Download BME 502: Handout on Synaptic Transmission #2

BME 502 / handout #5
BME 502: Handout on Synaptic Transmission #2
Synaptic Transmission II: Postsynaptic Mechanisms
relation between receptor binding, channel state, synaptic current, and synaptic potential:
hydrolysis(C )(O )
uptake
A
T
B

k1
+ R _ T  R _ T  R*

k2
diffusion
concentration of neurotransmitter, T, in the synaptic cleft that is available to bind to the
postsynaptic receptor, R, is dependent on the concentration of transmitter in the vesicles, the
number of vesicles released, and the cleft geometry
decline of neurotransmitter in the synaptic cleft is dependent on diffusion and either hydrolysis or
reuptake by pre- and postsynaptic elements and glial cells
one or more of the transmitter molecules will bind to the receptor, forming T • R, a bound but
closed state of the channel, (C)
binding to the receptor will occur with a rate constant of k1
unbinding from the receptor will occur with a rate constant of k2
the channel will make a transition from the closed state to the open state, (O), which is a different
state of the bound receptor, T • R*
the transition from closed to open will occur with a rate constant of 
the transition from open back to closed will occur with a rate constant of 
during the open state, the amount of current flowing through the channel is dependent on the
single-channel conductance and the driving force; the current per channel is given by:
Install Equation Editor and doubleclick here to view equation.
where g' is the single channel conductance, and I' is the current flowing through a single channel
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if all channels opened simultaneously and stayed open, then the total synaptic conductance would
be the sum of all of the individual channel conductances:
Install Equation Editor and doubleclick here to view equation.
thus, given multiple populations of channels, each with different properties, a multiple, parallel
conductance model is most appropriate:
considering again the a change in conductance for a single channel, when a channel is open
(switch closed), I' charges the membrane capacitance, Cm (current flows across the membrane
conductance, Gr )
the direction of I' , and thus the polarity of the potential developed across Cm , is determined by Es
relative to Er
when the transmitter concentration is low and the channels close (switches open), the potential
developed across Cm decays with a rate determined by m
for synaptic events faster than m, decay of the PSP potential is governed by m
for synaptic events slower than m, the decay of the PSP is governed by the kinetics of the
channel
the vast majority of excitatory postsynaptic potentials (EPSPs) are "fast" synaptic events, i.e.,
conductances mediated by channels that have a rapid transition rate from the closed to the open
state and from the open to the closed state
also true of the majority of inhibitory postsynaptic potentials (IPSPs)
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the consequence is that the membrane m is the primary determinant of the rise time and fall time
of the synaptic potential, resulting in the following relation between the synaptic current and the
synaptic potential:
where gs is the total synaptic conductance, shown here with a polarity reversed relative to that
traditionally used
synaptic inputs are classified as excitatory or inhibitory depending on whether or not they increase
or decrease the probability of action potential generation -- and not on whether or not they are
depolarizing or hyperpolarizing, because whether or not a given synaptic current results in
depolarization or hyperpolarization depends on Vm at the time of the synaptic input, and on E for
the ion species carrying the synaptic current:
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Excitatory Amino Acids: Glutamate
glutamate is the most prevalent exctitatory neurotransmitter in the nervous system
binds to three different subtypes of receptors, all of which have very different consequences
for cell excitability
receptor subtypes are identified by the agonists that bind most selectively to each:
AMPA (-amino-3-hydroxy-5-methyl-4-isoxazoleproprianate) subtype
fast kinetics; time-to-peak: 20 msec; duration: 50 msec
receptor linked directly to the channel to cause a change in conductance
conductance is carried by Na+ and K+ ions
antagonist: 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)
I-V curve constructed in the presence of agonist
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NMDA (N-methyl-D-aspartate) subtype
slow kinetics; time-to-peak: 70 msec; duration: 120 msec
receptor also linked directly to channel to cause a change in conductance
conductance is Ca2+, Na+ and K+ ions
