Why your brain reacts differently to progesterone: the ALLO theory of PMDD

If you live with PMDD, you've heard the standard explanation: your hormones fluctuate, and that causes your mood to crash in the two weeks before your period. It's tidy. It's also not the full picture.

Women with PMDD don't have abnormal hormone levels. If you compared blood work from someone with PMDD and someone without it, the progesterone, estrogen, and allopregnanolone levels would look the same. The difference is in how the brain responds to those hormones.

Researchers call this the allopregnanolone (ALLO) theory of PMDD. It's one of the most important shifts in how neuroscientists understand this condition, and it puts the brain, not the ovaries, at the center of the story.

First, a bit about ALLO

Progesterone is one of the key hormones that rises after you ovulate. Your body converts some of that progesterone into a neurosteroid called allopregnanolone, or ALLO.

ALLO's job is to boost the activity of GABA, the brain's main calming neurotransmitter. Think of GABA like the brakes on a car. When your brain is processing a lot of signals (stress, emotion, pain), GABA slows things down, helping you feel calm and steady. ALLO makes those brakes work better by binding to GABA-A receptors, the specific docking stations on neurons where GABA does its work. It's a similar mechanism to how anti-anxiety medications like benzodiazepines work, through a different part of the same receptor.

For most women, rising ALLO after ovulation means those brakes get a gentle boost. The system works as expected.

The paradoxical response

Researchers, including Sundström-Poromaa, Bäckström, and Bixo at Umeå University in Sweden, have spent decades trying to understand why some women's brains have the opposite reaction to a neurosteroid that should be calming.

Healthy volunteers who receive intravenous allopregnanolone become more sedated, which is what you'd expect from something that boosts the brain's calming system. Women with PMDD don't. During the luteal phase, their brains respond in the opposite direction: the ALLO that should calm them appears to contribute to anxiety, irritability, and low mood instead (Bäckström et al., 2014).

The pattern gets more specific. Bäckström and colleagues (2014) found that the relationship between ALLO concentration and negative mood follows an inverted U-shaped curve. Low ALLO levels have little effect. Very high levels (above what the body produces) activate the calming response as expected. But moderate levels, the levels that match what your body produces during the luteal phase, are the ones associated with the worst mood effects.

It's a bit like an allergic reaction. Pollen is harmless to most people's immune systems, but in someone with hay fever, the immune system overreacts to a normal substance. With PMDD, the brain overreacts to a normal neurosteroid. The substance isn't the problem. The response is.

Inside the receptor: why the building blocks matter

To understand why this response goes wrong, you need to look at the GABA-A receptor. These receptors are assembled from five protein subunits, like a combination lock with five tumblers. The specific mix of subunits determines how the receptor behaves and how sensitive it is to ALLO.

One subunit, the delta (δ) subunit, matters most for this story. Delta-containing GABA-A receptors sit outside the synapse (the gap between neurons) and are more sensitive to neurosteroids like ALLO than other types. When ALLO binds to delta-containing receptors, it produces a steady, background level of calm, like a thermostat keeping the emotional temperature in a comfortable range.

In animal studies, delta subunit expression increases during the equivalent of the luteal phase. Think of it as the brain turning up the sensitivity of the thermostat to match the rising ALLO levels, keeping everything balanced (Smith et al., 2007).

In PMDD, this adjustment appears to stall. Timby and colleagues (2025), publishing in Translational Psychiatry, measured GABA-A receptor subunit expression in women with PMDD compared to controls. Women with PMDD had lower expression of the delta subunit during the luteal phase, the exact point in the cycle when it should be going up.

The functional consequence was clear too. Women with the lowest delta expression also showed the highest amygdala activation, with the brain's emotional alarm system firing harder in response to emotional triggers. The thermostat wasn't adjusting, so the emotional temperature was climbing unchecked.

A receptor problem, not a hormone problem

The ALLO theory reframes how we think about PMDD. Your hormones aren't "wrong." A blood test won't show anything unusual. The difference lies in how your brain's GABA system adapts (or doesn't adapt) to normal hormonal shifts across the cycle. It's a receptor sensitivity issue.

This explains why hormone levels look normal in women with PMDD, because the hormones are normal. It explains why the same neurosteroid that calms one person can destabilize another, because their GABA-A receptors are composed differently. And it explains why treatments that suppress ovulation can work, not because the hormones were harmful, but because removing them stops the brain from having to respond to them.

The reframe also points toward targeted treatments. Bixo and colleagues (2017) tested Sepranolone, a compound designed to block ALLO's action at the GABA-A receptor, in women with PMDD. In their randomized controlled trial, women who received Sepranolone during the luteal phase showed a 75% reduction in their experience scores, compared to 47% with placebo, with an effect size of 0.7 (Bixo et al., 2017). A larger follow-up across 12 European centers confirmed the effect with very few side effects (Bäckström et al., 2021).

