synaptic transmission Flashcards
vesicular docking and fusion involves?
- interactions between synaptobrevin (in the vesicle membrane) and syntax in (in the plasma membrane) –> come together to form a SNARE complex –>facilitates exocytosis and membrane fusion
- Ca2+ triggers the vesicular fusion to terminal membrane and exocytosis
-vesicle buds off from endosome (filled with NT)
active zones contain high levels of voltage-gated Ca2+ channels, fusion and docking proteins. when AP arrives–> influx of Ca2+ –> stimulate vesicles to dock and prepare for exocytosis into the synaptic cleft.
spent vesicles coated with clathrin, undergo endocytosis and recycled back into endosome inside the cell.
what triggers NT release?
axon terminal depolarization activated voltage gated Ca2+ channels that stimulate the release of NT sufficient for a postsynaptic response
experiments?
need Ca2+ before depolarizing to get a response
Mg 2+ added before Ca2+ can block the response
depolarization without Ca2+==> no post synaptic response
Type I synapses
excitatory (asymmetric) synapses–> will depolarize
widee synaptic cleft (30-40nm) and active zone (1-2 micrometers)
Vesicles clustered around dense projections (area above the terminal membrane)
Well developed postsynaptic density:
a lot of receptors for the excitatory NT
lot of scaffolding proteins
and signal transduction (Ca2+, calmodulin, protein kinase)
ex: glutamate in neuromuscular junction–> excitatory
Type II synapses
Inhibitory (symmetric) synapses–> decrease likelihood of postsynaptic depolarization
More narrow synaptic cleft (20nm) & active zone ( inhibitory
En passant (nondirected synapses)
Often observed in the post ganglionic autonomic nervous system neurons (SNS and PNS)
Varicose swellings interspersed along length of axon terminal serve as sites of neurotransmitter release
Greater separation between varicosity & postsynaptic membrane (400 nm)
No postsynaptic specialization–> regulation by the presynaptic elements –> slower and more diffuse onset of response
Criteria to be a neurotransmitter
- Neuronal biosynthesis (neuron has to make it)
- Localized to axon terminal, released in response to appropriate stimuli and in amounts that trigger a postsynaptic response
- Postsynaptic effect mimicked by exogenous application (without stimulus, but added NT–> still get response)
- Rapid termination of action (uptake or degradation)
Catecholamine biosynthesis (which product, enzymes, cofactors, location)
dopamine, norepi and eli from phenylalanine precursor
phenylalanine–> tyrosine
enzyme: phenylalanine hydroxylase
* REQUIRES tetrahydrobiopterin cofactor
tyrosine –> dopa (enz: tyrosine hydroxylase)
*REQUIRES tetrahydrobiopterin cofactor
dopa–> domamine (enz: dope decarboxylase pyrodoxial phosphate)
dopamine–> norepi (enz: dopamine beta hydroxylase)
*occurs within the synaptic vesicle
norepi–> epi (enz: phenylethanolamine-N-methyltransferase)
*in brainstem and adrenal cortical cells ONLY
indoleamine biosynthesis
tryptophan–> 5-hydroxytryptophan (5-HTP) (by tryptophan hydroxylase)
–>Seratonin (5-hydroxytryptamine (5-HT)) (by 5-HT decarboxylase)
seratonin in the gut and in dorsal raphe in the brain (regulation of mood and arousal)
histamine biosynthesis
histadine–> histamine (enz: histadine decarboxylase)
usually in posterior hypothalamus
regulate homeostasis
Amino acid NT biosynthesis
- glutamate
Reductive amination of α-ketoglutarate (from Citric acid cycle) to form glutamate
predominantly excitatory in the CNS
found in large quantities in the CNS
into synaptic vesicles by BPN 1 (transporter) - GABA
Decarboxylation of glutamate to form GABA
predominantly inhibitory in the CNS - Glycine
Neuropeptide transmitters
Derived from posttranslational modification of peptide precursors–> A LOT!
Examples
β-endorphin derived from proopiomelanocortin
Somatostatin derived from preprosomatostatin
Precursor: prepropeptide molecule
can have several neuropeptides.
