A corticolimbic circuit including the amygdala and medial prefrontal cortex (mPFC) affects sensitivity to threat, related aspects of personality and risk for psychopathology. Serotonin (5‐HT) is a potent neuromodulator of this circuit, however, 5‐HT receptors mediating these effects and genetic sources of variability in 5‐HT receptor availability are not understood. We determined the association between 5‐HT1A and 5‐HT2A binding and the response to threat within this corticolimbic circuit using a multimodal neuroimaging strategy in humans in vivo. Corticolimbic circuit function was assessed with a threat‐related faces matching paradigm using functional magnetic resonance imaging (fMRI). Regional 5‐HT1A and 5‐HT2A binding was assessed with [11C]WAY100635 and [18F]altanserin PET, respectively. We evaluated the association between receptor binding and common polymorphisms (rs6295, rs6311 and 5‐HTTLPR) in 5‐HT related genes.In Study 1 we found that 5‐HT1A binding within the dorsal raphe nucleus was inversely associated with threat‐related amygdala reactivity. This is consistent with 5‐HT1A autoreceptors negatively regulating 5‐HT release, which within the amygdalapotentiates its response to threat. In Study 2 we found that mPFC 5‐HT2A binding was inversely associated with threat‐related amygdala reactivity and positively associated with amygdala habituation and amygdala‐mPFC functional connectivity. In Study 3 we found that mPFC 5‐HT1A binding significantly moderated the inverse association between mPFC 5‐HT2A binding and amygdala reactivity.These findings are consistent with the co‐localization of 5‐HT1A and 5‐HT2A on glutamatergic neurons within mPFC indicating the 5‐HT2A receptor is localized to facilitate regulation of the amygdala and the 5‐HT1A receptor is localized to moderate its effects within mPFC. In Study 4 we found that 5‐HTTLPR genotype predicted 5‐HT1A and 5‐HT2A binding in brain regions within this circuit such that the S and LG alleles were associated with reduced 5‐HT1A and 5‐HT2A binding.These findings provide novel insight into mechanisms that mediate the effects of 5‐HT signaling on the response to threat of a key corticolimbic circuit in humans. Our findings indicate that 5‐HT1A and 5‐HT2A receptors contribute significantly to threat‐related corticolimbic circuit function in humans. Furthermore, the 5‐HTTLPR may contribute to individual variability in neural and behavioral sensitivity to threat by biasing 5‐HT1A and 5‐HT2A availability
