PSYC-2900 Chapter 5

experimental ablation

surgical removal of tissue to reveal the function of structures, brain areas, and/or pathways

lesion

a wound/injury caused by damaging the tissue or disrupting its functioning, possibly from ablation. can be intentional, or naturally occurring (e.g. brain injury, stroke)

radio frequency lesion

a technique for ablation in which an electrical current is passed through a stainless-steel wire (insulated except for the tip), creating heat that kills surrounding cells (destroys everything)

excitotoxic lesion

a technique for ablation in which an excitatory amino acid (e.g. kainic acid) is injected using a cannula to create an area of excitotoxicity. the cell bodies die but axons passing through will stay intact

permanent lesion

a lesion causing cell death

temporary lesion

created by temporarily silencing neurons by the injection of anaesthetics or other drugs (e.g. muscimol), sometimes preferred over permanent lesion

stereotaxic surgery

a procedure involved in ablation that ensures precise placement and tightly controlled administration. the individual is positioned in a stereotaxic apparatus which positions them securely and precisely in a standard position. the device for administering ablation is secured and moved within a controlled way

stereotaxic atlas

a tool used in ablation, consulted to determine accurate positioning of the target. it is based on the distances from the bregma

bregma

the junction of the coronal and sagittal sutures of the skull that provides a reference point for what areas of the brain are underneath

sham lesion

a control condition in ablation that creates the physical damage of administration without the additional lesion process

histological methods

methods used in ablation to confirm the location of a lesion. the tissue is fixed, sliced, stained, and examined under a microscope.

• tissue must be preserved to protect from autolytic enzymes, decomposition, bacteria, and mould

• tissue is treated with a fixative

histological methods in animals

in ablation, the animal is usually perfused first to clear blood from tissue, euthanized with a large dose of anaesthetic, and injected with a fixing agent to clear blood from neural tissue

tissue sectioning

in histological methods of ablation, the tissue is mounted (placed on a material to provide stability (paraffin wax or frozen)) for sectioning/slicing, and is then placed onto a microscope slide

tissue staining

in histological methods of ablation, allows for additional features to be observed. different materials absorb different types of dyes, allowing features to be more prominent. a mounting medium (often an oil) is placed over the tissue to keep the coverslip in place and prevent the tissue from drying out

• methylene blue & cresyl violet stain cell bodies

• golgi staining stain the whole cell

• reduced silver staining stain nuclei and cytoskeletal proteins

• hematoxylin and eosin distinguished between the nucleus and cytoplasm

viewing the tissue sections

in histological methods of ablation, different types of equipment are required for different types of histology/tissue:

• light microscopy: cheapest, magnify 40-100x, allows viewing of whole cells, but not subcellular structures

• electron microscope: higher magnification, tissue must be thinly sectioned

  • transmission electron microscope: beams of electrons transmitted through the tissue, images reflect shadows that are casted, allowing small details to be observed

  • scanning electron microscope: less magnification, but 3D, scans tissue with a moving beam of electrons and reflection of the beam is detected

• confocal laser scanning microscope: provides high magnification in thick tissue sections, tissue is stained with a fluorescent dye, allows for selective staining to show specific structures/cells/molecules, laser and mirror is used to expose tissue to a specific wavelength of light

efferent pathways

neural pathways that carry outgoing signals from the structure.

• traced using an anterograde labelling method in which chemicals are taken up by dendrites or cell bodies and transported down the axon to the terminal buttons

• tissue is labelled with an immunocytochemical method, which is uses the labelled antibodies to attach to antigens (proteins/peptides) expressed in cells

afferent pathways

neural pathways that carry incoming signals coming to the structure

• traced using retrograde labelling methods to allow labelling in the backwards direction

• fluorogold injection is taken up by the terminal buttons and transported to the cell bodies by retrograde axoplasmic transport

combined tracing techniques

methods in which both anterograde and retrograde labelling techniques are used to allow a picture of how circuits may be wired

• often a single link of the chain

• other approaches can give wider view of connections

transneuronal tracing methods

a type of tracing method that identifies the series of neurons that form synaptic connections with each other. a virus is injected into the area of interest and spread from neuron to neuron along the pathway of synapses

structural techniques

a type of neuroimaging technique that reveals anatomical and physical attributes

computerized tomography (CT)

a structural imaging method in which the head is placed between an x-ray tube and a detector. radiation is passed through the head and detected on the other side, different material absorbing different amounts of radiation. a computer analyzes the data obtained to produce a 2D image

• cheaper, but involves radiation

magnetic resonance imaging (MRI)

a structural imaging method that produces a signal based on properties of hydrogen protons when placed in a high magnetic field, followed by the application of a radio frequency (RF) pulse. when the RF pulse is removed, hydrogen protons relax at different rates depending on the material they are located in (e.g. white/gray matter, CSF). different imaging sequences are used to sample the signal at different points of relaxation, generating different types of images that provide different details/information. contrast between grey and white matter can be found

diffusion tensor imaging (DTI)

a type of MRI that is sensitive to the direction of the movement of water and visualizes white matter tracts

functional methods

a type of neuroimaging technique that reveal information about neural activity

• (e.g. signals related to electrical signalling, and metabolism, including glucose/oxygen consumption, and changes in gene expression)

electrophysiological methods

a type of functional method that studies the brain in action using microelectrodes to produce a single-unit recording. signals are very small and require amplification