If the problem is how the brain receives the hormonal signal, you don't need to eliminate the signal. You can change how it's received.

Where neuromodulation fits in

Understanding the ALLO theory raises a question: if the problem sits in the brain's GABA system and the emotional circuits it regulates, could you address it by working with the brain itself?

This is where neuromodulation enters the picture. Brain stimulation techniques deliver gentle electrical currents to targeted brain regions, and researchers have shown that this can influence the same systems involved in the ALLO theory of PMDD.

The evidence comes from two directions. First, magnetic resonance spectroscopy (MRS) studies have shown that brain stimulation applied to the prefrontal cortex changes GABA concentrations in the stimulated region (Stagg et al., 2014). In other words, you can influence the brain's inhibitory signaling, the same GABAergic system that the ALLO theory identifies as dysregulated in PMDD, through targeted electrical stimulation. GABA-A receptors mediate these changes, the same receptor type where delta subunit expression falls short in women with PMDD.

Second, stimulating the prefrontal cortex has been shown to reduce amygdala reactivity. In a randomized clinical trial, Heeren and colleagues (2019) found that prefrontal stimulation reduced bilateral amygdala threat response while increasing activity in attentional control networks. Heightened amygdala activation is one of the downstream consequences of low delta subunit expression in PMDD. Prefrontal neurostimulation appears to strengthen the top-down control that helps keep the amygdala's alarm response in check, like giving the brain a better volume knob for emotional signals that are being amplified too loud.

No one is claiming that neurostimulation "fixes" GABA-A receptor composition or reverses the delta subunit issue. But the research suggests that brain stimulation can influence the same pathways, GABAergic tone, and prefrontal-amygdala regulation, that the ALLO theory identifies as disrupted in PMDD. It's addressing the downstream consequences of the receptor sensitivity problem through a different route.

The brain as an interpreter

PMDD has long been framed as a hormonal condition. Hormones are involved as the trigger. But the reason one woman's brain responds with calm and another's responds with distress lies in the brain: the composition of GABA-A receptors and how well they adapt to shifting neurosteroid levels.

Once you see PMDD through this lens, the question shifts from "what's in your blood" to "what's happening in your brain." And that's where researchers at Umeå and elsewhere are focusing. And it's why approaches that target the brain, whether through targeted pharmacology like Sepranolone or through neurostimulation, represent some of the most promising directions in PMDD research.

Knowing the mechanism won't make the luteal phase easier on its own. But it offers a clear, biological explanation for what you're experiencing. Your brain's GABA system responds to a normal hormonal signal in a way other people's brains don't. Researchers are learning how to address that, and the brain is where the answers are.

References

Bäckström, T., Bixo, M., Johansson, M., Nyberg, S., Ossewaarde, L., Ragagnin, G., Savic, I., Strömberg, J., Timby, E., van Broekhoven, F., & van Wingen, G. (2014). Allopregnanolone and mood disorders. Progress in Neurobiology, 113, 88–94.

Bixo, M., Ekberg, K., Poromaa, I. S., Hirschberg, A. L., Jonasson, A. F., Andréen, L., Timby, E., Wulff, M., Ehrenborg, A., & Bäckström, T. (2017). Treatment of premenstrual dysphoric disorder with the GABA-A receptor modulating steroid antagonist Sepranolone (UC1010), a randomized controlled trial. Psychoneuroendocrinology, 80, 46–55.

Bäckström, T., Ekberg, K., Hirschberg, A. L., Bixo, M., et al. (2021). A randomized, double-blind study on efficacy and safety of sepranolone in premenstrual dysphoric disorder. Psychoneuroendocrinology, 133, 105426.

Heeren, A., Billieux, J., Philippot, P., De Raedt, R., Baeken, C., de Timary, P., & Maurage, P. (2019). Effect of prefrontal cortex stimulation on regulation of amygdala response to threat in individuals with trait anxiety: a randomized clinical trial. JAMA Psychiatry, 74(12), 1299–1307.

Smith, S. S., Ruderman, Y., Frye, C., Homanics, G., & Yuan, M. (2007). Neurosteroid regulation of GABA-A receptors: Focus on the alpha4 and delta subunits. Pharmacology & Therapeutics, 116(1), 58–76.

Stagg, C. J., Bachtiar, V., & Johansen-Berg, H. (2014). tDCS-induced alterations in GABA concentration within primary motor cortex predict motor learning and motor memory: a 7 T magnetic resonance spectroscopy study. NeuroImage, 123, 281–287.

Timby, E., et al. (2025). Transcription of GABA-A receptor subunits in circulating monocytes and association to emotional brain function in premenstrual dysphoric disorder. Translational Psychiatry, 15, 220.

Gulinello, M., Gong, Q. H., Li, X., & Smith, S. S. (2001). Short-term exposure to a neuroactive steroid increases alpha4 GABA-A receptor subunit levels in association with increased anxiety in the female rat. Brain Research, 910(1–2), 55–66.