antagonist: D-5-aminophosphonovalerate (APV)
but channel conductance requires more than just the presence of the ligand, glutamate
also requires depolarization
not because channel is voltage-sensitive
but because Mg2+ ions bind to another binding site inside the channel so as to
block conductance by any other ions
the blockade is voltage-dependent, so that depolarization is required to remove
Mg2+ from the channel site
when both the requirements of glutamate and depolarization to remove the channel
block are met, channel opens and Ca2+ ions are allowed to flow
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thus, I-V curve substantially different than I-V curve for AMPA receptor-channel, and
shape of I-V curve markedly dependent on concentration of extracellular Mg2+
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time courses of AMPA and NMDA components are substantially different:
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relative contributions of each receptor-channel subtype to the amplitude-time course of the EPSP
depends on Vm :
major functional consequence is that magnitude of NMDA conductance is frequency-dependent
due to temporal summation
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at least one other requirement for NMDA channel conductance
a binding site on the receptor-channel complex exists for glycine (inhibitory
neurotransmitter in spinal cord; not in brain)
glycine must be present for NMDA channel to function at all, regardless of the amount
of glutamate or the level of depolarization
thus, multiple mechanisms for regulating the NMDA receptor channel; contrasts with relative
simplicity of AMPA receptor channel
other binding sites inside the NMDA channel
PCP, drug that has psychotomimetic effects
MK-801
Zn2+
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Metabotropic receptor (mGlu)
identified by most effective agonist, quisqualate
receptor is not linked to a channel, but instead has the effect of releasing calcium from
intracellular storage sites, i.e., mitochrondria, and endoplasmic reticulum
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second messenger mediated synaptic transmission: example of the glutamate
metabotropic receptor
fast, i.e., receptor-channel complex
slow, i.e., second messenger activated conductances
second messenger-activated conductances mediated by G-proteins, which bind guanine
nucleotides. G-proteins are trimers of three subunits, , , and 
the  subunits bind and hydrolyze guanosine triphosphate (GTP) and interact with
receptor and effector proteins
the  and  subunits anchor the  subunit to the membrane (and thus are required) in
resting state, GDP (guanine diphosphate) is bound to the  subunit and the three
subunits are associated as a trimer
when the receptor is activated by a neurotransmitter, GTP replaces GDP, resulting in
the dissociation of the  subunit, which then is free to diffuse through the membrane to
interact with various target proteins, which can be components of channels, enzymes
or pumps, eventually changing the conductance state of a channel
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G-proteins have been groups into classes depending on their target:
Gs
Gi
Gt
Gk
Gp
activates adenylyl cyclase
inhibits adenylyl cyclase
activates cyclic GMP phosphodiesterase
activates potassium channels
activates phospholipase C
Direct effects of G-protein -subunits on channel conductance
a number of instances have been identified in which the -subunit acts directly on a channel
protein to alter channel conductance
example 1:
ACh effect on cardiac muscle: closes K+ channels
(figure above applies, with ACh as neurotransmitter, and in this case with channel closing)
example 2:
similar effect in CNS with ACh effects on muscarinic subtype of ACh receptor
two ACh receptor subtypes: nicotinic and muscarinic
nicotinic is the type at the neuromuscular junction
permeable to both Na+ and K+
muscarinic receptors localized at other synapses in peripheral nervous system and at
CNS synapses
action is to close K+ channels -- voltage-dependent K+ channels that are normally open
at rest or that open rapidly at voltages near rest
referred to as M-current; rapidly activating, but very slow to no inactivation
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Effect of metabotropic receptor on intracellular stores of Ca2+ is an example of an indirect
effect of G-protein -subunits on channel conductance
G-protein activation of phospholipase C
activated -subunit hydrolyzes membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2),
producing inositol 1,4,5-triphosphate (IP3) and diacyglycerol (DAG)
IP3 releases Ca2+ from intracellular stores (endoplasmic reticulum):
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Inhibitory Amino Acids: GABA and Glycine
