Nociceptors Function Differently in Males versus Females – Whether in Mice, Monkeys, or People

A new study in multiple species reveals sex differences in prolactin and orexin B sensitization of nociceptors. The novel finding may help to explain sex differences in the prevalence of many chronic pain disorders – and migraine, too.

The evidence is clear: Women and men differ in the chronic pain conditions they are most likely to experience. For instance, women are more likely to have fibromyalgia, painful bladder syndrome, and irritable bowel syndrome, among others. The same is true for migraine, which is two to three times more prevalent in women than in men. But what drives these sex differences remains poorly understood.

Now, a new study has found key sex differences in how nociceptors function. Nociceptors are sensory neurons that detect potentially harmful stimuli, relaying electrical signals to the brain which, after further processing, can culminate in pain sensation.

The study, from Frank Porreca, University of Arizona, Tucson, US, and colleagues, found that prolactin, a hormone made by the pituitary gland, sensitized nociceptors from female but not male mice, consistent with previous findings from the same group. Conversely, orexin B, a neuropeptide made by the brain, sensitized male but not female nociceptors.

Of most novelty and note, the authors replicated their findings in nociceptors from nonhuman primates and postmortem human donors. They say that these findings may warrant re-examination of past failed clinical trials that did not assess the effects of pain-relieving drugs based on sex.

Ted Price, a pain researcher at the University of Texas at Dallas, US, who was not involved with the research, said the new work is the “first thorough comparison of sex differences in nociceptors across species,” and shows that “we cannot ignore the possibility that different approaches to pain treatment might be needed in men and women.”

The study appeared online ahead of print on June 3, 2024, in the journal Brain.

Prolactin sensitizes only female nociceptors
First author Harrison Stratton told Migraine Science Collaborative that while sex differences in pain processing could occur throughout the nervous system, “we theorized that there must be differences in nociceptors that could be driving pain differentially in men and women.”

There was reason from preclinical studies to support this idea, but researchers were missing a key piece of the puzzle to truly build the case for it.

“There was evidence that sensory neurons were different between males and females at the level of messenger RNA, but it was unknown if this corresponded to functional changes” in these neurons, said Porreca.

The study was grounded in previous findings of sex differences in prolactin signaling in the pain system. Prolactin circulates at higher levels in women than in men and had been shown to promote pain-like behaviors in female but not male rodents. Porreca’s group also recently showed that prolactin signaling is important for migraine signaling in female rats, but it was not tested in males in that study.

The team started by investigating whether prolactin had different sensitization effects in male versus female nociceptors. To do so, they used patch clamp electrophysiology, a method for assessing the electrical properties of neurons.

First, the investigators dissected dorsal root ganglia (DRG) tissue from male and female mice; the DRG is where the cell bodies of nociceptors reside. The next step was to culture the tissue in a laboratory dish, and then measure various electrical properties of the DRG neurons in response to prolactin. The researchers applied prolactin directly to the neurons overnight, and then measured changes in their excitability the next day.

Prolactin had a dramatic effect on DRG neurons from female mice, causing increased electrical excitability, but had very little effect on the excitability of DRG neurons from male mice. The team then repeated these experiments in DRG neurons collected from macaque monkeys, or from human donors. The plot remained the same: Prolactin sensitized nociceptors only from female but not male DRGs from these two other species.

“This shows that differences in the way pain may be produced in males and females are conserved across species and are of evolutionary significance. This is important because it means that any therapy developed using preclinical models such as mice has a high likelihood of also being effective in humans,” said Stratton.

Consistent with the patch clamp electrophysiology findings, the team used different techniques that revealed higher levels of the prolactin receptor in females, in both mouse and human DRG neurons, compared to males.

A different story line with orexin B
The authors also wanted to explore mechanisms that explained why some pain disorders are more prevalent in men than in women. A recent study that grabbed their attention showed that a clinically available orexin antagonist, suvorexant, reduced pain after nerve injury in male mice. Orexin is a neuropeptide involved in sleep, but also in pain, headache, and migraine.

The investigators again turned to patch clamp electrophysiology, this time applying orexin B to nociceptors from male and female mice immediately before measuring cell excitability. They found that orexin B had no effect on nociceptor excitability in female mice but caused rapid sensitization of nociceptors from male mice.

Like before, repeating these experiments with nociceptors from macaques or human donors showed that orexin B sensitized male but not female nociceptors across all the species. And, in line with all of these findings, the team saw higher levels of the orexin B receptor in mouse and human DRG neurons from males, compared to females.

Similar mechanisms likely explain sex differences in migraine, too
The results suggest that prolactin signaling may be a selective driver of chronic pain conditions in women, and orexin B a selective driver in men. Stratton said the data show that the “most fundamental building blocks of pain differ between men and women.”

While the study focused on somatic pain, Porreca told MSC that the study also has bearing on migraine, too, as “it is very possible that the sex differences observed in DRG nociceptors could also be relevant to trigeminal nociceptors innervating the cranial meninges and therefore relevant to headache disorders.”

Price agreed, saying that “migraine is where these sex differences seem to be the most profound,” referring to the striking disparity in migraine prevalence between men and women. He also pointed to another recent study from Porreca and colleagues showing that, according to the available evidence, gepants are effective for treatment of acute migraine only in women, with no proof that they work in men (see MSC related news coverage).

“I am hopeful that studying sex differences will become a big part of future migraine clinical trials. Maybe this will help us to get deeper knowledge of underlying mechanisms that cause people to get migraine in the first place. It is such a mysterious disease,” said Price.

Time to revisit failed clinical trials?
One of the major hurdles facing basic research on pain and migraine in the last decade has been the failure of many clinical trials that tested compounds with promising results in animals. Porreca said a key reason could be that the trials did not account for sex differences in the response to pain treatments under investigation in the trials.

A small clinical trial in 2015 showed that an orexin receptor antagonist called filorexant was not effective for migraine treatment. However, most patients in the trial were women, meaning positive effects of filorexant in men could have been missed. That trial is just one example of many where this logic applies.

Porreca said it’s important that future studies have the statistical power to detect drug responses in a specific sex, and to re-analyze old trials to identify compounds that failed because results may not have been separated by sex.

“Now that we have evidence showing nociceptors are different between men and women, it will be important to consider revisiting prior clinical trials to determine if there is no effect in both males or females separately or if an incorrect conclusion as to overall failure of efficacy might have been reached,” Porreca said.

Price said that Porreca’s recent studies have “set the precedent” to start re-examining past failed trials to look for sex differences in pain and migraine treatments.

“Frank [Porreca] and David Dodick already showed this for anti-CGRP antibodies in acute migraine where the efficacy is much better in females than males,” said Price, referring to their recent study reviewing the evidence for efficacy of the antibodies and gepants in men versus women for acute or preventive treatment. “There is also evidence for COX inhibitor efficacy being better in males than females in acute, experimental models of pain. Why such sex differences exist is definitely something we should know,” Price said.

Fred Schwaller, PhD, is a freelance science writer based in Germany. Follow him on Twitter @SchwallerFred

Image credit: 123RF Stock Photo.

Reference
Nociceptors are functionally male or female: from mouse to monkey to man.
Stratton et al.
Brain. 2024 Jun 3:awae179. Online ahead of print.