Increased c-Fos immunoreactivity in anxiety-related brain regions following paroxetine discontinuation

Selective serotonin reuptake inhibitor (SSRI) therapy cessation often induces a disabling discontinuation syndrome, including increased anxiety. We recently reported that SSRI discontinuation induced behavioural changes in mice, which we hypothesise arose from activated anxiety circuitry. Here, we investigated the effect of discontinuation from the SSRI paroxetine on the expression of the activity-dependent gene c-fos in selected anxiety-related midbrain and forebrain regions. Male mice were injected daily with paroxetine (10 mg/kg) or saline for 12 days, then treatment was either continued or discontinued for two or five days. Mice were then tested on the elevated plus maze (EPM) and tissue collected 90 min later. Brain sections including the dorsal (DRN) and median raphe nucleus, periaqueductal grey, hippocampus, prefrontal cortex, and amygdala were processed for c-Fos immunoreactivity. Two days after paroxetine discontinuation, when mice showed elevated anxiety-like behaviour on the EPM, increased c-Fos immunoreactivity was evident in the DRN and ventral hippocampus, but not in any other region examined, compared to saline-treated controls. Increased c-Fos in the DRN was evident in TPH2-immunopositive neurons as well as neurons doubled-labelled for TPH2 and VGLUT3, suggesting activation of 5-HT-glutamate co-releasing neurons. Five days after paroxetine discontinuation, increased c-Fos immunoreactivity was evident in the DRN, but mice no longer exhibited increased anxiety. These findings suggest that, under the current conditions, paroxetine discontinuation is associated with a short-lasting activation of anxiety-promoting circuitry limited to DRN 5-HT neurons and the hippocampus. This circuitry may contribute to symptoms such as anxiety that are a feature of SSRI discontinuation syndrome.