Impaired fear extinction in serotonin transporter knockout rats is associated with increased 5-hydroxymethylcytosine in the amygdala

Aims

One potential risk factor for posttraumatic stress disorder (PTSD) involves the low activity (short; s) allelic variant of the serotonin transporter‐linked polymorphic region (5‐HTTLPR), possibly due to reduced prefrontal control over the amygdala. Evidence shows that DNA methylation/demethylation is crucial for fear extinction in these brain areas and is associated with neuronal activation marker c‐Fos expression. We hypothesized that impaired fear extinction in serotonin transporter knockout (5‐HTT^−/−) rats is related to changes in DNA (de) methylation and c‐Fos expression in the prefrontal cortex (PFC) and/or amygdala.

Methods

5‐HTT^−/− and 5‐HTT^+/+ rats were subjected to fear extinction. 2 hours after the extinction session, the overall levels of DNA methylation (5‐mC), demethylation (5‐hmC), and c‐Fos in fear extinction and nonfear extinction rats were measured by immunohistochemistry.

Results

5‐HTT^−/− rats displayed decreased fear extinction. This was associated with reduced c‐Fos activity in the infralimbic PFC. In the central nucleus of the amygdala, c‐Fos immunoreactivity was increased in the fear extinction group compared to the no‐fear extinction group, regardless of genotype. 5‐hmC levels were unaltered in the PFC, but reduced in the amygdala of nonextinction 5‐HTT^−/− rats compared to nonextinction wild‐type rats, which caught up to wild‐type levels during fear extinction. 5‐mC levels were stable in central amygdala in both wild‐type and 5‐HTT^−/− extinction rats. Finally, c‐Fos and 5‐mC levels were correlated with the prelimbic PFC, but not amygdala.

Conclusions

In the amygdala, DNA demethylation, independent from c‐Fos activation, may contribute to individual differences in risk for PTSD, as conferred by the 5‐HTTLPR s‐allele.