GABA (gamma-aminobutyric acid) -- primary inhibitory neurotransmitter in brain; glycine is the
primary inhibitory neurotransmitter in spinal cord
almost always found in interneurons; cells having local connections with principal
neuron that provides the output from a structure; thus, acts to modify excitability
of principal neuron; involved in feedforward and feedback inhibitory circuitry
acts through two classes of receptors - GABAA and GABAB
GABAA receptor
conductance: Clconcentration of Cl- approx 10x greater outside than inside, so when channels opened,
inward flow of Cl- ions, resulting in hyperpolarization of neuron, IPSP
postsynaptic hyperpolarization associated with a marked decrease in membrane
resistance
pharmacology: receptor blocker: bicuculline; channel blocker: picrotoxin
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kinetics: fast kinetics; rapid onset and offset
time-to-peak: 50 ms; duration: 100-200 msec
when part of a negative feedback pathway,
IPSP follows directly after the repolarization of
the membrane by Ic and gk(v), so partially
merges with the hyperpolarization caused by
those currents
also partially overlaps with hyperpolarization of
AHP; "hump" in hyperpolarization
inactivation:
primary
inactivation is uptake
mechanism
for
location: postsynaptic
modulatory sites:
allosteric action of pentobarbital
pentobarbital increases the probability of channel opening to GABA, with little effect in
isolation of the agonist, similar to glycine action at the NMDA receptor channel
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allosteric action of hormones: progesterone derivatives, e.g., pregnanolone
functional role: feedback vs. feedforward circuits
feedback: how might feedback inhibition be revealed in the activity of a principal neuron?
assume circuit below: principal neuron #1 and #2 are excitatory; epsp time to peak in #2
and interneuron is 5 msec; ipsp in principal neuron caused by interneuron activation of
GABAA receptor has a time to peak of 10 msec and a decay time of 30 msec
summary: feedback inhibition will have a "low-pass filtering" effect on the activity of
neuron #2; or, the interneuron acts like a "high-pass filter" with respect to the
output of neuron #2
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paired pulse stimulation:
in the real nervous system, inhibitory feedback is rarely if ever distributed to only one
neuron; because principal neurons almost always out-number interneurons, the output
of principal neurons converges onto interneurons; output of the interneuron diverges
with respect to principal neurons
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thus, it is possible to have feedback inhibition to a cell which exhibits only a
depolarization EPSP to an excitatory input, i.e., which does not drive the feedback; the
excitatory input is suprathreshold to some other principal neuron of the population
consequence is that IPSP can limit the duration of a simultaneous EPSP, i.e.,
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precise effect of interneuron output on principal neuron depends to a large extent on
the intrinsic membrane properties of the interneuron as well as on the kinetics of the
channels, etc.
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feedforward inhibition:
have found that interneurons involved with feedforward inhibition often have lower
thresholds and thus, discharge with an earlier latency to same excitatory afferent that
drives principal neuron
thus, can "select" that subpopulations which will be susceptible to subsequent
excitatory signal
depolarizing ipsp's
apply GABA to cell body; observation is hyperpolarization; bicuculline blockade indicates
GABAA receptor mediation; picrotoxin blockade indicates Cl- conductance
apply GABA to dendrites; observation is depolarization; bicuculline blockade and picrotoxin
effects consistent with mediation by GABAA receptor; raises the question of whether or not
the GABA effect is inhibitory -- maybe it is excitatory which would be more consistent with
the depolarization; test hypothesis by stimulating excitatory input and applying GABA -effect is blockade of stimulation-induced action potential discharge
how to explain depolarization: leads back to concepts of driving force -- two relevant
variables are membrane potential and equilibrium potential
how to explain inhibitory effect: leads back to same concept -- effect of a given
conductance can be seen as "clamping" membrane potential at the equilibrium potential of
the ion carrying the conductance
activation of GABAA receptors usually associated with increased Cl- conductance and
resulting hyperpolarization
in some cases, however, have found that activation of GABAA receptors leads to a
