Effects of Hypoxia or Dexamethasone at Different Gestation Periods on Fear Conditioning in Adult Rats

Abstract

We report here the long-term effects of deep hypobaric hypoxia or dexamethasone administration (0.8 mg/kg) endured at the early prenatal period of gestation (days 14–16 or 17–19) on contextual and cued fear conditioning in postnatal ontogenesis of male rats. Fear responses conditioned to a context or a cue (tone) were comparatively evaluated in the male offspring of rats exposed to hypoxia or injected with dexamethasone at different gestational ages. Hypoxic exposures at any period of early prenatal ontogenesis were found to entail no statistically significant differences in the offspring relative to control. During contextual fear conditioning or extinction, the freezing time increased relative to control in animals born to females that were injected with dexamethasone on gestational days 17–19. In an analogous test on fear conditioning to a tone, the freezing time decreased relative to control in rats born to females injected with dexamethasone on gestational days 14–16 and increased in those whose mothers were injected with dexamethasone on days 17–19. We suggest that effects induced by dexamethasone administration at both gestation periods were mainly due to the involvement of the amygdala nuclei. The effect of dexamethasone on contextual fear conditioning, which seems to be due to functional changes in the hippocampus, was only detected upon its injection on gestational days 17–19.