Sex differences in effects of mild chronic stress on seizure risk and GABA_A receptors in rats

Abstract

Social stress is a common occurrence in our society that can negatively impact health. Therefore, we wanted to study the effects of a mild stressor designed to model social stress on seizure susceptibility and GABA_A receptors in male and female rats. The mild chronic stress of individual housing consistently decreased bicuculline (but not pentylenetetrazol, PTZ) seizure thresholds by 10–15% in both sexes. Housing conditions did not alter the anticonvulsant activity of diazepam or ethanol, although the anticonvulsant effect of ethanol was significantly greater against PTZ-induced seizures. Experiments testing the addition of an acute restraint stress unmasked sex differences in seizure induction. The acute stress also selectively decreased the potency of GABA to modulate GABA_A receptor-mediated chloride uptake in group-housed females. There were additional sex differences by housing condition for GABA_A receptor-gated chloride uptake but no differences in [³H]flunitrazepam binding. We also found significant effects of sex and housing on ethanol-induced increases in corticosterone (CORT) levels. In summary, there were complex and sex-selective effects of mild chronic stress on seizure induction and GABA_A receptors. Gaining a better understanding of mechanisms underlying interactions between sex and stress has important implications for addressing health concerns about stress in men and women.

Introduction

Stress plays an important, often overlooked, role in human health. Stress can operate at many levels and be acute/chronic or mild/severe, depending on how it is applied as well as by how it is perceived. In humans, stress has been suggested to contribute to risk for seizures (Lancman et al., 1993), heart disease (Kamarck and Jennings, 1991), high blood pressure (Snider and Kuchel, 1983), and emotional disorders, such as depression (Kessler, 1997). Several research laboratories have been investigating the effects of stress in a variety of animal models. Our interest has focused on social stress, as it is the form of stress most commonly experienced by humans in day-to-day living. Social stress generally results from a sense of lack of control over one's environment and interactions.

Also of interest is the increasing evidence that men and women may respond differently to stressors. Recent findings showed that women might react to stressful situations by “tend-and-befriend” behaviors Taylor et al., 2001, Troisi, 2001. In contrast, men may display more of the “fight-and-flight response” when in stressful situations. Sex differences have also been observed in several animal models of stress Blanchard et al., 2001, Brown and Grunberg, 1995, Palanza et al., 2001. For example, Brown and coworkers found that overcrowded living conditions decreased corticosterone (CORT) levels in female rats, whereas this increased CORT levels in male rats, suggesting that crowding stresses males but not females. In female mice, individual housing decreased the tendency to explore (i.e., increased anxiety), whereas in male mice it was group housing that decreased exploratory behaviors (Palanza et al., 2001). These results suggest that even something as simple as housing condition can model social stress and have a sex-selective impact on behaviors.

There is evidence that one's sex may influence basal brain excitability, differentially increasing the risk for harm from stress in males and females. Studies measuring seizure susceptibility found subtle sex differences in basal seizure risk between males and females as well as differences within females across the estrus cycle Finn and Gee, 1994, Kokka et al., 1992, Wilson, 1992. In contrast, we found minimal sex differences in basal bicuculline seizure susceptibility when testing intact females in estrus or diestrus compared with ovariectomized females (Devaud et al., 2000). Studies with humans have found that men and women can display gender differences in expression of epilepsy. Seizure risk and/or severity have been found to vary with the menstrual cycle in some women and this is called catamenial epilepsy (Schachter, 1988).

As levels of ovarian steroids, including progesterone, estradiol, and some of their neuroactive metabolites, fluctuate across the estrus/menstrual cycle, it is possible that they influence basal seizure susceptibility. However, the interplay among being female, risk for seizures, and the influence of stress on this risk has not been explored. Therefore, the aim of the present study was to determine whether use of the mild chronic stressor, individual housing, exerted sex-selective actions on seizure thresholds (seizure susceptibility) in intact male and female rats compared with the grouped, but not crowded, housing condition.

We also investigated the role of GABA_A receptors in these responses as a previous report showed that swim stress preferentially affected GABA_A receptor binding in whole brain from male but not female mice (Akinci and Johnson, 1993). A large number of studies have implicated GABA_A receptors as being involved in responding to stress. We have previously reported sex-selective alterations in seizure risk, GABA_A receptor function and level of expression of several GABA_A receptor subunits in our studies of ethanol dependence and withdrawal Devaud et al., 1996, Devaud et al., 1999, Devaud et al., 2000, Devaud et al., 2003. As ethanol dependence and withdrawal are stressful, observed alterations in GABA_A receptors could include effects of stress. In the present study, we directly tested the effects of a mild type of social stress in male and female rats. Furthermore, we tested the effects of several additional, acute stressors added onto the differential housing conditions. We found interesting and significant sex-selective effects that appeared to depend, at least in part, on the convulsant agent used. We also found significant chronic mild stress-induced differences in GABA_A receptor function between male and female rats.