depolarization, even though conductance still is carried by Clremains inhibitory because ECl below threshold; thus, activation of GABAA receptors
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effectively "clamps" membrane potential below threshold for action potential
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GABAB receptor linked to a K+ conductance
inhibition due to GABAA receptor activation illustrates a mechanism of inhibition that relies on
an influx of negatively-charged ions, into the cell
an alternative mechanism could involve the efflux of postively-charged ions, out of the cell
the inhibtion caused by the GABAB receptor subtype induces inhibition by increasing K+
conductance
conductance: K+ conductance; outward current, resulting in hyperpolarization of neuron, IPSP
pharmacology: receptor blocker: saclofen; receptor agonist: baclofen (see below)
kinetics: slow kinetics; time-to-peak: 200 msec; duration: 600 ms
very slow onset and offset due to the fact that the GABAB receptor induces
a change in K+ conductance indirectly through action of a G-protein
receptor is linked to a G-protein
inactivation: no distinction with respect to receptor subtype; inactivation
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location: pre- and postsynaptic
GABAB receptors found postsynaptically, but also found presynaptically
leads to inhibition of neurotransmitter release
can inhibit either excitatory or inhibitory neurotransmitters, and because can inhibit GABA
release, represents a form of auto-inhibition
action of saclofen (antagonist) and baclofen (agonist)
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Catecholamines
norepinephrine (NE) and dopamine (DA) as examples
1. biosynthetic pathway
in the case of NE, biosynthetic pathway consists of tyrosine  dopa  dopamine  NE, with
the specific enzymes responsible for the conversion of each protein, tyrosine hydroxylase for
conversion from tyrosine  dopa; dopa decarboxylase for conversion from dopa  DA;
dopamine--hydroxylase for the conversion from dopa  DA;
the synthetic enzymes, not the substrate proteins, are the rate-limiting step in the synthetic
process
2. synaptic morphology
different synaptic morphology than the characteristics of the other neurotransmitters
some synapses appear to be relatively normal in the sense of the presence of
a small synaptic cleft, vesicles, pre- and postsynaptic densities, etc
other synapses, however, can be identified only on the basis of the presynaptic
release site; no apparent postsynaptic receptor in the immediate vicinity
mechanism of inactivation includes both degradative enzymes and uptake; for NE,
degradative enzymes include monoamine oxidase (MAO) and catechol-o-methyl-transferase
(COMT), though uptake appears to be more important than the degradative enzymes
3. receptor subtypes
-receptors
1 receptors located postsynaptically lead to opening of K+ channels, and thus,
hyperpolarization of the neuron (act through IP turnover)
2 receptors are located presynaptically, and are inhibitory to NE release, so that blockade
of 2 receptors leads to prolonged release of NE in response to electrical stimulation, and
increased basal levels of NE release (acts to inhibit cAMP)
-receptors
stimulation of the  adrenergic receptor subtype catalyzes the formation of cAMP, which
then activates a substrate protein, protein kinase A (PKA)
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PKA acts on the gk(AHP) channel to block conductance of K+ ions
indirect second messenger systems
cAMP system
receptor ligand binding leads to activation of the -subunit, which then catalyzes the
formation of cAMP
cAMP activates a substrate enzyme, protein kinase A (PKA)
PKA acts on the channel to change conductance
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example: -adrenergic decrease in gK(AHP) channel conductance
left top panel: response of cell to intracellular current injection; note that cell eventually fails to
exhibit action potential (called accomodation) due to AHP (left bottom panel)
in the presence of NE, cell continues to respond (top right panel) because AHP is blocked (bottom
right panel)
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G-protein activation of phospholipase C
activated -subunit hydrolyzes membrane lipid phosphatidylinositol 4,5-bisphosphate
(PIP2), producing inositol 1,4,5-triphosphate (IP3) and diacyglycerol (DAG)
IP3 releases Ca2+ from intracellular stores (endoplasmic reticulum):
example 1: metabotropic receptor
example 2: NE activation of IP3 and DAG second messengers; increased Ca2+ due to
IP3 and DAG together activate protein kinase C, which leads to
decrease in voltage-dependent Ca2+ current
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