Fos expression in brain stem nuclei of pregnant rats after hydralazine-induced hypotension

Fos and dopamine beta-hydroxylase immunoreactivity were evaluated in the br ain stems of 21-day pregnant and virgin female rats injected with either hy dralazine (HDZ; 10 mg/kg iv) or vehicle. KDZ produced significant hypotensi on in both groups, although baseline blood pressure was lower in pregnant r ats (96 +/- 2.5 mmHg) than in virgin female rats (121 +/- 2.8 mmHg). There mere no differences in Fos immunoreactivity in the brain stems of pregnant and virgin female rats after vehicle treatment. HDZ-induced hypotension sig nificantly increased Fos expression in both groups; ho however, the magnitu de of the increases differed in the caudal ventrolateral medulla (CVL), the area postrema (AP), and the rostral ventrolateral medulla (RVL). Fos expre ssion after HDZ in pregnant rats was augmented in noncatecholaminergic neur ons of the CVL but was attenuated in the AP and in noncatecholaminergic neu rons in the RVL. These results are consistent with differences in the sympa thetic response to hypotension between pregnant and virgin female rats and indicate that the central response to hypotension may be different in pregn ant rats.