Sex matters: Exploring sex differences in opioid withdrawal mechanisms
Dan Kalamarides was the lead author on this study. Dan’s research interests are rooted in neuropsychopharmacology, that is, the intersection of brain physiology, drugs (both “good” and “bad”), and behavior. This interest has been applied in the context of preclinical models for several therapeutic areas including substance use disorders, pain, neuroinflammation, and depression. Dan is currently planning to transition to industry where he can leverage his neuroscience expertise in the pharmaceutical world to enhance treatment strategies for mental health disorders and brain diseases.
or technically,
Sex differences in VTA GABA transmission and plasticity during opioid withdrawal
[See Original Abstract on Pubmed]
Authors of the study: Daniel Kalamarides, Aditi Singh, Shannon Wolfmann, John Dani
Scientific research has long been biased on the basis of sex. From cells and tissues to animals and people, there is a long history of scientists including more male subjects in their studies. As a result, we don’t understand how female bodies respond differently to diseases or to treatments, and the quality of healthcare has suffered. The National Institutes of Health (NIH) and several scientific journals have started requiring researchers to consider sex in their science, but the progress towards equal representation of males and females has been slow.
Opioids - including heroin, fentanyl, morphine, and others - are one of many classes of drugs that affect men and women differently. For example, women are less responsive to the pain killing effects of opioids but more sensitive to affects the drugs have on respiration compared to men. This difference makes it a lot harder to safely and effectively treat women with opioids in the clinical setting, and it can make recreational opioid use more dangerous. Despite these differences in people, basic science research into the effects and mechanisms of opioids in females is still lacking compared to our understanding of the drugs in males.
One area of research on opioids that still has a lot of unanswered questions, related to sex differences and more generally, is opioid withdrawal. Scientists, including recent NGG graduate Daniel Kalamarides, want to better understand opioid withdrawal so that they can treat the withdrawal, help people feel better, and make it easier for people to stop using opioids. In his paper, Daniel and his fellow researchers wanted to learn more about how the brain changes during opioid withdrawal, while keeping in mind that these changes could look different in males and females. Specifically, he was curious about a brain region called the ventral tegmental area (VTA), which contains neurons responsible for releasing dopamine into another brain region (the striatum) involved in reward.
Previous studies have shown that the active effects of opioids (think the “high”) are in part caused by an increase in dopamine release from those neurons in the VTA. This happens because opioids remove a natural brake on the dopamine system. In an opioid-free brain, other inhibitory neurons in the VTA – known as GABAergic neurons because they release the neurotransmitter GABA – decrease the release of dopamine from the dopaminergic neurons. Opioids remove this brake by decreasing the activity of the inhibitory neurons. This makes the system go faster, or, more specifically, release more dopamine.
Your brain adapts if opioids are in the body for an extended period of time. In the VTA, this means that those inhibitory neurons amp up their control of the dopamine-releasing neurons so that, even in the presence of an opioid, a relatively normal amount of dopamine is released. This is fine until the opioids are removed. Now you have an overactive brake, and there’s not enough dopamine released into the reward-related brain regions.
Researchers have found, in male mice only, that the inhibitory neuron control of the dopamine-releasing neurons increases in withdrawal because the connections between them grow stronger. This increase in connectivity is known as long term potentiation (LTP) or plasticity, and it’s one of the primary mechanisms by which the brain changes depending on how it’s used and what it’s exposed to. Knowing that the effects of opioids can differ between males and females, Daniel explored whether a similar phenomenon occurs in female mice.
Daniel first induced opioid withdrawal in mice by giving them morphine for a week, then studied the properties of neurons in the VTA when the mice were in withdrawal. He used patch-clamp electrophysiology, a technique which allowed him to measure the electrical current flowing into or out of the neuron as he manipulated the voltage. By using this technique, he was able to learn about the strength of the connection between the inhibitory neurons and the dopaminergic neurons and compare that connection between male and female mice.
Daniel measured how likely the inhibitory neurons were to release GABA – and thus inhibit the dopamine-releasing neurons – spontaneously and when electrically stimulated. He found that, in male mice, morphine withdrawal increased the probability of GABA release (or increased the strength of that brake). This was a great result because previous studies had also found this phenomenon, which means that this science is replicable. When he looked at female mice, however, he didn’t see any difference between the morphine treated mice and the control mice. That’s a surprise!
Daniel also tried to experimentally force LTP to occur in the brains in morphine withdrawal so that he could learn more about how the probability of GABA release was changing. He stimulated the inhibitory neurons with a really high frequency of electrical current, which would cause LTP in a normal neuron. He found that he could cause plasticity in the female mice, but he couldn’t in the males. This result suggested that the increase in the probability of GABA release in males was due to LTP. The molecular components needed for LTP were all used up in the males, so Daniel couldn’t create more. The components were still available in the females, on the other hand, so Daniel was able to stimulate the neurons and cause LTP.
To be thorough, Daniel also asked if the male and female mice were experiencing a similar level of morphine withdrawal. If the female mice were going through less withdrawal, it could maybe explain the sex differences in plasticity in the VTA. Daniel measured the strength of withdrawal that the mice were experiencing by counting physical signs of morphine withdrawal, and he found that males and females displayed a similar number. After all of these experiments, we still don’t know for sure that the opioid withdrawal mechanisms in male and female mice are entirely different. If Daniel used a different dose of morphine or if he studied the brains at a different time into withdrawal, he might be able to observe the same plasticity in female mice that he saw in male mice. However, by running this control experiment, he was able to strengthen the argument that there is a true difference in how the male and female mouse brains changed in opioid withdrawal.
This research by Daniel and his fellow scientists reinforced the fact that opioids affect males and females differently, and they showed that we still don’t understand how female brains change in opioid withdrawal. Hopefully, this evidence will push other scientists to continue thinking about sex differences in opioid research and in neuroscience broadly. In the meantime, Daniel has led us a step closer towards developing treatments for opioid use disorder, and he’s contributed to reducing bias in science.
About the brief writer: Lyndsay Hastings
Lyndsay is a first year NGG PhD student broadly interested in the relationship between neurocircuitry and behavior.