Optogenetics (Disruptive) Technologies applied to systems neuroscience research Flashcards
What is an important aspect of systems neuroscience?
That it requires us to think of things at different spatial and temporal scales
What are the different levels of description to consider in systems neuroscience?
Psychological level
Systems level
Microcircuit level
Neuronal level
Intracellular level
Molecular level
What is the issue with different levels of description?
A single method will usually only provide information at one level of description when to understand an entire disease or drug treament we must consider many different levels
What can behavioural scientists do?
Combine different methodologies
What is a downside of Ca2+ imaging?
Invasive- may be best for preclinical studies
What are the constraints that must be considered?
Expertise
Facilities
Time
Money
Ethics
Different methods are affected in different ways by these constraints. e.g. consider blood pressure measurement in human vs. functional MRI in an animal model.
The extent to which constraints are insurmountable depends on what?
The importance of the question
(and your ability as a scientist to make your case)
It is possible to get grants and ethical approval so long as you justify the work - these are the appropriate methods to answer the question
To do good science with ethical approval what should we aim to do?
Minimise cost and maximise benenfit
What do we also need to consider to test a hypothesis?
What type of data we need to easily test the hypothesis
What does the best type of research bring together?
In modern neuroscience, it is often the case that the best research brings multiple techniques together in a single study
Multiple approaches and techniques together
What do we need to consider in visualising and stimulating the brain?
What methods are available?
What are their relative strengths / weaknesses?
How can these both affect the results and the conclusions drawn from the research
What is MRI?
MRI scanners use magnetism to ‘see’ the position of hydrogen atoms in water molecules inside the body
This allows it to build up an image of the internal structures.
So MRI allows you to build up a detailed picture of brain structure that is sensitive to the differing tissue types (can differentiate between white and grey matter)
Very high spatial resolution, but this is just brain ‘structure’ not function
How can we assess function?
fMRI
What is fMRI?
The principle is same as MRI, but you ‘tune’ your scanner to be sensitive to something that disturbs the way the energy is absorbed and the emitted
This is blood, because blood contains haemoglobin, which contains iron and iron is paramagnetic
The scanner can be made very sensitive to the effect of the iron in the blood on the way the energy given to tissue by the radiofrequency pulse is re-emitted
When haemoglobin is carrying oxygen, it ‘hides’ the iron, so actually fMRI is really picking up on the oxygenation of blood in the tissue
What is BOLD fMRI
picks up signal from blood flow in the tissue so not directly measuring neural activity
Activated brain cells can call up more (fresh, oxygenated) blood, so fMRI tells us about brain activity
What is the contrast agent in fMRI?
Blood is the contrast agent
What is PET imaging?
We make a contrast agent that is specifically targeted to the biological process we want to image
Get a chemical that binds to the target (e.g. oxygen, glucose, specific receptors)
Attach a radioisotope (radiation emitting molecule) to that chemical (specifically a positron emitter)
Inject this tracer (contrast agent) into the subject
Detect the emitted radiation and use a computer to work our where it is coming from (tomography)
What is PET excellent for?
Look at where the tracer goes to and where it collects so it is excellent for informing on specific biological processes (fMRI very limited in this respect)
What are two limitations of PET?
Spatial and temporal resolution poor compared to fMRI- limited in terms of what we can track in terms of processes
Uses radiation, so much more limited in research applications- means we muight have to avoid repeat measurement and means it cannot really be used in children
How can we overcome the limitations of PET?
Use PET-fMRI
-Combine advantages of PET and MRI/fMRI
What if we want to do molre direct recording of neurons in humans?
Use EEG and MEG instead
What is EEG?
EEG gives indication of regional brain activity underlying electrodes
What are strengths of EEG?
good temporal resolution
Signals are often separated into different frequency bands (slow waves or fast waves) - Different frequency bands appear to relate to distinct neurophysiological processes- greater insight
What are three limitations of EEG?
Poor spatial resolution- detecting signal but through the scalp, the skull and even the hair
Analysis is complex and takes a lot of time!
Needs to have subject in an electrically shielded environment to minimise electrical noise- otherwise can convolute the signal
What do we get from EEG?
As well as monitoring ongoing activity, EEG (like any of the techniques described) can be used to look at brain responses to a specific stimulus – and Event Related Potential (ERP)
What is MEG?
Magnetoencephalography
The electrical current of large numbers of cells and white matter tracts (bundles of axons, think wires) induces a magnetic field that can be detected with a very large machine
What is a limitation of MEG?
MEG signals are very small and hard to detect
What is a strength of MEG?
