Effect of microinjecting of 5-aza-2-deoxycytidine into ventrolateral orbital cortex on depressive-like behavior in rats

• Microinjection 5-aza into VLO exerts a depressive-like effect in rat FST.
• Microinjection 5-aza into VLO has no effect on rat locomotion.
• VLO hypomethylation may be involved in pathogenesis of depression.
• DNA methylation on depressive-like behaviors appear to be brain region-dependent.

DNA methylation and histone modification are two major epigenetic mechanisms involved in the pathophysiology of major depressive disorder (MDD) and the action of antidepressants. We and others have recently shown that epigenetic regulation through histone acetylation within ventrolateral orbital cortex (VLO) contributes to the antidepressant-like effects of histone deacetylase inhibitors [HDACi, such as valproic acid (VPA) and MS-275] observed in rats. However, there is so far no investigation focused on the effect of DNA methylation in VLO on depressive-like behaviors. Here, we examined the effects of the DNA methyltransferases (DNMTs) inhibitor 5-aza-2-deoxycytidine (5-aza) on rat forced swimming test (FST) and locomotor activity when microinjected into VLO. We found that bilateral intra-VLO injections of 5-aza increased the duration of immobility in FST in a dose-dependent manner compared to vehicle-treated controls. The effects of 5-aza observed in the FST paradigms could not be attributed to non-specific decreases in activity since the inhibition of DNA methylation in VLO did not cause general impairment in locomotor activity. These results add to the evidence that DNA hypomethylation in VLO is involved in regulating depressive-like behaviors, and suggest that the effect of DNA methylation on depressive-like behaviors appear to be brain region-dependent.