Sex differences and estrous cycle effects on foreground contextual fear conditioning

- Male and female rats were tested in paired and explicitly unpaired fear conditioning.
- Sex hormones influenced primarily unpaired conditioned responses in females.
- BDNF protein levels differed by sex but not by fear conditioning procedure.

Previous reports have suggested that the female estrous cycle modulates fear responses to contextual and discrete cue stimuli following fear conditioning, but conflicting results have been obtained across studies. We assessed sex differences between males and naturally cycling female rats using fear conditioning procedures that involve learning about a discrete cue (tone conditioned stimulus, CS) and either background (explicitly paired) or foreground (explicitly unpaired) contextual conditioning. We measured brain derived neurotrophic factor (BDNF) expression in amygdala, hippocampus, and medial prefrontal cortex, brain regions implicated in the fear circuitry. When comparing males to females without considering the estrous cycle phase, sex differences were found in unpaired conditioning only; females showed significantly less freezing than males during acquisition. Significant differences in unpaired but not paired conditioning were found between subgroups of females depending on the phase of the estrous cycle. Female rats that underwent unpaired conditioning in estrus showed significantly less freezing during acquisition and cued recall than males. In contrast, contextual recall in females that received unpaired training was not affected by estrous cycle phase. BDNF protein expression remained unchanged by fear conditioning but overall males expressed higher levels in all brain regions examined compared to females. Together, these results show that sex differences in fear behaviors following foreground contextual conditioning depend on the phase of the estrous cycle. These results are discussed in light of previous reports of sex and estrous cycle effects on fear learning and BDNF.