The study, published in Frontiers in Psychiatry by a team at TSUMURA & CO., a Japanese Kampo pharmaceutical company, addresses a condition that touches the vast majority of women of reproductive age. More than 90 percent of women aged 20 to 49 experience some premenstrual symptoms, according to a meta-analysis cited by the authors. When at least five symptoms are present, including at least one psychological and one physical symptom, the condition is diagnosed as premenstrual dysphoric disorder (PMDD), a severe form that can impair daily life, work, and relationships, and has been linked to an increased risk of perinatal depression.

Current first-line treatments leave many patients underserved. Selective serotonin reuptake inhibitors are recommended for moderate-to-severe affective symptoms, and hormonal therapies such as combined oral contraceptives are used to flatten ovarian hormone fluctuations, while lifestyle changes and cognitive behavioral therapy help some patients. But these approaches do not work uniformly and can carry adverse effects, which is why the search for additional options continues. Herbal medicines have attracted attention as complementary therapies for gynecological disorders, and Kamishoyosan is frequently prescribed for PMS after oral contraceptive treatment, with clinical reports describing improvement in irritability and headaches.

To dissect the mechanism, the researchers turned to the progesterone withdrawal model. Female Wistar rats received high-dose progesterone injections once daily for six days, followed by a 24-hour drug-free period. This abrupt withdrawal mimics the sharp hormonal decline that occurs just before menstruation and reliably induces anxiety- and depression-like behaviors in rodents. Liquid chromatography-mass spectrometry confirmed the expected hormonal rollercoaster: plasma and hippocampal levels of progesterone and its metabolite allopregnanolone spiked on the final injection day and collapsed to control levels after withdrawal.

Allopregnanolone is central to the story. This neurosteroid is a potent allosteric modulator of GABA type A receptors, the brain’s principal inhibitory machinery, and it normally exerts calming, anxiolytic, and antidepressant effects by enhancing GABA signaling. The prevailing view holds that it is not the absolute level of allopregnanolone but its wild fluctuations and the resulting impaired GABA receptor responses that drive PMS pathogenesis. Notably, in this study, allopregnanolone levels after withdrawal did not differ between treated and control animals, pointing the finger at altered receptor function rather than hormone levels per se.

That altered function has a specific molecular signature. GABA type A receptors are chloride channels assembled from combinations of 19 possible subunits, and their composition dictates their behavior and pharmacology. Receptors containing the alpha4 and delta subunits sit mainly outside synapses and mediate tonic inhibition. During progesterone withdrawal, expression of these subunits rises in the hippocampus, amygdala, and periaqueductal gray matter, a change associated with reduced GABA currents, increased seizure susceptibility, and heightened anxiety. Using in situ hybridization, the team showed that withdrawal rats had significantly increased alpha4 and delta subunit mRNA in pyramidal cells of the hippocampal CA1 region, with the alpha4 signal also appearing in outer layers of the pyramidal cell layer, hinting that GABAergic interneurons may be involved as well.

Behaviorally, the effects were clear. In the elevated plus maze, a standard test in which rodents that spend more time exploring open, elevated arms are considered less anxious, withdrawal rats showed reduced open-arm time, a higher anxiety index, fewer open-arm entries, and more time in enclosed arms. Kamishoyosan, delivered through the diet at an estimated dose of roughly 2,000 milligrams per kilogram per day starting four days before progesterone administration, attenuated these alterations. Open-arm entries increased significantly with treatment, while enclosed-arm entry counts were unchanged across groups, indicating that locomotor activity was not a confound. Effect sizes were large across behavioral measures, though the authors note that observed power varied and that a single behavioral paradigm is a limitation.

Crucially, the herb’s benefit did not come from smoothing the hormonal storm. Pharmacokinetic analysis showed that plasma progesterone peaked 30 minutes after the final injection and fell by more than half within 240 minutes, with hippocampal concentrations tracking plasma closely and allopregnanolone showing similar dynamics. Kamishoyosan-treated animals displayed essentially the same hormone trajectories, with no significant differences in peak concentrations or drug exposure. Gene expression analysis of GABA-synthesizing enzymes, the vesicular GABA transporter, the GAT-1 reuptake transporter, and EGR3, a transcription factor that regulates the alpha4 subunit gene, likewise revealed no significant differences among groups after batch-effect adjustment. The authors conclude that Kamishoyosan appears to act through a direct, plastic change in the GABAergic system rather than by altering steroid hormone levels or GABA production and transport.

This mechanism distinguishes Kamishoyosan from conventional GABA-targeting drugs. Benzodiazepines such as diazepam enhance GABA signaling by binding to the receptor, but chronic use carries risks of dependence, tolerance, falls, and delirium, and chronic benzodiazepine administration itself increases alpha4 subunit expression, a change implicated in tolerance. Sensitivity to these drugs also decreases after progesterone withdrawal. Because Kamishoyosan suppressed the withdrawal-induced rise in alpha4 and delta subunits rather than binding to the receptor, the authors suggest it may work in a fundamentally different way, one that could even offer a route to reversing tolerance caused by benzodiazepine-like drugs, though they caution that the exact mechanisms and whether tolerance reversal truly occurs remain to be studied.

The formula itself is a blend of ten crude drugs, several with documented anxiolytic or hormone-related activities: geniposide from Gardeniae Fructus has shown anxiolytic effects in social interaction tests, paeonol from Paeoniae Radix eased anxiety-like behavior in PMS model rats, saikosaponin D from Bupleuri Radix exhibits estrogen-like and hypothalamic-pituitary-adrenal axis regulatory effects, and isoliquiritigenin from Glycyrrhizae Radix acts as a positive allosteric modulator of GABA type A receptors. Kamishoyosan is an approved pharmaceutical in Japan, and clinical studies, including a double-blind placebo-controlled trial in menopausal women, have reported favorable safety with no serious adverse events. An open-label randomized controlled trial evaluating its efficacy and safety for PMS is currently underway. The authors acknowledge limitations, including the single dose tested, reliance on one behavioral test, and the need for a priori power planning, but they position the findings as evidence that Kamishoyosan reverses reduced GABA receptor sensitivity and may inform future PMS treatments that sidestep the drawbacks of existing options.

Subject of Research: Anxiolytic mechanism of the Kampo medicine Kamishoyosan in a progesterone withdrawal rat model of premenstrual syndrome

Article Title: Kamishoyosan reduces anxiety-like behavior by modulating GABAA receptor subunit expression in a premenstrual syndrome rat model

Article References: Harada-Nitta, S., Ogura, K., Konno, K., Nahata, M., Fujitsuka, N., & Tokita, Y. (2026). Kamishoyosan reduces anxiety-like behavior by modulating GABAA receptor subunit expression in a premenstrual syndrome rat model. Frontiers in Psychiatry, 17, Article 1799904. https://doi.org/10.3389/fpsyt.2026.1799904

Image Credits: AI Generated

DOI: 10.3389/fpsyt.2026.1799904

Keywords: premenstrual syndrome, PMDD, Kamishoyosan, Kampo medicine, GABA A receptor, allopregnanolone, progesterone withdrawal, hippocampus, anxiety, neurosteroids, alpha4 subunit, rat model