Finasteride inhibits the disease-modifying activity of progesterone in the hippocampus kindling model of epileptogenesis

Abstract

Progesterone (P) plays an important role in seizure susceptibility in women with epilepsy. Preclinical and experimental studies suggest that P appears to interrupt epileptogenesis, which is a process whereby a normal brain becomes progressively susceptible to recurrent, unprovoked seizures due to precipitating risk factors. Progesterone has not been investigated widely for its potential disease-modifying activity in epileptogenic models. Recently, P has been shown to exert disease-modifying effects in the kindling model of epileptogenesis. However, the mechanisms underlying the protective effects of P against epileptogenesis remain unclear. In this study, we investigated the role of P-derived neurosteroids in the disease-modifying activity of P. It is hypothesized that 5α-reductase converts P to allopregnanolone and related neurosteroids that retard epileptogenesis in the brain. To test this hypothesis, we utilized the mouse hippocampus kindling model of epileptogenesis and investigated the effect of finasteride, a 5α-reductase and neurosteroid synthesis inhibitor. Progesterone markedly retarded the development of epileptogenesis and inhibited the rate of kindling acquisition to elicit stage 5 seizures. Pretreatment with finasteride led to complete inhibition of the P-induced retardation of the limbic epileptogenesis in mice. Finasteride did not significantly influence the acute seizure expression in fully kindled mice expressing stage 5 seizures. Thus, neurosteroids that potentiate phasic and tonic inhibition in the hippocampus, such as allopregnanolone, may mediate the disease-modifying effect of P, indicating a new role of neurosteroids in acquired limbic epileptogenesis and temporal lobe epilepsy.

Introduction

Acquired limbic epilepsy or temporal lobe epilepsy (TLE) is caused by diverse precipitating factors, such as brain injury, stroke, infections, or prolonged seizures. The mechanisms underlying the development of acquired TLE are not very well understood. The term ‘epileptogenesis’ is used to describe the complex plastic changes in the brain that, following a precipitating injury, convert a normal brain into a brain debilitated by recurrent seizures [1], [2]. There is an unmet need for drugs that truly prevent the development of epilepsy (antiepileptogenic agents) or alter its natural course, delaying the appearance or severity of epileptic seizures (disease-modifying agents) in people at risk. Progesterone (P) plays an important role in women with epilepsy [3], [4]. Progesterone has long been known to have antiseizure activity in animal models [5], [6], [7] and in clinical studies [8], [9], [10], [11]. Therefore, it has been suggested that women with epilepsy are prone to seizures in response to decreased levels of P during perimenstrual periods [12], [13]. The antiseizure properties of P are due to its conversion in the brain to allopregnanolone (3α-hydroxy-5α-pregnane-20-one) and other structurally related neurosteroids [6], [7], [14], [15]. Recent studies show that neurosteroids are mainly present in principal neurons in many brain regions that are relevant to acquired TLE including the hippocampus [16]. Neurosteroids rapidly alter neuronal excitability through direct interaction with synaptic and extrasynaptic GABA-A receptors in the hippocampus [17]. Although they act on all GABA-A-receptor subtypes, they have pronounced effects on the extrasynaptic δ-subunit containing GABA-A receptors that mediate tonic inhibition [18]. Therefore, allopregnanolone and related 5α-reduced neurosteroids are powerful anticonvulsants in animal seizure models, and their synthetic analogues have been proposed for epilepsy therapy [19].

Neurosteroids are broad-spectrum anticonvulsants that confer seizure protection in various experimental models of evoked seizures in normal or healthy rats and mice [7], [20], [21], [22]. They protect against seizures induced by GABA-A receptor antagonists, 6-Hz model, pilocarpine-induced limbic seizures, and seizures in kindled animals. Unlike benzodiazepines, tolerance to their actions does not occur during chronic administration in rats [19], [23]. In addition to anticonvulsant activity, there is emerging evidence that neurosteroids play a role in regulating acquired epileptogenesis. Although the precise mechanisms underlying epileptogenesis remain unclear, epileptogenesis may involve an interaction of acute and delayed anatomic, molecular, and physiological events that are both complex and multifaceted. The initial precipitating factor activates diverse signaling events, such as inflammation, oxidation, apoptosis, neurogenesis and synaptic plasticity, which eventually lead to structural and functional changes in neurons [1], [2]. These changes are eventually manifested as abnormal hyperexcitability and spontaneous seizures. In the rodent brain, the 5α-reductase type I and II isozymes convert P to dihydroprogesterone ultimately leading to the production of allopregnanolone and related neurosteroids which may retard epileptogenesis by the interruption of one or more of the above events. In addition to catalyzing the rate-limiting step in the reduction of P, both isoforms of the 5α-reductase enzyme are responsible for the conversion of testosterone and deoxycorticosterone to dihydroprogesterone and dihydrodeoxycorticosterone, respectively [13]. Finasteride, an irreversible inhibitor of 5α-reductase, blocks the synthesis of all 5α-reduced neurosteroids, including testosterone-derived androstanediol [24] and deoxycorticosterone-derived allotetrahydrodeoxycorticosterone (THDOC) [25] and thereby may affect epileptogenesis.

As a precursor of neurosteroids, P is an appealing agent for the intervention of epileptogenesis. Three observations support this contention. First, P's inhibitory effect on kindling was observed in rats [26] and mice [27]. Second, following pilocarpine-induced status epilepticus, the neurosteroidogenic enzyme cytochrome P450 side chain cleavage (P450scc) was increased for several weeks, suggesting that it may be associated with the promotion of neurosteroidogenesis [28]. Cytochrome P450 side chain cleavage is a critical enzyme involved in the conversion of cholesterol into pregnenolone, a precursor for several neurosteroids. Third, inhibiting neurosteroid synthesis with finasteride accelerated the onset of spontaneous recurrent seizures [29], suggesting that endogenous neurosteroids play a role in restraining epileptogenesis. However, P has not been investigated widely for its potential disease-modifying activity in epileptogenic models. Using the mouse kindling model, we previously demonstrated that P inhibits the development of limbic epileptogenesis at doses that do not affect seizure expression and motor performance [27], indicating the disease-modifying activity of P. Kindling is a model of functional epileptogenesis, since seizures develop after repeated subthreshold stimulations applied in the hippocampus, amygdala or other brain regions, without induction of evident brain lesions [30]. However, the mechanisms underlying the protective effect of P against limbic epileptogenesis remain unclear.

In the present study, we sought to investigate the role of neurosteroids in the disease-modifying effect of P by using finasteride, a 5α-reductase inhibitor that blocks the synthesis of P-derived and 5α-reduced neurosteroids in the brain. Our results show for the first time that finasteride prevents P's inhibition of hippocampal kindling epileptogenesis, indicating that P-derived neurosteroids such as allopregnanolone may inhibit epileptogenesis, thus confirming the new role of neurosteroids in limbic epileptogenesis and TLE.