lecture 12: post synaptic mechanisms

Question 1

what is a neurotransmitter

Answer 1

• Neurotransmitters transmit messages between neurons, or from neurons to muscles.
• Electrical signals that have travelled along the axon are converted into chemical signals,
  through the release of neurotransmitters, which cause a specific response in the
  receiving cell.
• Communication between two neurons (or nerve and muscle) occurs at the synaptic cleft.
• Neurotransmitter can be excitatory, inhibitory or modulatory.
• The response is controlled by the available receptors on the postsynaptic cell.

Question 2

how are neuromodulators different to neurotransmitters

Answer 2

• 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

what are the four classes of neurotransmitters

Answer 3

1. type 1 = amino acids
2. type 2 = amines and purines
3. type 3 = neuropeptides
4. type 4 = gases

Question 4

type 1: amino acids “classical”

Answer 4

• glutamate (excitatory)
• glycine and GABA (inhibitory)
  –> found in small synaptic vesicles

Question 5

type 2: amines and purines

Answer 5

• acetylcholine
• catecholamines (noradrenaline and dopamine)
• histamine
• serotonin
  –> found in small synaptic vesicles

Question 6

type 3: neuropeptides

Answer 6

• opioids, substance P, neuropeptide Y..
  –> found in large dense core vesicles
• slower process have to go through translation, transcription etc

Question 7

type 4: gases

Answer 7

• NO, CO
  –> cannot be contained in lipid vesicles

Question 8

criteria for identifying neurotransmitters

Answer 8

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. Released in response to nerve terminal depolarization
5. Specific receptors must exist on the postsynaptic membrane
6. Action must be mimicked or inhibited by pharmacological agents
7. There must be uptake mechanisms or metabolising enzymes

Question 9

dopamine

Answer 9

• catecholamine (type 2 neurotransmitter)
• dopamine is now known to be involved in the regulation of movement, attention, mood, cognition, addiction, and reward

Question 10

what does dopamine loss result in

Answer 10

• a major contributor to parkinsons disease

Question 11

parkinsons disease

Answer 11

• 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 yrs
• 1 in 100 people > 60 yrs of age
• 2 in 100 people > 80yrs of age

–> neurons of the substantia nigra degenerate

Question 12

what is substantia nigra

Answer 12

• The substantia nigra is a critical structure located in the midbrain (part of the brainstem) and plays a key role in controlling movement and coordination
• releases dopamine

Question 13

what is the difference between someone healthy and someone with parkinsons in terms of substantia nigra

Answer 13

• neurons in the substantia nigra express high levels of a pigment = neuromelanin
  healthy brain:
• lots of dark stained regions
  parkinsons disease:
• loss of substantia nigra neurons = loss of dark stained regions

Question 14

functions of neuromelanin

Answer 14

• neuro protection
• inflammatory processes
• anti oxidant

Question 15

what causes loss of cells in the substantia nigra

Answer 15

a-synuclein
- found in “lewy bodies”
- forms toxic aggregates
- some familial forms of parkinsons disease have mutations in a-syn

Question 16

what cellular processes is a-synuclein involved in

Answer 16

• mitophagy, autophagy, inflammation
• ER-golgi transport, synaptic vesicles, mitochondria and lysosomes and other proteolytic machinery

Question 17

functions impacted due to forming aggregates of a-synuclein

Answer 17

• a synuclein regulates dopamine release
  –> synaptic vesicle exocytosis
  –> SNARE complex formation
  –> senses curved membranes
• involved in release of neurotransmitter
• when folded in aggregates it can’t do its job at the terminal bc it forms toxic oligomers

Question 18

how is dopamine present within synaptic vesicles

Answer 18

VMAT = vesicular monoamine transporter
- transporters imbedded in membrane, potential difference allows it to work so we can take dopamine into the vesicles

