Lecture 19- Neurotransmitters Flashcards
What are the four classes of neurotransmitter?
-Type I:- amino acids “classical”
-Type II:- amines and purines
-Type III:- neuropeptides
-Type IV:- gases
How are neuromodulators different to neurotransmitters?
Neuromodulators are different. They are not restricted to the synaptic cleft and can affect large numbers of neurons at once. Neuromodulators operate over a slower time
frame.
Where are the different classes of neurotransmitters found in terms of vesicle type?
-Type I:- amino acids “classical” +Type II:- amines and purines found in Small Synaptic Vesicles
-Type III:- neuropeptides found in Large Synaptic Vesicles
-Type IV:- gases not found in vesicles as just diffuse out membranes (lipid soluble)
What are the 7 criteria for something to be classed as a neurotransmitter?
- Present in the brain, in neurons
- Present within synaptic vesicles
- The enzymes for synthesis must exist in the presynaptic terminal or cell body
- Specific receptors must exist on the postsynaptic membrane
- Released in response to nerve terminal depolarization
- Action must be mimicked or inhibited by pharmacological agents
- There must be uptake mechanisms or metabolising enzymes
Why type of neurotransmitter is dopamine?
Type 2 neurotransmitter: Catecholamine
Who discovered dopamine?
Discovered by Arvid Carlsson in 1957
* Showed that circuits that controls movement
don’t work properly when dopamine levels are
decreased.
* Nobel Prize for Physiology and Medicine in
2000
What is dopamine invovled in and what disease is deficits in it highly associated with?
-Dopamine is now known to be involved in them regulation of movement, attention, mood, cognition, addiction, and reward
-Dopamine loss is a major player in Parkinson’s
disease
What is the synthesis pathway for dopamine? Is it just a precursor?
-Tyrosine to L-Dopa via Tyrosine Hydrolase
-L-Dopa is then converted to dopamine
-Dopamine goes to Noradrenaline and adrenaline
But despite this dopamine is still classed as it’s own neurotransmitter!
How does L-Dopa cross the BBB? Why is it so important to know this?
-Carrier mediated transport
-Dopamine can’t control the BBB therefore if need to replace it in the treatment of disease instead inject L-dopa and then this can be converted
What areas of the brain are invovled in the dopamine system?
-Neurons from the ventral tegmental area project to the frontal lobe
-Neurons in the substania nigra go to the striatum and then to other cortical areas
-The radiation/ widespread nature of the dopamine pathways help explain why damage has such vast effects.
What is Parkinson’s disease?
-Parkinson’s (mate pākenetana) is a
neurological condition that affects movement and coordination
-The first sign is often a tremor or slowness
of movement.
-Age of onset 55 to 65 years
-1 in 100 people >60 years of age
-Neurons of the substantia nigra degenerate to cause it
What differences are shown in the substania nigra for people with Parkinson’s versus people who don’t have the disease?
-In healthy individuals Neurons in the substantia nigra express high levels of a pigment, neuromelanin
-In Parkinson’s disease there is a loss of substantia nigra neurons: up to 80% (loss of dark stains)
How does dopamine meet the criteria to be a neurotransmitter and what techniques are used to show this?
-Dopamine is present in the brain (this shown via immunohistochemistry)
-It is present within synaptic vesicles, the enzymes for its synthesis exist in
the presynaptic terminal or cell body and there are Uptake mechanisms or metabolising enzymes invovled.
-Specific receptors exist on the postsynaptic membrane (immunohistochemistry used to show this where some receptors are being endocytosed to be added to membranes and some exocytosed to remove them, also some internal receptors)
-Released in response to nerve terminal depolarization
-Action is mimicked or inhibited by
pharmacological agents
What is DAB as a marker in immunohistochemistry? How is it used to identify dopamine?
-Remember that a marker is bound to a secondary antibody which is bound to a primary antibody which is bound to the target. It is the marker that makes the protein/ target visible (either via colour/ precipitate or fluorescence’s)
- DAB= diaminobenzadine stain and used as a marker for dopamine
-Dopamine= molecule of interest/ target, Anti-DA= primary antibody, Biotin tagged secondary antibody, Then Avidin which peroxidase attached (complex).
-The complex oxidizes DAB turning it into a brown pigment which precipitates out of solution as a brown solid located at the site of the antigen and thus allowing identification.
What are the enzymes invovled in the synthesis and uptake of SVs containing dopamine?
-Tyrosine hydroxylase
-Dopamine transporter, VMAT2 present on surface of vesicles (gets dopamine it into the synaptic vesicles)
What are synaptosomes?
-Particular biochemical preparation.
-Prepared from living tissue: processes sheared off to purify synaptic bouton
-Fragile enzymes and dendrites break off
-Plasma membrane repairs into balls: contains all the components of living tissue
-Can run tests on these like it was an actual membrane
-They will survive and can do things to encourage them to release neurotransmitter e.g. depolarize by potassium or calcium
-Can use these to show dopamine is released from synaptic vesicles (one of the criteria for being a neurotransmitter), plot time against dopamine release
How is dopamine’s action mimicked or inhibited by
pharmacological agents? i.e. what are the mimics and antagonists?
Agonist (mimics dopamine by activating)
e.g. Bromocriptine (Parlodel).
