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Human Biology
Malcolm Mac Kinnon

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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