Animal research assists in understanding fentanyl withdrawal symptoms

Overdose deaths related to synthetic opioids (primarily fentanyl) in the US have increased to over 73.000 thousand deaths per year [1]. This makes fentanyl the biggest contributor to drug overdose deaths. But how did we get here? And how is animal research contributing to understanding addiction and withdrawal? Keep reading this blog to learn more.

What is Fentanyl and why is it a problem?

Fentanyl is a synthetic opioid that was originally created to serve as a painkiller for surgeries. However, fentanyl has become problematic because drug dealers have been lacing and replacing drugs with fentanyl. Even though dealers have been doing this since the 70-ties, recently the occurrences have skyrocketed. Higher production in countries like China, Mexico and India in combination with the emergence of the dark web, has created an environment for illegal drug trafficking to flourish.

Lethal dose of Fentanyl

This has the effect that more people are knowingly and unknowingly consuming fentanyl. The drug is very potent there is a high risk of overdose and addiction. Therefore, it is important to know more about how exposure influences the behavior and physiology of users.

Research into withdrawal symptoms and recovery

Much like other opioids, fentanyl binds to the brain’s opioid receptors. These receptors are in the parts of the brain that control emotions and pain. Therefore, using fentanyl numbs the body’s responses and causes a rush of dopamine. Extended usage can also cause confusion, nausea and loss of consciousness.

How addiction and withdrawal progress

Because of its relatively short high and strong impact, it is very easy to get addicted to fentanyl. Addiction increases the tolerance of the user to individual doses, causing them to keep seeking more drugs, for a similar high. Besides the various health risks that come with addiction, extensive opioid usage carries a heightened risk of experiencing an overdose.

During an overdose the drug user will experience either slowed or stopped breathing as well as other numbing effects. This loss of oxygen can cause the person to suffocate and die. If a person is experiencing fentanyl overdose their breathing can be restored by administering naloxone.

Complications with treating Fentanyl

While opioid replacement therapies such as methadone and buprenorphine are effective in treating opioid use disorder, treating fentanyl addiction poses unique challenges. Its potency, fast-acting nature, and short duration of effect complicate both withdrawal management and relapse prevention. More preclinical research is necessary to uncover the neurobiological underpinnings of fentanyl addiction and identify potential therapeutic strategies.

Animal research gives insight into addicition

Researchers from the University of North Carolina developed a drinking in the dark (DiD) paradigm to model oral fentanyl self-administration [2].

Male and female mice were given access to increasing fentanyl concentrations over five weeks. This design mimics a voluntary oral consumption route, an increasingly common way fentanyl is taken due to counterfeit pills in the illicit drug supply. Here are some of their key findings:

• Fentanyl consumption increased with increasing concentrations even though water consumption was decreased.
• Naloxone-precipitated withdrawal revealed sex differences. Males displayed more severe withdrawal symptoms despite consuming less fentanyl. This indicates that withdrawal severity is not strictly dose dependent.
• Fentanyl impaired habituation and increased avoidance behaviors. Males spent less time in the light compartment during the light-dark test, while females showed altered adaptation in the open field.
• Ten days into abstinence, fentanyl-exposed mice showed impaired fear extinction learning, freezing more in response to conditioned cues compared to controls. This highlights a disruption in emotional regulation processes.
• Electrophysiological recordings revealed that basolateral amygdala (BLA) neurons became hyperexcitable, with an increased excitatory-to-inhibitory ratio. These changes likely contribute to the impaired fear extinction and heightened avoidance behaviors.

How EthoVision contributed

To measure the fear response and learning of the mice, the researchers used a fear conditioning chamber with a shock grid floor. Both the controlling of the hardware and analysis of behavior were done by EthoVision XT. EthoVision XT is the premier behavioral tracker used by researchers all over the world.

With this software, you can not only control a fear conditioning system, but many other systems. Furthermore, EthoVision XT allows you to automatically classify behaviors, track the nose, base and tail point, and analyze activity.

Its versatility allows for precise measurements in studies of addiction, anxiety, and learning, making it an essential tool for translational research.

Key take aways

• Long-term oral fentanyl exposure disrupts behavior and neural function in mice.
• Sex-specific differences emerged: males showed stronger withdrawal symptoms, while females consumed more fentanyl.
• Fear extinction learning was impaired, linking fentanyl withdrawal to emotional dysregulation.
• The basolateral amygdala was identified as a critical hub for these neural changes.
• EthoVision XT enabled precise measurement of fear responses and behavior, supporting robust translational research.

References

[1] National Institute on Drug Abuse, visited September 2025. https://nida.nih.gov/research-topics/trends-statistics/overdose-death-rates.

[2] Downs, A.M.; Kmiec, G.; McElligott, Z.A.(2024)
Oral Fentanyl Consumption and Withdrawal Impairs Fear Extinction Learning and Enhances Basolateral Amygdala Principal Neuron Excitatory-Inhibitory Balance in Male and Female Mice. Addict. Neurosci, 13, 100182.https://doi.org/10.1016/j.addicn.2024.100182.