Region-specific reduction in stress-induced c-fos mRNA expression during pregnancy and lactation

Hypothalamo-pituitary-adrenal (HPA) responses to stress are dramatically attenuated during lactation. To examine whether this is due to diminished stress-induced activation of specific areas of the brain involved in HPA responses, c-fos mRNA expression was employed as a marker of stress-induced neuronal activation. Regional levels of expression were quantified in female rats exposed to 30 min immobilisation stress during late pregnancy (days 19-21), early lactation (days 3-4) and mid-lactation (days 10-14), and compared with the levels in virgin females. Stress-induced levels of corticosterone were significantly lower in late pregnant and early lactating rats compared with the levels in virgin females, and this correlated with a marked attenuation of stress-induced c-fos mRNA expression in the parvocellular division of the PVN. This reduced activation suggests that neuroendocrine hyporesponsiveness during lactation may arise from an effect on afferent pathways to the PVN. Extrahypothalamic areas known to be important for HPA activation displayed three patterns of c-fos mRNA expression: (i) in the ventral tegmental area, dorsal vagal complex, pyriform cortex and all areas of the hippocampus (CA1, CA2, CA3, dentate gyrus), expression levels did not vary significantly with reproductive status; (ii) in the locus coeruleus (A6 catecholaminergic group), a peak of expression was detected in late pregnant animals; and (iii) in the medial amygdala, ventral part of the lateral septum and cingulate cortex expression was significantly reduced in pregnant and lactating animals, with a nadir in early lactation. The decreased expression of c-fos mRNA in these latter areas correlated with that in the parvocellular PVN, and suggests that their interaction may contribute to the reduced neuroendocrine responses of lactating rats.