Animal model diversity in neuroscience

One of the central goals of neuroscience is to understand how the human brain gives rise to the complex behaviors, thoughts, and emotions that humans experience at every instant. Achieving such an understanding has been monumentally difficult, in part due to the complexity of the human brain. The human brain contains about 100 billion neurons and trillions of connections between them. Furthermore, neurons are not all functionally or physiologically identical – for example, within the primate retina alone there exist over 60 distinct types of neurons. While we do not have similar understanding of the diversity of neurons in the rest of the brain, some estimate that the number of different neuron types may number from 100 to 1000.

To study a system as complicated as the brain, it is essential that we find a way to specifically manipulate and measure the system’s inputs and outputs. For example, we might want to kill or silence a certain population of neurons to assay their importance in decision-making, or we might want to record the activity in these same neurons while an individual makes decisions. For obvious ethical reasons, these manipulations cannot be done in humans, so instead many researchers study the brains and behaviors of animals such as mice and fruit flies. Research using these so-called model organisms has yielded a wealth of mechanistic insight into brain function.

Animal models in modern neuroscience

Work in rodents dominates modern neuroscience research. Data from the early 2000s suggest that nearly half of all neuroscientific studies were conducted in rodents1. In contrast, neuroscience research in all other mammals together accounted for less than ten percent of publications, as did research in all invertebrate species. The statistics of NIH funded grants are similar, and no data suggest that things have changed today, almost 20 years later. Beyond rodents, neuroscientists study a few other canonical model organisms – the fruit fly D. melanogaster, the zebrafish D. rerio, and the roundworm C. elegans.

There are two main reasons why a few model organisms dominate neuroscience research. First, these are organisms for which many experimental tools are readily available. For example, mice and fruit flies have been workhorses in genetics for a century, and multiple techniques have been developed to add or remove genes in these animals. The same cannot be said for non-traditional animals that few study. The second reason is specific to rodent and primate models – as these are mammalian models, their nervous systems are more similar to that of humans, so observations made in these models more directly translate to humans.

Given the significant advantages of using traditional model organisms, it’s fair to ask: is studying the brains of other animals worth the investment? Scientific funding is limited, so why should we allocate funds to research on non-traditional model organisms when so much can be done in traditional models? I argue that research using non-traditional model organisms has led to many fundamental discoveries about the brain and remain valuable even in modern neuroscience.

Fundamental neuroscientific discoveries made in non-traditional model organisms

An examination of the Nobel prizes awarded in neuroscience highlights the key role non-traditional model organisms have played in neuroscience research. The discoveries of how neurons fire action potentials, how neurons communicate with each other at synapses, and how some neurons encode information about the environment have all won Nobel prizes. One feature that all of these discoveries (and many other Nobel-prize winners) share is that they all were made in organisms that today would be considered nontraditional. The figure below comparing the distribution of modern model organisms to that of Nobel prize-winning discoveries reveals the relatively poor diversity of modern animal models.

Distribution of model organisms in which Nobel prize-winning discoveries have been made (top) and in which neuroscientific articles have been published between 2000 and 2004, adapted from reference 1 (bottom).

While the Nobel prize-winning discoveries mentioned above showcase the contributions of non-traditional model organisms to cellular neuroscience and neurophysiology, there are many examples of similar contributions to the study of neural circuits and behavior. Prominent examples include the elucidation of sound-localization circuitry in barn owls (ref. 2), motion detection in the beetle (ref. 3), and the mechanisms of contrast enhancement in the horseshoe crab eye (ref. 4). The principles uncovered by these studies have later been found to hold true in many other animals, including mammals. Clearly, research spanning the diversity of the animal kingdom has yielded numerous and important advances in neuroscience.

Advantages of using non-traditional model organisms

Why has the study of non-traditional model organisms been so beneficial to neuroscience? Arguably, part of the answer lies in the ability to leverage unique traits of different animal species to answer a particular biological question. The physiologist August Krogh expressed this idea in 1929, writing, “for…a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied” (ref. 5).

Krogh’s principle is exemplified by Hodgkin and Huxley’s choice of the squid as the model organism for studying the neuronal action potential (ref. 6). To understand the mechanisms underlying action potential generation, Hodgkin and Huxley needed to be able to record and manipulate the ionic currents flowing across the neuron’s cell membrane. Given the technology in the 1940s-50s, when Hodgkin and Huxley began their investigations, mammalian neurons were far too small for recordings and manipulations to be made. On the other hand, squid have a giant axon, the diameter of which can be up to a thousand times greater than even the largest axons in rodents and other mammals. The remarkably large size of this axon allowed Hodgkin and Huxley to perform the experiments essential for their discoveries. Before studying the squid giant axon, Hodgkin and Huxley studied similar questions in the frog sciatic nerve, a popular model at the time, but the small size of individual frog axons forced them to switch to the squid. Hodgkin and Huxley’s willingness to step away from a popular model organism and search for an animal better suited for their experiments was critical to their landmark discovery of how action potentials are generated.

