Qiu J, Bosch M A, Rønnekleiv O K, Kloosterboer H J, Kelly M J
Department of Physiology and Pharmacology, Oregon Health and Science University, Portland, OR 97239-3098, USA.
J Neuroendocrinol. 2008 Dec;20(12):1310-8. doi: 10.1111/j.1365-2826.2008.01789.x.
Tibolone is primarily used for the treatment of climacteric symptoms. Tibolone is rapidly converted into three major metabolites: 3 alpha- and 3beta-hydroxy (OH)-tibolone, which have oestrogenic effects, and the Delta 4-isomer (Delta 4-tibolone), which has progestogenic and androgenic effects. Because tibolone is effective in treating climacteric symptoms, the effects on the brain may be explained by the oestrogenic activity of tibolone. Using whole-cell patch clamp recording, we found previously that 17beta-oestradiol (E(2)) rapidly altered gamma-aminobutyric acid (GABA) neurotransmission in hypothalamic neurones through a membrane oestrogen receptor (mER). E(2) reduced the potency of the GABA(B) receptor agonist baclofen to activate G-protein-coupled, inwardly rectifying K(+) (GIRK) channels in hypothalamic neurones. Therefore, we hypothesised that tibolone may have some rapid effects through the mER and sought to elucidate the signalling pathway of tibolone's action using selective inhibitors and whole cell recording in ovariectomised female guinea pigs and mice. A sub-population of neurones was identified post hoc as pro-opiomelanocortin (POMC) neurones by immunocytochemical staining. Similar to E(2), we have found that tibolone and its active metabolite 3 beta OH-tibolone rapidly reduced the potency of the GABA(B) receptor agonist baclofen to activate GIRK channels in POMC neurones. The effects were blocked by the ER antagonist ICI 182 780. Other metabolites of tibolone (3 alpha OH-tibolone and Delta 4-tibolone) had no effect. Furthermore, tibolone (and 3 beta OH-tibolone) was fully efficacious in ER alpha knockout (KO) and ER beta KO mice to attenuate GABA(B) responses. The effects of tibolone were blocked by phospholipase C inhibitor U73122. However, in contrast to E(2), the effects of tibolone were not blocked by protein kinase C inhibitors or protein kinase A inhibitors. It appears that tibolone (and 3 beta OH-tibolone) activates phospholipase C leading to phosphatidylinositol bisphosphate metabolism and direct alteration of GIRK channel function. Therefore, tibolone may enhance synaptic efficacy through the G(q) signalling pathways of mER in brain circuits that are critical for maintaining homeostatic functions.
替勃龙主要用于治疗更年期症状。替勃龙可迅速转化为三种主要代谢产物：具有雌激素作用的3α-和3β-羟基（OH）-替勃龙，以及具有孕激素和雄激素作用的Δ4-异构体（Δ4-替勃龙）。由于替勃龙对治疗更年期症状有效，其对大脑的作用可能是由替勃龙的雌激素活性来解释的。我们之前使用全细胞膜片钳记录发现，17β-雌二醇（E₂）通过膜雌激素受体（mER）迅速改变下丘脑神经元中的γ-氨基丁酸（GABA）神经传递。E₂降低了GABA₍B₎受体激动剂巴氯芬激活下丘脑神经元中G蛋白偶联内向整流钾⁺（GIRK）通道的效能。因此，我们推测替勃龙可能通过mER产生一些快速作用，并试图使用选择性抑制剂和全细胞记录来阐明替勃龙在去卵巢雌性豚鼠和小鼠中的作用信号通路。事后通过免疫细胞化学染色将一部分神经元鉴定为阿黑皮素原（POMC）神经元。与E₂相似，我们发现替勃龙及其活性代谢产物3β OH-替勃龙迅速降低了GABA₍B₎受体激动剂巴氯芬激活POMC神经元中GIRK通道的效能。这些作用被雌激素受体拮抗剂ICI 182 780阻断。替勃龙的其他代谢产物（3α OH-替勃龙和Δ4-替勃龙）没有作用。此外，替勃龙（和3β OH-替勃龙）在雌激素受体α基因敲除（KO）和雌激素受体β基因敲除小鼠中能完全有效地减弱GABA₍B₎反应。替勃龙的作用被磷脂酶C抑制剂U73122阻断。然而，与E₂不同的是，替勃龙的作用不被蛋白激酶C抑制剂或蛋白激酶A抑制剂阻断。似乎替勃龙（和3β OH-替勃龙）激活磷脂酶C导致磷脂酰肌醇二磷酸代谢并直接改变GIRK通道功能。因此，替勃龙可能通过大脑回路中mER的G₍q₎信号通路增强突触效能，而这些回路对于维持稳态功能至关重要。
