NEUR 0010 - Chapter6 Flashcards
What does a neurotransmitter system include?
The NT molecule itself, molecular machinery for transmitter synthesis, vesicular packaging, reuptake, degradation, and action
What does cholinergic mean? Noradrenergic? Glutamatergic? GABAnergic? Peptidergic?
Neurons/synapses that use/release ACh; NE; glutamate; GABA; peptides…. You get the picture.
What are the three requirements for a molecule to be considered a NT?
Synthesized/stored in presynaptic neuron; released by presynaptic axon terminal upon stimulation; when experimentally applied, must produce a response in the postsynaptic cell that mimics the response produced by the release of the NT from the presyn neuron
What are the two most common methods for determining if a molecule is a NT?
Immunocytochemistry and in situ hybridization
What is immunocytochemistry?
Used to anatomically localize particular molecules in particular cells; NT candidate chemically purified, injected into bloodstream to stimulate immune response, antibodies bind to it; remove antibodies and chemically tag, apply to brain tissue; chemical tags of antibody+NT candidate will highlight brain areas where the NT candidate goes
What is in situ hybridization?
Used to confirm that a cell synthesizes a particular protein/peptide; complementary strand of mRNA (probe) for a particular nucleic acid sequence; probe is chemically labeled, binds to mRNA strand (hybridization), applied to brain tissue section, wash away extra probes that didn’t stick; search for neurons that contain the labeled probe
How do you test that an NT is released from the presyn neuron upon stimulation?
Bath brain tissue slice in high K+ solution (causes big depolarization, stimulates NT release from terminal to tissue), then in Ca++ (allows release of NT candidate into tissue slice after depolarization)
Why is it difficult to test that an NT is released from the presyn neuron upon stimulation?
Because following the K+ bath/Ca++ application to test for release of the NT candidate, it’s hard to tell whether the release is a secondary consequence or not; also hard to distinguish because many synapses with different NTs intermingle in the CNS (but not in the PNS)
How do you verify that the NT candidate evokes the same response as that produced by the release of naturally occurring NTs from the presyn neuron?
Microionophoresis
What is microionophoresis?
NT candidates dissolved in solutions to acquire net electrical charge; fine tipped pipette filled with ionized solution, inserted near postsyn membrane; pass electrical current through pipette to eject ionized solution; measure effects by microelectrode
What is the relationship between different NTs and different receptors? What can bind to many/only one of the other?
A NT can bond to many different receptors, but a receptor can only bind to one NT; “no two NTs can bins to the same receptor”
What are the two subtypes of cholinergic receptors? Why are they named as such?
Nicotinic and muscarinic; named for the agonists of each type
What are the antagonists for nicotinic and muscarinic cholinergic receptors?
Curare for nicotinic, atropine for muscarinic
What are three main subtypes of glutamatergic receptors?
AMPA, NMDA, kainate (each named for the agonists)
Of the three main subtypes of glutamatergic receptors, what substances can bind to what receptors?
Glutamate can bind to all three, but AMPA/NMDA/kainate can only bond to their respective receptors
What is a ligand?
Any chemical compound that binds to a specific site on a receptor; ligand = “to bind”
What is the ligand-bonding method?
Studying receptors using radioactively labeled ligands; can be agonist/antagonist/chemical NT itself
What are the two main NT receptor protein groups?
Transmitter-gated ion channels, G-protein coupled receptors (metabotropic)
What are the three main methods of studying different receptor subtypes?
Neuropharmacological analysis (how receptors respond to different drugs); ligand-bonding methods (radioactively tagging ligands to observe behavior); molecular analysis (identifying protein structure that makes up the receptors)
What is ACh derived from, and why is that different from usual NTs?
From acetyl CoA (byproduct of cellular respiration) and choline (important for fat metabolism); usually, NTs are derived from amino acids/amines/peptides
What is Dale’s principle?
The idea that a neuron has only one NT associated with it; often violated by peptide NTs
What group of NTs usually violates Dale’s principle? Why?
Peptide NTs: peptide-releasing neurons usually release the peptide NT and ALSO an amino acid/amine NT
What are co-transmitters?
NTs released from one axon terminal (according to Dale’s principle, there should only be one released from a given neuron)
What NT is present at all neuromuscular junctions? What does that imply about its synthesis?
ACh; synthesized by all motor neurons in the CNS
Where is ACh always present/synthesized?
In neuromuscular junctions; always synthesized by the motor neurons of the CNS
What enzyme is required for ACh synthesis? Where is it made?
