DOPAMINE MICRODIALYSIS IN THE NUCLEUS-ACCUMBENS DURING ACUTE AND CHRONIC MORPHINE, NALOXONE-PRECIPITATED WITHDRAWAL AND CLONIDINE TREATMENT

This study shows the effect of opiate withdrawal on dopamine (DA) in the nucleus accumbens (NAC). Microdialysis was used to detect variations in extracellular DA, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the NAC of freely moving rats during acute and chronic morphine treatment followed by naloxone-precipitated withdrawal with and without clonidine. Basal levels of extracellular DA did not change between sessions, but morphine (20 mg/kg, i.p.) caused a significant and identical increase in extracellular DA and metabolites in both the acute phase (day 1) and the chronic phase (day 7). On day 8, naloxone (20 mg/kg i.p.) caused a significant decrease in DA levels accompanied by typical withdrawal symptoms such as wet dog shakes and teeth-chattering. Clonidine pretreatment (200-mu-g/kg, i.p.) eliminated both the withdrawal symptoms and the DA decrease. These results support the view that morphine increases extracellular DA at times when the drug is rewarding and also suggest that the converse may be true; morphine withdrawal decreases DA release in association with the aversive state.