The dopamine D₁ receptor agonist, but not the D₂ receptor agonist, induces gene expression of Homer 1a in rat striatum and nucleus accumbens

Stimulation of dopamine receptors may induce striatal Homer 1a, an immediate-early gene (IEG) that is involved in the molecular mechanism for the signaling pathway of the group I metabotropic glutamate receptors. This study examined the effects of the agonists for dopamine D₁-like and D₂-like receptors on gene expression of Homer 1a, in comparison with the IEG c-fos expression, in the discrete brain regions of rats. The D₁-like agonist SKF38393 (20 mg/kg, s.c.) significantly increased the mRNA levels of Homer 1a in the striatum and nucleus accumbens, but not in the medial prefrontal cortex or hippocampus, 2 h after injection, whereas the D₂-like agonist quinpirole (1 mg/kg, s.c.) had no significant effect on Homer 1a mRNA levels in any brain region examined. Co-administration of SKF38393 and quinpirole significantly increased Homer 1a mRNA levels in the striatum, nucleus accumbens and hippocampus, while this effect was not significantly greater than that of SKF38393 alone. Any treatment did not affect the mRNA levels of other splicing variants, Homer 1b or 1c. In contrast, combination of both dopamine agonists produced a greater increase than SKF38393 did in the mRNA levels of c-fos in the nucleus accumbens, striatum and substantia nigra. These results suggest that stimulation of D₁-like receptors, but not D₂-like receptors, may induce gene expression of Homer 1a in the striatum and nucleus accumbens. However, in contrast to c-fos expression, it is unlikely that co-activation of both D₁-like and D₂-like receptors exerts a synergic action on Homer 1a expression in these regions.