Choline acetyltransferase enzyme (ChAT), made in the soma (only of cholinergic neurons, making it a good marker)
What do ChAT and ACh transporters do to help make ACh, and where?
In the cytosol of the axon terminal: ChAT assembles, transporters concentrate it into synaptic vesicles
How is ACh synthesized by ChAT in the axon terminal of cholinergic neurons?
ChAT transfers an acetyl group from acetyl CoA to choline, taken up from the ECF (choline is the rate-limiting step) through specific transporter
Cholinergic neurons make ACh and its degradative enzyme, AChE. How does AChE work?
AChE synthesized in cholinergic axon terminal (or other axon terminals, actually); secreted into synaptic cleft; degrades ACh into choline and acetic acid VERY QUICKLY; choline usually recycled for ACh synthesis
What are the catecholamines?
Dopamine, epinephrine, norepinephrine
What is the precursor to all three catecholamines?
Tyrosine (amino acid)
Where are catecholaminergic neurons usually found?
In the parts of the nervous system associated with regulation of movement, mood, attention, and visceral function
What enzyme is contained in all catecholaminergic neurons, since it catalyzes the first step of catecholamine synthesis? What does it do?
Tyrosine hydroxylase; converts tyrosine to “dopa;” rate-limiting factor for catecholamine synthesis
What are the rate limiting factors for ACh and catecholamine synthesis, respectively?
Choline; tyrosine hydroxylase
How does the catecholamine system exemplify end-product inhibition?
TH activity regulated by catecholamine release levels: if decreased catecholamine release, catecholamine levels increase in the cytosol, causing TH activity to decrease
What happens during high rate/prolonged periods of catecholamine release?
High rates of catecholamine release are accompanied by an elevation of Ca++ concentration, causing TH to increase activity to keep up; prolonged release also causes increased mRNA synthesis for TH enzyme
What happens once TH converts tyrosine into dopa, in the presence of dopa decarboxylase?
Dopa decarboxylase converts dopa into dopamine
What happens once TH converts tyrosine into dopa, in the presence of both dopa decarboxylase and dopamine beta-hydroxylase? Where is DBH found?
DBH converts dopamine into NE; DBH found in synaptic vesicles rather than in the axon terminal cytosol
Where is NE synthesized in noradrenergic axon terminals? Why?
In the synaptic vesicles: that’s where the DBH is (converts DA to NE)
What happens once TH converts tyrosine into dopa, in the presence of dopa decarboxylase, DBH, and PNMT?
PNMT converts NE into Epi
Where is Epi synthesized? Why is this kind of annoying?
In the axon terminal cytosol, because that’s where PMNT is. This is annoying and IMPRACTICAL because dopa hydroxylase converts dopa to DA in the cytosol, then DBH converts DA into NE in the synaptic vesicles (DBH only in vesicles), and then NE has to go back out into the cytosol to meet PNMT to be converted into Epi, and then repackaged into synaptic vesicles for release.
How are catecholamines taken up from the synaptic cleft? How does this differ from ACh reuptake?
Not very fast through degradation: selective uptake through Na+-dependent transporters on the axon terminal membrane
What is the precursor for serotonin?
Tryptophan (amino acid)
How prevalent are serotonergic neurons? What are they good for?
Not very; important for brain systems that regulate mood, emotional behavior, and sleep
Where do serotonergic neurons get the tryptophan from to make serotonin?
In the brain: ECF; in the body: in blood through diet
What are the two steps in serotonin synthesis?
Tryptophan converted to 5-HTP by tryptophan hydroxylase; 5-HTP converted to serotonin by 5-HTP dehyxdroxylase
What are the two enzymes involved in serotonin synthesis, and what do they do?
Tryptophan hydroxylase and 5-HTP dehydroxylase; tryptophan hydroxylase turns tryptophan into 5-HTP, and 5-HTP dehydroxylase turns 5-HTP into 5-HT (serotonin)
What does MAO do?
Monoamine oxidase: enzymatically destroys catecholamines and serotonin after reuptake
What are the three main amino acidergic neuron systems specific to?
Glu, Gly, GABA
What is the precursor for Glu and Gly?
Glucose and other precursors, made by action of enzymes that exist in all cells
What is the important distinction between glutamatergic and nonglutamatergic neurons?
Higher glutamate concentration in cytosol of glutamatergic neurons; also has glutamate transporter in axon terminal membrane that concentrates glutamate until about 50 mM in synaptic vesicles
What does the glutamate transporter do in glutamatergic neurons?
