CRF 1 and CRF 2 receptors are required for potentiated startle to contextual but not discrete cues

Corticotropin-releasing factor (CRF) peptides and their receptors have crucial roles in behavioral and endocrine responses to stress. Dysregulation of CRF signaling has been linked to post-traumatic stress disorder, which is associated with increased startle reactivity in response to threat. Thus, understanding the mechanisms underlying CRF regulation of startle may identify pathways involved in this disorder. Here, we tested the hypothesis that both CRF 1 and CRF 2 receptors contribute to fear-induced increases in startle. Startle responses of wild type (WT) and mice with null mutations (knockout, KO) for CRF 1 or CRF 2 receptor genes were measured immediately after footshock (shock sensitization) or in the presence of cues previously associated with footshock (ie fear-potentiated startle, FPS). WT mice exhibited robust increases in startle immediately after footshock, which was dependent upon contextual cues. This effect was completely absent in CRF 1 KO mice, and significantly attenuated in CRF 2 KO mice. In contrast, CRF 1 and CRF 2 KO mice exhibited normal potentiation of startle by discrete conditioned cues. Blockade of both receptors via CRF 1 receptor antagonist treatment in CRF 2 KO mice also had no effect on FPS. These results support an additive model of CRF 1 and CRF 2 receptor activation effects on potentiated startle. These data also indicate that both CRF receptor subtypes contribute to contextual fear but are not required for discrete cued fear effects on startle reactivity. Thus, we suggest that either CRF 1 or CRF 2 could contribute to the increased startle observed in anxiety disorders with CRF system abnormalities.