Faster heart rate, more stress?

The Problem

We know our heart rates speed up when we're anxious, excited, surprised, etc. But if our heart rate speeds up first, can it affect how we feel?

In today’s paper, researchers at Stanford tried to answer these questions by controlling a mouse’s heart rate and monitoring how it affected its behavior. To study this, the authors turned to a non-invasive technique to control heart rate without external intervention.

The Solution

The beating of your heart is from a large number of individual cells all syncing up and contracting at the same time. When they all contract at once, your heart beats. These cells are called cardiomyocytes.

The authors forced the mouse cardiomyocytes to create a light-sensitive protein that made the cells contract whenever the LED is on. Pulsing the LED at different speeds led to different heart rates, all controllable with a simple light switch.

At this point, they could reliably control heart rate with the LED without side effects and were interested in looking at how heart rate and anxiety are related. What next?

First, the mice were placed in a chamber with 2 sides. The LED lit up when the mice went in one of the sides but not the other. This kind of experiment by itself doesn’t stress the mice out at all. They’re just roaming around and exploring the chambers.

However, if the increased heart rate on the LED side resulted in anxiety, the mice would prefer the non-LED side and hang out over there. But they didn’t show any preference even with the LED. We can see this in the random patterns shown in the mouse tracking data above.

This tells us that an increased heart rate by itself isn’t enough to affect the mice.

Next, the mice were placed in a potentially stressful situation with the LED-induced elevated heart rate. To do this, the mice were placed in a simple maze shaped like a (+) sign with 4 arms. 2 of the arms were open while 2 were closed.

When they’re not stressed out, mice don’t care which of the arms they’re in and roam around without a preference. However, when the mice get stressed, they strongly prefer the safety of the closed arms. When their heart rates were raised by the LED in this experiment, the mice avoided the open arms and stayed in the closed arms. You can see this in the mouse tracking data below with the bottom chart being the “LED on” condition.

This indicates that the combination of raised heart rate + potentially stressful situation leads to a stressed state. It doesn’t matter that the mice didn’t have anything to be anxious about; they were stressed anyway (relatable).

This directly contrasts with the previous experiment where the mice were roaming around the chamber. In that not-stressful context, the mice didn’t care about the elevated heart rate. But the heart rate started mattering once a potential stressor was included.

Altogether, this shows that heart rate can induce anxiety in a context-dependent manner. The context being important implies that the brain is involved in interpreting the high heart rate based on other factors in the surroundings. There’s some flow of information from the heart to → brain that’s incorporated into the animal’s behavioral response based on other things the animal perceives. Context matters!

See you next week for more science,

Neil

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