* Approved to treat Parkinson’s disease and
* hyperprolactinemia and related conditions
Antagonists (inhibit the receptors)
e.g. Haloperidol
* Some antipsychotics are dopamine antagonists,
and as such they have found use in treating schizophrenia, bipolar disorder, and stimulantinduced psychosis
How is L-Dopa used in the treatment of Parkinson’s disease?
- L-dopa crosses into brain and converted
into dopamine - But note, this can only occur in surviving
neurons - L-DOPA treatment does not stop neurodegeneration
- Prolonged L-DOPA treatment often results
in side-effects - L-DOPA–induced dyskinesia
- Psychosis
How does dopamine meet neurotransmitter criteria 7 that there should be Uptake mechanisms or metabolising
enzymes?
- Dopamine synthesis via
tyrosine hydroxylase - Loaded into vesicles
(VMAT, vesicular
monoamine transporter) - Exocytosis
- Binding to receptors
- Re-uptake by dopamine transporters (DAT)
- Degradation via
monoamine oxidase - Recycling
- Further processed to
norepinephrine etc - Action terminated by reuptake by nerve terminal or glia
- Stimulants inhibit reuptake
- cocaine and amphetamines
-Thus, prolonging dopamine action
Are gases just neurotransmitters?
No can just be neuromodulators
How is nitric oxide an exception to the neurotransmitter list?
NO is a gas derived from arginine.
* As it is a gas, NO can not be stored in lipid vesicles.
* NO is not released by exocytosis
* NO does not bind to receptors
* NO is not metabolised by hydrolytic enzymes.
Doesn’t tick many boxes but is still classes as a neurotransmitter !
How is Control of the synthesis of NO key
to regulating its activity? Also what is it’s function?
NO is synthesized on demand
* neuronal nitric oxide synthase (nNOS) is
the principal isoform present in CNS
* NO diffuses from nerve terminals
* 40 – 300 μm
* Therefore, can act on both neuronal and non- neuronal cells within this range = modulator
* NO diffuses into cells
* Activates second messenger pathways
* Inactivated by interaction with substrate (an is short lived)
!NO plays many roles in the CNS and is an
important retrograde messenger involved in
the long-term potentiation model for memory
What may receptors be in terms of their location?/ features?
Receptors may be:
* postsynaptic
* presynaptic
* extra-synaptic
* excitatory or inhibitory
* fast or slow
What does the action of a neurotransmitter depend on?
-It’s receptors
-A single type of neurotransmitter may have multiple actions through multiple
different receptors subtypes located on different cell types.
What are the fast acting receptor type and how does it work? Example?
Ligand-gated ion channels:
* milliseconds
* have intrinsic ion channels (ligand gated ion channels)
* channel opens to allow influx or efflux of ions
* excitatory or inhibitory depending on ion involved and its direction of
movement
-Example: Glutamate
What are the slow acting receptor type and how does it work? Example?
G-protein coupled receptors:
* Metabotropic receptors
* 7-transmembrane domains
* Can directly regulate ion channels
* Generate second messengers
* Long-term metabolic change including altered
gene expression
* may have multiple actions
* excitatory or inhibitory
How might slow receptors also get a fast response?
If ion channel is in close proximity the G-protein can phosphorylate/ active it so can get fast response
but ultimately is mainly slow
What type of receptors are associated with dopamine?
G-protein coupled receptors (slow)…
D1-like receptors (D1 and D5)=
* Stimulation of adenylate cyclase
* Coupled to Gs
* Mediate excitatory neurotransmission
D2-like receptors (D2, D2, and D4)=
* Inhibition of adenylate cyclase
* Coupled to Gi/Go
* mediate inhibitory neurotransmission
Where are the sites of dopamine receptors and how does this effect function?
Dopamine has many sites of action….
Dopamine receptors are found throughout the brain including:
-substantia nigra to striatum circuit
-ventral tegmental area to cortex
-and limbic system
Reflects the vast array of functions and wide spread use of dopamine in the brain/ across systems
What is the ventral tegmental dopamine system?
-Projects to the nucleus accumbens
* Is a “reward” system which reinforces
desirable behaviours
* Common target of addictive drugs
* Opiates (heroin) and nicotine stimulate
VTA neurons
* Cocaine and amphetamines block
reuptake which increases DA at synapses
What does prolonged activation of the VTA result in?
downregulation
-Generates drug tolerance
-Removal of drug results in withdrawal symptoms –
system unable to function correctly in absence of
drug
What might the VTA be associated with? What other things can contribute to this?
-VTA also implicated in psychotic behaviour
-amphetamine addiction shows features of schizophrenia
-L-DOPA can also cause psychotic behaviour
What is the dopamine hypothesis of schizophrenia?
Schizophrenia
* A mental health condition characterised by
recurring episodes of psychosis
* Characterised by a disruption of thought,
perception, mood and movement
* Genetic and environmental causes
- Haloperidol is a typical antipsychotic
medication used in the treatment
of schizophrenia. - Haroperidol is a high affinity antagonist of
the dopamine D2 receptor
-Some antipsychotics block inhibitory D2
receptors
What can the treatment of parkinson’s result in? What does this suggest about parkinson’s?
- Treatment for Parkinson’s disease can result
in psychotic behaviour
-This suggests that enhanced dopamine
transmission may be responsible for some
schizophrenic symptoms
-This is far from the whole story other neurotransmitters particularly serotonin and
glutamate also implicated in schizophrenia
