Differential regulation of mu-opioid receptor mRNA in the nucleus accumbens shell and core accompanying amphetamine behavioral sensitization

Repeated amphetamine (AMPH) administration results in behavioral sensitizat ion. To investigate the participation of the opioid system in this phenomen on, we examined the effects of acute and repeated AMPH administration on mu -opioid receptor (MOR) mRNA levels in the nucleus accumbens (NAc) and stria tum (STR) of rats, by quantitative non-radioactive in situ hybridization. F ive injections of PAh IPH (1.5 mg kg(-1), i.p., once every other day), resu lted in a sensitization response profile and a significant down-regulation of MOR mRNA levels in the NAc shell, whereas no change was observed in MOR mRNA levels in the NAc core compared to the saline controls. Conversely, MO R mRNA levels were up-regulated in the rostral STR of AMPH-sensitized rats compared to saline controls. No changes in MOR mRNA levels were observed af ter acute AMPH treatment in any of the brain regions studied. These results suggest that the opioid system participates in the neurobiological underpi nnings of behavioral sensitization and that opioid receptor (OR) expression in the STR and NAc shell and core is differentially modulated by repeated AMPH exposure. (C) 1999 Elsevier Science B.V. All rights reserved.