Alzheimer's disease, Parkinson's disease, Huntington's disease, motor neurone disease and multiple sclerosis represent some of the most challenging conditions facing modern medicine. These progressive disorders affect tens of millions of people across Europe, and as populations age the urgency to understand and treat them grows. Animal research has been instrumental in revealing how these diseases develop and in creating potential treatments.  
Alzheimer’s disease research has been advancing rapidly during the last decades. Initial animal studies particularly using genetically altered rodents with mutations in human genes linked to Alzheimer’s disease, were crucial for understanding how beta-amyloid accumulates in Alzheimer's disease and its relationship to pathology and cognitive decline. Researchers at the California National Primate Research Center (CNPRC), USA, developed a model for early-stage Alzheimer’s in monkeys, whose brain similarity to humans makes them particularly valuable for capturing disease progression as it would occur in patients. 
In what marks the cusp of first-generation treatments for Alzheimer's disease, a monoclonal antibody treatment developed by US pharmaceutical company Eli Lilly, using mouse antibodies, that modifies the disease itself, has shown success in a clinical trial at slowing brain decline in Alzheimer's patients.  
Researchers at the German Center for Neurodegenerative Diseases developed a gene therapy that could help to treat Alzheimer’s disease by reducing the levels of tau – another key protein that accumulates in the brain and drives disease progression in humans. 
Some important discoveries also emerge from unexpected places and from the unknown. Researchers from the University of Cambridge, UK, discovered that a ‘cold shock’ protein found in cold water swimmers may prevent brain diseases, such as dementia, having previously identified this in mice studies. This illustrates how animal research enables exploration of biological phenomena that would be difficult or impossible to study directly in humans. 
Research also benefits from studying naturally occurring conditions in animals. University of Edinburgh, UK, researchers found that ageing cats with feline dementia show similar Alzheimer's hallmarks to humans, including toxic amyloid-beta protein accumulation at brain cell connections. This natural disease model opens possibilities for testing promising Alzheimer's treatments while potentially benefiting ageing pets. 
Recent technologies have improved how researchers study Alzheimer's in living mice. Scientists at the University of Strathclyde, UK, and the Italian Institute of Technology developed optical fiber implants that detect fluorescent amyloid plaques in freely moving mice. This technology enables long-term tracking of disease progression in deep brain regions while decreasing the number of animals required for these studies. 
For Parkinson’s disease, strategies to protect against its progression or symptoms have been underpinned by animal studies. A study at the University of Geneva, Switzerland, used both mice and fruit flies to identify a gene that can protect against the disease, while research in monkeys showed that motor co-ordination can be improved through infusing dopamine into the brain at the University of Lille, France. ​Research in monkeys by Neurolixis, USA, led to the development of a new drug combatting a side effect of a common Parkinson’s disease treatment which entered clinical trials in 2020. The drug reduced uncontrolled movement in marmoset monkeys with symptoms of Parkinson’s who were receiving a medicine that treats muscle stiffness and tremors, without affecting the efficiency of the treatment.​ 
Motor neurone disease, particularly amyotrophic lateral sclerosis (ALS), affects how the nervous system controls movement and leads to severe muscle weakness. Researchers at the Institut National de la Recherche Scientifique, Canada used zebrafish to visualise motor deficits in ALS, taking advantage of these animals' transparency to observedisease processes in living organisms. Meanwhile, international prize-winning research on how the nervous system controls movement used mice to understand how it can go wrong in not only ALS, but other brain conditions like Parkinson’s. 
Huntington's disease is an inherited neurodegenerative disease that leads to cognitive and motor impairment. Recently, gene therapy company uniQure announced a groundbreaking therapy for Huntington's disease that slowed disease progression by 75% in human clinical trials after years of studies in mice, pigs, rats and monkeys. 

Anxiety, depression, eating disorders, drug addiction and schizophrenia involve intricate relationships between brain chemistry, neural circuits and behaviour that are difficult to disentangle without studying intact, behaving organisms. 
Mice have been used to study depressive behaviours and symptoms, which in turn can help to identify targets in the brain for drugs that reduce the effects of depression. For example, research in mice and cells at EARA member the University of Helsinki, Finland, found that the psychedelic drugs LSD and psilocin can be used as antidepressants without triggering hallucinations. Scientists from the University of Hong Kong and Guangdong University of Chinese Medicine, China, used eye stimulation as a successful treatment for depression in mice. 
Schizophrenia research at EARA member the Max Planck Institute (MPI) for Biological Cybernetics, Germany, used rats to map cognition in the animals' brains and to better understand the abnormalities linked to schizophrenia.  
A study at EARA member Ghent University, Belgium, found that dogs with anxiety have similar behaviours and brain changes as humans with the condition, which could lead to better treatments for both humans and animals.Another study at the MPI of Neurobiology (now the MPI for Biological Intelligence) looked at mice to understand the body-brain interactions on the regulation of emotion and fear. Research at University College of Cork, Ireland, has investigated how signals between gut bacteria and the brain can influence the behaviour and emotions of mice, which could be helpful to understand anxiety or depression in people. 
A team at the University of Colorado Boulder, US,  identified a previously unknown brain circuit in a specific zone of the midbrain that controls how mice respond and adapt to sudden threats, like a predator’s approach, using real-time brain imaging and optogenetics – a technique combining light and genetic engineering to control specific neurones. These type of studies are essential to understand anxiety and other stress-related conditions. 
Researchers at Stockholm University and EARA member Karolinska Institute, Sweden, found that brain cells controlling maternal aggression remain inactive until pregnancy, then switch on through hormonal changes. Understanding how hormones interact with brain cell networks, how these interactions differ between sexes and how neural pathways activate during specific life stages requires observing the interplay between endocrine and nervous systems across an animal's development. 

