While opioids are incredibly powerful pain management tools, they can also be highly addictive. Since 2013, the rate of deaths from an overdose due to synthetic opioids, like fentanyl, has steadily increased, with 22.7 deaths per 100,000 people reported in 2022 in the United States.
Opioids induce neurobiological changes in reward pathways, which subsequently drive their continued use.1,2 Pain also alters these neural pathways, but how these pain-induced changes influence opioid use is not clear, limiting understanding about the early drivers of opioid misuse.3 Additionally, despite research demonstrating sex differences in reported pain, substance misuse, and overdose, the biological basis for these discrepancies remains underexplored.4,5
Now, Jose Morón-Concepcion, a neuroscientist at Washington University in St. Louis, and his group have shown that hormonal effects on neurons in the brain’s reward centers may play an important role in pain-induced opioid usage. In a study published in Neuron, the team showed that estradiol, a female sex hormone, decreased rats’ use of fentanyl in the presence of persistent pain.6 These findings highlight important sex differences in the risk of addiction following opioid use for pain management that could help researchers develop improved assessments and treatments for substance use disorder.
Jose Morón-Concepcion and Jessica Higginbotham explored the role of sex differences on pain experience and the likelihood of drug use in rats.
Yolanda Campos
Morón-Concepcion said that they began investigating whether there was an interaction between long-term opioid use for pain management and the risk of addiction. When the team looked at clinical studies to begin to explore this question, they noticed differences in the rates of opioid use and overdose between men and women.
However, “Studying the relationship between pain and opioid misuse is really hard when we look at clinical populations, just because it's kind of a chicken or the egg [scenario]: what came first?” said Jessica Higginbotham, a neuroscientist in Morón-Concepcion’s group and study coauthor. Long-term opioid use can lead to increased pain sensitivity, which in turn can make individuals more likely to use these drugs. So, the team turned to animal models where they could study the early drivers of opioid use and determine if heightened pain sensation was a contributing factor.
The team injected complete Freund’s adjuvant (CFA), an inflammatory stimulus, into one of the rats’ rear paws to induce pain and provided devices that allowed the rats to self-administer fentanyl. Although researchers confirmed that the inflammatory response and pain perception between male and female rats were similar, male rats used fentanyl more often than female rats in the context of pain.
“We often think that maybe people that would be abusing opioids, it might be because they're in more pain,” Higginbotham said. While they saw that the presence of pain was necessary for the opioid seeking in rats, the male rats’ greater opioid usage did not appear to be due to greater perception of pain. “The pain condition is causing alterations in reward pathway function, and it's the function of the circuits that is really driving the change in behavior,” she added.
Previously, Morón-Concepcion’s group showed that pain decreased activity in dopamine-producing neurons in the ventral tegmental area (VTA), a brain region involved in processing rewards, led to reduced motivation in animals to pursue rewards.7 Since opioids promote the release of dopamine in these same VTA neurons, the researchers considered whether pain may alter neuron function in a sex-specific manner that predisposes males to be more susceptible to increased fentanyl use.
“One of the main goals I had going into looking at this interaction between pain and opioid use was really looking at neural activity, like while the drug taking is actually taking place, as opposed to ex vivo or after the fact,” Higginbotham said. As opposed to looking at a single point in time, the team could consistently study the activity in the brain and how it changes with pain and fentanyl administration in real-time.
To explore how pain and sex differences influenced these VTA neurons in real-time, the researchers introduced a sensor that responded to activity in dopamine-producing neurons in the VTA. Using an optic fiber and novel wireless apparatus, the team measured these responses after pain induction and upon fentanyl administration.
While female rats maintained normal dopamine activity after CFA injection, this activity in male rats initially decreased before returning to normal after three weeks. Fentanyl self-administration initially reduced the dopamine responses in VTA neurons in male rats with pain, but these responses increased over the three-week study period, exceeding the activity observed in the other groups. Meanwhile, female rats’ dopamine responses remained consistent between animals with and without chronic pain. This suggested that pain caused changes to dopamine-producing neurons in the VTA that increased fentanyl use in a sex-specific manner.
