Influence of estrous cycle stage on acquisition and expression of fear conditioning in female rats.

It is unclear whether all animal models of anxiety-like states developed using males are appropriate for use in females. In females, tests involving a learning component might be influenced not only by estrous cycle stage on the test day but also by the stage during the conditioning process. We used two tests - conditioned freezing (CF) and fear potentiated startle (FPS) to compare responsiveness of male rats and females conditioned and/or tested in proestrus (P) or late diestrus (LD). For CF all rats displayed a similar freezing response regardless of sex or estrous cycle stage. In terms of FPS, males and females conditioned in P and tested in P or LD, and females conditioned in LD and tested in LD all showed potentiated startle. The response waned during the test session in males and in females conditioned in P, but not in those conditioned in LD. In contrast, FPS was not apparent in the first half of the test session in females conditioned in LD and tested in P but developed in the second half. We suggest that fear learning during P and LD is robust but may be initially be obscured in rats tested in P because of generalization to the CS due to high estrogen. Estrous cycle stage is an important consideration which must be taken into account in designing behavioural tests in females.