a dis-inhibitory way.