Immediate withdrawal from chronic “binge” cocaine administration increases μ-opioid receptor mRNA levels in rat frontal cortex

An increase in preprodynorphin (ppdyn) mRNA was detected in the caudate putamen of chronically cocaine-treated and 3-h withdrawn rats. An increase in μ-opioid receptor (MOP) mRNA levels was observed in the frontal cortex of 3-h withdrawn rats. Naloxone had no effect on the increase of MOP or ppdyn mRNA levels. The results indicate that the opioid system is altered during early withdrawal from chronic cocaine administration.