The influence of estradiol on neuronal excitability
There are numerous studies to date concerning the actions of estradiol in the CNS. The vast majority of studies suggest that estradiol influences neuronal excitability (i.e., action potential generation and/or synaptic function), but not always in the same direction. To discuss in more detail how estradiol may be able to increase or decrease excitability, we will focus on CA1 hippocampal pyramidal cells, because these cells (and the CA1 region) have been studied extensively. In addition, area CA1 is germane to seizure propagation in limbic circuits, and is relevant to temporal lobe epilepsy.
In area CA1 of hippocampus in the rat, estradiol appears to increase excitability in a several ways (Fig. 4), primarily involving synaptic structure and function rather than membrane properties (74). Most of the data are consistent with an ability of estradiol to increase neuronal discharge by enhancing glutamatergic transmission and also by depressing GABAergic inhibition. What are some of these actions of estradiol? One involves structural changes: estradiol increases the number of spine synapses (75), spine density (76), and spine shape (77), effects that are likely to increase glutamatergic synaptic transmission if one assumes that more spine synapses mean more synapses that mediate larger glutamatergic depolarizations (EPSPs)—and in fact this appears to be the case (78–81). Estradiol also can enhance actions of glutamate at ionotropic glutamate receptors; in area CA1, effects are mostly at NMDA receptors (78–84). Estradiol inhibits GABAergic transmission by decreasing the effects of GABA at GABAA receptors of CA1 pyramidal cells, although this effect may be transient, and accompanied by an increase in the duration of inhibitory postsynaptic currents (IPSCs) (which could mask/complicate the predicted disinhibitory effect on pyramidal cells; 81,85). Repolarization of action potentials is also impaired by estradiol because it decreases the slow afterhyperpolarization mediated by calcium-dependent potassium currents (86,87). Together, all of these effects would be expected to increase action potential generation. The type and magnitude of these cellular effects are likely to be regulated by factors such as estradiol dose and the time after estradiol exposure. Therefore, it may not be surprising that these effects have not been consistently reported across laboratories (79,88). In addition to estradiol dose and timing, differences in experimental preparation also may be an important issue. For example, it is difficult to compare results from in vitro experiments with those from intact animals; indeed, investigators have questioned what studies in culture—on neurons taken from embryonic brains—can tell us about estradiol actions in the mature female. Furthermore, many studies of estradiol action use male rats (which have a distinct distribution and composition of ERα and ERβ; 89). The food and housing of animals appears more important than previously anticipated. Thus, it is now clear that the degree of soy phytoestrogen in the diet of female rodents has a large influence on the effects of estrogen on hippocampal-dependent behavior (90,91). It is also clear that there is a substantial interaction of stress and the glucocorticoid system with effects of estrogen and seizures (87). Finally, the relationship of estradiol to excitability may not be a unidirectional: estradiol clearly influences excitability, but the converse may also be true (92). As a result, the type of experimental preparation and history of the animal may lead to very different effects of estradiol.
Figure 4. Effects of 17β-estradiol in area CA1 of rat hippocampus. A summary of potential actions of 17β-estradiol in area CA1 of rat hippocampus. Estradiol activates target genes by nuclear hormone receptors that act as transcription factors. Estradiol also acts by nongenomic mechanisms that involve membrane receptors. Its effects are not only on pyramidal cells but also on GABAergic neurons, cholinergic input, glia, and blood vessels. Glutamatergic transmission may be influenced in a number of ways, by pre- and postsynaptic mechanisms. In addition, ion channels on pyramidal cells are modulated by estradiol, influencing neuronal firing behavior. For further description and references, see text.
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Besides direct actions, estradiol also has a number of actions that indirectly modify excitability. For example, estradiol has been shown to increase acetylcholine release (93), and it interacts with the cholinergic system to alter NMDA receptor binding in CA1 (94). In hippocampus, the interactions of estradiol and the cholinergic system are largely attributed to septohippocampal projections (95), which express ERα presynaptically, and also express ERα on postsynaptic targets within hippocampus (96). The estradiol-cholinergic relationship appears to have a muscarinic component (94), and possibly a nicotinic component in area CA3 (97). The muscarinic contribution is important to bear in mind when comparing studies of estradiol effects on the seizures induced by the muscarinic convulsant, pilocarpine, with estradiol effects on seizures elicited by other convulsants (see ref. 98 for further discussion). For example, if estradiol and muscarinic cholinergic receptor activation increases NMDA receptor binding, estradiol might facilitate the seizures induced by pilocarpine, not because of direct estradiol effects, but because of its interaction with the particular choice of convulsant.
Other notable effects of estradiol on excitability are mediated by indirect mechanisms, and one example that is especially relevant to seizures is its regulation of the neurotrophin, brain-derived neurotrophic factor (BDNF). Estrogen has a response element on the BDNF gene (99), and BDNF potentiates several of the glutamatergic pathways in hippocampus and other brain regions (100). Using diverse animal models of epilespy, several laboratories have provided evidence that BDNF may be proconvulsant (101). Therefore, the estradiol surge during the periovulatory period may lead to a transient elevation in seizure frequency, particularly limbic seizures, because it induces BDNF (102). Furthermore, BDNF appears to induce neuropeptide Y (NPY; 103–105), and NPY has actions which are generally consistent with an anticonvulsant effect, most likely due to its presynaptic actions that depress neurotransmitter release in hippocampus (106). However, like estradiol, the effects of BDNF are also hard to reproduce in vitro, and both BDNF and NPY vary in their effects depending on dose, duration of exposure, and brain region examined.
The effects summarized above support the concept that estradiol can increase excitability, and that it is a proconvulsant agent. However, there is another perspective. Estradiol has been shown to increases the levels of glutamic acid decarboxylase (GAD), the synthetic enzyme for γ-aminobutyric acid (GABA; 81,107), and thus increase levels of GABA. ERα is expressed in subsets of GABAergic neurons (108). Estradiol also has other effects consistent with an “anticonvulsant action,” such as the ability to increase the expression of NPY (109,110), as indicated above. Furthermore, estradiol has been shown to be neuroprotective, an effect that most would not find easy to reconcile with a proconvulsant action (110,111).
How can these disparate results be interpreted? It is helpful to consider the studies that have used a nonphysiological endocrine state (such as gonadectomy) and supraphysiological doses of estradiol separately from studies that examine rats under physiological conditions (i.e., examine responses to the normal, physiological rise in estradiol at proestrus). Supraphysiologic doses may be misleading because estrogen-specific responses may actually decrease excitability if dose is high. In addition, supra-physiological concentrations of estradiol bind to the PR (112). However, estradiol levels are not always measured, and estradiol-progesterone interactions are not always considered. Moreover, it is important to remember that different studies use different ages of animals, treat animals at different times after ovariectomy, handle and feed animals inconsistently, and examine them with different endpoints. Under different conditions, it is likely that estradiol has very different—even opposite—effects.