Abstract
Dopamine receptors (DARs) in the nucleus accumbens (NAc) are critical for cocaine's actions but the nature of adaptations in DAR function after repeated cocaine exposure remains controversial. This may be due in part to the fact that different methods used in previous studies measured different DAR pools. In the present study, we used a protein crosslinking assay to make the first measurements of DAR surface expression in the NAc of cocaine-experienced rats. Intracellular and total receptor levels were also quantified. Rats self-administered saline or cocaine for 10 days. The entire NAc, or core and shell subregions, were collected one or 45 days later, when rats are known to exhibit low and high levels of cue-induced drug seeking, respectively. We found increased cell surface D1 DARs in the NAc shell on the first day after discontinuing cocaine self-administration (designated withdrawal day 1, or WD1) but this normalized by WD45. Decreased intracellular and surface D2 DAR levels were observed in the cocaine group. In shell, both measures decreased on WD1 and WD45. In core, decreased D2 DAR surface expression was only observed on WD45. Similarly, WD45 but not WD1 was associated with increased D3 DAR surface expression in the core. Taking into account many other studies, we suggest that decreased D2 DAR and increased D3 DAR surface expression on WD45 may contribute to enhanced cocaine-seeking after prolonged withdrawal, although this is likely to be a modulatory effect, in light of the mediating effect previously demonstrated for AMPA-type glutamate receptors.