Withdrawal from repeated morphine administration augments expression of the RhoA network in the nucleus accumbens to control synaptic structure

The nucleus accumbens (NAc) is a critical brain reward region that mediates the rewarding effects of drugs of abuse, including those of morphine and other opiates. Drugs of abuse induce widespread alterations in gene transcription and dendritic spine morphology in medium spiny neurons (MSNs) of the NAc that ultimately influence NAc excitability and hence reward-related behavioral responses. Growing evidence indicates that within the NAc small GTPases are common intracellular targets of drugs of abuse where these molecules regulate drug-mediated transcriptional and spine morphogenic effects. The RhoA small GTPase is among the most well-characterized members of the Ras superfamily of small GTPases, and recent work highlights an important role for hippocampal RhoA in morphine-facilitated reward behavior. Despite this, it remains unclear how RhoA pathway signaling in the NAc is affected by withdrawal from morphine. To investigate this question, using subcellular fractionation and subsequent protein profiling we examined the expression of key components of the RhoA pathway in NAc nuclear, cytoplasmic, and synaptosomal compartments during multiple withdrawal periods from repeated morphine administration. Furthermore, using invivo viral-mediated gene transfer, we determined the consequences of revealed RhoA pathway alterations on NAc MSN dendritic spine morphology. Our findings reveal an important role for RhoA signaling cascades in mediating the effects of long-term morphine withdrawal on NAc MSN dendritic spine elimination.