The history of neuroscience is full of examples like Hodgkin and Huxley’s experiments. Some of them are shown below:

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Examples of important discoveries made by leveraging unique traits of non-traditional model organisms, adapted from reference 7.

Non-traditional models in the 21st Century

Many of the research successes in non-traditional model organisms occurred in the early to middle 20th Century. Since then, relatively few model organisms have dominated neuroscience research – so what changed? For one, the development of powerful genetic tools in the late 20th Century has rendered rodents and flies, under Krogh’s principle, the most “convenient” animals in which to study neuroscientific questions. This argument has some merit – genetic access allows for recording and perturbation with specificity not usually attainable. The ability to manipulate specific neuronal subtypes has provided insights into circuit function missed when using non-specific methods. Furthermore, as more and more discoveries have been made in rodents and flies, the experimental tools have become even more advanced, making these organisms appear to be the best suited for modern neuroscience studies.

However, we can, and in my opinion should, start developing these same tools for other animal models. The advent of powerful genome-editing tools like the CRISPR/Cas9 system, as well as high-throughput genome sequencing, represents an unprecedented opportunity for the study of non-traditional model organisms. We can now start to generate transgenic animals from non-traditional species and use modern neuroscience tools, like calcium indicators for neuronal recordings or channelrhodopsins for optogenetic manipulations. Over time, the technological playing field will level between non-traditional and traditional model organisms, and the diversity in these different organisms will once again play a central role in the choice of which animal to study for a particular biological question.. For example, the advancement of microscopy technologies and indicators of neural activity has led researchers to look for animals that would be particularly accessible to functional neuroimaging. The observation that larval zebrafish are completely transparent and have relatively small brains quickly led to its becoming an important model in systems neuroscience in the early 2000s, when researchers showed that they could image its entire brain during complex behaviors. More recently, researchers have looked for a similarly accessible mammal, settling on the Etruscan shrew, whose brain might be small enough to also image in its entirety. Only time will tell whether technology continues to drive diversity in the use of model organisms.

Conclusion

Throughout the history of neuroscience, non-traditional model organisms have catalyzed fundamental scientific discoveries. While work in rodents and a few other organisms currently dominate the field, promising technological advances have opened new opportunities for leveraging the diversity of the animal kingdom to tackle neuroscientific problems. I believe that these new opportunities will yield breakthroughs on par with the landmark advances of the past century that relied on non-traditional model organisms. Just as important, I think that research on non-traditional model organisms will reveal beautiful and previously unappreciated biological innovations across the animal kingdom.

Further reading

I am certainly not the first person to point out the benefits of studying non-traditional model organisms in neuroscience. Here is an inspiring paper (ref. 6) with a similar message: http://science.sciencemag.org/content/sci/358/6362/466.full.pdf 

References

  1. Manger, P., Cort, J., Ebrahim, N., Goodman, A., Henning, J., Karolia, M., Rodrigues, S.L. and Strkalj, G., 2008. Is 21st century neuroscience too focussed on the rat/mouse model of brain function and dysfunction?. Frontiers in neuroanatomy2, p.5.

  2. Knudsen, E.I. and Konishi, M., 1979. Mechanisms of sound localization in the barn owl (Tyto alba). Journal of Comparative Physiology133(1), pp.13-21.

  3. Hassenstein, B. and Reichardt, W., 1956. Systemtheoretische analyse der zeit-, reihenfolgen-und vorzeichenauswertung bei der bewegungsperzeption des rüsselkäfers chlorophanus. Zeitschrift für Naturforschung B11(9-10), pp.513-524.

  4. RATLIFF, F., HARTLINE, H.K. and Lange, D., 1974. The dynamics of lateral inhibition in the compound eye of Limulus. I. Studies on Excitation and Inhibition in the Retina: A Collection of Papers from the Laboratories of H. Keffer Hartline, p.463.

  5. Krogh A (1929). The progress of physiology. American Journal of Physiology 90:243-251.

  6. Hodgkin, A.L. and Huxley, A.F., 1952. A quantitative description of membrane current and its application to conduction and excitation in nerve. The Journal of physiology117(4), pp.500-544.

  7. Yartsev, M.M., 2017. The emperor’s new wardrobe: rebalancing diversity of animal models in neuroscience research. Science358(6362), pp.466-469.