Less interference by scalp & skull than electrical signals so can offer better spatial resolution than EEG - this can compensate for the small signals being hard to detect
What can we use to stimulate the brain ?
TMS or TDCS
What is TMS?
Transcranial Magnetic Stimulation
arguably less physiological which is something to consider for the aims of research
Induces electrical current in brain tissue which disrupts the ongoing activity
Used in research to ‘turn off’ parts of the brain so that their role in a cognitive function can be assessed
What is a limitation of TMS?
Hard to target precisely
What is a strength of TMS?
Some evidence of clinical potential (e.g. in treating depression)
What is TDCS?
Transcranial Direct Current Stimulation
Pass (mild) current through the brain, between the positively charged anode and the negatively charged cathode
Can excite or inhibit underlying brain tissue, which may be useful experimentally
What are some limitations of TDCS?
Arguably low cost and accessible but its true effects arent fully known yet
Early evidence for cognitive enhancement effects and possible clinical benefits, but simplicity of device may risk mis/over-use
What are other psychophysiological measures?
External or internal stimuli that are of survival significance will act through the brain to trigger the sympathetic nervous system
It is probably a uniquely human phenomenon that ‘thoughts’ can also achieve these responses
Through this, it means that aspects of brain function can be inferred by measuring systemic (non-CNS) factors
What are some non-CNS factors?
Skin conductance (sweating)
Heart Rate
Blood Pressure
Pupil Dilation
Muscle Tension
Body Language?
When choosing a method, other than spatial and temporal resolution, what do we need to consider?
Invasive vs non-invasive method
In research what is there a tradeoff between?
Trade-offs between spatial resolution, temporal resolution, invasiveness and cost.
What are invasive methods typically be used for?
Animal models
These are Strictly regulated (by the Home Office in the UK)
Requires a careful justification of how likely benefits of the research for either other animals or humans outweigh the costs
What are the three guiding principles for all research involving animals? (3Rs)
Replacement (can another method be used)
Refinement (can it be done in a better way that further maximises the cost:benefit equation)
Reduction (can it be done with a smaller number of animals)
What animals are used in research in the UK?
Mice- 82.6%
Fish- 15%
Rats- 1.5%
Stats from: Animal Research Statistics for Great Britain, 2021
What is possible if we use invasive methods that is not possible in humans?
Make direct measurements of the activity of brain cells.
Direct measurement of activity of human brain cells is possible, but only in rare circumstances
e.g. although EEG measures electrical activity of neurons the spatial resolution is poor.
What are the different types of recording invasively?
There are several different methods of recording activity of brain cells
Some target single cells (intracellular recording, also termed unicellular recording)
Some record from larger numbers of cells (extracellular recording)
What does intra vs extra -cellular refer to?
Whether the tip of the electrode is inside the cell itself (intra), where it is relatively insulated from the activity of surrounding cells
Other than direct measurements, what else can we examine with invasive methods?
Determine connectivity between structures, flow of information
Stimulating electrodes can be inserted into one part of the brain (including into single cells) and recording electrodes inserted into another region (or single cell)
The effect of stimulating the first region (or cell) on the second can then be determined
What is a retrograde tracer ?
Tracer travels backwards along axon towards cell body in region
What is an anterograde tracer?
Tracer travels forward along axon towards next synapses
What else can invasive methods allow us to do?
-Disruption
Disrupt connectivity between structures to determine effects upon circuit function
Measure effects on activity in downstream structures
Measure effects on behaviour
This gives us some understanding of causation in brain circuits (or behaviour), but results can still be difficult to interpret
What else can invasive methods allow us to do?
-Lesions
Lesion specific structures to inform us about what function that structure performs
This gives us some understanding of function of specific structures but again results can be difficult to interpret
Might be able to lesion a structure but not specific cell types but does still give us an idea of functiojn
What is psychopharmacology?
A field that investigates the effect of drugs on thoughts or behaviour
Psychoactive drugs such as these and other (illegal) compounds often have complex actions on many brain regions, acting on different types of receptor on different cell types
From a research perspective, is it often necessary to use specially designed compounds that have much more targeted (and known) actions
What is pharmacological research methods?
Different drugs will act at different points in the process of neurotransmission, e.g.
Some will mimic the neurotransmitter to act on the receptor
Some will block the receptor
Some will prevent manufacture of the neurotransmitter
Some will block re-uptake
Others….
How can pharmacological studies be developed?
Pharmacological manipulations can be combined with other methods:
Administer drug…..
Stimulate a region
Block the receptor thought to be involved along a hypothesisd pathway
…and see if the resulting effects on Region B, or behaviour are similar to effects of a lesion of structure A (or the pathway)
