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Summary: Purpose: Progestins can have profound effects on seizure processes. However, the effects and mechanisms of progestins to modulate seizures have not been systematically investigated. The present studies were designed to characterize the effects of progestins to modulate pentylenetetrazole (PTZ)-induced seizures in female rats.
Methods: In Experiment 1, ictal activity and plasma and hippocampal progesterone (P) and 5α-pregnan-3α-ol-20-one (3α,5α-THP) levels of proestrous rats were compared with those of diestrous and ovariectomized (ovx) rats. Experiments 2 and 3 examined effects of ovx and replacement with vehicle, P, or 3α,5α-THP, systemically (Experiment 2) or to the hippocampus (Experiment 3) on seizures and plasma and hippocampal P and 3α,5α-THP concentrations.
Results: Proestrous rats had reduced ictal activity and increased levels of P and 3α,5α-THP in plasma and hippocampus compared with diestrous or ovx rats (Experiment 1). Rats administered systemic P or 3α,5α-THP had significantly reduced ictal activity and increased plasma and hippocampal P and 3α,5α-THP levels compared with vehicle-administered rats (Experiment 2). Administration of P or 3α,5α-THP to the hippocampus of ovx rats significantly reduced seizure activity and increased hippocampal, but not plasma, levels of P and 3α,5α-THP compared with vehicle administration (Experiment 3).
Conclusions: Together, these data suggest that P can have antiseizure effects, and these effects may be due in part to actions of its metabolite, 3α,5α-THP, in the hippocampus.
Progesterone (P) and its 5α-reduced metabolite, 5α-pregnan-3α-ol-20-one (3α,5α-THP), have demonstrated antiseizure effects in some, but not all, reports (1). First, endogenous variations exist in seizure susceptibility of women and rodents, such that seizures are typically decreased during phases of the cycle when progestin levels are high compared with when they are low (2–7). Second, P or 3α,5α-THP administration to women with epilepsy, or to female rodents in seizure models, generally decreases ictal activity (8–16). Third, inhibiting the formation of 3α,5α-THP, by coadministering finasteride, a 5α-reductase inhibitor, with P increases seizures in people and animal models (3,10,17–19). Thus progestins may modulate seizures of women and rodents.
The antiseizure effects of progestins may be related to the formation of 3α,5α-THP in brain areas such as the hippocampus. The hippocampus is a target of progestins' actions. For example, P can protect the hippocampus from adrenalectomy (ADX)-induced neurodegeneration (20) and enhance neuroplasticity in the hippocampus (21). As well, 3α,5α-THP levels are highest in the hippocampus, compared with other brain areas examined (22). Although many brain areas are involved in the modulation of seizure processes, the hippocampus may be particularly sensitive to seizures. Seizures can result in degeneration of the hippocampus. Hippocampal neuron loss has been reported among children with severe seizures (23). Hippocampal damage also is reported in pilocarpine-, kainate-, pentylenetetrazole (PTZ)-, perforant pathway stimulation–, and kindling-induced ictal activity (14,24–26). Some evidence suggests that P or 3α,5α-THP administration can abrogate the effects of perforant pathway stimulation–induced seizures on neuron loss in the hilar region (10,24), although this is not the case in all reports (14). These data suggest that actions of progestins in the hippocampus may play a role in modulating ictal activity.
These data support a role of progestins in mediating seizures; however, whether actions of 3α,5α-THP in the hippocampus are involved in the antiseizure effects of progestins has not been directly investigated. Thus the current studies were designed to investigate the effects of progestins in the hippocampus to mediate PTZ-induced seizures in female rats. We hypothesized that if the antiseizure effects of P occur in part through actions of 3α,5α-THP in the hippocampus, then endogenous hormonal milieus or exogenous progestin regimens that increase 3α,5α-THP levels in the hippocampus would reduce PTZ-induced seizure activity.
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The present findings supported our hypothesis that actions of progestins in the hippocampus mediate the antiseizure effects of progestins in the PTZ seizure model. First, PTZ-induced ictal activity was decreased with endogenous hormonal milieus characterized by high levels of P and 3α,5α-THP in plasma and hippocampus. Second, systemic administration of a progestin regimen that produced high concentrations of P and 3α,5α-THP in plasma and hippocampus resulted in decreased seizure activity. Finally, intrahippocampal progestin administration had effects to decrease ictal activity similar to those seen after systemic administration but increased concentrations of P and 3α,5α-THP only in the hippocampus (not in plasma). Together, these data suggest that progestins can have antiseizure effects and that these effects may be due in part to actions of progestins in the hippocampus.
