Neuroscientist Dr Alice Witney of Trinity College Dublin, Ireland, and Professor Malcolm Burrows at the University of Cambridge, UK, together with MRI physicists Dr Christian Kerskens of Trinity College and Dr Salman Shahid, Indiana University School of Medicine, US, report the first use of diffusion magnetic resonance imaging (dMRI) at a micro scale, which has enabled this important imaging technique to be applied to an insect brain. Imaging live locusts, the researchers revealed internal brain structures and inferred the connectivity of major neural tracts. Their work provides an additional way to study the brains of these insects which are known to differ widely between swarming or solitary types. This is the first step towards using this technique to aid understanding of the locust swarming behaviour that regularly devastates crops in Africa.
Diffusion magnetic resonance imaging (dMRI) is a widely used technique for non-invasive imaging of the human brain and revealing connectivity between brain regions. Dr Christian Kerskens’ team at the Trinity College Institute of Neuroscience have developed state-of-the-art micro diffusion MRI techniques to enable this imaging to be applied at a much smaller scale. Now, Dr Alice Witney of Trinity College Dublin, Ireland, collaborating with locust expert Professor Malcolm Burrows at the University of Cambridge, UK, has worked with MRI specialists Salman Shahid of Indiana University School of Medicine in the US and Christian Kerskens of Trinity College, to pioneer the use of micro dMRI modelling to investigate insect neurobiology.
Why research insects?
Although there are differences in brain physiology and function between species, the basic organisation of the brain in insects shows similarity with other species. For example, early researchers found similarities in how humans and insects process visual information. Like the brains of humans, insect brains show a modularity of function – separate brain regions which interact with one another to produce a sophisticated set of behaviours. Insect models have therefore become a useful tool in neuroscientific research to learn about the link between brain and behaviour. Insects’ short lifespans also make them particularly useful for investigating changes in the brain over time during ageing. Further, the nervous system of the insect has been used by researchers to study the biology of axonal damage and neurodegenerative changes. Due to their simpler nervous systems, insects have also provided useful insights into general principles of the linkage between brain and behaviour.
dMRI: a new application
The insect brain is usually imaged by electron microscopy, a technique which produces valuable anatomically precise, high-resolution images, but it cannot be used on live animals. In contrast, magnetic resonance imaging – MRI – is used extensively for non-invasive live imaging of mammalian brains and can therefore enable researchers to understand changes in an animal’s brain across its life. In their recent study, Witney and her colleagues investigated whether dMRI could offer even greater benefits to biological understanding.
dMRI has been widely used in research on humans, with quantitative imaging metrics that represent difference in diffusivity being found to be valuable for predicting the severity of neurodegenerative disease. The technique records changes in the diffusion of water molecules in brain tissues, which can be used to infer underlying anatomy. It also means that small changes in brain structure resulting from ageing, brain damage, and disease can be detected. As dMRI can be used to infer connectivity of neural tracts, it has led to a widespread interest in understanding the ‘connectome’ of the brain – the colourful visualisations modelling the connectivity between brain regions that subserve different functions. In these visualisations – which can be seen on www.humanconnectomeproject.org – the colours represent different orientations of fibres as inferred through the diffusion modelling. From this, dMRI has the potential to reveal adaptive plasticity – the way brain connections change in response to environmental factors. This high sensitivity to small changes in brain architecture means dMRI is valuable for providing physiological as well as anatomical insights into the brain over time, for example in response to environmental change.
However, much uncertainty remains about the biological underpinnings for these quantitative metrics, so imaging a simpler nervous system may help provide insights. Until recently, however, only large and very complex brains could be visualised by dMRI. The new micro-scale dMRI method has changed this and Witney and her colleagues are the first to apply dMRI methods to the brains of live insects. Their study shows that dMRI can be applied on a brain as small as a locust which opens the possibility for investigating the significance of the established, but not entirely understood, diffusion metrics in a simpler nervous system.
“Insect models could improve our understanding of how ageing, disease, and environmental change impact brain functions and behaviour.”
Locusts’ pivotal role
The desert locust is a particularly important species for neuroscientific investigations because its brain is relatively large for an insect. The locust is also an especially interesting species because, unusually, its phenotype can switch between two different phases throughout its lifetime: the non-migratory solitary phase and the migratory gregarious phase. ‘This transformation is an extreme form of phenotypic plasticity which is known to be associated with large changes in the brain,’ Witney explains. ‘Plasticity in the brain is a topic of widespread interest in neuroscience so the locust could be the ideal animal to help us understand this important process.’
As dMRI is specialised for detecting changes in brain connections over time, Witney and colleagues believe it could be the first method to enable us to understand neurobiological changes between phases in the locust brain. This is a timely research objective, given the disruption to locust migratory behaviour due to climate change, and environmental stress which is likely to exacerbate locust swarming.
Future promise of dMRI
Overall, Witney and her team have shown that dMRI methods can be applied to small species such as insects. This method could therefore be used in the future to advance our understanding of insect neurobiology, including dynamic changes in neural circuits and how neuronal structures influence behaviour.
Shahid, first author of the study, explains that dMRI is preferable to other methods of brain imaging because it is non-invasive, so it can be used repeatedly over an animal’s lifetime. However, dMRI will not replace other methods of brain imaging; rather this new study aims to use dMRI to provide additional information, including physiological as well as anatomical insights. Though as the researchers note, it may not be possible to apply this technique yet in smaller insects such as flies, which are also frequently used in neuroscientific research.
Using micro dMRI in such insect model systems could open new research avenues for scientists investigating the nervous system of other species, including how they are affected by environmental stresses.
Locusts exhibit an extreme form of phenotypic plasticity – this plasticity represents the animal’s ability to adapt to its environment by changes in physiology, anatomy, and behaviour. Interestingly, confocal microscopy studies have previously revealed that the brains of swarming locusts show large differences compared with those of solitary locusts. dMRI offers a technique that could be applied to image the locust brain and gain insights into the physiological as well as anatomical changes that occur in the brain of this notorious crop pest.
Further reading
- Shahid, et al, (2021) Elucidating the complex organization of neural micro-domains in the locust Schistocerca gregaria using dMRI, Scientific Reports, 11(1), 1–12.
10.26904/RF-146-4103172231
Research Objectives
Alice Witney and colleagues test the viability of using diffusion magnetic resonance imaging (dMRI) on locusts to provide a new technique for insect neurobiology.
Collaborators
- Salman Shahid
- Christian Kerskens
- Malcolm Burrows
Bio
Dr Alice Witney is a neuroscientist interested in sensorimotor control. After an undergraduate degree in psychology and a PhD and post-doctoral work in human motor control, she undertook a Wellcome Trust training fellowship in insect neurobiology. Witney’s work aims to integrate comparative neuroscience approaches to understand key principles in sensorimotor neurobiology.
Contact
E: awitney@tcd.ie
T: +353 852 799 049
W: Alice G Witney (0000-0002-3726-8479) (orcid.org)
W: Alice Witney – Google Scholar
