Reduced stress responsiveness in pregnancy: relationship with pattern of forebrain c-fos mRNA expression

Abstract

Stress-induced activation of the hypothalamo-pituitary-adrenal (HPA) axis is known to be attenuated during late pregnancy and throughout lactation. To investigate the neural basis of this stress hyporesponsiveness we examined the changes in the restraint-induced HPA response and accompanying forebrain expression of c-fos mRNA that occur in rats between days 16 (D16) and 19 (D19) of gestation, times associated with declining levels of progesterone, a potential mediating factor. Compared to D16, the D19 group showed a significantly attenuated release of ACTH following 30min restraint. This reduced HPA response was accompanied by significantly lower levels of restraint-induced c-fos mRNA expression in the hypothalamic paraventricular nucleus. Other areas of the forebrain, including medial amygdala, piriform cortex, and ventrolateral septum, showed low c-fos mRNA expression in non-stressed (control) animals and a large increase following restraint, the magnitude of which was similar between D16 and D19 animals indicating no involvement in the differential HPA response to stress. However, a markedly different pattern of c-fos mRNA expression was observed in other brain areas, including barrel cortex and CA1 ventral and CA3 regions of the ventral hippocampus: D19 animals had low control expression which was increased by restraint, but D16 control animals had raised c-fos mRNA expression which was not further elevated by stress. These data demonstrate that region-specific changes in basal and stress-induced cellular activity occur during a period of late gestation coincident with attenuated HPA responsiveness. These changes in neuronal activity may contribute to the adaptive processes that prepare the mother for parturition and lactation.

Fig. 1

HPA and PVN activation by restraint stress. (A) Mean (± SEM) plasma ACTH concentrations in control animals (open bars) or following a 30 min period of restraint (closed bars) at day 16 (D16) and day 19 (D19) of gestation. (B) Mean (± SE, n = 7–8) integrated optical density (OD) measurements of c-fos mRNA expression in the PVN of control animals (open bars) and animals following a 30 min restraint (closed bars). *significantly different by post-hoc Tukey's test, P < 0.05. (C) Representative autoradiograms showing the hybridized c-fos mRNA signal throughout frontal sections at the level of the PVN from non-restrained (i) and restrained (ii) rats at day 16 of pregnancy, and from non-restrained (iii) and restrained (iv) rats at day 19 of pregnancy. Arrows indicate the positions of the PVN. Note that in all cases the presence of the PVN was confirmed after staining with cresyl violet.

Fig. 2

Autoradiograms showing the hybridized c-fos mRNA signal throughout forebrain sections at the level of the ventrolateral septum (LSV) (A) and ventral hippocampus (B). Sections are from non-restrained (i) and restrained (ii) rats at day 16 (D16) of pregnancy and from non-restrained (iii) and restrained (iv) rats at day 19 (D19) of pregnancy. BC, barrel cortex, PC, piriform cortex.

Fig. 3

Optical density (OD) measurements of c-fos mRNA expression within layer V of the cortex (barrel cortex, A), and the CA1 ventral (B) and CA2 (C) subfields of the ventral hippocampus of control animals (open bars) or animals subjected to a 30-min period of restraint (closed bars) on day 16 (D16) or day 19 (D19) of gestation. Values are the mean ± SE of integrated optical density measurements (n = 7–8). *Significantly different by t-test, P < 0.05. Statistical values show the effect of stage of pregnancy (two-way ANOVA).

Fig. 4

Optical density (OD) measurements of c-fos mRNA expression within the piriform cortex (A), ventrolateral septum (B), medial amygdala (C), and principal nucleus of the bed nucleus of the stria terminalis (D) of control animals (open bars) or animals subjected to a 30-min period of restraint (closed bars) on day 16 (D16) or day 19 (D19) of gestation. Values are the mean ± SE of integrated optical density measurements (n = 5–8).