Transcript Nerve Cells

Biochemistry of
Neurotransmission: A Type of
Cell-Cell Signaling
Biochemistry is fun
Biochemistry of Cell Signaling
Fig. 19-1
Study Guide
• Contrast resting, ligand-gated, voltage-gated, and signal-gated ion channels
• How do voltage gated ion channels monitor the voltage?
• What is the neurotransmitter at the vertebrate neuromuscular junction? The
crayfish neuromuscular junction?
• What is the chief excitatory neurotransmitter in the mammalian brain? The
chief inhibitory neurotransmitter? What vitamin is required for the synthesis
of the chief inhibitory brain neurotransmitter? What is the role of PyP in
catecholamine synthesis? What is the role of tetrahydrobiopterin in second
messenger synthesis (3 answers)?
• How is the action of acetylcholine terminated? Serotonin?
• What is Parkinson's disease, and what is the mechanism for its
development? How is parkinsonism treated?
• How are neurotransmitters released at the synapse? What proteins are
involved? Name the calcium ion sensor
• Describe the Otto Loewi experiment and explain its significance.
• What is myasthenia gravis and what is the mechanism for its development?
• Describe the molecular components and actions of G-proteins. How many
transmembrane domains do receptors that interact with G-proteins possess?
• What is the role of cyclic GMP in vision?
Overview
• The human brain contains about 1012 neurons, and
some neurons make 1000 connections
– Dendrites, cell body, axon
– The cell body contains the nucleus, and this is where
almost all protein synthesis occurs
– The cell body also contains nearly all of the lysosomes
– Proteins and other molecules are transported from the
nucleus via axoplasmic transport
– Axons are long processes specialized for the conduction
of action potentials
• The nervous system also contains glial cells that
support and nourish the neurons (Schwann cells in
the peripheral nervous system)
• Types of neurons: sensory neurons, interneurons,
motor neurons
Neuroanatomy
Fig. 19-2
Anatomy of
the Neuron
• Arrows indicate the
direction of conduction of
the action potential
• A motor neuron typically
has a single axon
• The axon of the sensory
neuron branches after it
leaves the cell body
– Both branches are
structurally and
functionally axons
– The cell body is located
in the dorsal root
ganglion near the spinal
cord
Signaling within the Neuron
• The axon carries an electrical impulse called the
action potential.
– These move at speeds of 100 m/s
– The action potential originates in the axon hillock
– An axon can be 1 meter and longer (from spinal cord to
the big toe)
• Dendrites receive signals and convert them into
small electric impulses and transmit them to the
cell body
The Action Potential
• AP: transient depolarization of the membrane followed by
repolarization to about – 60 mV
• Below: 1 action potential every 4 msec
• Invasion of the synapse results in release of
neurotransmitter that bind to postsynaptic receptors and
activate them
– This can be excitatory (depolarization)
– This can be inhibitory (hyperpolarization)
Synapses
• Specialized Sites where neurons communicate with other cells
– Neurons
– Muscle cells
– Endocrine cells
• Types of synapses
– Chemical (vast, vast majority)
• Presynaptic cell contains vesicles
• The neurotransmitter (NT) interacts with postsynaptic cell within 0.5 ms
– Electrical (a curiosity)
• Connected by gap junctions
• The next slide illustrates various synapses
– Hippocampal interneurons which makes about 1000 synapses (orange red dots)
– Electron micrograph of a CNS synapse
Synapses
The Action Potential and the
Conduction of Electric Impulses
• An electric potential exists across the plasma membrane
because of ion gradients
• Resting potential is about – 60 mV owing to the large
number of open potassium channels
• Voltage-gated channels allow the transmission of the
electrical impulses
• Action Potential
– Na+ channels open allowing Na+ to enter the cell and depolarize it,
