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.

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