Download Presynaptic Questions

Neuroanatomy: Biochemistry and Presynaptic Mechanisms Study Questions
1. What category of small molecules appears to be the precursors of virtually all neurotransmitters (and in some
cases functions as transmitters themselves)?
Amino acids (GABA, glutamate, aspartate, glycine)
o Tyrosine is the precursors for catecholamines (dopamine, NE, Epi)
o Other biogenic amines: serotonin (tryptophan), histamine (histidine)
o Peptide NTs
Side Note: 6 Major Classes of NTs
o Cholinergic: ACh
o Biogenic amines:
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Adrenergic: dopamine, NE, epi
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Serotonin
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Histamine
o Amino Acids
o Purigenic: ATP, adenosine
o Peptides
o Non-Conventional: dissolved gases (NO and CO); not packaged in vesicles, but rather synthesize in
response to Ca++
2. What synthetic enzyme would you measure in the brain if you suspected a defect in acetylcholine synthesis?
Choline acetyltransferase (ChAT)
o Catalyzes: acetyl Co-A + choline  ACh + CoASH
ChAT levels are abnormally low in Alzheimer’s Disease in some specific brain regions (hippocampus and cortex)
3. What enzymatic step is bypassed in catecholamine synthesis when a Parkinson’s patient is treated with L-DOPA?
Giving L-DOPA bypasses the rate limiting step of synthesis (tyrosine  L-DOPA using the tyrosine hydroxylase
enzyme)
4. What is the objective (in terms of NT levels) of treating a Parkinson’s patient with L-DOPA?
L-DOPA is given in the hopes of raising the patient’s dopamine levels (L-DOPA  dopamine using the DOPA
decarboxylase enzyme)
5. What do we mean by the “rate-limiting enzyme” in neurotransmitter synthesis? In what ways has the amount or
activity of the rate-limiting enzyme in NE synthesis been shown to be regulated?
Rate-limiting enzyme is the enzyme who’s activity ultimately determines the rate at which NT is made
Regulation of RLE in NE Synthesis:
o End Product Inhibition: NE binds tyrosine hydrozylase and decreases its affinity for biopterin;
ultimately decreases the activity of tyrosine hydroxylase
o Covalent Modification: phosphorylation of tyrosine hydroxylase; occurs in response to Ca-dependent
or cAMP-dependent kinase

Ca++ activates PKC  TH becomes phosphorylation  Phosphorylated TH is less sensitive to
end-product inhibition and has a higher affinity for biopterin cofactor (TH more active)

NT activates adenylate cyclase  Increases cAMP, activates kinase  TH phosphorylated
and more active
o Enzyme Induction: intense neuronal activity causes increase in the amount of TH

5’ non-coding end of TH has a cAMP response element (CRE)

cAMP-dependent kinase phosphorylates CRE binding protein (CREB), which binds to CRE to
stimulate transcription (more TH enzyme)
6. What NTs is dopamine the precursor for?
NE and Epi
o Dopamine  NE (dopamine beta-hydroxylase)
o NE  Epi (PNMT)
7. What is the function of the signal peptide in the structure of a neuropeptide precursor?
The signal peptide serves as a signal for newly synthesized proteins to enter the cisternae of the RER
8. What types of amino acids (and how many?) generally signal a site in a neuropeptide precursor at which it will be
cleaved by an endopeptidase?
Dibasic amino acids generally signal cleavage sites
o Lysine (K) and arginine (R)
o Sequences are generally KK, KR or RR
9. In what ways can the same DNA sequence code for more than one type of neuropeptide? What is being spliced to
what when we talk about “alternate splicing” in the synthesis of substance P and K?
Alternate splicing at the gene and peptide level can lead to a large variety of peptides even from the same gene
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10.
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Example: POMC produces 7 different products based on alternative splicing
Substance P and K:
o Both come from the same gene, but alternative processing yields different mRNAs (and therefore
different proteins)
o Alternative splicing generates two prohormones that give rise to the tachykinins (substance P and K)
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One prohormone includes exons for both substance P and K
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One prohormone skips over the substance K exon and only encodes for substance P
Criticize the statement: The neurotransmitter vesicles in a neuron contain only one type of NT and no other
molecules.
Neurons can contain more than on NT; they frequently contain a peptide and one of the other types of NTs
What ion is critical in vesicular release of NT? How might the role of this ion explain the proposed mechanism of
the Lambert-Eaton Syndrome?
Vesicular release of NTs is a Ca-dependent process
o Ca++ results in the binding and opening of vesicles at the presynaptic membrane
Lambert-Eaton Syndrome: antibodies form against voltage-gated calcium channels in the pre-synaptic
membrane; decrease the amount of Ca++ entering and therefore less NT can be mobilized (vesicles cannot fuse
and release contents)
What type of molecules on the surface of vesicles and the inner side of the cell membrane mediate the fusion of
vesicles with the presynaptic membrane? What type of interaction between these molecules provides the energy
to “zip” them together during vesicle fusion?
V-SNARE proteins on the vesicles bind to t-SNARE proteins on the presynaptic membrane
o V-SNARE is VAMP (synaptobrevin)
o T-SNARES include syntaxin and SNAP-25
Hydrophobic interactions between VAMP on the vesicular membrane and synatxin and SNAP-25 on the
presynaptic membrane “zip” them together and provide energy to initiate fusion of the vesicle membrane with
the target membrane
SNARE interactions are Ca-dependent (dependence is due to the other vesicle proteins)
o Synapsin I anchors the vesicles to the cytoskeleton; this binding is broken by phosphorylation of
synapsin by Ca-calmodulin-dependent protein kinase
o Synaptotagmin requires Ca to bind to pre-synaptic membrane phospholipids and VAMP
What are the mechanisms of action of botulinum toxin and tetanus toxin on NT release?
Botulinum toxin: inhibits release at cholinergic endings (especially neuromuscular control of respiration and
cholinergic PS terminals)
o Mechanism: toxin is taken up into the pre-synaptic ending (action is not immediate)

