IL1A and post-traumatic stress disorder: The causal relationship between IL-1 and PTSD was suggested by one mechanistic study, which found that in the stress-enhanced fear learning model of PTSD, IL-1 signaling is required for the increased fear response which was reduced by IL-1ra or by morphine treatment that reduced IL-1β expression in the hippocampus (Jones et al., 2018; Szczytkowski-Thomson et al., 2013).