Chapter 6- Neurotransmitter Systems
Introduction:
Three major classes of neurotransmitters: amino acids, amines, and peptides
First molecule identified as a neurotransmitter was acetylcholine, Ach
Cholinergic- cells that produce and release Ach
Noradrenergic- neurons that use the amine neurotransmitter norepinephrine (NE)
Glutamatergic- synapses that use glutamate
GABAergic- synapses that use GABA
Peptidergic- synapses that use peptides
Ach and all the molecular machinery associated with it are collectively called cholinergic
system
Studying Neurotransmitter Systems:
Certain criteria must be met to distinguish a molecule as a neurotransmitter:
o The molecule must be synthesized and stored in the presynaptic neuron
o The molecule must be released by the presynaptic axon terminal upon stimulation
o The molecule must produce a response in the postsynaptic cell
Localization of Transmitters and Transmitter-Synthesizing Enzymes:
Hints that a particular molecule may be a neurotransmitter:
o Molecule is concentrated in the brain tissue
o Application of the molecule to certain neurons alters their action potential firing rate
To confirm the molecule is a neurotransmitter, the molecule must be localized in and
synthesized by particular neurons
Two techniques used are immunocytochemistry and in situ hybridization
Immunocytochemistry:
Immunocytochemistry- a method used to anatomically localize particular molecules to
particular cells
o Once the neurotransmitter candidate has been chemically purified, it is injected into the
bloodstream of an animal, where it stimulates an immune response
o The response is the generation of large proteins called antibodies
Antibodies can bind tightly to specific sites on the foreign molecule such as the
transmitter candidate
Best antibodies for this method bind very tightly to the transmitter of interest, and
bind very little or not at all to other chemicals in the brain
o This method can be used to localize any molecule for which a specific antibody can be
generated
In Situ Hybridization:
Is also useful for confirming that a cell synthesizes a particular protein or peptide
Recall: proteins are assembled by the ribosomes according to instructions from specific mRNA
molecules
A unique mRNA molecule for every polypeptide is synthesized by a neuron
If the sequence of nucleic acids in a strand of mRNA is known, it is possible to construct in the
lab a complementary strand that will stick to the mRNA molecule
o Complementary strand is called a probe
o Process by which the probe bonds to the mRNA molecule is called hybridization
In order to see if the mRNA for a particular peptide is localized in a neuron, we chemically label
the appropriate probe so it can be detected, apply it to a section of brain tissue, allow time for
the probes to stick to any complementary mRNA strands, then wash away all the extra probes
that have not stuck; finally we search for neurons that contain the label
In situ hybridization, probes are usually labelled by making them radioactive
o Since we cannot see radioactivity, hybridized probes are detected by laying the brain
tissue on a sheet of special film that is sensitive to radioactive emissions
o After exposure to the tissue, the film is developed like a photograph, and negative images
of the radioactive cells are visible as clusters of small dots
This technique for viewing the distribution of radioactivity is called
autoradiography Immunocytochemistry is a method for viewing the location of specific molecules, including
proteins, in sections of brain tissue
In situ hybridization is a method for localizing specific mRNA transcripts for proteins
Both methods put together, enable us to see whether a neuron contains and synthesizes a
transmitter candidate
Studying Transmitter Release:
Most regions of the outer central nervous system (CNS) contain a diverse mixture of
intermingled synapses using different neurotransmitters
Read Pg. 137-138!
Studying Synaptic Mimicry:
Knowing that a molecule is localized in, synthesized by, and released from a neuron is still not
sufficient to qualify it as a neurotransmitter
A 3 criterion must be met:
o The molecule must evoke the same response as that produced by the release of naturally
occurring neurotransmitter from the presynaptic neuron
To asses the postsynaptic actions of a transmitter candidate a method called microionophoresis
is used
o Microionophoresis- a method of applying drugs and neurotransmitters in very small
quantities to cells
Read this section on Pg. 138
Studying Receptors:
Each neurotransmitter exerts its postsynaptic effects by binding to specific receptors
o As a rule no two neurotransmitters bind to the same receptor; but one neurotransmitter
can bind to many different receptors
Each of the different receptors a neurotransmitter binds to is called a receptor subtype
Ach act
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