Methamphetamine-induced changes in AP-1 binding and dynorphin in the striatum: correlated, not causally related events?

Activation of D1 dopamine (DA) receptors in the striatum increases the expression of the opioid neuropeptide, dynorphin (DYN). While cAMP is generally accepted as a second messenger in this signal transduction pathway, the role of Fos/FRA proteins and AP-1 binding in mediating D1 receptor-induced changes in DYN expression remains uncertain. In this study, DYN peptide and mRNA levels, as well as Fos/FRA proteins and AP-1 DNA binding activity, were measured in individual animals following acute challenge with the psychostimulant drug methamphetamine (METH). METH caused an initial decrease in striatal levels of DYN A, reflecting increased peptide release. Six hours postinjection, DYN mRNA levels were significantly elevated by METH as compared to vehicle-injected controls. At the same time, these drugs increased the expression of Fos/FRA-ir proteins, in particular the 35- and 40-kDa molecular weight species, and increased binding to the AP-1 DNA element. Analyses of the time course of METH's effects revealed that DYN mRNA levels, Fos/FRA proteins and AP-1 binding activity showed variable increases by 3 h but all were significantly elevated above control levels by 6 h posttreatment. The D1-specific antagonist, SCH 23390, completely blocked the METH-induced changes in DYN peptide and mRNA levels while a D2 receptor antagonist (sulpiride) had little or no effect. These data indicate that stimulant drugs such as METH increase the expression of DYN and AP-1 factors in the striatum via a D1 receptor-mediated mechanism. However, the finding that AP-1 binding merely paralleled, but did not precede, the increase in DYN expression, as would be expected if it were mediating increased gene transcription, suggests these may be correlative, not causally related events.