Synaptic Transmission

Question 1

Intro

Answer 1

• synapse
  
  • 1897 Charles Sherrington
• electrical synapse
  
  • proven in 1959 by Furshpan and Potten in crayfish
• chemical synapse
  
  • evidence in 1921 by Loewi
  • Katz demonstrated fast transmission at NMJ chemically mediated
  • by 1951, Eccles studied synaptic transmission in mammalian CNS using glass microelectrode
  • during last decade, new methods of studying the molecules revelease that synapses are far more complex

Question 2

Otto Loewi

Answer 2

• 1873-1961
• discovered acetylcholine
  
  • Nobel prize laureate in physiology/medicine 1936

-most famous experiment (using frogs) came from his dream in the night of Easter Sunday 1921 and found vagusstoffII turned out to be acetylcholine showing that synaptic signalling used chemical messengers

Question 3

Electrical synapses

Answer 3

• 6 connexins form a channel (connexon)
• 2 connexons form a gap junction channel
  
  • many channels comprise one gap junction
• pore of channel 1-2nm
  
  • all major cellular ions to pass through directly from cytoplasm of one cell to another
• cells electrically coupled
• very fast
  
  • AP in presynaptic neuron can produce AP in postsynaptic neuron instantaneously

-in invertebrates, electrical synapses found between sensory and motor neurons in pathways mediating escape refelxes

Question 4

Electrical synapses in vertebrate brain

Answer 4

• common in every part of mammalian CNA
• among electrically coupled neurons, APs in presynaptic neuron can cause small amount of ionic current flow across gap junction into other neuron producing postsynaptic potential (PSP)
• PSP generated by single electrical synapse in mammalian brain is usually small, and may not be large enough to produce AP in postsynaptic cells

-roles vary by brain region

Question 5

Synapse categories

Answer 5

1. Connectivity
   
   • which part of neuron is postsynaptic to axon terminal
2. Synapse anatomy
   
   • size and shape
     1. Grays Type 1: assymetrical (postsynaptic membrane is thick)
   • usually excitatory
     2. Grays type II: symmetrical
   • usually inhibitory

-larger synapses have more active zones

Question 6

Requirements for chemical synaptic transmission

Answer 6

1. Mechanism for synthesizing and packing NTs into vesicles
2. Mechanism for causing vesicle to release NT into cleft in response to AP
3. Mechanism for producing electrical or biochemical response to NT in postsynaptic neuron
4. Mechanism for removing NT from cleft
5. Must occur very rapidly to be useful for sensation, perception, control of movement

Question 7

Types of NTs

Answer 7

1. Amino acids
   
   • stored in synaptic vesicles
   • small organic molecules
   • GABA, glutamate, glycine
2. Amines
   
   • small organic molecules
   • stored in synaptic vesicles
   • Ach, DA, Histamine
3. Peptides
   
   • short AA chains
   • stored in secretory granules

• fast transmission uses AA or Ach
• slow transmission may use any of 3 types

Question 8

NT synthesis and storage

Answer 8

1. AA and Amines
   
   • enzymes transported to axon terminal and convert precursor molecules into NT molecules in cytosol
   • transporter proteins load NT into vesicles in terminal where they are stored
2. Peptides
   
   • synthesis of precursor peptide (long) in rough ER in cell body
   • split in Golgi apparatus to yield active peptide
   • secretory vesicles with the peptide bud off from golgi
   • secretory granules transported down axon to terminal where peptide is stored

Question 9

NT transport

Answer 9

• transporters
  
  • proteins in vesicle membrane
  • take up and concentrate the AA and amine NTs inside vesicle

Question 10

NT release

Answer 10

1. AP enters axon terminal
2. Voltage gated Ca2+ channels open
   
   • Ca2+ activates proteins in vesicle and active zone
3. Activated SNARE protein complex guide synaptic vesicles to dock and fuse with presynaptic membrane active zone
   
   • NT released by exocytosis
4. Vesicle membrane recovered by endocytosis
   
   • recycled vesicle refilled with NT

Question 11

Calcium triggered vesicle fusion

Answer 11

• SNAP25 function in normal dA transmission
• ASD, ADHD, Schizophrenia - hyperactivity, anxiety, may arise from altered dA, 5HT Glut circuitry

-epilepsy may arise from abnormalities in calcium transients caused by a SNAP25 deficient in modulating presynaptic VG calcium channels

Question 12

Secretory granules

Answer 12

• also release peptide NT by exocytosis
• calcium dependent
• typically not at active zones
• requires high-frequency trains of AP and more calcium influx
• slower (50msec vs 0.2ms)

Question 13

Auto-receptors and presynaptic inhibition

Answer 13

• NT receptors commonly found in membrane of presynaptic terminal
  
  • autoreceptors
• autoreceptors are GPCRs

-activation lead to inhibition of NT release

Question 14

Curare

Answer 14

• arrow-tip poison

- binds tightly to ACh receptors and blocks actio nof ACh at NMJ

Question 15

Female Black Widow spider

Answer 15

• venom (a-latrotoxin)
• affects ACh transmission
• affects sympathetic neurotransmission
• acts presynaptically to release Ach, NE, GABA from sensory nd motor neurons and endocrine cells