then they close for a refractory period
– K+ channels open permitting efflux of K+ which hyperpolarizes the
membrane
• As these channels close, the membrane returns to its resting potential
Ion Channels
• (c, d) are located on dendrites and cell bodies
• d is coupled to a NT receptor via a G-protein
Origin of the Resting Potential
• Sodium pump or
sodium/potassium ATPase
generates these gradients
• Na+ is extracellular
• K+ is intracellular
• A- represents protein
• The open potassium
channels and the potassium
gradient are responsible for
the resting potential
Myelination Increases the
Velocity of Impulse Conduction
• Myelin is a specialized membrane
– Derived from Schwann cells in the PNS
– Derived from oligodendrocytes (glia) in CNS
• Contains protein and lipid
• Action potential jumps from node to node (saltatory
conduction), and this greatly increases the velocity of AP
conduction
• Less energy is required to transmit an action potential in a
myelinated nerve
• More energy is required to transmit an action potential in
unmyelinated nerves
• Most nerves are myelinated
Myelin Sheath
• (a) Myelinated peripheral nerve surrounded
by a Schwann cell that produces the myelin
• (b) Sciatic nerve axon is surrounded by a
myelin sheath (MS)
Myelinated and
Non-Myelinated
Nerves in
Dental Pulp
Structure of a Peripheral Myelinated
Axon
Saltatory Conduction from Node to Node
• Saltatory refers to
the jumping of the
action potential from
node to node
• The nodes are the
only regions along
the axon where the
axonal membrane is
in direct contact
with the
extracellular fluid
Molecular Properties of Voltage-Gated Ion Channels
• Voltage-gated K+ channels are assembled from four similar
subunits, each of which has six membrane-spanning alpha
helices and a nonhelical P segment that lines the ion pore; 24
TM segments total
• Voltage-gated Na+ and Ca2+ channels are monomeric proteins
containing four homologous domains each similar to a K+
channel subunit; 24 TM segments total
• The S4 alpha helix acts as a voltage sensor
• Voltage-sensing alpha helices have a lysine or arginine every
third or fourth residue; outward movement toward the negative
extracellular space in response to depolarization opens the
channel
• Voltage-gated K+, Na+, and Ca2+ channel proteins contain
cytosolic domains that move into the open channel thereby
inactivating it
• Non-voltage gated K+ channels and nucleotide-gated channels
lack a voltage-sensing alpha helix, but otherwise their structures
are very similar to the voltage-gated K+ channels
Transmembrane Structures of
Gated Ion-Channel Proteins
• The voltage-gated K+
channel consists of four
identical subunits and six
transmembrane alpha helices
– Helix 4 is the voltage
sensor
• cAMP and cGMP-gated ion
channels are made of four
identical subunits that lack a
voltage sensor
– These occur in the
olfactory and visual
systems, respectively
Voltage-gated
+
Na
Channel
• All voltage-gated channels contain four transmembrane
domains (each with 6 TM segments), and each domain
contributes to the central pore
• In the resting state, the gate obstructs the channel
• There are four voltage-sensing alpha helices which have
positively charged side chains every third residue
– When the outside of the membrane becomes negative (depolarized)
the helices move toward the outer plasma membrane surface
causing a conformational change in the gate segment that opens the
channel as shown in b
– Shortly afterwards, the helices return to the resting position as
shown in c
– The channel inactivating segment (purple) moves into the open
channel preventing further ion movement as shown in c
Structure and Function of the Voltagegated Na+ Channel
Transmembrane Structures of
Gated Ion-Channel Proteins
• Voltage-gated Na+ and Ca+
channels are monomers
– These form a channel
similar to that of the K+
channel
– There are 24 transmembrane
segments
• These channels contain
regulatory portions, not
shown here
