Sex differences in sensitivity to seizures elicited by pentylenetetrazol in mice

Abstract

Sex differences in sensitivity to seizures elicited by intraperitoneally injected pentylenetetrazol (PTZ) were studied in 240 (120 males and 120 females) adult Swiss mice. Animals were separated into four groups according to the dose that was injected: 40, 50, 60 and 70 mg/kg. Seizure severity was expressed by the following scoring scale: (0) no abnormal behavior; (1) myoclonus; (2) running bouncing (RB) clonus; (3) tonic hind limb extension (THE). The analyses of the dose–response curves indicated that females were more susceptible than males when the 50- and 60-mg/kg doses were used. Specifically, females often displayed RB clonus, while males frequently displayed only myoclonus or no abnormal behavior. No significant sex differences were demonstrated when either the 40- or the 70-mg/kg doses were used. These data indicate that, for a specific range of doses, sex differences in seizure susceptibility can be clearly demonstrated with the use of intraperitoneally injected PTZ. In this sense, this method could be used as a tool to investigate the role played by sexual hormones in regulating the sensitivity of the γ-aminobutiric acid (GABA_A) receptor complex (GRC).

Introduction

Sex differences in behavior have been extensively demonstrated both with Jung et al., 1999, Pericic et al., 1986 and without (Schmidt et al., 1999) the use of drugs as part of the experimental design. The former can be illustrated by dependence to alcohol (ethanol) in both humans (McGue et al., 1997) and animals (Lancaster et al., 1996). Ethanol acts on the γ-aminobutiric acid (GABA_A) receptor complex (GRC) (Crews et al., 1996), which is also known to be the site of action of many convulsants, such as picrotoxin, bicuculline, methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM) and pentylenetetrazol (PTZ) Macdonald and Olsen, 1994, Olsen, 1981.

The effects of gender and sexual hormones on incidence and severity of seizures are evident in humans. Such effects can be illustrated by the influence of the estral cycle on seizures in women (Backstrom et al., 1990). In addition, some types of epilepsy are clearly affected by gender. For instance, it is well known that childhood absence epilepsy affects more girls than boys (Holmes et al., 1987). On the other hand, epilepsy with myoclonic absences affects more boys than girls (Commission on Classification and Terminology of the International League Against Epilepsy, 1989). Several lines of research now point to the fact that seizure susceptibility is intimately related to the modulation of the activity of the GRC by sexual hormones Finn and Gee, 1994, Pericic et al., 1996, Wilson, 1992.

The study of sex differences in animal models of epilepsy in which the chemoconvulsant acts on the GRC is complicated by the fact that the aforementioned receptor presents distinct binding sites for different convulsants Macdonald and Olsen, 1994, Olsen, 1981. In fact, several reports suggested that each binding site of the GRC could be affected by sex in a way that favors males in some cases and females in others. For example, Pericic et al. (1986), using both rats and mice, have studied sex and species differences with GABA_A-related convulsants, such as bicuculline and picrotoxin. These authors demonstrated that female mice are more sensitive to intraperitoneal administration of bicuculline, while males are more sensitive to intraperitoneal administration of picrotoxin. On the other hand, male and female rats were equally sensitive to bicuculline, while female rats were more sensitive to picrotoxin.

PTZ, which also acts on the GRC Macdonald and Olsen, 1994, Olsen, 1981, is the most frequently used chemoconvulsant in experimental models of epilepsy (Löscher et al., 1991). However, studies concerning sex effect in response to PTZ specifically used rats in their experimental design. For instance, Kokka et al. (1992) reported “a small but significant sex difference” in intravenously injected PTZ seizure threshold, males being considered more susceptible than females. On the other hand, using the same route of injection, Finn and Gee (1994) showed that male rats had a similar response to females and, additionally, these authors demonstrated that the estrus cycle did not significantly affect susceptibility to PTZ. Likewise, Kubová et al. (1993), using PTZ (100-mg/kg dose) subcutaneously injected, did not observe a significant difference between sexes.

To our knowledge, sex differences in sensitivity to PTZ have not been systematically studied in mice. The high genetic variability of the mouse allows the development of many epilepsy-prone and epilepsy-resistant strains Kosobud and Crabbe, 1990, Löscher and Schmidt, 1988. Furthermore, several Mendelian traits have been related to susceptibility to seizures, indicating that genetic factors play an important role in seizure disorders Kosobud and Crabbe, 1990, Puranam and McNamara, 1999. In addition, the investigation of sex differences in response to chemoconvulsants in rodents can be used to understand how sexual hormones and brain sexual dimorphisms influence seizure susceptibility. Here, we study the effect of sex on the susceptibility to PTZ seizures in the mice.