​There are several different types of rare cancer that start in childhood, including neuroblastoma that mainly affects children under five-years-old and starts in nerve cells called a neuroblast, often in the adrenal glands located on the top of each kidney. Several studies in mice have contributed to improving survival rates and developing treatments with less severe side effects, for example a team at the University of Gothenburg, Sweden, were able to find an effective drug that worked against one of the two gene mutations that cause neuroblastoma.  
Mice were also used in a study of the rare eye cancer retinoblastoma (which mainly affects young children) at the St Jude’s Research Hospital, USA, to confirm that the cancer’s cause was not a series of mutations, like most cancers, but the loss of a single gene – explaining why it develops so quickly.

ADHD, autism spectrum disorder and intellectual disability begin during brain development in the embryo, making them particularly challenging to study. Understanding how these conditions emerge requires observingdevelopmental processes that unfold over time in living organisms. 
A wide range of animals have contributed to our understanding of autism, including zebrafish, to understand its symptoms, as well as roundworms and fruit flies to look at gene mutations linked to the disorder, and even fish and frogs. Using gene editing, a study led by the Institute of Science and Technology Austria (ISTA), looked at the effect of ‘starving’ the brain of essential amino acids in mice, and found that the animals had smaller brains than healthy mice, which was also linked to behavioural changes similar to those seen in neurodevelopmental disorders such as autism. 
Researchers at University College London, UK, are working to develop a mouse model of ADHD to study the common features of the condition, including behaviour, and to identify ways to treat both symptoms and the underlying condition in people. 
Researchers from EARA member University of Milan, Italy, showed that feeding young rats a ketogenic diet after prenatal stress exposure protected them from behavioural issues like reduced grooming and decreased sociability. This suggests dietary interventions might help treat long-term effects of prenatal trauma, demonstrating important connections between nutrition and brain health. 

er went to human trials, from research at Auburn University, USA, while a factor from the blood of pigs has historically been used as a treatment for specific types of haemophilia (such as haemophilia A), to successfully stop bleeding episodes in severe cases.

Blindness, hearing loss, paralysis and other sensory problems involve both how the brain processes information and its connections to the body. Restoring these functions requires understanding and potentially repairing both components. 
Scientists at the Netherlands Institute for Neuroscience (NIN), Amsterdam, created a brain implant that could restore some vision from blindness. The team developed high-resolution implants and then inserted these into the visual cortex of two sighted monkeys – the part of the brain that processes visual information. 
Treatment of paralysis caused by injury or disease has benefited from studies in mice, for example at Ruhr University Bochum, Germany, which stimulated the spinal cord of paralysed mice to allow them to walk again, and in the testing of a drug designed to regrow nerves, at EARA member the University of Padua, Italy. More recently, Swiss researchers have developed an implant that allows patients with a complete spinal cord injury to stand and walk again. The device by École Polytechnique Fédérale de Lausanne (EPFL), and Centre Hospitalier Universitaire Vaudois were based on years of studies using monkeys and rats. 
Meanwhile, researchers at the University of Pittsburgh and Carnegie Mellon University, both USA, tested a technology that enabled one patient to move their arm again after it was paralysed by a stroke. This breakthrough involved studies in animals such as rats and monkeys to investigate how to repair spinal cord injuries.​ 

Many people with another genetic brain disease, Rett syndrome, which causes mental and physical disability, can live to adulthood, but certain complications such as pneumonia and problems with heart rhythms, can mean some die during childhood. Mice have been important for testing gene therapies for Rett syndrome, particularly by allowing researchers to study it in females, since it affects females more than males. 

For some neurological diseases, the fruit fly has proved to be a valuable species, playing a considerable role in not only our understanding of how these diseases work, but also in the discovery of rare diseases. It was thanks to investigating a specific gene mutation generated in fruit flies that researchers at Baylor College of Medicine, USA, first identified Mitchell syndrome(previously a mystery) to understand how it caused progressive loss of various body functions, including movement and hearing, in people with the mutation.