These findings confirm and extend past research that suggests that progestins can have antiseizure effects in various animal models of seizure disorder. Proestrous rats and ovx rats administered P or 3α,5α-THP had reduced ictal activity compared with relative controls. Notably, levels of 3α,5α-THP produced by our systemic and intrahippocampal replacement regimen produced 3α,5α-THP concentrations in the hippocampus well within physiologic ranges (less than that seen in proestrous rats) and decreased ictal activity (greater than that of proestrous rats). Previous reports demonstrated that progestins can decrease forebrain- as well as brainstem-mediated seizures; however, these studies used progestin regimens that produce pharmacologic progestin concentrations (40,41). The present data suggest that progestin regimens that produce physiologic concentrations of 3α,5α-THP in the hippocampus are sufficient to decrease PTZ-induced tonic–clonic seizures.
Although the endogenous and exogenous hormonal milieus that produced antiseizure effects in these studies were within the physiologic range, other endogenous factors also must be considered. For example, in Experiment 1, hippocampal progestin levels of ovx and diestrous rats were similar; however, ictal activity of diestrous rats was not as robustly increased as that of ovx rats, compared with proestrous rats. It is possible that the recent exposure (3–4 days previously) of diestrous rats to high levels of progestins may have had some protective effects, whereas ovx rats had been exposed to few or no progestins for 7 days. Thus recent prior exposure to progestins may reduce vulnerability to PTZ-induced seizures. As well, other hormones produced by the ovaries of intact rats, adrenals of ovx rats, or de novo synthesis in glial cells may account for some of the variability in the antiseizure effects of progestins that were seen in latencies of ovx groups in Experiment 2 compared with Experiments 1 and 3. Estrogen also is increased on proestrus and typically has proconvulsant effects [although not in all cases (i.e., 14,42)], which might explain the discrepancy between progestin levels and ictal activity seen in proestrous rats, compared with ovx, progestin-replaced rats, in the present studies. Further, among the ovx control rats, hippocampal levels of 3α,5α-THP were slightly higher than hippocampal P levels. Although it is possible that de novo production of 3α,5α-THP in the hippocampus may account for this, a more likely explanation is that a portion of the hormone measured in the hippocampal tissues of ovx rats represents the 5α-reduced metabolites of corticosterone. Rats in the present studies were not adrenalectomized, and the antibody used for radioimmunoassay cross-reacts with these metabolites. Notably, 5α-reduced metabolites of corticosterone have effects similar to those of 3α,5α-THP on γ-aminobutyric acid (GABA)ergic transmission (43). Thus although these data support a role of 3α,5α-THP in mediating seizures, other endogenous factors also may be important for modulating seizure processes.
Our current findings that P and 3α,5α-THP similarly reduce ictal activity suggest that some of the effects of P to modulate seizure processes may be due in part to actions of the 5α-reduced metabolite of P, 3α,5α-THP, in the hippocampus. Systemic or intrahippocampal administration of P or 3α,5α-THP similarly reduced ictal activity compared with vehicle-administered rats. Notably, whereas endogenous and systemic hormonal milieus produced higher plasma progestin levels, direct hippocampal progestin administration resulted in increased hippocampal, but not plasma, progestin levels. As well, direct implants of steroids to the hippocampus result in limited diffusion (∼2 mm) of the steroid, which would preclude progestins having actions in other brain regions to produce their antiseizure effects in Experiment 3. Further, when P or 3α,5α-THP was administered directly to the hippocampus, 3α,5α-THP appeared to have greater effects, albeit not significantly different, to reduce seizures compared with those of P. Notably, P has a high affinity for intracellular progestin receptors. However, in physiologic concentrations, 3α,5α-THP does not (44). 3α,5α-THP is a very potent modulator of GABAA receptors (45) and can alter the function of N-methyl-d-aspartate (NMDA) receptors (46). Thus the extent to which the antiseizure effects of progestins may be due in part to actions of 3α,5α-THP at intracellular progestin, GABAA and/or NMDA receptors in the hippocampus (or other brain areas) require further investigation.
In summary, these data support a role of progestins' actions in the hippocampus to modulate seizure processes are intriguing and have clear clinical relevance. In the United States, 2.5 million people have epilepsy; however, 30% of people with epilepsy do not respond to traditional pharmacotherapies used to control seizure activity. Further investigation of the effects and mechanisms of the ability of gonadal hormones to modulate ictal activity is necessary to expand the therapeutic options available to those with epilepsy.