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

Kathleen S. Curtis, J. Thomas Cunningham, and Cheryl M. Heesch
Am. J. Physiol. 277 (1999) 

Pregnancy in humans and other animals may cause changes in baroreflex function. Specifically, the ability to increase sympathetic nerve activity (SNA) in response to a hypotensive challenge appears to be decreased during pregnancy. This could be due to the fact that the control of sympathetic outflow is altered by hormonal changes associated with pregnancy. 3a-OH-DHP (reproductive hormone) has been shown to increase the activity of inhibitory GABAergic pathways that play an important role in the baroreflex. When 3a-OH-DHP was injected into the RVLM of non-pregnant rats, they showed inability to increase SNA during hypotensive challenges, similar to what is seen in pregnant rats. This attenuated increase in SNA in response to a hypotensive challenge during pregnancy may indicate that there are differences in the central processing of information related to control of sympathetic outflow.

This study examined Fos immunoreactivity in pregnant rats after acute decreases in blood pressure to determine whether the activation of brain stem areas known to be involved in the central control of SNA is altered by pregnancy. Immunolabeling for dopamine b-hydroxylase (DBH) was also performed to determine whether pregnancy differentially affected the activity of catecholamine-containing neurons in response to acute decreases in blood pressure. To test these points pregnant and virgin rats were injected with either hydralazine (HDZ; induces hypotension) or an isotonic saline vehicle (VEH; control) and compared.

Among other things, this study found that Fos expression in the RVLM of HDZ-treated pregnant rats was less than that of HDZ-treated virgin female rats, and the difference was specific to noncatecholaminergic neurons (DBH negative). Additional experiments are necessary to determine the phenotype and projections of the neurons affected by pregnancy. The attenuated SNA in HDZ-treated pregnant rats could be attributed to reproductive hormones augmenting GABAergic inhibitory pathways, or attenuating ANG II-mediated excitatory pathways.

- Ben Huber