In turn, through investigating potential treatments for the syndrome, again using fruit flies, members of the same team were then also able to apply their insights to other neurological diseases such as multiple sclerosis (which is more common, but includes the common feature of brain inflammation).

Meanwhile, in a study led by EARA member Uppsala University, Sweden, researchers concluded that routinely used blood-thinning drugs could be a hugely beneficial option for people with cerebral cavernous malformations (abnormal groups of blood vessels in the brain that can lead to bleeding, resulting in seizures), by studying mice with mutations that reflected the human condition, as well as human tissues. 

Brain tumours, epilepsy, sleep disorders and other neurological conditions represent diverse challenges where animal research continues providing crucial insights. 
For instance, animal research can provide insights into both how brain damage can occur and how to stop it. A group at the Institute of Science and Technology Austria found that starving the brain of certain dietary nutrients can impact brain development in mice, while in zebrafish, a team at Ludwig Maximillian University, Germany, were able to prevent the damage caused by brain scarring. Scientists activated existing immune cells in the brain called microglia, to study their effects – microglia are key to forming scar tissue. 
Epilepsy research at Stanford University in the USA achieved a significant milestone by successfully switching brain cells 'on and off' within mouse brains, preventing seizures using optogenetics. This precision approach, impossible without intact neural circuits in living animals, offers hope for more targeted treatments with fewer side effects than current medications. 
Brain tumours present particular treatment challenges due to their location and aggressive growth. A study in mice at EARA member the University of Zurich, Switzerland, has allowed for the testing and refinement of treatments for aggressive brain tumours by showing that a protein-antibody combination treatment can slow or reverse the growth of cancer cells. Another study at the Hospital for Sick Children, US, and EARA member University of Toronto, Canada, used gene editing in mice to discover why childhood brain tumours resist radiation therapy, identifying genes that could be targeted to restore treatment effectiveness. 
The link between the nervous system and digestive system has been investigated in mice, for example at EARA member the Champalimaud Foundation, Portugal, which looked at feeding behaviour in mice to provide insights into understanding and treating obesity, demonstrating how neurological research addresses conditions not traditionally viewed as brain disorders. 
Sleep disorders often connect to other brain conditions, as shown in research at the University of Queensland, Australia, which found that a condition called sleep apnoea was associated with a higher risk of developing Alzheimer’s disease. ​ 
The brain and spinal cord also have an important role in sexual behaviour and premature ejaculation. Researchers at EARA member Champalimaud Centre for the Unknown, Portugal, and University of Bordeaux, France, discovered that specific spinal cord neurones and a type of brain cells control ejaculation and arousal in mice and rats, suggesting that rodents may be a good model to study premature ejaculation and human arousal regulation. 

Rodents represent the most commonly used species in neuroscience, as the complexity of their brains is similar enough to humans to give a good overview of brain processes. Mice can also be genetically modified with relative ease, meaning that researchers can look at the effect of individual genes on the way disease progresses. The ability to study complete neural circuits (e.g., interactions between different types of neurones across brain regions) in behaving animals makes rodents invaluable for connecting changes at the cellular and molecular level to changes in memory, motor abilities and behaviour. 
As mice and rats have a shorter life span than other mammals, it is possible to use them to study diseases over a longer period of time, or in ageing animals. These rodents can also be used to look at the effects of additionalconditions (comorbidities), such as obesity or diabetes, on neurological diseases, as well as how molecules such as sex hormones can impact the brain and behaviour – see our #TransparencyThursday with Dario Aspesi, at the University of Guelph, Canada. 
Recent refinement methods have improved brain research in living mice. Stanford University, US, researchers developed a technique using ampyrone to temporarily make mouse scalps transparent, enabling real-time observation of brain development and activity over time in the same animal without invasive procedures, and refining traditional methods requiring brain tissue examination. 

Zebrafish are also becoming more widely used for neuroscience research because researchers can directly observemolecular changes in the brain and firing neurones while they are alive. Studies in zebrafish are ongoing to understand how these animals react to stress and depression and are used to test potential treatments or highlight new mechanisms to study in humans. 
​Because zebrafish also possess the ability to regenerate their tissue and organs, for example when they become damaged, these fish are also providing insights into how to repair the brain after injury, trauma or infection. 

Zebrafish have therefore contributed to the study of various genetic diseases, including Batten disease, an incurable disease that results in several symptoms, such as vision loss, seizures and problems with cognition. By using tools such as CRISPR gene editing, researchers can initiate in zebrafish the same gene mutations as people with the condition (mostly children), thereby replicating key aspects and testing the effect of possible drugs.