Neurotransmitters (NTs)
• Impulses are transmitted by the release of NTs from the axon terminal
of the presynaptic cell into the synaptic cleft. NTs bind to specific
receptors on the postsynaptic cell causing a change in the ion
permeability and the potential of the postsynaptic plasma membrane
• Classical NTs are imported from the cytosol into synaptic vesicles by
a protein-coupled antiporter, a V-type ATPase that maintains a low
intravesicular pH (V = vesicle)
– The V-type ATPase pumps protons into the synaptic vesicle
– Then protons leave the vesicle in exchange for the NT which is transported
inward; this is antiport
– Catecholamines (DA, NE, EPI) are unstable at pH 7; they are stable at pH 5 in
the intravesicular space
• Excitatory receptors lead to depolarization thereby promoting
generation of an action potential
• Inhibitory receptors lead to hyperpolarization thereby inhibiting
generation of an action potential
• Ligand-gated receptors induce rapid (msec) responses
Neurotransmitters (cont)
• G-protein coupled receptors (GPCR) induce responses that last for
seconds or more
• Removal of transmitters is by hydrolysis (metabolism), diffusion away
from the synapse, or most commonly by uptake
– ACh by hydrolysis
– Nearly all other NTs by uptake
• A single postsynaptic cell can amplify, modify, and compute
excitatory and inhibitory signals received from multiple presynaptic
neurons
• Postsynaptic cells generate action potentials in an all-or-nothing
fashion
• At electric synapses, ions pass directly from the pre to the
postsynaptic cell through gap junctions
• Impulse transmission at chemical synapses occurs with a small time
delay but is nearly instantaneous at electric synapses
Small Molecule Neurotransmitters
• Acetylcholine (ACh)
– Vertebrate neuromuscular junction
– Pre and postganglionic parasympathetic
nervous system
– Preganglionic sympathetic nervous system
– Central nervous system (CNS)
• Glycine: chief inhibitory NT
in the spinal cord
• Glutamate: chief excitatory
NT in the CNS
• Dopamine (DA): selected
CNS neurons; parkinsonism
• Norepinephrine (NE)
– Postganglionic sympathetic NS
– Selected CNS neurons
Small Molecule Neurotransmitters (cont)
• Epinephrine
– Selected CNS
– Adrenal medulla
• 5-Hydroxytryptophan (5-HT), or
serotonin: CNS (Prozac, Zoloft,
SSRIs, selective serotonin
reuptake inhibitors)
• Histamine (mast cells)
• GABA (gamma aminobutyric
acid): chief inhibitory NT in the
CNS
Selected Neurotransmitters
ACh at the vertebrate nm junction
Glutamate at the invertebrate nm junction
(crayfish and lobster)
Acetylcholine
• Grandfather of all neurotransmitters
• Sites of action
– Vertebrate neuromuscular junction: nicotinic
– Pre-and post-ganglionic parasympathetic: nicotinic and
muscarinic, respectively
– Pre-ganglionic parasympathetic: nicotinic
– Present in CNS (both Muscarinic and Nicotinic
receptors)
• Inactivated by hydrolysis (the only classical
neurotransmitter that is inactivated by
metabolism)
• Pathology
– Alzheimer (?)
Acetylcholine Metabolism (Fig.
19-15, 19-16)
• ACh is inactivated by hydrolysis
Acetylcholine Congeners (Fig.
19-17)
Catecholamines
Catecholamine Biosynthesis
• Tyrosine hydroxylase
– First and rate-limiting
– Activated by PKA and other
PKs
– Uses tetrahydrobiopterin as
cofactor
• Aromatic Amino Acid
Decarboxylase (AAD) uses
PyP (B6) as cofactor
• Dopamine beta hydroxylase
(DBH) uses vitamin C, or
ascorbate
Parkinsonism
• A slowly progressive neurological disease characterized by
– a fixed inexpressive face
– a tremor at rest, slowing of voluntary movements
– a gait with short accelerating steps, peculiar posture, and muscle
weakness
• It is caused by degeneration of the basal ganglia, and by low
production of the neurotransmitter dopamine
• Most patients are over 50, but at least 10 percent are under 40
• Also known as paralysis agitans and shaking palsy
• Treatment is by medication, such as levodopa and carbidopa
(Sinemet)
– Levodopa is converted to dopamine; levodopa is able to pass the blood