To determine whether this effect was mediated by sex hormones, the researchers removed female rats’ ovaries (OVX). OVX female rats self-administered more fentanyl than male rats, even in the absence of chronic pain. The loss of gonadal hormones also increased the fentanyl-driven dopamine neuron activity in OVX rats, resembling the activity of male rats. The findings pointed to a protective effect of female sex hormones in neural rewiring following pain that otherwise sets animals up for increased fentanyl use.
Since estradiol, a gonadal hormone elevated in female animals of reproductive age, influences dopamine activity, the researchers administered this hormone to OVX rats and male rats in the third week that the animals had access to fentanyl.8 Estradiol decreased dopamine activity in response to fentanyl in male rats with pain as well as their fentanyl intake but did not affect these responses in OVX rats. However, in the absence of pain, estradiol increased OVX rats’ use of fentanyl and their fentanyl-driven dopamine responses.
“This is actually showing, for the first time, hormonal regulation is actually involved in the effects of pain on drug escalation,” said Morón-Concepcion. The team concluded that pain in fact changes dopamine neurons in the VTA to become more susceptible to fentanyl use, but estradiol protects against this effect in intact female rats. While estradiol could protect males against this increased drug use, it failed to protect ovariectomized rats with pain, and in fact increased their fentanyl use in the absence of pain.
These findings stood out to Morón-Concepcion because of their potential implications for opioid risk assessments and interventions. For example, their findings suggest that estrogen supplementation may not be effective in menopausal women, even though this hormone is protective at earlier periods and in men. “That means that there's something going on,” he said. “It's not just a single hormone. It's a combination of the balance of these hormones in the body.” He and his team intend to explore the interactions of these other hormones in future work.
While previous research demonstrated that estradiol influenced drug-seeking behavior, John Mantsch, a neuroscientist at the Medical College of Wisconsin who was not involved in the study, noted that, “This [study] takes it one step further and really applies this in the context of what is a really, really relevant model for understanding contributing factors to opioid use [in the context of pain].” 9
While Mantsch said hormonal therapy may not be a treatment for substance use disorder, he said that understanding the role of estrogen in the pathway, “is certainly going to reveal some interesting mechanisms that may be able to guide future therapeutic interventions.”
- Galaj E, et al. Dissecting the role of GABA neurons in the VTA versus SNr in opioid reward. J Neurosci. 2020;40(26):8853-8869.
- Matsui A, et al. Separate GABA afferents to dopamine neurons mediate acute action of opioids, development of tolerance, and expression of withdrawal. Neuron. 2014;82(6):1246-1256.
- Higginbotham JA, et al. Endogenous opioid systems alterations in pain and opioid use disorder. Front Syst Neurosci. 2022;16:1014768.
- Kim JA, et al. Sex-differences in network level brain dynamics associated with pain sensitivity and pain interference. Hum Brain Mapp. 2020;42(3):598-614.
- Kaplovitch E, et al. Sex differences in dose escalation and overdose death during chronic opioid therapy: A population-based study. PLoS ONE. 2015;10(8):e0134550.
- Higginbotham JA, et al. Estradiol protects against pain-facilitated fentanyl use via suppression of opioid-evoked dopamine activity in males. Neuron. 2025;113(9):1413-1429.e5.
- Markovic T, et al. Pain induces adaptations in ventral tegmental area dopamine neurons to drive anhedonia-like behavior. Nat Neurosci. 2021;24(11):1601-1613.
- Vandegrift BJ, et al. Estradiol increases the sensitivity of ventral tegmental area dopamine neurons to dopamine and ethanol. PLoS ONE. 2017;12(11): e0187698.
- Donchek EM, et al. Estradiol regulation of the prelimbic cortex and the reinstatement of cocaine seeking in female rats. J Neurosci. 2021;41(24):5303-5314.
