As is well known, frightening situations can get our pulse rate skyrocketing – or our heart “almost stops”. Is that also the case with other living beings? Researchers have now used fruit flies to study how the insects’ special cardiovascular system reacts to danger. Apparently fear can also trigger palpitations in them – as well as the opposite. This depends on whether the fly decides to flee or whether it goes into shock. Surprisingly, a kind of autonomous nerve reaction also seems to be at work in insects. This result is of importance for basic research, say the scientists.
Just the sound of a quickening heartbeat can give you goose bumps – it’s a symbol of fear. In addition to the increased heart rate as a reaction to danger, the opposite effect is known in humans and other vertebrates: If fear triggers a kind of paralysis rather than an escape reaction, the pulse rate can drop significantly. In extreme cases, this leads to dropouts, which are reflected in the phrase “my heart almost stopped”.
“We know that in vertebrates, in the event of a threat, the so-called autonomic nervous system comes into action and causes the changes in heart activity known to us in a flash. This system does not exist in insects, however, and it was unclear what changes in cardiac activity they show when there is danger, ”says Marta Moita from the Champalimaud Center for the Unknown in Lisbon. In order to close this knowledge gap, she and her colleagues have now carried out studies on the famous research model insect: Drosphila melanogaster.
Fly hearts in sight
In their experiments, the scientists observed the heart of the flies through their slightly transparent exoskeleton using special lighting and fluorescence effects. The test animals were immobilized, but were still able to show escape reactions in the form of “running away”: They stood on a tiny ball that they moved with their feet. To frighten the flies, the researchers presented them with a growing shadow on a screen that looked like an approaching threat.
“Surprisingly for us, the heart of the flies also changed its activity depending on which defense reaction was accepted,” reports Moita: When she decided to flee, the heartbeat accelerated. If, on the other hand, the animal fell into a state of fear – an inconspicuous immobility – the heart slowed down, the studies showed. “This finding is very interesting for us neuroscientists,” emphasizes Moita. “Since flies do not have an autonomous nervous system like that of vertebrates, this means that there is obviously a different mechanism at play here. The question now is whether there is a structure similar to the autonomic nervous system that we do not yet know, or whether there is a completely different mechanism that has yet to be discovered, ”explains Moita.
As the researchers also report, they also made interesting detailed discoveries relating to a special feature of the insect’s heart: it can pump in two directions. In contrast to our hearts, the organ is a kind of tube made up of two rows of cells. Through certain contraction patterns, the insect heart can generate a flow of blood in the direction of the upper or lower body. The experiments now showed that the fly’s heart pumps more actively towards the front part of the body in both reactions to danger – escape and paralysis.
New research potential
“At least when trying to escape, it makes sense to pump more nutrients into the front part of the body, because that’s where the brain, legs and wings are located. The reaction to the immobility, on the other hand, was rather surprising. We suspect that the fly, even though it freezes, is preparing for action, ”explains Moita. Through further studies, the researchers also found indications that this state of readiness is associated with increased energy consumption. “This finding contradicts the assumption that rigidity is a passive, energy-saving behavioral state. An open question is what the flies are preparing for, ”says Moita.
The most important research topic that the researchers now want to pursue, however, will be the investigation of the neural structure that controls the heart reactions in the flies at risk. The scientists hope that the findings in flies – as in other cases – can uncover basic principles in living beings. “Ultimately, this research could contribute to a better understanding of how the nervous system controls behavior in other animals, including humans,” concludes Moita.
Source: Champalimaud Center for the Unknown, specialist article: Current Biology, doi: 10.1016 / j.cub.2021.10.013