Dibasic residues serve as cleavage sites.
serve as the N or C termini for bioactive peptides.
Whether expressed or not is based on post translational proteolytic cleavage
ACTH=important intermediary –> stimulates cortex to produce cortisol in anterior Pituitary
Hypothalamus=proteases will break ACTH into alpha-MSH (regulation of energy balance) and CLIP
beta-LPH also divided to form gamma-LPH and beta-endorphin
Combinations of NT can be formed by one precursor
expression depends on the array of proteases in the cells and tissues
tightly regulated tissue specific process.
Acetylcholine biosynthesis
Synthesized by choline acetyl-transferase (CAT)
(rxn: choline +AcCoA–> CoA+ACh)
Transported into vesicles via VAChT
in brain and neuromuscular junctions
G-protein
Guanosine nucleotide binding protein comprising α, β & γ subunits
α subunits possess GTPase activity
Transmitter binding triggers nucleotide exchange and dissociation of α subunit
-without ligand, GDP is bound to the alpha subunit.
-once NT is bound, GDP–>GTP and alpha is dislocated from the beta-gamma subunit
5 G protein families: Gs, Gi/o, Gt, Gq & G13
Dopamine receptors and interactions
D1 and D5 receptors interact with Gs–> stimulate adenylyl cyclase, cAMP and PKA
D2, D3, and D4 receptors interact with Gi/o which: inhibits adenylyl cyclase, cAMP formation, and PKA (activates K+ channels and inhibits Ca2+ channels)
Norepinephrine and epinephrine receptors and interactions
Interact with two subtypes of α receptor and three subtypes of β receptor
α1 receptors interact with Gq that activates phospholipase C (PLC)
PLC forms diacylglycerol (DAG) and inositol triphosphate (IP3) from phosphatidyl inositol bisphosphate
DAG activates protein kinase C
IP3 mobilizes intracellular stores of Ca2+
α2 receptors couple to Gi/o
decrease adenylyl cyclase –> inhibit Ca2+ influx
β1-3 receptors couple to Gs
increase adenylyl cyclase and increase PKA dependent phosphorylation
Serotonin (5-HT) Receptors
Numerous 5-HT receptor subtypes
5-HT1 receptors couple to Gi/o
inhibit AC–> decrease cAMP-> decrease PKA –> open K+ channels, decrease Ca2+ influx
5-HT2 receptors couple to Gq
The 5-HT3 receptor is a ligand-gated ion channel (like glutamate receptors)
histamine receptors
H1 receptors couple to Gq
H2 receptors couple to Gs
H3 receptors couple to Gi/o
Glutamate receptors
Majority are ionotropic receptors (ligand-gated ion channels)
Three subtypes of ionotropic glutamate receptors:
1. N-methyl-D-aspartate (NMDA) receptors
2. α-Amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptors
3. Kainate receptors
NMDA receptor
Two primary subunits (NR1 & NR2) Each subunit has: Extracellular N-terminus Intracellular C-terminus Three transmembrane domains Pore-forming loop (TM2)
Activation of the NMDA receptor:
Increases Na+ & Ca2+ influx
Increases K+ efflux
*Glycine requirement (positive allosteric modulator)
Mg2+ channel block
-rest, Mg2+ in the pore and glutamate ineffective
-depolarized==> Mg2+ kicked out of pore–> allows the movement of ions and depolarization of membrane potential
*PCP and ketamine=non competitive inhibitors (open channel blocker)
enter the pore in open state and produce noncompetitive inhibition
AMPA and Kainate Receptors
Four AMPA receptor subunits (GluR1-4)
Five kainate receptor subunits (Glu5-7, KA1, KA2)
Structural characteristics similar to NMDA receptor subunits
The TM2 domain contains the Q/R site which Determines Ca2+ permeability of AMPA receptors
• Exchange of arginine for glutamate during mRNA processing at the Q/R site will render GluR2 impermeable to calcium.
o The N-terminal S1 domain, along with S2 domain linking the TM3 and TM4 domains, are critical for glutamate binding.
