Well, to be clear, the animals didn't volunteer. We made the choice for them. Some of these mice in this experiment were wild-type, others were actually specifically bred to have a certain genotype (the P17 knockout) that made them relevant to this experiment. We created them for this particular chance to understand something about the brain and how to help it, and we kill them after we learn what we wanted.
This is a complex, nuanced topic, and yes, you're right, it is cruel to the mice (although, if you read the paper, they took steps to _minimize_ the mouse's suffering -- the mice were anaesthetized during the concussion, and they were euthanized after the experiment). In order to perform an experiment in animals in a research setting, you have you propose your experiment to a ~5-person committee called an IACUC (every institution doing research on invertebrates has to have one), and they review your experiment to ensure that it meets their standards of being ethical, humane, and of sound experimental design.
Because of this system, we are able to work with animal models (often mice) that teach us a lot about how our own body works, how proposed drugs could be used to treat illnesses and injuries. We do this because we really don't have a better system of learning about bodies.
We do have many _worse_ systems of learning about bodies. Centuries of experiments (many in recent memory) of experiments on humans. People experimenting with convicts, impoverished people, and people who someone decided are sub-human and are therefore ok to experiment on. That's not to say that it's ok to cause suffering to animals because we also caused suffering to humans. I bring it up because I actually derive some hope from it, that we are more aware of suffering and take more steps to reduce the suffering we cause in pursuit of knowledge. I sincerely hope that that trend will continue.
In my day job, I'm a wildlife photographer. I spend a lot of time watching animals. I see animals kill and eat eachother. I see animals get injured and sick, I see many animals die prolonged, painful deaths. Causing animal suffering is not an exclusively human domain -- much of this also happens right next to our houses, just out of our sight or awareness. I guarantee, wherever you live, that many animals are killed and eaten in your house every day, mostly without your knowledge.
Of course, we cause our share of that suffering, as well, more through neglect than intent. I see countless animals dying, entangled in plastic or with guts full of plastic chunks. I see tens of thousands of fish in my city's rivers impaired or dying from chemicals in my city's runoff. I watch otters and bald eagles happily eat those animals -- they're easy to catch, after all. It's harder to be angry at the people who caused this suffering, because, well, we don't know who they are, and often _they_ don't know who they are. They just chucked a plastic bag into the road, or made a mistake on a jobsite, or were just passive participants in a very flawed system.
Sometimes, we cause that suffering through intent. Victor, the mousetrap company, has sold over a billion mousetraps, which we use because we find mice in our homes and cities to be a nuisance, or gross, or whatever the imagined offense is. Take a walk through the pest control aisle at your local big box and take a moment to imagine the unmitigated suffering that each of those chemicals cause when it reaches its intended, or unintended, target.
I also volunteer in a wildlife rehab in my state -- whenever a wild animal is found injured, it ends up in this particular building (I live in a small state). Each year, we get thousands of animals, often unintentionally but painfully injured by people. Possums that get into the rat poison and show up bleeding from their face and hands. Waterfowl that show up with lead poisoning from eating lost fishing weights. More animals hit by cars than I can count. Some animals are intentionally injured by people -- lots of bloody birds peppered with bullets, often airsoft or other non-lethal bullets that someone just shot because they were bored. We treat these animals. Many get better, many don't.
We euthanize the animals that don't using the same method the scientists use in this paper -- isoflurane to anaesthetize and a second method to ensure that the animal has died painlessly. I feel fine about this practice -- the goal of a rehab is to reduce harm in the wild population and to return healthy animals to the wild. Not all injured animals get better, and the cold calculation is that it's kinder to kill the animal humanely and painlessly than to keep it alive in a state of suffering and stress.
My point is, the scale of animal suffering right outside your door is vast. It's a noble effort to want to reduce that amount of suffering. However, _most_ of the suffering that's around us in cities is pointless, un-intended suffering, and nobody gains anything from it -- the mouse in the glue trap or the fish stuck in a plastic bag. It's true that these scientists are causing animal suffering, but it's also true that there is a point to it. They are doing these experiments to understand and reduce future suffering, in people as well as animals (basically all the drugs we use in the wildlife rehab, for example, are the exact same drugs we use in human hospitals). The suffering the scientists are causing is a drop in the bucket compared to what is happening each day, unseen, in the square block around your house. It's easy to criticize their work because they are being honest about their methods -- that's part of what being a scientist means -- but in my opinion, if your goal is to reduce animal suffering, you're missing the forest for the trees. Scientists are not the target you're looking for, both in terms of the scale of animals or the magnitude of suffering.
I will say, we have made dramatic improvements in how we understand medicine's effect on the body. Computational chemistry and drug discovery has made giant strides in the past several decades, and that lets us learn by computation what we could previously only learn from animal trials. Perhaps in the future we will be able to better understand complex mechanisms in the body and represent them in software, letting us try some or all of these ideas for fixing problems painlessly, in software, rather than in meatspace. Maybe you'll help create that future.
I'll end with what I feel is a touching piece of art -- in Siberia, there is a statue called the Monument to the Laboratory Mouse (https://en.wikipedia.org/wiki/Monument_to_the_laboratory_mou...). It stands tribute to the countless mice who gave their lives in pursuit of knowledge, and the statue, if you look at it, is a blend of a mouse and a scientist, recognizing the link between their lives and the knowledge that comes from both those lives. It's a complex piece of art that acknowledges a complex topic, I appreciate it, and maybe you will, too.