Chapter 6 - Neurotransmitter systems

Question 1

What are the 11 elements of neurotransmitter systems?

Answer 1

1. Presynaptic axon terminal
2. Neurotransmitter-synthesizing enzymes
3. Synaptic vesicle transporters
4. Reuptake transporters
5. Degradative enzymes
6. Transmitter-gated ion channels
7. G-protein-coupled receptors
8. G-proteins
9. G-protein-gated ion channels
10. Second messenger cascades
11. Postsynaptic dendrite

Question 2

What are cholinergic, noradrenergic, glutamnatergic and peptidergic synapses?

Answer 2

Synapses that use acetylcholine, norepinephrine, glutamate, and peptide, respectively.

Question 3

What are the criteria for a molecule to be considered a neurotransmitter?

Answer 3

1. Must be synthesized and stored in the presynaptic neuron
2. Must be released by the presynaptic axon terminal upon simulation
3. When experimentally applied, must produce a response in the postsynaptic cell that mimics the response produced by the release of the neurotransmitter from the presynaptic neuron.

Question 4

What is immunocytochemistry?

Answer 4

Once the neurotransmitter candidate has been chemically purified, it is injected under the skin or into the bloodstream, where it stimulates an immune response. It can be used to localize any molecule for which a specific antibody can be generated, including the synthesizing enzymes for transmitter candidates.

1. Molecule of interest is injected into an aminal, causing an immune response and the generation of antibodies.
2. Blood is withdrawn, and antibodies isolated from the serum.
3. Antibodies are tagged with a visible marker and applied to sections of brain tissue. The antibodies label only those cells that contain the neurotransmitter candidate.

Question 5

What is in situ hybridization?

Answer 5

A synthetic probe is constructed containing a sequence of complementary nucleotides that will allow it to stick to the mRNA. If the probe is labeled, the location of cells containing the mRNA will be revealed.

Question 6

Tell a little about studying transmitter release.

Answer 6

Once a transmitter candidate has proven to be synthesized by a neuron and localized to the presynaptic terminal, it must be shown that it is actually released upon stimulation.

Even when it has been shown that a transmitter candidate is released upon depolarization in a calcium-dependent manner, we can’t be sure that the molecules collected in the fluids were released from the axon terminals; they may have been released as a secondary consequence of synaptic activation.

Question 7

Tell a little about studying synaptic mimicry.

Answer 7

The third criterion for neurotransmitters is that the molecule must evoke the same response as that produced by the release of the naturally occurring neurotransmitter from the presynaptic neuron.

Through microiontophoresis, the transmitter candidate’s effects on the membrane potential can be measured.

Question 8

Tell a little about studying receptors.

Answer 8

The receptors for various neurotransmitter systems need to be researched.

Three approaches are very useful. Neuropharmacological analysis of synaptic transmission, ligand-binding methods, and molecular analysis of receptor proteins.

Question 9

What is neuropharmacological analysis?

Answer 9

1. Analysis of what effects different drugs have on different parts of physiology.
2. Using selective antagonists, to distinguish receptor subtypes.

Question 10

What are ligand-binding methods?

Answer 10

Any chemical compound that binds to a specific site on a receptor is called a ligand for that receptor. The technique of studying receptors using radioactively or nonradioactively labeled ligands is called the ligand-binding method.

Question 11

What is molecular analysis?

Answer 11

The structure of the polypeptides that make up many proteins has been determined, and this has shown how much diversity there can be in them. This is a field of molecular neurobiology.

Question 12

What is Dale’s principle?

Answer 12

The idea that a neuron has only one neurotransmitter. Many peptide-containing neurons violate this. When two or more transmitters are released from one nerve terminal, they are called co-transmitters.

Question 13

What are cholinergic neurons?

Answer 13

Neurons that have the neurotransmitter Acetylcholine ACh. Its synthesis requires a specific enzyme, ChAT. They also manufacture AChE for degrading ACh.

Question 14

What are catecholaminergic neurons? Where are they found?

Answer 14

Neurons that contain neurotransmitters that have a chemical structure called a catechol. These are dopamine (DA), norepinephrine (NE) and epinephrine (adrenaline).

These neurons are found in the parts of the nervous system involved in the regulatio nof movement, mood, attention, and visceral function. They all contain the enzyme tyrosine hydroxylase (TH), and dopa decarboxylase.

Question 15

What are serotonergic neurons?

Answer 15

Neurons containing the amine neurotransmitter serotonin. They are relatively few in number, but appear to play an important role in the brain systems that regulate mood, emotional behavior, and sleep.

Question 16

What are amino acidergic neurons?

Answer 16

Neurons with the neurotransmitters glutamate (Glu), glycine (Gly) and gamma-aminobutyric acid (GABA), that serve at most CNS synapses.

Question 17

What are some other neurotransmitter candidates and intercellular messengers?

Answer 17

Adenosine triphosphate (ATP) which is key in cellular metabolism, is also a neurotransmitter.

