Fear conditioning with five footshocks is sufficient to produce stress-enhanced fear learning in rats but does not produce physiological characteristics of stress.

Stress-enhanced fear learning (SEFL) is widely used in rodent studies to understand how prior experience with stress amplifies subsequent fear conditioning. SEFL procedures using rats often implement prolonged stress exposure (15 unsignaled footshocks) and sex differences are rarely, if ever, observed. Here, we first investigated how a 5-shock contextual fear conditioned procedure (unsignaled footshocks) influences subsequent fear learning, with the hypothesis that females would be more sensitive to SEFL under these conditions following similar work showing female mice are sensitive to SEFL with more mild procedures. 5 inescapable footshocks prior to subsequent fear conditioning with a single context-footshock pairing produced robust enhancement of later contextual freezing, and this effect was observed in both males and females to a similar extent. Further experiments demonstrated that SEFL remained intact even when the initial footshocks were signaled by a cue. While experience with 5 unsignaled footshocks did not increase freezing following a single-trial of delay conditioning, it did enhance freezing to a novel cue, consistent with stress-enhanced fear responding (SEFR). Both 5 and 15 shocks increased expression of the immediate early gene zif268 in several brain regions associated with stress, including the prelimbic and infralimbic cortices, the anterior and posterior retrosplenial cortex, and the paraventricular nucleus. While zif268 expression in the basolateral amygdala was increased following 5 shocks, this was not the case following 15 shocks. Serum corticosterone was selectively increased in females following 15 footshocks, but in neither sex following 5 footshocks. Together, these findings demonstrate that experience with 5 footshocks is sufficient to produce a SEFL-like effect despite not showing physiological changes identical to a 15-footshock stress procedure.