Background Exposure to uncontrollable stressors often increases anxiety-like behavior in both humans and rodents. In rat, this effect depends upon stress-induced activity within the dorsal raphé nucleus (DRN). However, the role of serotonin in DRN projection regions is largely unknown. The goals of the current study were to 1) determine if DRN activity during a post-stress anxiety test is involved in anxiety-like behavior, 2) assess the effect of uncontrollable stress on extracellular serotonin in the basolateral amygdala during the anxiety test, and 3) determine the role of the serotonin 2C receptor (5-HT2C) in uncontrollable stress-induced anxiety. Method Rats were exposed to tailshocks that were uncontrollable. On the following day anxiety-like behavior was assessed in a JSE test. BLA extracellular serotonin concentrations were assessed during JSE by in vivo microdialysis 24 h after uncontrollable stress, controllable stress or no stress. In separate experiments drugs were administered before the JSE test to inhibit the DRN or to block 5-HT2C receptors. Results Exposure to uncontrollable shock reduced later social exploration. Prior uncontrollable stress potentiated serotonin efflux in the BLA during social exploration, but controllable stress did not. Intra-DRN 8-OH-DPAT and systemic and intra-BLA 5-HT2C receptor antagonist SB 242084 prevented the expression of potentiated anxiety in uncontrollably stressed rats. Intra-BLA injection of the 5-HT2C agonist CP 809101 mimicked the effect of stress. Conclusion These results suggest that the anxiety-like behavior observed after uncontrollable stress is mediated by exaggerated 5-HT acting at BLA 5-HT2C receptors.

BACKGROUND Safety signals exert a powerful buffering effect when provided during exposure to uncontrollable stressors. We evaluated the role of the sensory insular cortex (Si) and the extend amygdala in this “safety signal effect.” METHODS Rats were implanted with microinjection cannula, exposed to inescapable tailshocks either with or without a safety signal and later tested for anxiety-like behavior or neuronal Fos expression. RESULTS Exposure to the uncontrollable stressor reduced later social exploration, but not when safety signals were present. Temporary inhibition of Si during stressor exposure, but not during later behavioral testing, blocked the safety signal effect on social exploration. The stressor induced Fos in all regions of the amygdala, but safety signals significantly reduced the number of Fos immunoreactive cells in the basolateral amygdala and ventrolateral region of the bed nucleus of the stria terminalis (BNSTlv). Inhibition of BNSTlv neuronal activity during uncontrollable stressor exposure prevented the later reduction in social exploration. Finally, safety signals reduced the time spent freezing during uncontrollable stress. CONCLUSIONS These data suggest that safety signals inhibit the neural fear or anxiety response that normally occurs during uncontrollable stressors and that inhibition of the BNSTlv is sufficient to prevent later anxiety. These data lend support to a growing body of evidence that chronic fear is mediated in the basolateral amygdala and BNSTlv and that environmental factors that modulate fear during stress will alter the long-term consequences of the stressor.