Endocannabinoids can be released from postsynaptic neurons and act on presynaptic terminals, making them retrograde messengers that do retrograde signaling.

Endocannabinoids are not packaged in vesicles, but manufactured rapidly and on demand. They are small and membrane permeable, and can diffuse rapidly across the membrane of their cell origin to contact neighboring cells. They also bind selectively to the CB1 type of cannabinoid receptor, mainly located on certain presynaptic terminals.

Nitric oxide (NO) is also a chemical messenger proposed for intercellular communication. It may be another example of a retrograde messenger.

Question 18

How is the operation of a neurotransmitter system like a play with two acts?

Answer 18

Act I: Presynaptic, culminates in the transient elevation of neurotransmitter concentration in the synaptic cleft.
Act II: Generation of electrical and biochemical signals in the postsynaptic neuron. Main players are transmitter-gated channels and G-protein-coupled receptors.

Question 19

What is the basic structure of transmitter-gated channels and what are they?

Answer 19

Very small channels (11 nm), regulating the flow of large currents with great precision.

E.g., the ACh receptor is a pentameter that forms of five protein subunits arranged like the staves of a barrel.

These subunits consist of alpha helices that are packed together.

Question 20

What are amino acid-gated channels?

Answer 20

They mediate most of the fast synaptic transmission in the CNS. Their properties:

1. Pharmacology of the binding sites: which transmitters affect them
2. Kinetics of the transmitter binding process and channel gating: duration of effect
3. Selectivity of ion channels: whether they produce excitation or inhibition, whether Ca2+ enters the cell in significant amounts
4. Conductance of open channels: magnitude of the channels’ effects

Question 21

What are glutamate-gated channels?

Answer 21

AMPA, NMDA, and kainate channels.
AMPA: Permeable to both Na+ and K+
NMDA: Permeable to Ca2+ and inward ionic current through NMDA-gated channels is voltage dependent
kainate: functions not clearly understood, exist throughout the brain

Question 22

What are GABA-gated and Glycine-gated channels?

Answer 22

GABA mediates most synaptic inhibition in CNS. Synaptic inhibition must be tightly regulated in the brain: too much is loss of consciousness/coma, too little is seizure.

GABA_A receptor has several other sites where chemicals can dramatically modulate its function.

GABA_A receptor has modulary binding sites for an unknown reason, but works well for modern drugs.

Question 23

What are G-protein-coupled receptors and effectors?

Answer 23

G-protein-coupled receptors involves (1) binding of the neurotransmitter to the receptor protein, (2) activation of G-proteins, (3) activation of effector systems.

Most G-protein-coupled receptors are simple variations of a common plan, single polypeptide containing seven membrane-spanning alpha helices.

Question 24

What are G-proteins?

Answer 24

Guanosine triphosphate (GTP) binding protein, a diverse family of about 20 types. There are more transmitter receptors than G-proteins: some types of G-proteins can be activated by many receptors.

Question 25

How do G-proteins operate?

Answer 25

(1) Alpha-subunit of the G-protein binds GDB.
(2) When activated by a G-protein-coupled receptor, GDP is exchanged for GTP.
(3) Activated G-protein splits, both Galpha and Gbetaypsilon become available to activate effector proteins.
(4) Galpha subunit slowly removes phosphate from GTP, converting GTP to GDP and terminating its own activity.

Question 26

What is the shortcut pathway?

Answer 26

G-protein gated ion channel.

Example: G-proteins in the heart muscle are activated by ACh binding to muscarinic receptors, and then the activated Gbetaypsilon subunit directly induces a potassium channel to open.

Question 27

What are second messenger cascades?

Answer 27

Activation of an enzyme by a G-protein that triggers a series of biochemical reactions. Neurotransmitter coupled via multiple steps to activation of a downstream enzyme.

Question 28

What is phosphorylation and dephosphorylation?

Answer 28

Protein kinases transferring phosphate from ATP floating in the cytosol to proteins: phosphorylation

Enzymes called protein phosphatases act rapidly to remove phosphate groups. The degree of channel phosphorylation at any moment therefore depends on the dynamic balance of phosphorylation by kinases and dephosphorylation by phosphatases.

Question 29

What is the function of signal cascades?

Answer 29

The activation of one G-protein-coupled receptor can lead to the activation of many ion channels. Signal amplification.

Question 30

What is divergence and convergence in neurotransmitter systems?

Answer 30

Divergence: Rule among neurotransmitter systems. One transmitter can activate multiple receptor subtypes. Can occur at any stage in the cascade of transmitter effects.

Convergence: Many various receptors affect the same effector system.

Neurons integrate divergent and convergent signaling systems, resulting in a complex map of chemical effects. The wonder is that it works, the challenge is to understand how it does.

Question 31

List the criteria that are used to determine whether a chemical serves as a neurotransmitter.
What are the various experimental strategies you could use to show that ACh fulfills the criteria of a
neurotransmitter at the neuromuscular junction?

