The Contribution of Normal Pregnancy to Eclampsia

Abstract

Eclampsia, clinically defined as unexplained seizure in a woman with preeclampsia, is a life threatening complication unique to the pregnant state. However, a subpopulation of women with seemingly uncomplicated pregnancies experience de novo seizure without preeclamptic signs or symptoms, suggesting pregnancy alone may predispose the brain to seizure. Here, we hypothesized that normal pregnancy lowers seizure threshold and investigated mechanisms by which pregnancy may affect seizure susceptibility, including neuroinflammation and plasticity of gamma-aminobutyric acid type A receptor (GABAAR) subunit expression. Seizure threshold was determined by quantifying the amount of pentylenetetrazole (PTZ) required to elicit electrical seizure in Sprague Dawley rats that were either nonpregnant (Nonpreg, n = 7) or pregnant (Preg; d20, n = 6). Seizure-induced vasogenic edema was also measured. Further, activation of microglia, a measure of neuroinflammation (n = 6-8/group), and GABAAR δ- and γ2-subunit protein expression in the cerebral cortex and hippocampus (n = 6/group) was determined. Seizure threshold was lower in Preg compared to Nonpreg rats (36.7±9.6 vs. 65.0±14.5 mg/kg PTZ; p<0.01) that was associated with greater vasogenic edema formation (78.55±0.11 vs. 78.04±0.19% water; p<0.05). The % of active microglia was similar between groups; however, pregnancy was associated with downregulation of cortical GABAAR-δ and hippocampal GABAAR-γ2 expression. Overall, pregnancy appears to be a state of increased seizure susceptibility that is not due to neuroinflammation, but rather is associated with reduced expression of GABAAR subunits and greater edema. Understanding neurophysiological changes occurring in normal pregnancy could allow for better prevention and management of de novo seizure, including pathologic states such as eclampsia.

Fig 1. The effect of normal pregnancy on seizure threshold and susceptibility.

(A) Representative EEG tracings from nonpregnant (Nonpreg) and late-pregnant (Preg) rats during timed-infusion of pentylenetetrazol (PTZ). Black arrows indicate when PTZ infusion started and the onset of spike-wave discharge indicative of electrical seizure. (B) Seizure threshold was significantly lower in Preg rats compared to Nonpreg rats. (C) Preg rats scored higher on the seizure susceptibility scale compared to Nonpreg rats. ** p < 0.01 vs. Nonpreg by Student’s t-test.

Fig 2. The effect of pregnancy on seizure severity.

(A) Representative EEG tracing of a recurrent seizure during the 30-minute post-pentylenetetrazol (PTZ) infusion time period of a nonpregnant (Nonpreg) rat. Left black arrow indicates seizure onset and right black arrow seizure cessation. (B) Number of recurrent seizures in the 30-minute post-PTZ infusion period was similar between Nonpreg and late-pregnant (Preg) rats. (C) The percent of time spent in seizure during the 30-minute post-PTZ time period was similar between Nonpreg and Preg rats.

Fig 3. Basal activation state of microglia in cerebral cortex of nonpregnant (Nonpreg) and pregnant (Preg) rats.

(A) Representative photomicrographs of Iba 1⁺ microglia in the cerebral cortices of Nonpreg and Preg rats. (B) There was no difference in the number of microglia in the cerebral cortices of Nonpreg and Preg rats. (C) The percent of Iba 1⁺ microglial cells in each activation state was similar in the cortices of Nonpreg and Preg rats.

Fig 4. The effect of pregnancy on GABA_AR δ- and γ2-subunit protein expression in the hippocampus and cerebral cortex.

(A) Representative Western blots showing protein expression of the GABA_AR γ2-subunit and δ-subunit in the cerebral cortex of nonpregnant (Nonpreg) and pregnant (Preg) rats. (B) GABA_AR γ2-subunit protein expression was significantly lower in the hippocampus from Preg compared to Nonpreg rats, and GABA_AR δ-subunit expression trended towards being decreased in the hippocampus of Preg compared to Nonpreg rats. (C) Cortical GABA_AR γ2-subunit expression was similar in Preg and Nonpreg rats, however, GABA_AR δ-subunit expression was decreased in the cerebral cortex of Preg compared to Nonpreg rats. ** p < 0.01; * p < 0.05 vs. Nonpreg using Student’s t-test.

Fig 5. The effect of seizure on vasogenic edema formation in nonpregnant (Nonpreg) and late-pregnant (Preg) rats.

Percent water content of the posterior cerebral cortex was significantly higher after seizure in Preg compared to Nonpreg rats. * p < 0.05 vs. Nonpreg using Student’s t-test.