The Effects of Three Types of Stress on Fos Expression in the Hypothalamic Paraventricular Nucleus, Hippocampus and Amygdala in Female Rats at Different Stages of Preg- nancy

Using immunohistochemistry to reveal the Fos protein (a marker of neuronal activation), the present experiments examined whether there were differences in the responses of the paraventricular nucleus (PVN), hippocampus, and amygdala of pregnant rats exposed to three types of stressors (restraint, immobilization, and communication-box stress), all having inherently different severities, at three pregnancy stages (6 days into pregnancy, or P6, early-pregnancy), P12 (mid-pregnancy) and P18 (late-pregnancy). The parvocellular PVN was activated by all three stressors almost equally at all pregnancy stages. The magnocellular PVN appeared to become more active without stressors in mid-pregnancy than in early-pregnancy. The stress responses of the hippocampal dentate gyrus to immobilization and communicationbox stress, and that of the basolateral amygdala to immobilization stress in pregnant females, were greater in mid-pregnancy than at the other pregnancy stages. The magnocellular PVN was more sensitive to communication-box stress in late-pregnancy than at the other pregnancy stages, while the hippocampal CA3 and medial amygdala were less sensitive to communication-box stress and to restraint stress in late-pregnancy than at the other pregnancy stages, respectively. These results suggest that the responses of the PVN, hippocampus, and amygdala in pregnant female rats to different stressors were variable at different pregnancy