Systemic endotoxin induces gene expression of inducible nitric oxide synthase in fetal rat brain

Few studies have examined the response of the fetus under stress, such as with maternal infection. Recent work has indicated that nitric oxide (NO) modulates corticotropin-releasing hormone (CRH) secretion by the hypothalamus, but details of the action of NO on the fetus remain unclear. Therefore, we investigated the expression of inducible nitric oxide synthase (iNOS) mRNA and the response pattern following lipopolysaccharide (LPS) loading using a rat model of fetal infection.

Fetuses were delivered by cesarean section on day 20 of gestation and immediately placed in a chamber maintained at 37 degrees C and 100% relative humidity. The LPS group (n=12) was given 400 microg of LPS/100 g body weight, and the physiologic saline group (n=12) was given physiologic saline. Fetuses were then incubated for a further 3 hours. Fetuses were decapitated, the trunk blood was collected immediately after cesarean section or after 3 hours of incubation, and the fetal brains were fixed in formaldehyde and cryopreserved. Coronal cryosections of the brains were prepared, and a (35)S-uridine triphosphate-labeled antisense RNA probe for iNOS was then prepared. In situ hybridization was performed, and iNOS expression was evaluated semiquantitatively on the basis of optical density. In both groups, plasma corticosterone levels were determined with radioimmunoassay.

Expression of iNOS mRNA was not noted in the physiologic saline group (3 hours postpartum). In the LPS group, iNOS mRNA expression was observed in the subfornical organ, but not in the paraventricular nucleus. Plasma corticosterone levels were significantly elevated in the LPS group.

In 20-day-old rat fetuses, the hypothalamic-pituitary-adrenal axis was already mobilized in response to LPS-induced stress. These results suggest that iNOS is not involved in the acute response of the hypothalamic-pituitary-adrenal axis to LPS challenge in 20-day-old rat fetuses.