-prevents relaxation of muscle, causing tetany (constant, strong, painful muscle contraction) and rigidity

Question 16

Synaptic integration

Answer 16

-process by which multiple synaptic potentials combine within one postsynaptic neuron

• spatial summation
  
  • 2 or more presynaptic inputs are active at same time, their individual EPSPs add together
• temporal summation
  
  • when same presynaptic fibre fires APs in quick succession, individual EPSPs add together

Question 17

Patch clamp

Answer 17

-permit measurement of ion movement through a single channel

Question 18

Contribution of dendritic properties to synaptic integration

Answer 18

• current must spread down dendrite and some and cause of spike-initiation zone to be depolarizer beyond threshold for AP
• effectiveness of excitatory synapse in triggering an AP depends on how far the synapse is from the spike-initiating zone and properties of dendritic membrane

-amount of depolarization falls off exponentially with increasing distance

Question 19

Shunting inhibition

Answer 19

• inhibiting current flow from soma to axon hillock
• IPSPs generated when ion channels are opened causing hyperpolarization of membrane
  
  • when GABA, glycine opens Cl- channels

-inward movement of Cl- anions will negate outward flow of positive ions, inhibiting current flow from soma to axon hillock

Question 20

Excitatory/Inhibitory synapses

Answer 20

1. Excitatory synapses (glutamate)
   
   • grays type 1
   • spines: excitatory synapses
   • wide synaptic cleft
   • large active zone
2. Inhibitory (GABA, glycine)
   
   • grays type 2
   • clustered on soma or axon hillock
   • narrow synaptic cleft
   • small active zone

Question 21

Modulation

Answer 21

-modify effectiveness of EPSPs generated by other synapses

• NE B receptor
  
  • binding of NE to receptor triggers cascade of events to produce cAMP
  • causes potassium channel to close by attaching phosphate group

Question 22

Chemical synaptic transmission

Answer 22

• rich diversity allows for complex behaviour
• provides explanations for drug effects
• defective transmission is basis for many neurological disorders
• key tp understanding neural basis of learning and memory

Question 23

Optogenetics

Answer 23

• light switch for the brain
• 2011
  
  • use of nanoparticles and magnetic pulses to activate specific cell types without such invasive measures
• combines genetic recombination and optics to control neurons in intact tissue/behaving animals
• light-sensitive opsin genes discovered in species of algae/microbes and are inserted into neural networks to study how and when neurons fire
• can activate or inhibit a signal, allowing for key insight to both normal and abnormal brain function
• neurons can be turned on/off with high specificity and temporal precision

Question 24

Optogenetics basic concepts

Answer 24

• express a light-activated ion channel (opsins isolated from bacteria) in a specific sub-population of neurons
• illuminate neurons to modulate activity
• modulation depends on type of channel
  
  1. Channelrhosopsins: light-gated ion pumps, non-specific cation channels, neural activation
  2. Halorhodopsin: light gated ion pumps, specific for Cl-, neural silencing

Question 25

Parameters for optogenetics

Answer 25

- contains 1. Viral vector 2. Promoter gene 3. Ops in gene 4. Reporter gene - light activated ion channel expressed in specific sub-population of neurons - Illuminate neurons to modulate activity - modulation depends on type of channel - record output (electrophysiology, fMRI)

Question 26

Viral vector

Answer 26

-Lentivirus and adeno-associated virus are most common

Question 27

Opsins

Answer 27

- 4 categories 1. Fast excitation 2. Fast inhibition 3. Step function 4. Biochemical modulation (slower)

Question 28

Optogenetics experiment steps

Answer 28

1. Create a genetic construct (promoter and opsin gene) 2. Insert construct into virus 3. Inject virus into animal brain (pain is expressed in targeted neurons) 4. Implant fibre-optic cable coupled with an electrode 5. Laser light of specific wavelength opens ion channel in neurons 6. Record electrophysiology and behavioural results -photoactivation of inhibitory Vgat BNST->LH circuit produces immediate voracious feeding behaviour of a palatable energy dense food

Question 29

Mapping aggression in the brain

Answer 29

- photo activation of neurons in ventromedial hypothalamus, ventrolateral subdivision (VMHvl) causes male mice to attack inanimate objects as well as other mice - neurons activated during attack are inhibited during mating, suggesting a potential neural substrate for competition between these opponent social behaviours

Question 30

Advantages of optogenetics

Answer 30

- significant experiment control - high resolution - spatial and temporal - wide variety of opsins available - can be applied to more fields than just neuroscience - potential disease models

Question 31

Disadvantages of optogenetics

Answer 31

- damage to surround tissue - light absorption = heat = damage tissues or affect physiological events - viral infection of exogenous proteins can lead to unwanted alterations in cell capacitance, physiological activity, structural abnormalities, and toxicity - second and third degree currents can confound actual effects - causality can not always be proven