Systems level methodologies

Systems level methodologies

1. Neuroanatomy

   • No neuroanatomy = no circuits, no circuits = no function
     • Function is an emergent properties of signal processing in a neural network
   • Good knowledge of neuroanatomical connections essential for interpreting the results of systems-level investigations e.g. modern neuroimaging studies
   • Methods
     • Basic structure (location of cell groups and major fibre tracks) of the brain discovered using histological procedures involving stains for different elements - cell bodies, and fibre tracts (myelin)
     • Tract-tracing: Anterograde
       • Transport from neuronal cell bodies > axon terminals
     • Tract -tracing: Retrograde
       • Transport from axon terminals > neuronal cell bodies
       • Different cell morphology - Different processing capacities
     • Bidirectional tracers
       • Friend or foe?
         • Major pitfall/advantage….dye can go retrograde then anterograde along branched collateral giving spurious anterograde tracing – i.e. it might be concluded – wrongly – that area A projects to area C
       • WGA-HRP
       • Biotinylated dextran
       • Choleral toxin subunit B
     • Simultaneous anterograde/retrograde
     • Chemical neuroanatomy
       • Anterograde tracer BDA + TH Immunohistochemistry (identifies dopamine neurones)

2. Class A experiments

   • Some behavioural, physiological or pharmacoloigcal variable is manipulated and the consequent effects on brain structure/activity are determined.
   • Some of the most important measures of brain structure/activity are
     • Effects of manipulations/variables on structure
       • Behavioural/physiological manipulation - enriched environment / schizophrenia / learning
       • All shown to cause changes in the dentritic structure of cortical cells and orientation of processes in cells of the hippocampus.
     • Effects on metabolic activity
       • Increases in neuronal firing increases demand for nutrients, especially glucose. This increased demand can be detected using the radioactive 2-deoxy glucose technique.
     • Local haemodynamic changes
       • more neural activity - more blood
     • Immediate early gene
       • E.g. c-fos
       • Neural activation can induce expression of immediate early genes
       • Can be detected by in situ hybridisation or immunohistochemistry
     • Change in transmitter release
       • Increases in neural activity – increases in release of neuro-transmitters and their breakdown products.
         • Detectable in
           • Cerebrospinal fluid (lumbar puncture)
           • Local extracellular space
             • Microdialysis : Chemicals diffuse across dialysis membrane – measured by biochemical assay
             • Electrochemistry: Amperometry or Fast scan cyclic voltammetry - measures change in electrical potential when specific neurochemicals are oxidised.
     • Changes in electrical activity
       • EEG gives indication of regional brain activity underlying electrodes
         • Good temporal resolution
         • Poor spatial resolution
         • Good for detecting signs of epilepsy
     • Local field potentials - deep brain EEG electrodes
       • Exact dimensions of region contributing to record uncertain
     • Single unit electrophysiology
       • Single unit recordings give good temporal resolution (e.g. response latency/duration but poor spatial resolution (recording is from only one cell)
         • problems when trying to assess population coding of brain function
     • Multichannel - spike sorting

3. Class B experiments

   • Some aspect of brain structure (lesion) or activity (stimulation/inhibition) which is manipulated and the consequent effects on behaviour/physiology /endocrinology which is determined
   • Some of the most important manipulations of brain structure/activity are
     • Surpession of neural activity
     • Lesions/Inactivation
       • Look at ppt for this bit
       • 1. Mechanical
         • (e.g. lobotomy/stroke/tumour/trauma)
       • 2. Aspiration
         • (suction; mainly for surface structures)
       • 3. Electrolytic
         • local damage to deep structure via implanted electrode
       • 4. Excitotoxic
         • chemicals like kainic acid or ibotenic acid damage cells only when injected into local regions of brain
       • 5. Knife cut
         • wire knife cuts used to make fine transections of major fibre bundles.
       • 6. Chemospecific
         • certain chemicals have neurotoxic properties for specific cell groups : e.g. 6OHDA only kills cells which use the transmitters dopamine or noradrenaline…MDMA (ecstasy) neurotoxic to serotonergic neurones
       • 7. Pharmacological receptor blockers
         • drugs which block neurotransmission in virtually all known neurotransmission systems are now available. e.g. the anti psychotic phenothiazines block dopamine receptors.
       • 8. Optogenetic suppression of neural activity
         • cell-specific photic silencing of neurones
       • Look at ppt
     • Activation of neural signals
       • Stimulation of the brain
         • 1. ECT
           • stimulating electrodes are placed on the outside of the skull….effective treatment for psychotic depression
         • 2. Electrical stimulation
           • via implanted electrode – deep brain stimulation for Parkinson’s disease
         • 3. Chemical stimulation
           • systemic injections (subcutaneous, intraperitoneal, intravenous - injections into cerebral ventricles
         • 4. Local chemical injections
           • via cannulae into local regions of brain
         • 5. Microiontophoresis
           • application of a weak positive or negative current to a pipette (needle) containing a solution of oppositely charged particles repels (drives out) the charged ions into the brain – used in conjunction with single unit electrophysiology.
         • 6. Optogenetic stimulation
           • cell-specific photic stimulation of neurones

4. Critical evaluation

   • Behavioural/ physiological/ pharmacological specificity
     • Class A experiments
       • Are there are adequate controls to ensure the observed changes in brain activity/structure are produced only by the claimed behavioural/ physiological/ pharmacoloigcal manipulations?
     • Class B experiments
       • Are the effects of brain manipulation specific to the claimed physiological /endocrinological / behavioual changes ?....e.g. akinetic effects of lateral hypothalamic lesions originally misinterpreted as aphagia (unwillingness to eat)
   • Anatomical specificity
     • Class A experiments
       • Are measured changes in brain structure/activity specific to the claimed regions of the brain ?
     • Class B experiments
       • Is the used brain manipulation specific to the intended neural structures - fibres of passage ?.... or specific to the claimed cell-type?
   • Pharmacological specificity
     • Class B experiments only
       • Does the used drug have effects which are specific to the intended receptor system?
       • Where is the drug acting to produce observed effects ?
         • Drug side effects

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