NEUROTRANSMITTER SYSTEMS

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NEUROTRANSMITTER SYSTEMS
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Studying Neurotransmitter Systems

• Identifying Neurotransmitter - three criteria
• Synthesis and storage in presynaptic neuron
• Released by presynaptic axon terminal upon
  stimulation
• When experimentally applied, produces response in
  postsynaptic cell that mimics response produced
  by release of neurotransmitter from the
  presynaptic neuron

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Studying Neurotransmitter Systems

• Localization of Transmitters and
  Transmitter-synthesizing enzyme
• Immunocytochemistry
• Anatomically localize particular molecules to
  particular cells

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Studying Neurotransmitter Systems

• Studying Transmitter Localization
• In situ hybridization
• mRNA strands can be detected by complementary
  probe
• Probe can be radioactively labeled -
  autoradiography

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Studying Neurotransmitter Systems

• Studying Transmitter Release
• Loewi and Dale identified Ach as a transmitter
• CNS contains a diverse mixture of synapses that
  use different neurotransmitters
• impossible to stimulate a single population of
  synapses
• Brain slice as a model (ex vivo, brain in a dish)
• Kept alive in vitro ? Stimulate synapses, collect
  and measure released chemicals (mixture)
• Often stimulated by high K solution to cause
  massive synaptic release
• Ca2 dependency of the release has to be
  confirmed
• Direct or secondary consequence of activation ?

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Studying Neurotransmitter Systems

• Studying Synaptic Mimicry
• Qualifying condition Molecules evoking same
  response as neurotransmitters naturally released
• Microionophoresis
• Chemicals in solution acquire charge
• Glass pipette filled with drug is positioned near
  postsynaptic membrane
• Current passing cause the drug to be ejected by
  electrical repulsion
• Measures effects on membrane potential

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Studying Neurotransmitter Systems

• Studying Receptors
• No two transmitters bind to the same receptor
  however one neurotransmitter can bind to many
  different receptors
• Receptor subtypes
• Neuropharmacology
• Subtype specific agonists and antagonists
• ACh receptors

Skeletal muscle
Heart
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Studying Neurotransmitter Systems

• Studying Receptors

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Studying Neurotransmitter Systems

• Studying Receptors
• Ligand-binding methods
• Drugs that interact selectively with
  neurotransmitter receptors were used to analyze
  natural receptors
• Solomon Snyder and opiates
• Identified receptors in brain
• Subsequently found endogenous opiates
• Endorphins, dynorphins, enkephalins
• Enormously important for mapping the anatomical
  distribution of different neurotransmitter
  receptors in brain

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Studying Neurotransmitter Systems

• Studying Receptors
• Molecular analysis
• Molecular cloning of genes that encode
  neurotransmitter receptors
• Revealed many isoforms and paralogs
• Vast diversity of receptor subunits!
• e.g. GABAA receptors are composed of 5 subunits
  a, b, g, d, r
• a 1-6, b 1-4, g 1-4
• 151,887 possible combination in theory

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Neurotransmitter Chemistry

• Dales Principle
• One neuron, one neurotransmitter
• Peptide-containing neurons could be exceptions
• Co-transmitters
• Two or more transmitters released from one nerve
  terminal
• An amino acid or amine plus a peptide

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Neurotransmitter Chemistry

• Cholinergic (ACh) Neurons

good marker for cholinergic neurons
Rate-limiting step of Ach synthesis
Secreted from the axon terminal and associated
with axon terminal membrane
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Neurotransmitter Chemistry

• Cholinergic (ACh) Neurons

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Neurotransmitter Chemistry

• Catecholaminergic Neurons
• Involved in movement, mood, attention, and
  visceral function
• Tyrosine Precursor for three amine
  neurotransmitters that contain catechol group
• Dopamine (DA)
• Norepinephrine (NE, noradrenaline)
• Epinephrine (E, adrenaline)

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Neurotransmitter Chemistry
Marker for catecholaminergic neurons

• Rate limiting, regulated by physiological signals
• Low-rate release - increased catecholamine conc.
  - inhibit TH activity
• High-rate release - increased Ca2 influx - boost
  TH activity

Present in the synaptic vesicles
Present in the cytosol
Released from the adrenal gland as well
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Neurotransmitter Chemistry

• Serotonergic Neurons
• Serotonin (5-HT,5-hydroxytryptamine) is derived
  from tryptophan
• Regulates mood, emotional behavior, sleep
• Synthesis of serotonin
• Limited by the availability of blood tryptophan
  (diet)
• Selective serotonin reuptake inhibitors (SSRIs)
• Antidepressants

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Neurotransmitter Chemistry

• Amino Acidergic Neurons
• Amino acid neurotransmitters Glutamate, glycine,
  gamma-aminobutyric acid (GABA)
• Glutamate and glycine
• Present in all cells - Differences among neurons
  are quantitative NOT qualitative
• Vesicular transporters are specific to these
  neurons
• Glutamic acid decarboxylase (GAD)
• Key enzyme in GABA synthesis
• Good marker for GABAergic
  neurons
• One chemical step difference
  between major excitatory
  transmitter and major
  inhibitory transmitter