From electrified frogs to salmon in MRI machines, aquatic animals have been at the cutting edge of neuroscience for decades.
More than two centuries ago, we discovered that electricity can cause muscular contractions. Fifty years ago, the Nobel Prize in Medicine was awarded for the description of how this ‘biological electricity’ is generated in a single cell, known as a neuron. We have since developed techniques to record electrical activity not just in one neuron but simultaneously from thousands of nerve cells. Today, we are on our way to realising the seemingly impossible task of mapping the entire human neural network. What do all these scientific advancements have in common? The host of unlikely underwater creatures that made them possible.
Often, we are confronted with the image of a laboratory animal as a rodent, farm animal or primate. It's no surprise that this should be the case when these very animals are entrenched in our scientific lingo: You are a 'lab rat' for a task, or someone's 'guinea pig', able to be manipulated at whim. But here's the catch: More scientific experiments in Australia are carried out on fish, amphibians and other aquatic animals than on rats, guinea pigs, rabbits and primates combined. More and more neuroscientists are getting hooked on the idea of using marine wildlife to elucidate the workings of the human brain. And while this may sound a little fishy, this isn't a new thing — these slippery creatures have been making waves in neuroscience for more than 200 years.
In 1792, Luigi Galvani, an Italian physician and researcher, made a remarkable discovery. Whilst dissecting a frog's leg, Galvani's metal scalpel touched the amphibian's exposed sciatic nerve. Curiously (and probably quite disturbingly for Galvani), the dead frog’s leg twitched. Galvani repeated his experiment, and each time the frog was shocked, so was Galvani: The metal scalpel, acting as a conductor of electrical charge, was ‘re-animating’ the frog’s muscles again and again.
The Galvani frog experiments sparked international interest, both about electricity as well as the essence of life: Had Galvani, by reinstating movement in the frogs, brought them back to life? This outlandish idea is even rumoured to have galvanised Mary Shelley, a contemporary of that time, to write about electrical re-animism in her seminal novel, Frankenstein.
The dead frog legs had hinted that our nerves were capable of ‘storing’ electricity, but how were they doing this? Galvani had heard of a contraption called a ‘Leyden jar’, where generation of a positive charge on one side of the jar and negative charge on the other was able to store a net ‘electricity’ in the vessel. He hypothesised that this, too, was how nerve fibres in a muscle stored their energy.
Unfortunately for Galvani, there was no way to prove his theory. Measuring charge across two surfaces involved placing recording needles on either side of the surface; trying to stick needles into microscopic nerve fibres without rupturing or damaging the membrane was nigh impossible. It wasn’t for another 150 years that we realised that instead of trying to make the recording needles smaller, we should be searching for something bigger.
Squid have evolved a distinctive neural circuitry underlying their escape behaviour, a common feature of which is enlarged ‘escape neurons’, with thicker fibres than other neurons. Larger fibres conduct signals more quickly than smaller fibres, and so activation of the ‘escape neurons’ when evading oncoming predators means the squid can scoot away at high speeds.
In the 1950s, Alan Hodgkin and Andrew Huxley realised that the squid was the only animal with a nerve fibre large enough to successfully insert recording electrodes. In a series of groundbreaking experiments for which they were awarded a Nobel Prize, Hodgkin and Huxley detailed how the flow of positively and negatively charged ions across a neuron’s membrane generates an electrical signal, just as Galvani had hypothesised.
Frogs and squids had revealed to us the mechanism by which a single neuron generates electrical signals, but what about the functioning of the brain as a whole? The same year that the Hodgkin-Huxley experiments were published, the Nobel Prize in Physics was awarded for the discovery of nuclear magnetic resonance — the property of atoms to orientate themselves in a magnetic field. In the 1970s, researchers realised that they could image this orientation, and hence image living creatures in a non-invasive manner. The first living creature ever imaged in such a way was not an ordinary lab rat, but another underwater creature: a clam. The technique was so successful that it was soon installed in all major research facilities and hospitals to diagnose medical conditions in humans. Today, we know this imaging technique as magnetic resonance imaging, or MRI.
Last decade, a US-based research team led by neuroscientist Craig Bennett undertook an MRI study looking at brain regions involved in social judgements. Participants were to be shown images of various social situations and asked to identify how people in the images would be feeling. Before the study began in earnest, however, a routine preliminary test on the MRI was performed to check that the settings were picking up the appropriate level of detail and contrast. Bennett wanted to test a structure that would have similar features to a human brain — with fat, bone and muscle. A trip to the local deli procured the perfect test subject: a full-length Atlantic salmon.
Bennett and colleagues placed the dead salmon in the MRI and ran through the experimental procedure, displaying pictures of social situations and waiting while the fish ‘responded’. Lo and behold, the salmon’s brain lit up with activity! Had Bennett, just like Galvani, somehow briefly reinstated ‘life’ into this salmon? No, of course not. The salmon had been dead for days before it turned up on that deli doorstep. Bennett knew that this was a red herring, dressed up as an Atlantic salmon.
To measure the activity in one region of an MRI image, the signal in that specific region must be compared to the signals of all other regions. Doing this for an entire image equals a heck of a lot of comparisons. Bennett knew that when many comparisons are statistically analysed, it just so happens that every so often statistics gets it wrong. It churns back out a small proportion of ‘false’ answers – or regions that are mistakenly identified as being active or inactive in comparison to others. This was all the salmon’s ‘activity’ was — the introduced errors.
Bennett’s salmon highlighted the undeniable importance of proper statistical correcting, winning the ‘Ig Nobel’ Prize in 2012 — the award for science that “makes you laugh, then makes you think”. From Nobel to Ig Nobel, all manner of underwater animals were driving the currents of mainstream neuroscience research.
And the next big fish in the neuroscience sea seems to be one of the smallest: the tiny tropical zebrafish. For the first few weeks of their lives, zebrafish are translucent. Just like MRI, this means that we can see what is going on in a zebrafish nervous system in a living and freely-behaving animal. However, these zebrafish can be imaged at much higher resolutions, including the structure and behaviour of individual cells. Better yet, zebrafish are cheap, breed quickly and don’t take up much space. It’s no wonder that schools of thought are moving in the direction of schools of fish.
Neuroscientist Florian Engert, leader of his own research lab at Harvard University, has taken the bait and works almost exclusively on zebrafish. His team are tackling one of the most perplexing challenges of modern neuroscience: modelling whole-circuit brain function. His lab was the first to record activity from every single neuron in a living organism simultaneously, and has succeeded in painstakingly mapping each neuron’s position and connections. Engert’s work represents a huge step forward in being able to create comprehensive models of whole-brain function at a cellular level.
The history of neuroscience is swimming with many and varied underwater creatures that have underpinned some of the greatest triumphs in research. If the next big splash in neuroscience isn’t due to our watery companions, I wouldn’t flounder — after all, there are plenty more fish in the sea.
Edited by Andrew Katsis