Research questions which have perplexed scientists for centuries are now starting to be answered. The mechanisms of learning and memory are being revealed in an astonishing amount of detail. However some of the most critical questions remain unanswered. This is both exciting and daunting, when progress is being made in so many areas, how can we think of new ways to address the outstanding issues? With many novel resources available, both clinical and laboratory based, it is essential to conduct research in a translational manner, interpreting novel findings from patients and animals contemporaneously. For many years a degree of segregation between clinical research and bench research has existed. The divide here is shrinking, however there is still some way to go. In this article, we will look at an area of neuropsychiatric research which has tested clinical and laboratory hypotheses to uncover a largely comprehensive mechanistic understanding of the theory. We will use the example of fear conditioning and extinction as an example of how the neurobiology of a behavioural phenomenon can be revealed using complementary clinical and laboratory methods.
I have conducted laboratory based and clinical research, at the same time and separately, and this experience has allowed me to see the merits and limitations of the available modalities. While there is a vast amount of overlap in techniques and research questions, there are many avenues of investigation which can be utilized for the simultaneous progression of the field.
Clinical science is strong in cognitive and symptomatic assessment; the development of molecular imaging techniques such as positron emission tomography (PET) has allowed researchers to look in much more detail at the molecular aspects of psychiatry. However, at this time, the resolution of clinical research is relatively limited. The resolution of magnetic resonance imaging (MRI) has improved since its invention, however to call the structural discriminatory ability high would be an overstatement. We are in a time of testing in the laboratory where details can be viewed, in a living animal, on the scale of individual synapses . Animal research provides us with many useful tools to study the systems disrupted in psychiatric disorders and can inform the clinical context in a complementary way. It is possible to look at neuronal circuitry, as well as manipulate activity in real-time to observe the resulting synaptic, transcriptional and behavioural changes. It is essential to be cautious when interpreting the results of animal research related to clinical disorders. However many researchers unfairly disregard very informative animal data unnecessarily.
The idea of translational research is not a new concept, it is the use of basic science for practical application to improve the health and wellbeing of society. Using all the approaches at our disposal, it is important to understand the pathology of psychiatric disorders, as well as understanding the workings of the healthy brain. From this understanding, we can then identify potential therapeutic targets for disorders, which at present are treated with limited success.
Research about fear conditioning and the subsequent extinction of a conditioned fearful stimulus provides a good example of how a concept, from initial observations, can be translationally investigated to understand the fundamental mechanisms governing a behavioural phenomenon. Pavlov’s experiments in the 1920s, and others following, demonstrated distinct fearful behavioural responses to very specific contextual cues.
Aversive stimulus (e.g. loud noise) + environmental stimulus (e.g. a white lab rat being presented) = fear conditioning to aversive and environmental stimuli
The aversive stimulus (loud noise) causes an initial fearful response, which becomes conditioned to the environmental stimulus (presentation of the rat). Hence re-presentation of the environmental stimulus, without the aversive stimulus, would cause a fearful reaction. Watson’s experiment on an infant in 1920 used a loud noise to condition a fearful response to the presentation of a white lab rat . Over time, with re-presentation of the environmental stimulus, the fearful response reduces until it is considered extinct. This work ultimately lead to the identification of the amygdala as the ‘fear centre’ in the brain . Figure 1 demonstrates the main cortical regions and connecting pathways contributing to the network involved in this behaviour.
Subsequent integration of this theory with functional MRI (fMRI) techniques allowed investigators to demonstrate changes in amygdala based responses to conditioned and unconditioned stimuli. Researchers were able to condition a fearful stimulus in subjects and image the brain activity associated with the degradation of the conditioned stimulus. At this stage of understanding, fear conditioning had been explored in humans to identify the amygdala. However from there, researchers went from the clinical investigation of fear responses with fMRI back to the animal laboratory to dissect the functional networks surrounding this behaviour. The network involvement of the prefrontal cortex and the hippocampus could then be identified as regulators of the observed behaviour (Figure 1.). The prefrontal cortex is responsible for complex cognition and executive function and the hippocampus is the memory centre of the brain. When combined with the prefrontal cortex and amygdala, the hippocampus is able to produce fearful memories and this circuit is thought to underlie context specific anxiety.
In rats it has been shown that stimulating activity in the frontal cortex (with implanted stimulating electrodes) can make fear extinction more efficient by reducing activity in the amygdala . A recent study demonstrated how fear conditioned stimuli could be modulated in humans with cortical stimulation (a magnetic field generator produces local activity via electromagnetic induction, i.e. think of the school experiment where you pass a magnet through a coil of wire and measure the voltage). Subjects were conditioned to respond to fearful stimuli and the extinction of this response was measured (the response was removed by repeatedly presenting the environmental stimulus without the aversive stimulus). Using the cortical magnetic stimulation method, researchers were able to alter the extinction so that discrimination between the aversive and environmental stimulus was no longer apparent . The study was relatively small and in healthy volunteers, however the results suggest investigation in patients undergoing cognitive therapy for anxiety could be a useful future application.
The progression of research regarding the amygdala and fear conditioning/extinction is an example of how clinical and basic research can inform each other and provide mutual feedback to progress understanding of both contexts more thoroughly. The final stage of translation, where this knowledge can be applied beneficially in the clinic, has not yet been made but it is promising that such progress has been made to understand the fundamentals of a behavioural concept. While clinical imaging may not provide the resolution achievable in the laboratory, and animal experimentation doesn’t fully represent disordered cognition, in combination a more complete model can be formed. It is difficult to model psychiatric illness in animals, it is not possible to tell if a rat or mouse is clinically depressed or psychotic, as clinical diagnosis relies greatly on investigative questions and interviews to reveal specific symptoms. However tissue changes, identified from patient investigations, can be modelled in the laboratory to provide more opportunities to understand the consequences in terms of brain region interaction. One hugely powerful tool in the laboratory which isn’t accessible clinically is genetic manipulation, where the genome of an animal can be selectively modified to produce variants seen in human populations of patients. Identifying genetic risk factors in patient populations can be taken from the clinical setting and applied to animals to look at the ways individual genes, and even combinations of genes, can affect the nervous system. Further to this, the way genes then interact with environmental cues can provide a huge insight to the pathophysiology of disorders.
In the current state of research, apart from a number of exceptions, clinical and animal research are remarkably segregated. The two areas can benefit greatly from collaboration and scientists from each discipline offering comment and interpretation of results. The British Association of Psychopharmacology is particularly successful in addressing such issues and I feel they have a cohesive and progressive approach in this area. In my experience of their meetings and publications, there is a drive towards the translation of clinical and animal findings, which will no doubt lead to some unique and potentially very rewarding findings.
- Holtmaat, A., et al., Imaging of experience-dependent structural plasticity in the mouse neocortex in vivo. Behavioural Brain Research, 2008. 192(1): p. 20-25.
- Watson, J.B. and R. Rayner, Conditioned emotional reactions. Journal of Experimental Psychology, 1920. 3: p. 1-14.
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- Guhn, A., et al., Medial prefrontal cortex stimulation modulates the processing of conditioned fear. Frontiers in Behavioral Neuroscience, 2014. 8.