Concentrates glutamate in synaptic vesicles; distinctive feature of glutamatergic vs nonglutamatergic
How does synthesis of GABA differ from synthesis of Glu and Gly?
Specific to GABAnergic neurons, whereas Glu and Gly are synthesized by most cells since they’re common amino acids
What is the precursor for GABA? What enzyme is involved?
Glutamate: enzyme is glutamic acid decarboxylase (GAD)
What is so interesting about the conversion of glutamate to GABA?
Glutamate is highly excitatory, and GABA is highly inhibitory!
What happens to GABA after reuptake through Na+-dependent transporters?
Metabolized by GABA transaminase enzyme
What enzyme is responsible for metabolizing GABA after reuptake?
GABA transaminase
Why do researchers suspect that ATP is a NT?
Concentrated in synaptic vesicles in the CNS/PNS, and released into cleft by presynaptic spikes in a Ca++-dependent way; usually packaged in vesicles with other classic NTs (co-transmitters)
What effect does ATP-as-an-NT have on some neurons?
Excitatory: gates a cation channel; binds to purinergic receptors (may be transmitter-gated, but also lots of G-protein coupled)
What are endocannabinoids?
Small lipids; can be released from postsyn neurons and act on presyn terminals! (retrograde signaling”
What is interesting about endocannabinoid signaling?
Retrograde: goes from postsyn to presyn terminal!
What is the function of retrograde signaling?
Serve as feedback system to regulate conventional forms of synaptic transmission
What is the mechanism of endocannabinoid signaling?
Vigorous AP firing in postsyn neuron; causes voltage-gated Ca++ channels to openl influx of Ca++, which stimulates synthesis of endocannabinoid molecules from membrane lipids
What are three unusual qualities of endocannabinoids?
Not packaged in vesicles (manufactured on demand); small and membrane permeable (rapid diffusion); bind selectively to CB1 type of cannabinoid receptor located on certain presyn terminals
What are CB1 receptor?
For endocannabinoids; G-protein coupled receptors; reduce opening of presyn calcium channels (impairs ability of presyn neuron to release its NT)
What is the precursor to NO?
Arginine (amino acid)
What does NO do in the body?
Helps regulate blood flow; in NS, may be another retrograde messenger
How are endocannabinoids and NO similar, and why is this useful?
Both small and membrane-permeable, so they can diffuse easily across membrane (are also both retrograde messengers)
What is one of the “downsides” to NO as a neurotransmitter?
It’s evanescent: breaks down very rapidly
Do neurotransmitters exist outside of the nervous system?
Yeah duh. They just usually serve different purposes (amino acids used to make proteins, ATP as energy source, NO to relax smooth muscle in blood vessels, ACh in the cornea, serotonin in blood platelets, etc.)
What is the subunit arrangement of the nicotinic ACh receptor at skeletal muscle in neuromuscular junctions?
Five subunits: alpha(2)/beta/gamma/delta
What is required for a nicotinic ACh receptor at skeletal muscle in neuromuscular junctions for the channel to open?
Simultaneous binding of ACh to the binding sites on each alpha subunit
How do the nicotinic ACh receptors differ for neurons vs skeletal muscle at the neuromuscular junction?
Both pentameters, but at neuron, it’s usually alpha(3)beta(2) instead of alpha(2)beta(1)gamma(1)delta(1)
Give a brief overview of the structural similarities of subunits for different transmitter-gated ion channels.
Segments M1, M2, M3, M4 are hydrophobic coiled alpha helices (thread back and forth through membrane); M1-3 are pretty similar in layout, but M4 can be closer/further along the amino acid chain and is towards the end
How do glutamate-gated ion channels differ from other transmitter-gated ion channels?
Probably tetramers; M2 region forms a hairpin instead of spanning the membrane (resembles K+ channels)
What kind of transmitter-gated ion channels mediate fast synaptic transmission in the CNS, for the most part?
Amino acid-gated channels
What are the three glutamate-gated ion channels? Which two mediate most of the fast excitatory synaptic transmission in the brain
AMPA, NMDA, kainate; AMPA and NMDA mediate most of the fast synaptic transmission in the brain, and kainate isn’t very well understood yet
What ions are AMPA-gated channels permeable/impermeable to? What is the overall effect of this?
