Elsevier

Brain Research

Volume 1116, Issue 1, 20 October 2006, Pages 1-11
Brain Research

Research Report
Acute estradiol application increases inward and decreases outward whole-cell currents of neurons in rat hypothalamic ventromedial nucleus

https://doi.org/10.1016/j.brainres.2006.07.104Get rights and content

Abstract

Acute estradiol (E2) can potentiate the excitatory responses of hypothalamic ventromedial nucleus (VMN) neurons to neurotransmitters. To investigate the mechanism(s) underlying the potentiation, the whole-cell patch voltage clamp technique was used to study VMN neurons in hypothalamic slices prepared from female juvenile (3–5 weeks) rats. A voltage step and/or ramp was applied every 5 min to evoke whole-cell currents before, during and after a treatment with E2 (10 nM), corticosterone (10 nM) or vehicle for up to 20 min. Acute E2 increased inward currents in 38% of neurons tested. Their average peak inward current amplitudes started to increase within 5 min and reached the maximum of 163% of pretreatment level (Pre) at 20 min of treatment before recovering toward Pre. These increases are significantly greater than the Pre and corresponding vehicle controls and non-responsive neurons. Outward currents were decreased significantly by E2 in 27% of E2-treated cells, down to 60% of Pre levels. E2 also appeared to affect the kinetics of the inward and outward currents of estrogen-responsive neurons. Whenever observed, the effects of acute E2 were reversible after a 5- to 10-min washing. Probability analysis indicates that E2 affected the inward and the outward currents independently. The E2 effects are specific in that they were not produced by similar treatment with vehicle or corticosterone. Pharmacological characterizations using ion replacement and channel blockers showed that the inward currents were mediated practically all by Na+ and the outward currents mainly by K+. Thus, acute E2 can enhance inward Na+ and attenuate outward K+ currents. Since both effects will lead to an increase in neuronal excitability, they may explain our previous observation that E2 potentiates the excitation of VMN neurons.

Introduction

In addition to their classical genomic actions, estrogens are now also widely recognized to have non-genomic, acute membrane-initiated actions (Simoncini and Genazzani, 2003). In a series of studies, we have demonstrated that the non-genomic actions can potentiate the genomic actions in a neuronal cell line (Vasudevan et al., 2001), a breast cancer cell line (Devidze et al., 2005) and in hypothalamic cells governing reproductive behavior (Kow and Pfaff, 2004). In female rats, acute membrane-limited estrogen actions on the membranes of hypothalamic ventromedial nucleus (VMN) neurons could potentiate obligatory genomic actions in the induction of lordosis behavior (Kow and Pfaff, 2004). To find the membrane action(s) that could be involved in this potentiation, we studied the effects of acute estrogen on VMN neurons and found that estradiol (E2) applied in the bath could both rapidly enhance the excitatory, and abbreviate the inhibitory responses of VMN neurons to two transmitter receptor agonists, histamine and N-methyl-d-aspartate (NMDA) (Kow et al., 2005).

The abovementioned acute E2 effects on VMN neurons may be achieved by E2 modulating membrane currents because both agonist actions involve changes in ion fluxes. NMDA excites neurons by causing inward cation currents through the activation of NMDA receptors, which are ligand-gated cation channels (Dingledine et al., 1999). Histamine, on the other hand, depolarizes neurons to increase their firing activity by inhibiting potassium currents or by activating certain sodium influxes (Brown et al., 2001). Therefore, estrogen could enhance the excitations caused by NMDA and histamine by acting directly or indirectly on ion channels to increase inward cation and reduce outward potassium currents, respectively. These possibilities were evaluated in the present study using whole-cell patch recording to determine whether acute estrogen application can modulate membrane currents and, in turn, whether such effects can explain our previous findings of E2's potentiation of neuronal excitation.

In the present study, the whole-cell current, rather than isolated currents, was examined for the following reasons. First, we did not know which current(s), if any, would be affected by acute E2 and therefore did not want to miss the modulated current by isolating the wrong one. Secondly, since acute E2 can affect the responses to both NMDA and histamine, we expected it to affect more than one type of current. Focusing too early on one isolated current might abrogate the opportunity to unveil estrogen effects on other types of currents. A portion of the results have been reported in preliminary form (Kow et al., 2003).

Section snippets

Treatments with vehicle (Veh)

In control experiments, neurons were treated with vehicle (ACSF containing 10 3% ethanol) in attempts to chart spontaneous variations of the amplitudes of inward and outward currents. The results for normalized inward and the outward current amplitudes are illustrated in Fig. 2, Fig. 3, respectively. Vehicle treatments had no effect on either set of currents over time [inward current: F(3,27) = 0.691, p > 0.56; outward current: F(3,27) = 1.22, p > 0.31. Post hoc tests: all p values > 0.30 for both

Discussion

Using whole-cell patch clamp recordings, we have found that acutely applied E2 could modulate membrane currents of VMN neurons within 5 min. It enhanced inward currents and attenuated outward currents. These effects were not duplicated by a different steroid hormone, corticosterone, and hence, appeared to be specific to estrogen. Further pharmacological analyses showed that the estrogen-sensitive inward currents essentially are sodium currents and the outward currents are due to potassium.

Animals

Intact female Sprague–Dawley rats, 3–5 weeks old were used. These young females were chosen because they provided healthy brain tissue slice preparations, and because they are already estrogen-responsive behaviorally and their hypothalamus rich in estrogen receptors (ER) (Bogun et al., unpublished observations). Since VMN is essential for E2 induction of lordosis, these rats are relevant for exploring biophysical responses of VMN neurons to E2, as in the current study. The rats were housed in

Acknowledgment

The authors want to thank Mr. Yong Lu for his advice on statistical analyses.

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