Lecture 19- Neurotransmitters

Question 1

What are the four classes of neurotransmitter?

Answer 1

-Type I:- amino acids “classical”
-Type II:- amines and purines
-Type III:- neuropeptides
-Type IV:- gases

Question 2

How are neuromodulators different to neurotransmitters?

Answer 2

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.

Question 3

Where are the different classes of neurotransmitters found in terms of vesicle type?

Answer 3

-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)

Question 4

What are the 7 criteria for something to be classed as a neurotransmitter?

Answer 4

1. Present in the brain, in neurons
2. Present within synaptic vesicles
3. The enzymes for synthesis must exist in the presynaptic terminal or cell body
4. Specific receptors must exist on the postsynaptic membrane
5. Released in response to nerve terminal depolarization
6. Action must be mimicked or inhibited by pharmacological agents
7. There must be uptake mechanisms or metabolising enzymes

Question 5

Why type of neurotransmitter is dopamine?

Answer 5

Type 2 neurotransmitter: Catecholamine

Question 6

Who discovered dopamine?

Answer 6

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

Question 7

What is dopamine invovled in and what disease is deficits in it highly associated with?

Answer 7

-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

Question 8

What is the synthesis pathway for dopamine? Is it just a precursor?

Answer 8

-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!

Question 9

How does L-Dopa cross the BBB? Why is it so important to know this?

Answer 9

-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

Question 10

What areas of the brain are invovled in the dopamine system?

Answer 10

-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.

Question 11

What is Parkinson’s disease?

Answer 11

-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

Question 12

What differences are shown in the substania nigra for people with Parkinson’s versus people who don’t have the disease?

Answer 12

-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)

Question 13

How does dopamine meet the criteria to be a neurotransmitter and what techniques are used to show this?

Answer 13

-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

Question 14

What is DAB as a marker in immunohistochemistry? How is it used to identify dopamine?

Answer 14

-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.

Question 15

What are the enzymes invovled in the synthesis and uptake of SVs containing dopamine?

Answer 15

-Tyrosine hydroxylase
-Dopamine transporter, VMAT2 present on surface of vesicles (gets dopamine it into the synaptic vesicles)

Question 16

What are synaptosomes?

Answer 16

-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

Question 17

How is dopamine’s action mimicked or inhibited by
pharmacological agents? i.e. what are the mimics and antagonists?

Answer 17

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

Question 18

How is L-Dopa used in the treatment of Parkinson’s disease?

Answer 18

• 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

Question 19

How does dopamine meet neurotransmitter criteria 7 that there should be Uptake mechanisms or metabolising
enzymes?

Answer 19

• 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

Question 20

Are gases just neurotransmitters?

Answer 20

No can just be neuromodulators

Question 21

How is nitric oxide an exception to the neurotransmitter list?

Answer 21

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 !

Question 22

How is Control of the synthesis of NO key
to regulating its activity? Also what is it’s function?

Answer 22

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

Question 23

What may receptors be in terms of their location?/ features?

Answer 23

Receptors may be:
* postsynaptic
* presynaptic
* extra-synaptic
* excitatory or inhibitory
* fast or slow

Question 24

What does the action of a neurotransmitter depend on?

Answer 24

-It’s receptors
-A single type of neurotransmitter may have multiple actions through multiple
different receptors subtypes located on different cell types.

Question 25

What are the fast acting receptor type and how does it work? Example?

Answer 25

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

Question 26

What are the slow acting receptor type and how does it work? Example?

Answer 26

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

Question 27

How might slow receptors also get a fast response?

Answer 27

If ion channel is in close proximity the G-protein can phosphorylate/ active it so can get fast response
but ultimately is mainly slow

Question 28

What type of receptors are associated with dopamine?

Answer 28

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

Question 29

Where are the sites of dopamine receptors and how does this effect function?

Answer 29

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

Question 30

What is the ventral tegmental dopamine system?

Answer 30

-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

Question 31

What does prolonged activation of the VTA result in?

Answer 31

downregulation
-Generates drug tolerance
-Removal of drug results in withdrawal symptoms –
system unable to function correctly in absence of
drug

Question 32

What might the VTA be associated with? What other things can contribute to this?

Answer 32

-VTA also implicated in psychotic behaviour
-amphetamine addiction shows features of schizophrenia
-L-DOPA can also cause psychotic behaviour

Question 33

What is the dopamine hypothesis of schizophrenia?

Answer 33

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

Question 34

What can the treatment of parkinson’s result in? What does this suggest about parkinson’s?

Answer 34

• 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