Permeable to Na+ and K+; impermeable to Ca++; Overall Effect = Na+ influx to depolarize (same excitatory transmission at CNS as nicotinic at neuromuscular)
Why do most glutamate-mediated EPSPs have components contributed to by both AMPA and NMDA receptors?
Because AMPA and NMDA receptors often cohabitate the same synapses
In what two ways do NMDA receptors differ from AMPA receptors?
NMDA are permeable to Ca++; inward ionic current through NMDA is voltage-dependent (magnesium block at normal negative resting membrane potentials)
Explain how NMDA receptors’ ion current is voltage-dependent.
At normal negative resting membrane potential, binding of glutamate opens the channel, but it is then blocked by Mg++ ions; Mg++ ions only pop out during depolarization (usually requires AMPA receptors to have opened)
Why is it super convenient that AMPA and NMDA receptors are often found in the same synapses?
Because NMDA’s ion current is voltage-dependent, and will only eliminate the Mg++ block after AMPA opening causes depolarization
What kind of synaptic transmission do GABA and glycine mediate in the brain?
Inhibitory: gate a chloride channel (but otherwise, have structure similar to excitatory cation-gated nicotinic ACh receptors)
What two drugs affect GABA-gated channels (GABA A, specifically), and how do they do that?
Benzodiazepines (When GABA present: increase channel opening frequency) and barbiturates (When GABA present: increase channel opening duration)
What is the result of the “amplifying” effects of benzodiazepines/barbiturates on GABA-A gated channels?
Increased inhibitory Cl- current, stronger IPSPs: enhanced inhibition
What two substances can also affect GABA-A-gated channel activity, besides benzodiazepines and barbiturates?
Ethanol and neurosteroids
What does the name G-protein say about the structure?
Short for GTP-binding protein; uses GDP/GTP to function
What is the basic mode of operation for G-proteins?
Resting state: G-alpha subunit binds GDP and floats along membrane; Bumps into right receptor with bound transmitter: GDP released, GTP picked up from cytosol; Activated G-protein: G-alpha plus GTP splits from G-beta+G-gamma complex, they all go do their thing with effector proteins; G-alpha breaks the GTP into GDP again; all three subunits reunite
What is the resting state for a G-protein?
GDP bound to alpha subunit, float along the membrane interior
What happens when a G-protein bumps into the proper receptor, if the receptor has its transmitter bound to it?
Alpha subunit swaps its GDP for a GTP from the cytosol and detaches from the other two subunits, and they all float off to do their own thing with effector proteins
What are the two types of effector proteins that G-proteins interact with?
G-protein-gated ion channels, and G-protein-activated enzymes
What is the G-protein shortcut pathway?
G-protein-gated ion channels
How do G-protein-gated ion channels work?
G-protein bumps into right receptor and does its swap/split routine; Beta/gamma subunit goes and hooks up with an ion channel to open it
What are the advantages of G-protein-gated ion channels over G-protein-activated enzymes?
Very fast (but not as fast as transmitter-gated ion channels), and localized (only nearby channels can be affected)
How do G-protein activated enzymes work?
G-protein bumps into right receptor and does its swap/split routine; Alpha subunit goes and activates certain enzymes, triggering a cascade of enzymes that produce second messengers: “second messenger cascade”
How does the cAMP secondary messenger cascade from NE-beta activation work?
NE-beta receptor activates G-protein S; G-protein S activates adenylyl cyclase; adenylyl cyclase converts ATP to cAMP; cAMP activates protein kinase A, which can phosphorylate K+ channels (this whole thing is inhibited when NE-alpha2 receptors are activated)
Why are protein kinases important for ion channels?
Can phosphorylate them: enhance or inhibit their propensity to open/close
What molecules reverse the effects of protein kinases on ion channels?
Protein phosphatases: remove phosphate groups that the protein kinases attached
What are the advantages to G-protein coupled transmission (gated ion channels or activated enzymes)?
Signal amplification: activation of one G-protein coupled receptor can activate many ion channels; this can have long lasting chemical changes, and provides many sites for regulation if needed
In terms of transmitter activity, what is divergence?
The ability of one transmitter to activate more than one subtype of receptor and cause more than one type of postsyn response; the general rule of neurotransmitter systems
What does transmitter divergence allow for, in terms of effects?
By activating different receptor subtypes: can affect different neurons/parts of neurons; can lead to different G-protein/effector system behavior per subtype
In terms of transmitter systems, what is convergence?
The ability of multiple NTs, affecting their own transmitter type, to affect the same effector systems