EARA member the Luxembourg Centre for Systems Biomedicine (LCSB), at the University of Luxembourg, was able to identify three possible early markers of Batten disease, using zebrafish larvae which had advantages over using mice or other mammals, which do not develop the disease until adulthood. 

"Our new model may also facilitate drug screening to identify compounds that could slow or halt disease progression, a breakthrough that the 14,000 patients living with this still uncurable disease are eagerly waiting for," said Dr Carole Linster, LCSB, Luxembourg.

Meanwhile in work at Sidra Hospital, in Qatar, researchers are using genetically modified zebrafish to gain key insights into the biology and symptoms of a range of rare childhood diseases (including inherited blood disorders such as sickle cell anaemia), in turn paving the way for precision medicines. 

Another benefit of zebrafish larvae is their transparent bodies which makes them particularly suited to research into rare skeletal disorders, such as idiopathic scoliosis (abnormal spine curvature arising after birth). Their transparency means that genes linked to the development of such conditions can be easily seen under a microscope, as well as the bones and skeleton itself, to understand the fundamentals of bone formation and where it can go wrong. Adult fish can also develop similar bone complications to people (they have shown osteoporosis-like features, for example), but are also able to repair and renew their skeletal tissue, which can offer insights into possible treatment strategies.

For cancer, the benefit of genetic manipulation has led to zebrafish that have specific gene changes linked to the disease. For example, in a study at the University of Birmingham and University of Warwick, both UK, zebrafish were edited to have a so-called GATA2 deficiency – a hallmark of a rare bone marrow disorder that is tied to the development of blood cancer – to shed light on how this deficiency increases the risk of the cancer. 

Having such a close genetic relationship to humans, monkeys are one of the most valuable animal models used in research. Less than 0.2 per cent of the animals used in research in the EU are monkeys, however their impact in providing the most reliable information for what is happening, or what is going to happen, in humans cannot be underestimated.  
​All animal experiments are strictly regulated and reviewed by ethical committees before they are allowed to proceed and the use of monkeys in research is only permitted when there is no other animal or non-animal model that could provide the same answer. In addition, in Europe, research with great apes such as chimpanzees – the animal that is the most closely-related to humans – is prohibited. 
While there are understandable ethical worries about using monkeys, they continue to be an essential model for studying the function of the brain due to the similarity in structure and composition with humans. The prefrontal cortex structure in non-human primates more closely resembles humans than does the rodent brain, making monkeys essential for studying complex behaviour, emotion, vision and higher cognitive functions. 
Much of what we know today about complex behaviour and emotion, vision and higher cognitive function has been gained from the study of monkeys – as well as insights into how to treat these functions when they go wrong, such as in vision loss, paralysis and stroke.  
An international task force published a position paper in NPJ Parkinson's Disease highlighting how non-human primates remain critical for Parkinson's research and understanding ageing. Monkey studies have enabled the mapping of brain circuits and improved understanding of deep brain stimulation, translating directly into patient therapies. Their genetic, anatomical and behavioural similarities to humans make them irreplaceable for treatment development. 

"Lots of neurological ph lifetime, which is more difficult to achieve in larger animals and humans. For example, research at the Institute for Research in Biomedicine and SJD Paediatric Cancer Center Barcelona, both Spain, engineered fruit flies to express a variant of a cancer gene that causes a rare bone cancer called Ewing sarcoma, which has a survival rate of only just over 50% in teenagers, and less than 30% after the cancer has spread to other parts of the body. This resulted in the same tumour development as seen in humans, and was the first time the disease was modelled in an animal, after failed attempts to do so in mice. 

The causes of the rare neurodevelopmental disorders, Harel-Yoon syndrome and Yoon-Bellen syndrome, were both identified as due to specific genetic mutations thanks to studying fruit flies, by researchers at Baylor College of Medicine, USA. Although this gene mutation had been linked to neurological symptoms in humans, it was only by determining the effect in fruit flies that researchers could confirm the cause – paving the way to targeted treatments. 

Meanwhile, in a fruit fly study at the University of Sydney, Australia, researchers were able to pinpoint how a mutation underpinned a key mechanism behind episodic ataxia, a condition that severely affects balance and co-ordination. 

Fruit flies reach adulthood in just two weeks, so using them in research opens doors to exploring the long-term effects of a disease and how it progresses over a lifetime, which is more difficult to achieve in larger animals and humans. For example, research at the Institute for Research in Biomedicine and SJD Paediatric Cancer Center Barcelona, both Spain, engineered fruit flies to express a variant of a cancer gene that causes a rare bone cancer called Ewing sarcoma, which has a survival rate of only just over 50% in teenagers, and less than 30% after the cancer has spread to other parts of the body. This resulted in the same tumour development as seen in humans, and was the first time the disease was modelled in an animal, after failed attempts to do so in mice.