By utilizing a behavior in zebrafish where they exhibit a tendency to ‘give up’, along with new imaging techniques for their brains and a unique virtual reality setup, a research team has pinpointed the action of ketamine in the zebrafish brain. Remarkably, ketamine targets supporting cells known as astroglia, instead of the neurons themselves.
The long-established anesthetic ketamine has the potential to revolutionize the treatment for severe depression; however, there remain many uncertainties about its specific mechanisms, particularly how it interacts with brain cells and their circuits.
In their quest for answers, scientists are looking to an unexpected source: tiny larvae of zebrafish.
These small, transparent fish might not undergo depression in the same way humans do, but they do display a behavior resembling “giving up”: when faced with obstacles, they cease swimming in frustration – a passive reaction that researchers use to explore depression in animals.
By leveraging this behavioral trait, the ability to visualize the entirety of the zebrafish brain, and a specialized virtual reality apparatus, a research team from HHMI’s Janelia Research Campus, Harvard, and Johns Hopkins discovered that ketamine acts on astroglia in the zebrafish brain rather than on neurons.
Earlier studies by Janelia scientists indicated that astroglia function as a counter that signals the fish when to abandon their efforts. When the fish realizes they are not making progress, they increase their swimming intensity, and the activity of astroglia intensifies accordingly. Once this activity surpasses a certain threshold, the astroglia send signals to the neurons, instructing the fish to stop swimming.
The latest research shows that brief exposure to ketamine disrupts the “giving up” behaviors by excessively stimulating astroglia. This excess stimulation appears to result from ketamine activating noradrenergic neurons that trigger astrocytes, leading to a reduced sensitivity in the astroglia counter. Consequently, the fish continue swimming as usual, even when they perceive no advancement.
“Our findings indicate that astroglia, a type of non-neuronal cell, play a crucial role, and that the primary impacts of these antidepressant drugs likely involve alterations in astroglial physiology,” explains Alex Chen, a joint PhD student in the Ahrens Lab at Janelia and the Engert Lab at Harvard, and one of the lead authors of the study.
The research team’s discoveries, which also demonstrate similar astrocyte activation in mice, could provide researchers with a clearer understanding of how antidepressants function in the brain. This may contribute to the creation of safer and more effective treatments for depression. The intricate workings of antidepressants at the molecular level have bewildered scientists for years, with much of the focus directed at their influence on neurons.
“Our study suggests that targeting astrocytes for new treatments could be a promising avenue,” remarks Marc Duque RamÃrez, a PhD student in the Engert Lab and a co-lead author of the paper.
Using zebrafish to evaluate ketamine
The project initiated when Duque and Chen aimed to determine if zebrafish could be used to test antidepressants previously shown to be effective in humans and rodents. The small size and transparency of zebrafish allow for the imaging of their entire brains, enabling researchers to monitor the effects of the drugs with precision.
The Ahrens Lab’s prior investigations established that zebrafish demonstrate a phenomenon known as futility-induced passivity, or the “giving up” behavior, which has also been observed in rodents. In a virtual reality environment, the researchers held the fish steady and presented them with various visual cues. When shown a pattern that simulated moving backward, the zebrafish reacted by moving their tails as if they were swimming forward. However, when the visual representation shifted to one that imitated being stuck, the fish initially struggled before yielding, becoming passive and ceasing to swim.
In this recent study, the researchers found that ketamine significantly reduced this giving up behavior for more than a day. While the fish still exerted effort when their swimming proved ineffective, they were less likely to surrender easily and displayed less passivity.
The authors also examined other rapid-acting antidepressants, including certain psychedelic compounds, finding a similar reduction in passivity as observed with ketamine. Conversely, stress-inducing agents, like chronic glucocorticoids, exacerbated the giving up behavior.
Imaging illustrates action on astroglia
Next, the team focused on how the drug affected the fish’s brains. Previous findings from the Ahrens Lab indicated that the tendency to give up is linked to a specific type of glial cell called radial astrocytes.
Using full-brain imaging, the researchers discovered that ketamine raised the calcium levels in the astrocytes, indicating that the drug significantly activated these cells for several minutes post-administration. They believe that while brief or rapid surges in astroglial calcium might incite the giving up behavior, the prolonged effects of the ketamine-induced calcium surge may weaken the astroglia’s responsiveness to the futility signal, making the fish more resistant to giving up in similar situations in the future.
“It’s desensitized because during ketamine it was so hyperactivated,” explains Misha Ahrens, a Senior Group Leader at Janelia and a senior author of the paper. “It’s akin to taking a cold shower – afterward, you’re less sensitive to the cold, but this is happening at a cellular and molecular level.”
The team’s findings also showed that this same mechanism operates in mammals. Eric Hsu, a graduate student at Johns Hopkins and co-lead author of the paper, noted that astrocytes were similarly activated in mice, both during their “giving up” episodes and when they were administered ketamine.
“This evidence of a consistent mechanism across species strengthens the possibility that similar processes occur in humans,” adds Dwight Bergles, a neuroscience professor at Johns Hopkins and a senior author on the paper.
This study illustrates that ketamine impacts astrocytes by enhancing norepinephrine levels; however, the precise methods through which this occurs and how it subsequently alters neuronal and astroglial physiology remain unclear. Nonetheless, these findings highlight a potential role for astroglia in depression and may guide future research directions.
“We should be cautious not to overstate these results, yet they could serve as a model for certain aspects of the mammalian brain,” Ahrens concludes.
