Estrogen may spur the body to make opioids after injury

The female sex hormones estrogen and progesterone may help relieve pain, an animal study finds.

The new research, conducted in lab mice, found that the two hormones work together to direct immune cells located near the spinal cord. These cells then make their own painkillers — namely, an opioid called enkephalin. The opioids are released in response to nerve injuries, but only in females, the study revealed.

In the absence of an injury, both male and female mice make a baseline amount of enkephalin using these special immune cells, said first study author Elora Midavaine, a postdoctoral fellow at the University of California, San Francisco (UCSF). "But after injury, the levels go up in females but not in males," she said.

In this way, the immune cells that make the painkillers, called regulatory T cells (Tregs), are interrupting pain signals triggered by neurons in the spinal cord, and this essentially prevents those signals from ever reaching the brain.

Related: Is there really a difference between male and female brains? Emerging science is revealing the answer.

"We weren't expecting those cells [Tregs] to play an active role in pain processing at all, and specifically not in a very sex-specific manner," Midavaine told Live Science. "It puzzled us for quite a while, and we at first kind of doubted those findings."

Down the line, this research could help unravel sex differences in human pain perception, as well as how pain perception shifts during pregnancy and menopause, when the body's levels of estrogen and progesterone change dramatically. Eventually, the work could point to new ways of treating pain, the study authors say.

Immune cells in pain processing

Many studies point to differences in how men and women experience pain.

In general, research finds that, compared with men, women show higher sensitivity to pain — meaning they feel the sensation more intensely when their pain receptors are activated. Research also finds they also have lower pain thresholds, so less stimulation is needed to activate the receptors in the first place.

These patterns are seen across different types of pain, including that caused by extreme temperatures, mechanical pressure and inflammation, and they've also been reported in studies of various male and female animals, hinting at biological underpinnings.

Women also experience higher rates of chronic pain than men do. Some differences in human pain may be cultural, tied to how each gender is socialized to cope with pain and how likely they are to seek help for it. However, research also points to biological differences in how pain is processed in the male body versus the female body.

In the new study, Midavaine, Dr. Sakeen Kashem, an assistant professor of dermatology at UCSF and co-senior author of the new study, and their colleagues wanted to better understand how immune cells might be talking to the nervous system to change pain signaling, and whether there were any sex differences in that process. Work from other labs had suggested that T cells play a role in pain perception in female mice, for instance, while pain in male mice relies on a different type of cell.

Tregs are a type of T cell, so the scientists wondered how they fit into the picture.

"We started looking at Tregs just because they're the brakes of the immune system," Kashem told Live Science. "So the question was, 'What if we just remove them?'"

An unexpected difference between the sexes

In their new study, published Thursday (April 3) in the journal Science, the researchers focused on Tregs located within the meninges — membranes that cover and protect the brain and spinal cord. These membranes house immune cells and route signals from the peripheral nerves, which sense pain, to the brain, and vice versa.

Using a toxin, the team deleted the vast majority of Tregs from the meninges of male and female mice, finding that only the females became more sensitive to mechanical pain caused by pressure. In another experiment, the team subjected both sexes of mice to nerve injuries and also depleted their Tregs. Again, they found that the depletion increased the females' — but not the males' — reactions to the pain.

Then, the team used a different technique to increase the number of Tregs in the mice. This had the opposite effect in the females — it decreased their pain — while it had no effect on the males.

The researchers wondered if sex hormones might play a role in this female-specific mechanism. They used various techniques to mess with the estrogen and progesterone levels in the mice. They found that blocking the effects of both hormones seemed to make Tregs work less well in females, leading them to become more pain-sensitive. Conversely, increasing the levels of both hormones amplified the pain desensitization mediated by Tregs.

Separately, the team has also collected some early data in pregnant mice: They've found that, as the levels of female sex hormones go up, so too does the pain-relieving activity of the Tregs.

Through further experiments, the team identified the opioid enkephalin as the mediator of this effect, finding that it blocked pain neurons from sending signals. What they don't know yet is how the sex hormones instruct immune cells to make the opioid.

"There is for sure a signal that activates those Tregs," Midavaine said. "We haven't pinpointed that signal just yet."

Pain research facing uncertain future

The study raises questions about what might happen in menopause, when the body's production of female sex hormones plummets. It also raises questions about what alternative mechanism for pain relief exists in males, given that they lack the mechanism described in the new study, said co-senior author Allan Basbaum, professor and chair of the UCSF Department of Anatomy.

Looking ahead, the work could inspire new ways of treating pain, Kashem suggested. For instance, could there be ways of increasing the number of opioid-releasing Tregs around the spinal cord, or delivering lab-made enkephalin to perform the same function?

There are many directions the team could take in future studies, the researchers noted. But crucially, they wouldn't have uncovered this female-specific mechanism if they had not included female mice in their research.

Related: 'Let's just study males and keep it simple': How excluding female animals from research held neuroscience back, and could do so again

The study was funded, in part, by the National Institutes of Health (NIH), which has long upheld a policy that requires grantees to include both male and female animals in their research, unless they have very strong scientific justification to include only one sex. Recently, though, the NIH appeared to archive that policy, and it is unclear whether it will continue to be upheld.

This potential policy shift comes at a time when executive orders have prompted major science institutions to reconsider projects containing certain words, such as "women" and "female."

"There's a concern now that because of all the stuff coming out of Washington, that's a no-no," Basbaum said. "I'm curious to know what would happen had we started this study now."

In the context of pain research, there are established differences between the sexes in the rates of pain disorders, the effects of pain-relieving drugs, and the way pain processing works at the cellular level, Midavaine noted.

"It really is fundamental to study both sexes," she emphasized. Without major funders like the NIH requiring scientists to study both sexes, "I think many labs may just decide to perform research on males, as it's been done for many years before. And you might miss a lot of very critical findings."