Protease that cleaves SNAP-25, VAMP and syntaxin
o Can be used clinically to treat muscle spasms (produces localized chemical denervation muscle
paralysis for up to 3 months)
Tetanus toxin: reduces release from GABA and glycine synapses by specific proteolysis of VAMP
What are the three typical ways in which NT action is terminated?
Diffusion: away from the synapse; peptides often do this
Enzymatic Degradation: examples include peptidases (break down peptides) and AChE (breaks down ACh)
o AChE: ACh  acetate + choline

Choline is taken back up by presynaptic ending for resynthesis of ACh
Reuptake: into the pre-synaptic neuron; examples include NE and serotonin
What is the typical mechanism by which a nerve gas is thought to poison the function of cholinergic synapses?
Nerve poisons blocked AChE causing ACh to build up in the synapse, causing over-excitation
What type of NT mechanism is inhibited by Prozac?
Reuptake of serotonin (Prozac is an SSRI- selective serotonin reuptake inhibitor)
What role can glial cells play in the reuptake and recycling of GABA?
GABA is taken up by glial cells from the synapse and converted to glutamate (GABA transaminase) and then to
glutamine (glutamine synthetase)
The glutamine is then transferred back to the pre-synaptic nerve ending where it is used to produce new GABA
What enzymes are involved in the degradative metabolism of catecholamines?
Monoamine oxidase and catechol-o-methyltransferase
Catecholamines can be further degraded in the presynaptic ending using these enzymes if not reused for
neurotransmission
19. What are the major brain locations (but usually by no means the only brain locations) of the following NTs (think
about where the cell bodies are and also about what they typically innervate; also, about how levels change in
some diseases)

Dopamine (how does this change in Parkinson’s?):
o Substantia nigra, pars compacta (A9); Area tegmentalis lateralis (A8)
o All give extensive innervations to striatum
o A8 and A9 degenerate in Parkinson’s Disease
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Acetylcholine (which areas decrease during Alzheimer’s? Which do not?):
o Nucleus basalis of Meynert: at least 90% of neurons here cholinergic; 70% of cortex ChAT here
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Provides major cholinergic projection to entire neocortex
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Adjacent cells in basal forebrain go to olfactory cortex and hippocampus
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These projections severely reduced in Alzheimer’s Disease (however, ACh replacement
therapies have not been successful at treating AD)
o Nucleus caudatus, putamen and nucleus accumbens are short axon neurons that are unaffected by AD
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Norepinephrine:
o All neurons making NE are in the pons and medulla
o Most prominent in locus ceruleus (contains ~50% of all NE neurons in the brain)
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Innervates the entire neocortex, cerebellum and spinal cored
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Serotonin:
o Raphe Nuclei: however, the 5-HT neurons do not constitute the majority of neurons in these areas
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Also contain dopamine, NE, GAB< CCK etc.
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Innervate virtually the entire CNS (comprise the most expansive central neuronal network)
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Functions: block pain input and facilitate motor output in the spinal cord; coordinating and
modulating role; improves mood (brain)
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Opioids:
o Highest concentration in the globus pallidus
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Followed by (in descending order of concentration) the rest of the telencephalon,
dienceplon, pons, mesencephalon and cerebellum
 However, very low in hippocampus and cortical areas
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Constitute 15-20% of neurons in the neostriatum and give rise to an extensive striopallidal
projection
o Huntington’s Chorea: plexus of fibers in lateral pallidal segment degenerates
o Function: endogenous opiods believe to be involved in regulation of response to stress (mechanism
unclear)