Question 19

the enzymes for its synthesis exist in the presynaptic terminal or cell body

Answer 19

tyrosine –(tyrosine hydroxylase)–> L-DOPA –(Dopa decarboxylase)–> dopamine
- using fluorescent microscopy tyrosine hydroxylase +ve cells were found (bright green) so its likely to make dopamine

Question 20

how are synaptosomes formed

Answer 20

• start with axon terminal and denrite in vivo
• gently shear the tissue in the brain so we are pulling off the projection, causing axons and dendrites to break but bubbles of synapses will remain
• if you increase the power of this shear the axon side will remain intact but the dendrite side will be completely ripped off except for the post synaptic density
• all of the molecules in charge of aligning the pre synapse with the post synapse are really tightly aligned so they can sustain the shearing force
• lipids can reform and create little bubbles if we have increased force we also shear off the post synaptic region

Question 21

three types of receptors

Answer 21

1. synaptic = would be within the post synaptic density
2. perisynaptic = close to the post synaptic density, just outside it
3. extra synaptic = somewhere else, further away

Question 22

what is the role of receptors

Answer 22

we can release neurotransmitters but need to have receptors for it to bind to

Question 23

how is synaptic location important

Answer 23

• binding to these receptors is going to impact on the ability of synaptic transmission to occur in this pathway
• important to understand localization of the receptor because this will tell us about its function and the function of the neurotransmitter

Question 24

what is the role of presynaptic dopamine receptors

Answer 24

regulatory:
- dopamine synthesis and release
- other neurotransmitters such as GABA and glutamate

Question 25

what is the role of extrasynaptic dopamine receptors

Answer 25

regulatory: - delayed effect - prolonged effect

Question 26

dopamine receptors

Answer 26

- are G-protein coupled receptors - metabotropic receptors (impact metabolism in different ways) - 7 transmembrane domains (slower effect on cells) - ligand receptor interaction is mediated by: --> G proteins - trimeric GTP-binding proteins - dissociate into two components upon activation (Ga and B/Y)

Question 27

how do mimics and antagonists work to mimic/inhibit an action by pharmacological agents

Answer 27

- really important to have agonist or antagonist to help understand the processes that are driven by a neurotransmitter --> dopamine receptor agonist eg: Bromocriptine (Parlodel) - approved to treat parkinsons disease - stimulate nerves that control movement --> dopamine receptor antagonists e: Haloperidol - antipsychotics - treatment of schizophrenia, bipolar disorder, and stimulant induced psychosis

Question 28

how is L-DOPA used to treat parkinsons disease

Answer 28

- L-DOPA crosses the BBB and can be converted into dopamine - 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 29

uptake and metabolism of dopamine

Answer 29

- reuptake by dopamine transporters (DAT) - lvls of dopamine can be controlled by the uptake of dopamine from these synaptic regions, this is done by transporters, they take dopamine up out of the synaptic cleft - this has a modulatroy effect as well because its going to reduce the availability of dopamine in the active regions - action terminated by reuptake by nerve terminal or glia - stimulants inhibit reuptake cocaine and amphetamines --> thus prolonging dopamine action --> euphoria, wakefulness, enhanced cognition

Question 30

where are dopamine receptors found

Answer 30

- dopamine has many sites of action - dopamine receptors are found throughout the brain including: - substantia nigra to striatum circuit - ventral tegmental area (VTA) to --> nucleus accumbens --> frontal cortex --> limbic system

Question 31

how does synaptosome show us that neurotransmitters are released in response to nerve terminal depolarization

Answer 31

- Synaptosomes are isolated nerve terminal preparations that retain many of the structural and functional characteristics of intact neurons. - They are typically obtained by homogenizing brain tissue and then centrifuging the mixture to isolate synaptic vesicles and the surrounding presynaptic membrane. - By using synaptosomes, researchers can study neurotransmitter release in a controlled environment, without the complexity of the full neuronal network.

Question 32

metabotropic receptors

Answer 32

Metabotropic receptors are a type of neurotransmitter receptor that work through G-protein coupled mechanisms to initiate intracellular signaling cascades, rather than directly altering the ion flow across the membrane