GABA receptors and glycine receptor
GABA A receptor:
Pentameric ligand-gated Cl- channel
Comprises α, β, γ, δ & ε subunits
Numerous allosteric modulatory sites that alter channel function
throughout brain and spinal core and NS of the gut
GABA A bound–> open–> Cl- into cell–> hyper polarize/inhibitory response
o Benzodiozepines, barbiturates, alcohol and neurosteroids can all potentiate this receptor thru positive allosteric modification of the GABAA receptor.
GABA C receptor
Also a pentameric ligand-gated Cl- channel–> leads to hyper polarization and inhibition
Different arrangements of three ρ subunits
GABA B recptor
Metabotropic receptor that couples to Gi/o
Glycine receptor
ligand-gated Cl- channel
posy synaptic neurons in brain stem and spinal neurons
pentamer of alpha and beta subunits. GABA binds–>inhibits
Muscarinic acetylcholine receptor
Interacts with two subtypes: muscarinic & nicotinic
Muscarinic acetylcholine receptors:
Five different subtypes (M1-M5)
M1, M3 (gut) & M5 couple to Gq
M2 (heart) & M4 (gut) couple to Gi/o
nicotinic acetylcholine receptor
Nicotinic acetylcholine receptor:
brain and autonomic ganglia
pentameric ligand-gated mixed cation channel
Pentameric arrangement of α1, β, δ, ε & γ subunits
(epsilon in adults or gamma in embryonic tissue)
pore lining =M2 domain
Large N-terminus in the extra cellular matrix
Four membrane-spanning domains (M1-M4) in each of the 5 subunits
Short C-terminus in the extracellular matrix
Interacts with a number of intracellular scaffolding proteins
–>Rapsyn–> promotes the insertion of nicotinic receptor in the plasma membrane and stabilizes it there.
The acetylcholine binding site
The binding of two acetylcholine molecules required for full receptor activation
Involves N-terminal interactions between the α1 & δ subunits, and the α1 & γ (or ε) subunits
Critical residues
Y93, Y190, Y198, C192, C193 & W149 on the α1 subunit
Negatively charged residues on the δ & γ (or ε) subunits help stabilize the positively charged ACh molecule
5 M2 domains form 4 rings of negative charge to determine absolute and relative ion selectivity
extracellular ring = Na+ selectivity for influx (absolute)
intracellular ring =K+ efflux (absolute)
intermediate ring= determines the relative rate of ion transport
relative filter for ion transport
central ring
Coformational change–>Na+ inf and K+ out
Collective response is large
end plate potential =60-70 mV
activation of the nicotinic ACh receptor leads to:
Suprathreshold depolarization of the postsynaptic membrane
Activation of voltage-gated Na+ & Ca2+ channels (excitation/contraction coupling)
Action potential formation
Muscular contraction
–> resembles the fast EPSP (reverses polarity at 0mV, complete inactivation w/in 100 msec)
Multinodal distribution with .4 mV in between
sometimes spontaneous without stimulation
mini-EPSPs in the absence of extracellular Ca2+ response of a single quantum of NT
Spontaneous could occur sporadically but only a few NT
A single quantum can depolarize the post synaptic membrane .4 mV
given a normal EPSP, or end plate potential (60-70 mV) can get idea of how many quantum are released
Fast IPSP
Arises from the activation of ligand-gated Cl- channels
e.g., GABA (A or C) & glycine receptors
At rest, fast IPSP produces transient
Cl- normally flows into the cell–>hyperpolarization of the postsynaptic membrane
Reverses polarity at ECl
full activation=very fast
complete inactivation within 100msec
ECl=-60mV
at a Vm of -70–> Cl- channel opening will have a depolarizing effect
IPSPs used for the inhibition of antagonist muscles when an agonist is contracting by renshaw cells (inhibitory interneurons of the spinal cord)
slow IPSP
Arises from the opening of a K+ channel modulated by Gi/o protein-coupled receptors
e.g., dopamine D2, 5HT1, α2 adrenergic & μ-opioid receptors
Channel modulation occurs via direct interactions with the βγ complex
