NAc can be sub-divided into two regions: the core and the shell. Both regions have different input and output projections (Zahm, 2000) and thought to play different roles in reward pathway (Ito et al., 2004). Recent studies have also reported on different dendritic compartments specifically the proximal and distal (Spruston, 2008). Cocaine regulation of dendritic spines can only be observed in thin, highly motile spine (Kasai et al., 2010) which were thought to be relevant to learning (Moser et al., 1994; Dumitriu et al., 2010) and addiction (Shen et al., 2009; LaPlant et al., 2010). Cocaine exposure caused an increase in spine density in the shell region but a decrease in proximal MSNs in the core region which is seen to be far more enduring (Dumitriu et al., 2012). This enduring change in core reciprocates the idea that shell is involved in addiction development while core in the learning of the addiction or long-term potentiation (Di Chiara, 2002; Ito et al., 2004; Meredith et al., 2008). In a study done by Kourrich and Thomas (2009), however, showed an increase in core MSNs and a decrease in shell MSNs raising the possibility that spine regulation may be compensating the changes in MSNs or spine regulation may be causing a homeostatic tuning of MSNs excitability. Few studies showed homeostatic increase in MSNs excitability following spine downregulation (Azdad et al., 2009; Ishikawa et al., 2009; Huang et al., 2011) supporting the latter but the clear association between both processes is still unknown. A possible mechanism behind the selective downregulation of core MSNs could be dopamine since cocaine withdrawal decreases dopamine levels (Parsons et al., 1991; Baker et al., 2003). Further support to this could be from the higher convergence of the dual glutamatergic and dopaminergic pathways in the core (Zahm and Brog, 1992).

Other than that, a study has found that there is also structural plasticity in input regions to the nucleus accumbens. These inputs include the ventral tegmental area (VTA) which is thoug