Research reportRegulation of extinction of cocaine-induced place preference by midkine is related to a differential phosphorylation of peroxiredoxin 6 in dorsal striatum

- We studied the effects of cocaine in MK and PTN genetically deficient mice.
- Acquisition of cocaine-induced CPP was similar in MK−/−, PTN−/− and wild type mice.
- MK is a novel genetic factor with a role in the extinction of cocaine CPP.
- Phosphorylation of striatal Prdx6 may be involved in the extinction of cocaine CPP.

The neurotrophic factors Midkine (MK) and Pleiotrophin (PTN) have been suggested to modulate drugs of abuse-induced effects. To test this hypothesis, cocaine (10 and 15 mg/kg)-induced conditioned place preference (CPP) was rendered in PTN knockout (PTN−/−), MK knockout (MK−/−) and wild type (WT+/+) mice, and then extinguished after repeated saline injections (distributed in 4 extinction sessions). Cocaine induced a similar CPP in all the three genotypes. We found a significantly increased percentage of MK−/− mice that did not extinguish cocaine CPP at the end of the extinction sessions. Particularly, 40% of MK−/− mice did not extinguish cocaine (15 mg/kg)-induced CPP compared to WT+/+ and PTN−/− mice (∼0-6%). Interestingly, we found that a greater magnitude of extinction of CPP after the first extinction session (5 days after last administration of cocaine) correlates with increased tyrosine phosphorylation of the enzyme peroxiredoxin 6 in the dorsal striatum of MK−/− mice. On the other hand, a greater magnitude of CPP extinction correlates with increased tyrosine phosphorylation of aconitase 2 in the prefrontal cortex of WT+/+ mice. In contrast, a lower magnitude of CPP extinction correlates with increased phosphorylation of aconitase 2 in the prefrontal cortex of PTN−/− mice, suggesting that the correlation between the tyrosine phosphorylation levels of aconitase 2 and magnitude of CPP extinction depends on the genotype considered. The data demonstrate that MK is a novel genetic factor that plays a role in the extinction of cocaine-induced CPP by mechanisms that may involve specific phosphorylation of striatal peroxiredoxin 6.