results in a hyperpolarization more prolonged than the fast IPSP
Reverses polarity at EK
Few seconds to reach peak
full decay can be up to minutes
Dependent on the time constant intrinsic to the postsynaptic cell
fast EPSP
Arises from the activation of ligand-gated mixed cation channels
e.g., AMPA & NMDA receptors (glutamate activates) or 5HT3
At rest, fast EPSP produces transient depolarization of the postsynaptic membrane
Reverses polarity at 0 mV, in between ECa, ENa & EK
NMDA = more sluggish, and blocked by Mg under resting conditions
at rest, only AMPA is activated. as depolarizes, Mg2+ removed from NMDA and this is also activated leading to further depolarization
*long-term potentiation
AMPA receptor-mediated depolarization relieves Mg2+ block of the NMDA receptor
Long-lasting NMDA receptor-mediated Ca2+ influx leads to synaptic adaptations that include:
protein phosphorylation
Retrograde messenger formation
More efficient glutamate release
Potentiation of the AMPA receptor-mediated fast EPSP
Cellular learning
==> increase synaptic strength from pre and post synaptic membranes=learning
late, slow EPSP
Arises from the closing of a passive K+ channel modulated by Gq protein-coupled receptors
e.g., muscarinic M1, 3, 5 receptors, gonadotropin-releasing hormone (GnRH) receptors
Activation–> activate PKC–>phosphorylate a leak channel, closing the leak channel–>decrease K+ efflux–> depolarization for longer
Channel modulation occurs indirectly via channel phosphorylation
results in a long-lasting depolarization
Reverses polarity at EK
activate =up to 1 minute
complete deactivation =up to 10 minutes
Slow EPSP=long lasting changes in post symnaptic excitability
pre-synaptic inhibition
Occurs at axo-axonic synapses
Neurotransmitter released from the upstream terminal decreases release from the a downstream neuron
Arises from the activation of Gi/o-coupled receptors that:
potentiate (increase) K+ currents and attenuate (decrease) Ca2+ currents
Presynaptic facilitation
Occurs at axo-axonic synapses
Neurotransmitter released from the upstream terminal increases release from the other (downstream neuron)
Arises from the activation of Gs-coupled receptors that:
increase N-type Ca2+ currents into the nerve terminal
increase of duration of AP at nerve terminal–> increase in response
catecholamine termination
(dopamine, serotonin, histamine)
Occurs through a combination of reuptake & metabolism
Metabolism via extracellular catechol-O-methyl transferase (in post synaptic neurons and glial cells) & mitochondrial monoamine oxidase (after monoamine NT transporter reuptake)
Reuptake via a monoamine neurotransmitter transporter
Requires co-transport of Na+ and Cl-
metabolites conjugated and excreted
serotonin termination
• Occurs through a combination of reuptake & metabolism
• Reuptake via a monoamine neurotransmitter transporter
o Requires co-transport of Na+ and Cl-
• Metabolism via mitochondrial monoamine oxidase to form 5-hydroxyindoleacetic acid (5-HIAA).
• May also be converted to melatonin (in pineal gland) or conjugated
histamine termination
• Occurs through a combination of reuptake & metabolism
• Reuptake via a monoamine neurotransmitter transporter
o Requires co-transport of Na+ and Cl-
• Metabolism via histamine-N-methyltransferase.
o Converts histamine to methylhistamine
• Further modified by MAO to yield methylimidazole acetic acid.
Glutamate termination
• Removed from synaptic cleft via Na+-dependent glutamate transporter which is tied to Na+/K+ ATPase activity.
GABA & glycine termination
Removed via active transport reuptake
Requires co-transport of Na+ and Cl-
GABA deaminated by GABA transaminase to succinic semialdehyde.
neuropeptide transmitter termination
rapid termination due to proteolytic cleavage into smaller peptide fragments
acetylcholine termination
rapid due to hydrolysis via acetylcholingesterase nto choline and acetate
