Transcript The Synapse - University of Toronto

Why Glutamate Receptors are
Important in Neurology:
Glutamate is present in millimolar quantities in most cells,
including neurons and glia
Glutamate is the main excitatory neurotransmitter in the
mammalian CNS
Glutamate is released in large quantities during
• Stroke
• Trauma
• Epilepsy
• Possibly in chronic neurological disorders
Why Glutamate Receptors are
Important in Neurology:
Excess glutamate is released at the synapse through
• Synaptic activity
• Reverse operation of glutamate transporters
• Reduced re-uptake (due to reduced ATP levels)
Glutamate levels may rise at the synapse to hundreds of
micromolar, which is enough to cause excitotoxicity
What happens to neurons with
excess glutamate?
Normal Neuron
What happens to neurons with
excess glutamate?
• Cell Swelling
• Dendritic
Beading
• Axons: no
change (?)
Glutamate
Excess glutamate kills neurons through Ca2+ overload
Calcium Homeostasis
Ca ions are ubiquitous intracellular 2nd messengers
responsible for a multitude of cellular functions including
• Activation of numerous enzymes responsible for
• Gene expression
• Protein structure
• Metabolic functions (libids, carbohydrates)
• The control of differentiation, polarity, synaptogenesis
• Synaptic efficacy – neuronal function & activity
Calcium Homeostasis
For these reasons cells maintain a
very tight control of Ca ions
• [Ca2+]I : [Ca2+ ]
20,000
e
is 1 :
• Ca2+ ions are sequestered into
intracellular organelles
• Ca2+ ions are actively pumped in
and out of cellular
compartments
• Cells contain diverse Ca2+
buffering molecules to restrict
the diffusion of Ca2+ ions.
Calcium Neurotoxicity
“Ca2+ Excess” is felt to be deleterious to neurons
How much is too much remains controversial
It is likely that Ca2+ ions activate distinct 2nd
messenger signaling pathways in neurons that
cause them to die.
Excitotoxicity causes “Ca2+ Excess”.
Hypothetical
Scheme Leading to
Ca Excess:
The Case of
Neurons
Scheme Leading to Ca Excess:
The case of axons (white matter)
Neurotoxic Phenomena
triggered by Calcium Excess
• The formation of free radical species
• Nitric Oxide formation
• Calcium Activated Proteases
• Endonucleases, Apoptosis, Necrosis
• Mitochondrial Damage
• Acidosis
Free radicals
• Free radicals are reactive oxygen species having a
single unpaired electron:
• e.g.: Superoxide (O2-), hydroxyl (OH-)
• Free radicals produce damage by reacting (oxidizing)
with critical cellular elements, usually structural
proteins, membrane lipids, DNA.
• Free radicals are produced mostly in mitochondria.
Mitochondrial e- transport
Superoxide production:
Although molecular oxygen is reduced to water in the terminal
complex IV by a sequential four-electron transfer, a minor proportion
can be reduced by a 1e addition that occurs predominantly in complex
III but also in complex I. A chance exists that this second electron can
be transferred to molecular oxygen, generating the superoxide anion
O2·.
Thus- normal mitochondria produce a small amount of superoxide.
This superoxide is
normally scavenged by
superoxide dismutase
(SOD)
Excitotoxicity and ROS:
Calcium loading of isolated mitochondria increases the
production of O2·
Excototoxicity causes mitochondrial Ca loading.
.
O2
Mitochondria
[Ca2+]
MnTBAP
.
- SOD
O2
ca
tal
as
e
H 2O
H2O 2
Fe2+
OH
.
IDE
S
IN
Ca2+
IDE
S
T
OU
Ca2+
Mitochondrial membrane potential upon NMDA exposure
Nitric Oxide Production
NO is a gas with a half-life of 6s.
It is produced in:
- Vascular endothelium (vasorelaxant)
- Glial cells
- Neurons
It is considered by many to be a neurotransmitter
associated with processes related to synaptic plasticity,
learning and memory.
NO toxicity:
NO is a relatively innocuous gas.
However, when combined with superoxide:
NO + O2- = ONOO-
ONOO- is a highly reactive free radical species that
produces damage in neurons.
Nitric Oxide & free radical
Production
-. SOD
.
O2-
O2
Mitochondria
NO
[Ca2+]
ca
tal
as
e
H 2O
H 2O 2
Fe 2+
Arachidonic acid
.
OONO -
NOS
Ca 2+-CaM
PLA 2
OH
E
D
I
INS
ROS
COX
Ca2+
IDE
S
T
OU
Ca2+
Calcium activated proteases
MAP2 immunofluorescence
Controls
NMDA
Recovery
Calcium activated proteases
(Caplains)
Role unclear – felt by most to mediate neuronal damage in
stroke.
However, some research suggests the reverse- that they
may be necessary for neuronal recovery from stroke.
Calcium-activated proteases
Calpain Inhibitor
No Calpain Inhibitor
Calcium-dependent proteolysis contributes to recovery of dendritic
structure after NMDA exposure. Calpain activation is not necessarily
detrimental and may play a role in dendritic remodeling after neuronal
injury.
Endonucleases, apoptosis,
necrosis
Necrosis: Acute cell death characterized by cell & organelle swelling. Is
generally rapid, and occurs due to massive insults.
Apoptosis: Slower cell death, characterized by cell shrinkage, nuclear
fragmentation, and may be mediated by a “death sequence” dictated
by a genetic program.
Endonucleases, apoptosis,
necrosis
Endonucleases are thought to be calcium-activated
enzymes that cleave DNA
May be responsible in triggering apoptosis.
How to treat stroke?
Concept of
therapeutic window:
Increases with increased
flow.
Exact time unknown for
humans.
How to treat stroke?
1. Repair the plumbing
2. Make the tissue
more resilient to
poor plumbing.
How to treat stroke?
The plumbing:
Best treatment of plumbing failure is
prevention.
Risk factors for atherosclerosis
- Diabetes
- High blood pressure
- Hypercholesterolemia
- Smoking
The plumbing:
Benefit of carotid
endarterectomy in
patients with symptomatic
moderate or severe
stenosis. North American
Symptomatic Carotid
Endarterectomy Trial
Collaborators. N Engl J
Med 1998 Nov
12;339(20):1415-25
Plumbing After Stroke Onset:
Tissue plasminogen activator for acute ischemic stroke. The
National Institute of Neurological Disorders and Stroke rtPA Stroke Study Group. N Engl J Med 1995 Dec
14;333(24):1581-7
“treatment with intravenous t-PA within three hours of the
onset of ischemic stroke improved clinical outcome at three
months.”
Plumbing After Stroke Onset:
Intra-arterial pro-urokinase for acute ischemic stroke: The PROACT II
Study: A randomized controlled trial. JAMA (282) 21, December 1, 1999
Last resort plumbing::
Last resort plumbing::
Last resort plumbing::
Last resort plumbing::
Last resort plumbing::