Answer 31

Three certain criteria must be met for a molecule to be considered a neurotransmitter.
1) The molecule must be synthesized and stored in the presynaptic neuron.
2) The molecule must be released by the presynaptic axon terminal upon stimulation.
3) The molecule, when experimentally applied, must produce a response in the postsynaptic cell that
mimics the response produced by the release of the neurotransmitter from the presynaptic neuron.
Immunohistochemistry shows where specific molecules are localized, and in situ hybridization shows
where specific mRNA transcripts for specific proteins are located. These methods could be used to
demonstrate the presence of ACh in the presynaptic terminal at the neuromuscular junction. It would be
useful to show that the synthesizing enzyme is present as well.

Question 32

What are three methods that could be used to show that a neurotransmitter receptor is
synthesized or localized in a particular neuron?

Answer 32

Three methods are used to study the receptors of various neurotransmitters:
neuropharmacological analysis of synaptic transmission, ligand-binding methods, and molecular analysis
of receptor proteins. Neuropharmacological analysis studies the actions of different drugs. Ligand-binding
methods can be used to identify the location of receptors by labeling ligands that bind to them, such as
specific agonists, antagonists, or chemical neurotransmitters. Molecular analysis studies the protein
molecules and the subunits that form the neurotransmitter receptors, such as transmitter-gated ion channels
and G-protein-coupled receptors. This method may also be used to examine the genes that encode these
proteins and the consequences of altering the genes or the gene products.

Question 33

Compare and contrast the properties of (a) AMPA and NMDA receptors, and (b) GABAA
and GABAB receptors.

Answer 33

(a) AMPA and NMDA are glutamate receptor subtypes; both are activated by glutamate, but the
drug AMPA acts only on the AMPA receptor and the drug NMDA acts only on the NMDA receptor.
AMPA and NMDA are chemical agonists used to differentiate the glutamate receptor subtypes. Their
antagonists can also distinguish receptor subtypes, for example, the antagonist for AMPA is CNQX and
the antagonist for NMDA is AP5. The differences in the receptors are related to slight differences in the
protein. An important property of the NMDA receptor is that it is only active in the presence of glutamate
and sufficient depolarization in the postsynaptic neuron. (b) GABAA and GABAB are GABA receptor
subtypes; both respond to GABA but muscimol is the agonist for the GABAA receptor, and the agonist
for GABAB is baclofen. The antagonist for GABAA is bicuculline whereas the antagonist for GABAB is
phaclofen.

Question 34

Synaptic inhibition is an important feature of the circuitry in the cerebral cortex. How would
you determine whether GABA or Gly, or both, or neither, is the inhibitory neurotransmitter of the cortex?

Answer 34

Synaptic inhibition is represented by inhibitory postsynaptic potentials in the postsynaptic neuron
of an inhibitory synapse. To determine whether GABA or Gly or both are inhibitory neurotransmitters,
you could record IPSPs in response to GABA or Gly application in an in vitro preparation. You might
also examine the nature of the postsynaptic receptors. Both GABA and Gly receptors gate a chloride
channel, which when opened, would help hyperpolarize the postsynaptic cell and make that neuron less
likely to fire an action potential.

Question 35

Glutamate activates a number of different metabotropic receptors. The consequence of
activating one subtype is the inhibition of cAMP formation. A consequence of activating a second subtype
is activation of protein kinase C. Propose mechanisms for these different effects.

Answer 35

The subtype of glutamate metabotropic receptor that inhibits cAMP formation may activate Gi.
This is the mechanism used by the NE receptor subtype called a2, which inhibits adenylyl cyclase and,
consequently, inhibits cAMP formation. The other subtype of glutamate metabotropic receptor might
activate a G-protein that stimulates the enzyme phospholipase C (PLC). PLC splits the membrane
phospholipids PIP2 into two parts: DAG and IP3. DAG stays in the plane of the membrane and activates
the downstream enzyme protein kinase C (PKC). (IP3, on the other hand, diffuses away and causes
organelles to discharge their calcium stores.)

Question 36

Do convergence and divergence of neurotransmitter effects occur in single neurons?

Answer 36

Diverging neurotransmitter effects are represented by the multitude of consequences a single
neurotransmitter may have because it affects many different receptor subtypes in postsynaptic neurons.
This effect may occur in a single neuron that possesses G-protein-coupled receptors with two or more
intracellular functions or, potentially, single neurons that elaborate different types of receptors in different
parts of the neuron. Convergence occurs when several transmitters affect a single effector system. This can
occur in a single cell at the level of the G-protein, the second messenger cascade, or the type of ion
channel. Neurons integrate divergent and convergent signaling systems, resulting in a complex map of
chemical effects.

Question 37

Ca2+ ions are considered to be second messengers. Why?

Answer 37

Ca2+ ions are considered to be second messengers because elevations of Ca2+ ions in the cytosol
can have widespread and long-lasting effects on the neuron. An example of this is Ca2+ activation of the
enzyme calcium-calmodulin-dependent protein kinase (CaMK), which is important in molecular
mechanisms of memory.