Marijuana is supposed to mellow people out, yet for some users a hit brings dread instead of calm. A new mouse study points to why: a single dose of a cannabis-like drug made fear-related brain cells fire harder, and switching those cells off stopped the drug from driving the animals to avoid a threatening smell. Freezing in fear, though, kept happening even with those cells shut down, pointing to two separate brain pathways for two different panic responses.
Researchers at Northwestern University, writing in the journal Nature Communications, traced the effect to a small group of neurons in the central amygdala, a deep brain region built for processing threats. A cannabinoid drug weakened the natural calming signal that normally keeps these cells quiet, making the fear cells far easier to excite and setting off bursts of activity tied to approaching, fleeing and freezing behavior.
Beyond the mouse cage, the findings carry weight for people. Cannabis use has climbed sharply in recent years, and a study tracking 12 million people found that cannabis-related emergency room visits were linked to about three times the risk of a later medical visit for an anxiety disorder. This new research cannot prove cannabis causes anxiety in humans, but it lays out, cell by cell, one way the drug could push a calm high toward sudden panic.
How Scientists Linked Cannabis and Anxiety in the Lab
To study this, the team used a lab-made drug called CP55940, which activates the same brain receptor as THC, the main mind-altering chemical in marijuana. Mice received different doses of the drug or a plain control solution, then were exposed to a chemical that mimics predator odor, a standard way to trigger natural fear in rodents without using an actual predator. Researchers tested both male and female mice, with roughly 18 to 22 animals per dose.
Researchers tracked how much the mice moved, whether they froze, and how much time they spent avoiding the area with the strongest smell. A low dose made the mice slightly more active, while the highest dose slowed them down. Higher doses increased freezing and avoidance across the board. Even so, the animals largely kept their ability to move, so the extra freezing and avoidance weren’t simply the drug slowing them down.
Fear Cells Light Up After Cannabis Exposure
A closer look came next. Researchers used a tiny camera mounted on each mouse’s head, letting them watch individual brain cells fire in real time while the animal moved freely. They focused on cells in the brain’s fear center that release a signaling chemical called somatostatin, nicknamed SOM cells, which earlier work had already tied to fear, avoidance, and freezing.
After the mice got the cannabinoid drug, these SOM cells fired in stronger bursts, and at the highest dose, more of them switched on overall. These cells also stopped acting in sync, splitting into competing activity patterns rather than firing as one unit.
When the mice were then exposed to the predator smell, these same cells showed busier, more layered responses tied to approaching, fleeing, and freezing. A computer model trained on this brain activity could predict, just by watching the cells fire, whether a mouse had taken the drug, especially as it approached the smell. These cells also began tracking more information at once, such as the mouse’s location combined with whether it was moving, and this effect grew stronger at higher doses.
How Cannabis Loosens the Brain’s Natural Brake
To confirm these fear cells were driving the behavior, not just along for the ride, researchers used a tool that blocks these SOM cells from sending signals, then compared those mice with a control group.
With those cells silenced, the cannabinoid drug could no longer make mice avoid the smell more. But freezing told a different story: the drug still increased freezing even with the cells switched off. That split points to more than one brain route for processing a threat. The drug appears to boost avoidance through this group of cells, while its effect on freezing seems to run through other brain circuits.
Lab experiments on thin slices of brain tissue revealed how. Normally, these fear cells receive a steady calming signal from neighboring cells, carried by a brain chemical called GABA, which acts like a built-in brake and keeps them from firing too easily. The cannabinoid drug weakened that brake. With less resistance holding them back, the fear cells became easier to excite. When researchers blocked the GABA brake directly with a separate chemical, they saw the same jump in cell activity that the cannabinoid drug produced, and combining both treatments didn’t push activity any higher than either one alone, pointing to that same natural brake as the route behind the drug’s effect.
Source : https://studyfinds.com/how-cannabis-can-flip-brains-calm-into-sudden-panic-new-study-shows/