brain barrier, but dopamine is not able to pass the BBB
– Carbidopa is an inhibitor of aromatic amino acid decarboxylase in the
periphery; carbidopa does not enter the CNS
Serotonin Metabolism (Fig. 19-19)
NOS (Fig. 19-23)
Recycling of Synaptic Vesicles
Selected Synaptic Proteins
• Synapsin
–
–
–
–
A vesicle protein
Recruits vesicles to the synaptic region
Binds to the cytoskelton
Phosphorylation by PKA and CaM Kinase II releases synapsin
from vesicles and allows them to move into the active region
• v-SNARES for vesicle-(Soluble NSF Attachment protein
REceptors) and NSF refers to N-ethylmaleimide Sensitive
Factor
– VAMP: vesicle associated protein
• Also called synaptobrevin
• t-SNARES for target
– Syntaxin
– SNAP25 (synaptosomal associated protein MW 25 kDa)
Selected Synaptic Proteins II
• Synaptotagmin: the calcium ion sensor
– Exocytosis is triggered by Ca2+
• Rab3A is a G protein found on vesicles and is
required for fusion with the plasma membrane and
exocytosis
• Formation of a VAMP-syntaxin-SNAP25 complex
occurs with vesicle fusion and exocytosis
– NSF (N-ethylmaleimide sensitive factor), alpha- beta-,
and gamma-SNAP dissociate the VAMP-syntaxinSNAP25 complex (ATP dependent) after fusion
– The proteins return to their initial state (in the vesicle or
on the target membrane)
• Action potential opens Ca2+ channels in the synaptic region
which triggers exocytosis
Vesicle Docking and Fusion
Excitation and Inhibition
• Top: frog skeletal muscle
• Bottom: frog heart
• The Loewi experiment
provided proof that
neurotransmission is chemical
in nature (as opposed to
electrical)
– Vagusstuff (ACh)
– Accelerinstuff (NE)
– Learn this experiment
Neurotransmitter Receptors
•
•
•
Ligand-gated receptors are fast; GPCRs are slow
ACh and the nicotinic receptor at the neuromuscular junction is ligand gated
and promotes the flux of both sodium and potassium
Nicotinic receptor and other ligand-gated receptors consists of 5 subunits
– There are four candidate membrane-spanning regions for each subunit
– An M2 alpha helix lines the ion channel
– NT binding triggers a conformational change leading to channel opening
•
Glutamate
– NMDA, AMPA, and kainate receptors are ionotropic
• The receptor is made of five subunits
• Segments 1,3, and 4 of each are transmembrane segments
• Segment 2 courses into, but not through ,the membrane from the cytosolic face
– Activation of NMDA requires depolarization and glutamate binding
– There are three classes of metabolotropic glutamate receptors (7 TM)
•
GABA and glycine receptors are ligand-gated Cl- channels
– Five subunits per receptor
– Intricate
– Four candidate transmembrane segments
Neurotransmitter Receptors II
• ACh and muscarinic receptors in heart
– Causes dissociation of a heterotrimeric G
protein
– G beta, gamma binds to and opens a K+
channel, and this leads to hyperpolarization
(inhibition)
• G-protein coupled catecholamine receptors
lead to elevated cAMP
Ligand-gated Ion Channel
Receptors
• Note that Cl- is responsible for hyperpolarization
• Note that Na+ is responsible for depolarization
• These receptors are made up of 5 subunits each
with 4 TM segments: 5X4 = 20 TM segments
Neurotransmitter Receptors
Nicotinic Receptor and
the nm Junction
•
•
•
•
•
•
•
•
•
The formation of autoantibodies against this
receptor produces myasthenia gravis
Myasthenia gravis (MG) is a chronic neuromuscular
disease characterized by varying degrees of
weakness of the skeletal or voluntary muscles of the
body
The muscle weakness increases during periods of
activity and improves after periods of rest.
MG most commonly occurs in young adult women
and older men but can occur at any age
Although MG may affect any voluntary muscle,
certain muscles including those that control eye
movements, eye lids, chewing, swallowing,
coughing, and facial expressions are more often
affected
Weakness may also occur in the muscles that
control breathing and arm and leg movements.
