How Brain Injuries Affect Sleep-Related Neurons Differently in Male and Female Mice
Rebecca Somach was the lead author on this study in the Cohen lab. Rebecca is currently doing her postdoctoral fellowship at Swarthmore College. Her current research uses planarians as a model system to understand how pesticides affect developmental neurotoxicity. She's interested in helping students understand neurobiology and helping them achieve their research goals.
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Mild Traumatic Brain Injury Affects Orexin/Hypocretin Physiology Differently in Male and Female Mice
[See Original Abstract on Pubmed]
Authors of the study: Rebecca T Somach, Ian D Jean, Anthony M Farrugia, Akiva S Cohen
A traumatic brain injury (TBI) occurs when an external source, such as a strong hit to the head, damages the brain. TBIs can be mild or severe, with mild TBIs (mTBI) being especially common and accounting for a large portion of TBI cases. Both TBI and mTBI can negatively impact brain physiology and lead to problems like sleep issues. Surprisingly, despite a large portion of TBI patients experiencing disordered sleep, there is very little research focusing on how these injuries may affect neural circuits that regulate sleep. Understanding the link between TBI and disrupted sleep is an important step in improving treatments for TBI patients.
Orexin/hypocretin neurons are specialized brain cells found in a brain region called the Lateral Hypothalamus. These neurons play a critical role in sleep and wake regulation. When they are not working properly or if they die off, it can lead to sleep-related issues, such as narcolepsy (a disorder where people fall asleep unexpectedly) or excessive sleepiness during the day. Some research suggests that TBIs can disrupt orexin, which could explain why many TBI patients have sleep troubles.
However, the Cohen lab found that sleep problems after mTBI can happen even when the number of orexin neurons does not decrease due to the injury. This raised an interesting and important question: is it the activity of these neurons after injury, rather than how many there are overall, that is important? To tackle this question, the Cohen lab looked at how active orexin neurons were after mTBI by labeling them for cFOS, a protein that is produced when neurons are active. They found that instead of orexin neurons dying off, the activity of the overall population was actually reduced (less cFOS in the population), suggesting that the number of neurons alone is not the only factor to consider after TBI. These findings were important, as only two other studies have looked at how orexin neuron activity changes after brain injury.
Rebecca Somach, an NGG graduate from the Cohen lab, decided to further expand on this work by incorporating other fascinating neuroscience techniques. These included electrophysiological recordings, which measure electrical activity in the brain, and a mouse that has orexin neurons tagged with a glowing protein, making it easier to study and see these neurons in the brain. Rebecca also wanted to examine the effects of brain injury might be different in male and female mice, since sleep can vary between sexes. This is particularly important because of the little research on the effects of TBI, specifically mTBI, on orexin in female animals.
Rebecca specifically examined whether mTBI changes how orexin neurons might respond to electrical signals or how often they fire in male and female mice. To do this, they used current clamp recording, a neuroscience technique used to see how neurons react when they are injected with electrical current. In female mice, they found that orexin neurons became more negatively charged after mTBI, which is a process called hyperpolarization. In contrast, they found that in male mice, orexin neurons had a reduction in action potential threshold after mTBI, making it easier for them to fire. These seemingly small changes in how neurons behave after mTBI can have a big impact on how they communicate to other areas of the brain.
While orexin neurons are found in the Lateral Hypothalamus and talk to neurons adjacent to them, they are also connected to many different parts of the brain. Because of this, Rebecca wanted to examine whether mTBI could affect the connections these neurons receive from other brain regions. Orexin neurons receive both excitatory and inhibitory signals. To only focus on excitatory signals, Rebecca recorded neurons while applying a chemical called bicuculline, that blocks inhibitory signals. This allowed them to study how only excitatory signals may have changed after brain injury in male and female mice. They looked at two types of excitatory post-synaptic currents - spontaneous (sEPSC) and miniature (mEPSC), which can be thought of as the orexin neurons’ responses to excitatory inputs from other neurons. In short, the stronger the EPSCs, the more likely the neurons are to become active. After injury, they saw that these signals were less frequent and weaker in both male and female mice, meaning that the orexin neurons were getting less excitatory activity.
Next, Rebecca did the reverse experiment and isolated only the inhibitory signals to examine if there were mTBI-induced changes. She did this by recording orexin neurons with AP5 and CNQX, which are chemicals that remove excitatory currents. Rather than looking at sEPSCs and mESPSCs, she then focused on recording spontaneous and miniature inhibitory currents (sIPSC, mIPSC) in male and female mice. After injury, she saw reduced time between sIPSCs, meaning they occurred more often. Interestingly, she only saw stronger sIPSCs and mIPSCs in female mice, which could help explain why sleep disturbances after TBI can vary depending on sex.
Overall, while previous research has shown an effect of mTBIs on orexin neurons, not many studies looked at how brain injury impacts the activity of these neurons. Rebecca addressed this gap by exploring how the activity and connections of orexin neurons are altered after brain injury in both males and females. She shows that mTBI significantly alters how orexin neurons respond to inputs from other neurons and that although these changes appear similar between male and female mice, the underlying mechanisms are not the same. Ultimately, her work provides insight into how brain injury may disrupt sleep-related circuits and informs us why TBI patients may experience disordered sleep.
About the brief writer: Diana Pham
Diana is a Neuroscience Ph.D. student in Dr. Brett Foster’s lab. She is broadly interested in how memories are encoded and consolidated in the brain.