Serotonin 2C receptors within the basolateral amygdala induce acute fear-like responses in an open-field environment.

Several studies indicate a role for the serotonin 2 subfamily (5-HT(2A), 5-HT(2B), 5-HT(2C)) in mediation of fear and anxiety responses. The current study began to examine the effects of stimulating 5-HT(2C) receptors within the basolateral (BLA) or central nucleus of the amygdala (CeA) on acute fear-like responses in rats. Bilateral intra-BLA infusions of mCPP (3-3000 pmol), a mixed 5-HT(2) agonist, produced ultrasonic vocalization and reduced exploratory behavior while increasing the latency to investigate a novel object. These responses were attenuated by SB-242084, a 5-HT(2C)-specific antagonist. Furthermore, a selective 5-HT(2C) agonist, IL-639, produced a similar repertoire of behavioral effects with the exception of effects on time spent in the center of an open-field arena. Finally, infusions of mCPP into the CeA produced no anxiogenic behaviors suggesting that 5-HT(2C) receptors primarily within the BLA are responsible for many of the acute fear-like responses reported here. To characterize further the neural circuits associated with 5-HT(2C)-mediated fear responses, we assessed c-fos mRNA expression after intra-BLA infusions of mCPP, IL-639 or their vehicles. Whereas the overall regional pattern of c-fos mRNA induction for the two compounds was distinct, c-fos activation was confined primarily to limbic nuclei with the medial prefrontal cortex as a common regional target of each drug. These results suggest that activation of 5-HT(2C) receptors within the BLA influences the activity of limbic circuits involved in the expression of acute innate fear responses.