Whether from the jab of a vaccine needle or the touch of a hot pan, we all experience pain. But how much pain you feel and how your body responds to it varies depending on your sex.
In a new study published in Neuron, researchers reported that activating a specific brain circuit in male mice relieved their pain, but activating the same circuit in female mice did not (1). Instead, activating this circuit while female mice experienced pain prompted them to move more — a very different response. Understanding how neural circuits regulate pain in the sexes will help researchers develop better pain management therapeutics for both women and men.
Sex matters when it comes to pain. For example, women are more likely to experience chronic pain than men. Pain after surgery, neuropathic pain, and chronic pain during childhood all vary based on sex.
Neuroscientists have not always considered sex differences when studying pain. “It wasn't a nefarious, ‘We're not gonna bother with those females.’ It was just trying to reduce variability,” said Margaret McCarthy, a neuroscientist at the University of Maryland School of Medicine, who was not involved in the new study, but who wrote a commentary on the research (2).
“There were some papers that came out in the 1990s that showed that synapse formation in the hippocampus varied across the rodent equivalent of a menstrual cycle,” she explained. “All the scientists freaked out and said, ‘Well, I’m not doing females anymore!’ And it was a quite reasonable thing to do at the time.”
What might have seemed like controlling for variability between the sexes actually masked important differences in how male and female bodies deal with pain.
Jeffrey Mogil, a neuroscientist at McGill University, and his team discovered that inhibiting the function of immune cells called microglia relieved pain in male mice, but not in female mice (3). This study pesented one of the first examples of sex-differences in pain. The team only discovered this because they regularly use both male and female mice in their research.
Due to the increasing evidence of sex differences in pain, the National Institute of Health (NIH) enacted a policy in 2016 that requires researchers to include both male and female subjects in their studies.
Meanwhile, Thomas Kash and members of his lab at the University of North Carolina learned that activating specific dopamine-producing neurons in a region of the midbrain called the ventrolateral column of the periaqueductal gray (PAG) and dorsal raphe (DR) reduced pain in male mice (4). This brain region is important for pain relief and regulating the body’s response to pain.
Waylin Yu, first author of the new study, wanted to make sure his research on the neuroscience of pain was as inclusive as possible, so he tested if activating this same neural pathway in female mice also relieved their pain.
“We began this study with the intention of just reproducing this effect in both sexes, and I fully expected to see more or less the same analgesic or pain-relieving effects happen in both sexes,” Yu said.
What he found was much more puzzling.
When the scientists shined a hot laser or poked a male mouse’s paw with a thin nylon filament while activating this neuronal circuit, the mouse’s pain response lessened. It didn’t draw its foot away from the heat or filament as quickly as it did when the circuit was not activated.
When the team tested female mice under the same conditions, they moved their feet away from the painful stimuli at the same rate, regardless of neural circuit activation.
Curious about a response to a long-lasting inescapable pain, Yu and his colleagues tested if activating this circuit could reduce chronic pain in male and female mice. To do that, they injected mice with acetic acid to trigger pain and looked for changes in behaviors such as nesting, writhing, and locomotion when they activated the circuit.
Both male and female mice stopped nesting while in pain, but the researchers noticed that when they activated the circuit in female mice, they writhed and moved more than before the circuit was activated, suggesting that they still experienced pain. The team also noticed that when in pain and with the activated neural circuit, the female mice moved more in the presence of a male mouse or when in a new environment.
The question now facing Yu and his colleagues is why. “That was really hard to grasp,” said Yu.
The team reasoned that because the PAG and DR are important for controlling adaptive behaviors, perhaps this circuit is important for how female mice deal with potentially stressful situations, like encountering a stranger or a new environment.
“There are different coping mechanisms,” Yu said. Citing earlier pain research performed in rats, Yu explained that female rats are “more likely to run away from the painful stimulus, rather than staying still and taking the pain, whereas males are more likely to do that.” Perhaps Yu’s mice coped with pain in a similar way.
When the researchers looked deeper into the different functional aspects of the circuit, such as synaptic connections and the transmission of the signal to downstream neurons in the mice, they also saw sex differences.
“That just studying this one brain pathway can confer so much complexity, that was pretty amazing to me,” said Yu.
McCarthy also wonders why this pathway works so differently in females. “It tells you something really foundational about the nervous system, about the brain, and how the brain develops,” she said. “It's kind of like a contrast agent. If the doctor wants to see the arteries in your heart, he injects the contrast agent and then they become visible. Well, that's what happens when you compare males and females: You see things you never would have seen.”
Mogil, who was not involved in this study, is excited to see another example of a brain circuit that is undeniably sex-specific.
“Example after example after example keep getting published, and so there's just no denying anymore. The question in my mind — that's the big one — is, ‘is there something about pain that is particularly sex specific, or is everything in the brain that sex specific?’”
McCarthy added that just like any good scientific study, Yu and his colleagues’ research raises more questions than answers, “And unfortunately,” she said, “in this case, the answers are all relevant to males leaving all the questions for females.”
1. Yu, W. et al. Periaqueductal gray/dorsal raphe dopamine neurons contribute to sex differences in pain-related behaviors. Neuron 109, 1365-1380.e5 (2021).
2. McCarthy, M. M. SexX matters when it comes to pain. Neuron 109, 1253-1254 (2021).
3. Sorge, R.E. et al. Different immune cells mediate mechanical pain hypersensitivity in male and female mice. Nat Neurosci 18, 1081-1083 (2015).
4. Li, C. et al. Mu Opioid Receptor Modulation of Dopamine Neurons in the Periaqueductal Gray/Dorsal Raphe: A Role in Regulation of Pain. Neuropsychopharmacol 41, 2122–2132 (2016).