Therapies include medications such as
anticholinesterase agents, prednisone, cyclosporine,
and azathioprine
Thymectomy
Plasmapheresis, a procedure in which antibodies are
removed from blood plasma
Nicotinic ACh Receptor
• Most of the protein mass is extracellular
• There are two acetylcholine binding sites
• There are four membrane TM segments (M1, M2, M3, M4) in each of
the five subunits (5X4=20)
– Five M2 helices form the pore
• Aspartate and glutamate side chains at both ends of the pore exclude
anions
Pore-lining M2 Helices
• Closed state: kink in the center of each M2
helix constricts the passageway
• Open state: kinks rotate to one side so that
helices are farther apart
• Only 3 of the 5 M2 helices are shown
Nicotinic
Receptor and the
nm Junction
(Fig. 19-18)
NMDA and Non-NMDA Glu Receptors
• NMDA is blocked by
Mg2+
• Depolarization of several
non-NMDA receptors
leads to depolarization
and removal of Mg2+
• Ca2+ as well as Na+
traverse the NMDA
receptor
• This leads to an
enhanced response in the
postsynaptic cells
• This is long-term
potentiation that results
from a burst of
stimulation
ACh-induced Opening of K+ Channels in
Heart
• ACh leads to activation of
the muscarinic receptor
• This leads to the exchange
of GTP for GDP in the
heterotrimeric G-protein
• The beta-gamma subunits
activate a K+ channel
• The outward flow of K+
leads to a more negative
intracellular potential, or
hyperpolarization, and a
decreased rate of
contraction
G-Protein Coupled Receptors
(GPCRs)
G-Protein Linked Receptors
G-Protein Cycle
Actions of Heterotrimeric G-proteins
• Stimulate adenylyl cyclase: Gs
• Inhibit adenylyl cyclase: Gi
• Activate phospholipase C leading to IP3 and
diacylglycerol production: Gq
Inactivation of NTs
• Uptake (most prevalent form)
–
–
–
–
–
–
DA, NE, EPI
5-HT
Glu
Gly
Almost all NTs except ACh and neuropeptides
Julius Axelrod at the NIH discovered norepinephrine
reuptake and transformed the field
• Hydrolysis
– Neuropeptides
– ACh
GABA Metabolism
An Electric Synapse
• The plasma membranes of the pre-and post-synaptic cells are
linked by gap junctions
• Flow of ions through these channels allows electric impulses
to be transmitted directly from one cell to the next
• Unusual in mammals
• Occur in fish (goldfish)
Transmission Across Electric and
Chemical Synapses
• Transmission across an
electrical synapse is fast
(microseconds)
• Transmission across a chemical
synapse occurs on the order of
milliseconds
– This was the evidence that
convinced everyone that
neurotransmission in the CNS is
chemical and not electrical in
nature
Sensory Transduction
• Converts signals from the environment into electric signals
–
–
–
–
–
Light: G-protein
Odor: G-protein
Taste: gated
Sound: gated
Touch: gated
• Vision
–
–
–
–
Stimulated rhodopsin activates transducin, a G-protein
Transducin alpha-GTP activates PDE
PDE lowers cGMP
cGMP-gated Na+/Ca2+ are closed, membrane hyperpolarization occurs, and less NT is
released
• Each sensory neuron in the olfactory epithelium expresses a single
type of odorant receptor
– Golf are coupled to and activates adenylyl cyclase
– cAMP opens gated channels causing depolarization of the cell membrane and generation
of an action potential
– The thousand or so olfactory receptors are intronless
Rod Cell
Hyperpolarization of the RodCell Membrane
• This system works
“backwards”
• Light causes
hyperpolarization
and decreased
released of a NT
(Glutamate)
Rhodopsin Metabolism
Actions in the Rod Cell
• In the dark, the rod cell is hyperpolarized owing to the
activation of a sodium channel by cGMP
• Light activates rhodopsin, a 7 transmembrane segment
light receptor (the first 7 TM domain protein to be
described)
• The heterotrimeric G-protein becomes activated
• The active a-subunit of the G-protein binds to the gsubunit of phosphodiesterase (abg) to form a complex
• The ab–complex of PDI is now active
• cGMP levels fall, the sodium channel is closed, and the
cell becomes less depolarized (i.e., more polarized or
hyperpolarized, and less Glu is released)
Role of Transducin (Fig. 19-26)
Color Vision and Spectra
• Color vision uses three opsin
pigments; opsins are proteins
• These correspond to the three classes
of cones
– Blue
– Green
– Red
• Opsins differ, but the pigment is the
same
• Red and green opsins are on
chromosome X
– Owing to recombination, X
chromosomes with only a red or a
green opsin gene is formed
– 8% of human males leads to redgreen blindness
Olfactory
Epithelium
• The human olfactory
epithelium expresses about
1000 different odorant
receptors
–
–
–
–
These are G-protein linked
Golf
Activate adenylyl cyclase
cAMP-gated channel
induces depolarization
• (b,c) Odorant cells
expressing the same
receptor project to the same
point in the olfactory bulb
Channel Summary
• Resting, always open
• Voltage gated K+ and cyclic nucleotide gated
– Four proteins with 6 TM segments = 24 TM segments
• Voltage-gated Na+ and Ca+ channels
– 24 TM segments
• Ligand gated (ACh, Glu) Na+ channels
– Five subunits with four TM segments = 20 TM
segments total
• Ligand gated (GABA, Gly)
– Five subunits with four TM segments = 20 TM
segments total