Tibolone rapidly attenuates the GABA_B response in hypothalamic neurones

Tibolone is primarily used for the treatment of climacteric symptoms. Tibolone is rapidly converted into three major metabolites: 3α- and 3β-hydroxy (OH)-tibolone, which have oestrogenic effects, and the Δ4-isomer (Δ4-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 17β-oestradiol (E₂) rapidly altered γ-aminobutyric acid (GABA) neurotransmission in hypothalamic neurones through a membrane oestrogen receptor (mER). E₂ 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 E₂, we have found that tibolone and its active metabolite 3βOH-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 182780. Other metabolites of tibolone (3αOH-tibolone and Δ4-tibolone) had no effect. Furthermore, tibolone (and 3βOH-tibolone) was fully efficacious in ERα knockout (KO) and ERβKO mice to attenuate GABA B responses. The effects of tibolone were blocked by phospholipase C inhibitor U73122. However, in contrast to E₂, the effects of tibolone were not blocked by protein kinase C inhibitors or protein kinase A inhibitors. It appears that tibolone (and 3βOH-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.