Some rats freeze when they anticipate danger. Others make a run for it
Fear has more than one gear. Why do some rats freeze while others spring into action when they anticipate danger? New research explores the brain mechanisms behind the responses, with implications for PTSD treatment.
A haunting song comes on the radio. A stranger on a bus has the same voice as an intimidating bully from your past. Whatever the trigger is, it brings back something you’d rather forget as the past takes over, blurring out the present.
Maybe you freeze in your tracks until someone next to you gives you a gentle nudge — hey, you okay?
Or maybe you get the urge to get away.
This response is known as conditioned fear.
To understand it better, scientists study this process in animals. They pair a harmless cue with an unpleasant experience until the cue alone triggers fear.
For decades, researchers using rats as subjects primarily looked at freezing — pressing pause on all rodent business and staying still — as evidence that the animal learned to be afraid of a particular prompt. But as a Northeastern University team confirmed, there’s another possibility. The exact opposite behavior — darting from corner to corner — can also signal learned fear.
A study by Julia Mitchell, who recently earned her doctorate in psychology at Northeastern, and psychology professor Rebecca Shansky examined both responses and the brain mechanisms behind them.
As they report in a recent issue of Oxford Open Neuroscience, darting occurs more often in females. They also identified a new role for a brain region called the infralimbic cortex in shaping conditioned fear responses by rats learning to anticipate a threat.
Rats provide a useful model for this work. The basic brain circuitry involved in learning about threats parallels that of humans pretty closely. Unlike more complex mammals, such as monkeys, however, their brains are easier to manipulate.
The findings have promising implications for treating post traumatic stress disorder (PTSD) in humans. By revealing how the brain shapes different reactions to learned fear, researchers can create tailored interventions that set people free from the grip of past trauma.
But to understand why those findings matter, it helps to step back. What is fear, and how do scientists study it?
There are two basic types, Mitchell said.
In addition to the learned response, there’s also an instinctual gut reaction that kicks in when you’re faced with sudden danger, be it a rabid possum on the loose, a strange noise in the middle of the night or a flower pot falling from a windowsill overhead.
With no time to lose, evolution has hardwired just a handful of options. There’s fight, flight, freeze or, more recently, fawn — the attempt to get into a potential attacker’s good graces. The options are limited for a reason. When danger strikes, it’s time to pick your lane and stick with it.
Conditioned fear, on the other hand, attaches one of those primal reactions to a specific warning sign. For example, a loud noise could work as a trigger. The sound itself isn’t dangerous, but the individual learns to be wary of over time.
As Mitchell explained, this is why people with post traumatic stress disorder (PTSD) react strongly to sounds associated with previous trauma.
The original threat could have been gunfire during combat, an intruder pounding on the door or a moose charging at a car. Months or even years later, an ordinary bang may bring the fear rushing back even when no danger is present.
That distinction between instinctive and learned fear lies at the heart of the experiments in Shansky’s lab. In their latest study, rats heard a 30-second tone paired with a half-second foot shock from an electrical charge in the cage floor, which was uncomfortable but not overwhelming. The researchers varied the amount of time between tone-shock pairings. As a result, the rats couldn’t predict when the round would occur.
Over time, the harmless tone effectively inherited the fear originally triggered by the zap.
“It’s not that you’ve learned how to have the reaction, it’s that you’ve learned to associate something that should be an innocuous stimulus with an aversive one,” Mitchell explained.
Some rats froze when they heard the tone. Others darted — “a rapid little dash from one side of the chamber to the other,” as Shansky described it.
A single dash was enough to put a rat in the darter group. Throughout the experiment, darters were generally more active and showed a stronger physical reaction to the foot shock.
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These initial findings brought on a slew of new questions.
What makes some animals dart instead of freeze, and why are females more prone to do it? And what brain mechanisms help explain the divide?
The search for answers led to the infralimbic cortex, which helps regulate emotion and stress.
Shansky explained that scientists had already linked this region to a process known as extinction — “unlearning” the conditioned fear response when the tone no longer leads to a shock. Perhaps it could also clear the way for darting?
When the researchers tinkered with the infralimbic cortex by stimulating and suppressing its activity, they found that it helped shape both responses — but not as a simple on-off switch. Instead, it’s more like a complex control panel.
The researchers don’t yet know the full mechanism that tips an individual rat toward one response to conditioned fear over the other.
As Thomas McHugh, deputy director of the RIKEN Center for Brain Science, a research center, told Northeastern Global News, multiple factors might be at play. Brain wiring, genetics and even body size could make a difference, he said.
What’s important is that darting seems to be a more active strategy, Shanksy suggested. Scientists once suspected that darters simply feared the initial shock more. Now, they’re considering whether darting might offer an evolutionary advantage.
“It’s a proactive response, trying to escape, as opposed to just freezing in place,” Shansky said.
While humans don’t have a region called the infralimbic cortex, they have comparable areas, which have been linked to PTSD, Shansky said.
That connection opens up intriguing possibilities for treatment down the line.
Previous experiments found that darters appeared to “unlearn” the tone-shock association faster once the threat disappeared. If springing to action reflects a more flexible fear response, understanding the brain mechanism behind it could offer new directions for therapy.