single-unit recording

a type of electrophysiological method in which a thin wires used to record electrical activity from an individual neuron (microelectrode) are surgically implanted and bonded to the skull to allow recording in an awake animal/nature state without anaesthesia

macroelectrodes

in electrophysiological methods, record changes in electrical potentials from the population of neurons. can be implanted or placed on the surface of the brain or scalp

electroencephalogram (EEG)

a type of electrophysiological method used to diagnose epilepsy and study stages of sleep that uses macroelectrodes

magnetoencephalography (MEG)

a type of functional technique that observes small magnetic fields generated with electrical currents produced along axons by using an array of superconducting quantum interference devices (SQUIDs). has excellent temporal resolution (precise timing of when the activity is occurring)

immunocytochemical methods

techniques used to record metabolism to identify cells containing Fos proteins, and determines cells that are active in particular behaviours

2-deoxyglucose (2-DG)

a molecule that, when radioactive, allows observing of areas of increased glucose metabolism used in metabolism recording methods

autoradiography

a metabolism recording method allows visualization of areas of increased glucose metabolism by sectioning tissue to reveal areas where radioactive 2-DG has accumulated

positron emission tomography (PET)

a functional imaging technique that uses a 2-DG injection and a PET scanner, which detects 2-DG accumulation in areas of higher glucose metabolism

• expensive, limited by the half-life of the tracer, limited temporal and spatial resolution

• allows quantification

functional magnetic resonance imaging (fMRI)

a functional imaging technique that is sensitive to changes in blood oxygenation, allowing for observations of areas of neural activity (which have increase blood flow/supply of oxygenated blood), by the blood oxygenation level-dependent (BOLD) effect

• excellent spatial resolution, good temporal resolution (those based on electrical sources of signal are better)

electrical stimulation

a method of stimulating neural activity that passes an electrical current through a wire inserted into the brain

chemical stimulation

a method of stimulating neural activity that injects a small amount of excitatory amino acid into the brain

• more localized and activates cell bodies, but not axons

transcranial magnetic stimulation

a type of electrical stimulation in which electrical coils within a wans are applied to a specific area of the skull to target a brain region of interest. the electrical current through the coils generates a magnetic field that can be used to stimulate or inhibit activity in a localized brain area

optogenetic methods

a neural stimulation technique that uses photosensitive proteins that can be stimulated or inhibited by specific wavelengths of light by exposure to harmless viruses

• when light is removed, action is removed

• turned on/off by implanted LEDs

finding neurochemicals

can be localized by:

• targeting the chemicals themselves

  • peptides/proteins can be imaged using immunocytochemistry

• targeting the enzymes that produce them

finding receptors

1. determine the location of the cells that contain the receptors of interest (autoradiography, 2-DG labels the cells that were metabolically active)

2. a radioactive ligand is used to bind to the receptor (immunocytochemistry can label receptors because they are proteins), which indicates where the postsynaptic cells are

microdialysis

a method of measuring chemical release that measures the amount of neurotransmitter in interstitial fluid by injecting a dilute salt solution, which picks up molecules, and then by extracting them through a small piece of tubing

PET scanners

a tool for measuring chemical release that can localize any radioactive substance that emits positrons

concordance rate

compared in twin studies to observe trait inheritance patterns in pairs of twins

• if both twins inherit the trait, they are concordant

• if only one inherits the trait, they are discordant

• if the trait has a strong genetic component, the rate will be higher in monozygotic twins than dizygotic

adoption studies

compares individuals who were adopted early in life with their biological (genetic influence) and adoptive (environmental influence (e.g. physical, social) family

• must locate biological parents to measure the trait of interest

genome

the DNA that is the code for genetic information

allele

an individual gene

genomic studies

aims to associate physical and behavioural traits with certain genes and locate them in the genome

in situ hybridization

a technique for studying genes in which the sample (tissue) is exposed to a small labelled probe sequence that binds to the complementary sequence. structures that the labelled probe finds reveal that the sequence was available for binding (i.e. being used by the cell for transcription/translation)

linkage studies

genomic studies that focus on individuals with a particular phenotype and compares their genes with known genetic markers of that phenotype

genome-wide association studies

genomic studies that allows the comparison of whole/parts of genome(s) with the presence/absence of a trait/disease (i.e. detects correlations). it is possible as the whole human genome has been sequenced

gene mutations

produces a change in regular functioning, produced in a laboratory and inserted into the chromosomes of mice when studying targeted mutations. the aim can be to make the gene defective or produce a new protein/increase production

knockout gene

in targeted mutations, making the gene defective, therefore causing the gene to fail to make the protein it’s supposed to

knock-in gene

in targeted mutations, producing a new protein or increasing production

conditional knockouts

in targeted mutations, mutations that don’t take effect util there is a trigger

antisense olgionucleotides

a modified strand of RNA/DNA that binds with a specific molecule of mRNA. it prevents the production of a particular protein and can produce changes in behaviour

clustered regularly interspaced short palindromic repeat (CRISPR-Cas) methods

a method for genomic studies that alters the production of proteins through changes to DNA by breaking both strands at a target site, and causing cells to repair.

• can inactivate the gene, creating a gene knockout

• can create a new genetic sequence, which can be inserted into DNA

used to model neurodegenerative disease caused by genetic mutation