Chapter 6: Neurotransmitter Systems

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9 Apr 2012
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Chapter 6- Neurotransmitter Systems
Introduction
o Cholinergic: Dale; describes cells that produce and release Ach
o Noradrenergic: Dale; describes neurons that use amine norepinephrine NE
o Glutamatergic: use glutamate
o GABAergic: use GABA
o Peptidergic: use peptides
o Cholinergic system: Ach and all the molecular machinery associated with it
Studying Neurotransmitter Systems
o Criteria
Molecule must be synthesized and stored in presynaptic neuron
Must be released by presynaptic axon terminal upon stimulation
When experimentally applied must produce a response in postsynaptic cell that mimics
response produced by release of neurotransmitter from presynaptic neuron
Localization of Transmitters and Transmitter-Synthesizing Enzymes
o Immunocytochemistry
Used to anatomically localize particular molecules to particular cells
Neurotransmitter is purified and injected into bloodstream where it stimulates immune
response that generates specific antibodies
Can then recover antibodies and tag them then use the tagged antibodies to see which cells
contain the transmitter candidate
Can be used for any molecule for which a specific antibody can be generated
Can also be used to localize synthesizing enzymes
o In Situ Hybridization
Create a probe (complementary strand of mRNA) chemically label (by making it
radioactive)it and allow it to stick to mRNA for a particular peptide
Hybridization: the process by which the probe binds to the mRNA
Then wash away probe and search for neurons containing label
Autoradiography: Detect radioactivity by laying tissue on sheet of film sensitive to
radioactive emissions; film is then developed and image of cells appear as clusters of small
dots
Studying Transmitter Release
o Sometimes can stimulate cells while taking sample of fluid bathing targets then test samples to
see if it mimics effect then chemically analyze it to reveal structure
o In CNS however it impossible to stimulate single population of synapse with only one
neurotransmitter
Take brain slices kept alive in vitro and bath with high [K] that causes depolarization
stimulating transmitter release
Must also only be released when Ca is present
Still cannot be sure may have been released as a secondary consequence of synaptic
activation
Most difficult to prove in CNS
Studying Synaptic Mimicry
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o Microionophoresis
Dissolve neurotransmitter in solution so it acquires net electric charge and fill fine glass
pipette with ionized solution and inject small amount next to postsynaptic membrane
Use microelectrode in postsynaptic neuron to measure effects of transmitter candidate on
membrane potential
If it causes changes that mimic effect of transmitter release at synapse and satisfies other
criteria then it is considered the neurotransmitter
Studying Receptors
o No two neurotransmitters bind to same receptor but one neurotransmitter can bind to many
receptors
o Receptor subtype: each of the different receptors a neurotransmitter binds to
o Neuropharmalogical Analysis
Can distinguish different receptor subtypes by actions of different drugs
Ex. Nicotinic Ach receptors have agonist nicotine and muscarinic Ach receptors have
agonist muscarine
Curare is antagonist at nicotinic receptors and atropine antagonizes at muscarinic
receptors
Glutamate receptors named after agonists ex. AMPA receptors, NMDA receptors and
kainate receptors
NE receptors: alpha and beta
GABA receptors: GABAA and GABAB
o Ligand-Binding Methods
Solomon Snyder studies Opiates (effects pain relief, euphoria, depressed breathing and
constipation)
Found that radioactive opiates bind to specific sites on membranes of some but not all
neurons in brain
Ligand binding method: technique of studying receptors using radioactively labelled ligands;
(can be agonist, antagonist or chemical neurotransmitter itself)
Important in mapping receptor distribution
o Molecular Analysis
Divided receptors into two types: transmitter gated ion channels and G protein coupled
receptors
Have determined structure of polypeptides and discovered a that there is alot of diversity in
subunits
Neurotransmitter Chemistry
o Most are amino acids, amines or peptides
Exception is Ach made from acetyl co A and choline
o Dale's principle: the idea that a neuron has only one neurotransmitter; peptide containing
neurons violate this principle as they release either an aa or amine and a peptide
o Cotransmitters: when two or more transmitters are released from one nerve terminal
Cholinergic Neurons
o Acetylcholine (Ach): made by all motor neurons in spinal cord and brain stem
o Choline acetyltransferase (ChAT): synthesises Ach; is made in soma and transported to axon
terminal where it makes Ach
Is found only in cells that release Ach therefore its a good marker
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