LECTURE 5 - indirect synaptic transmission

Question 1

What are metabotropic receptors?

Answer 1

G protein coupled receptors that use secondary messenger systems
The G protein has 3 subunits, the receptor does not
Examples:
Autonomic NS: heart, smooth muscle, glands
CNS

actions are classified by HOW they occur and WHERE
Timecourse: >100’s ms

Question 2

What are the 3 main pathways stimulated by G-protein coupled receptors?

Answer 2

1. Cyclic AMP pathway
2. Phospholipase C pathway (PLC)
3. Direct channel modulation

Question 3

Describe the cyclic AMP pathway

Answer 3

• via alpha subunit
• G protein subtypes (Gs and Gi) –> opposite effects on adenylyl cyclase, either increase or decrease cAMP
• cAMP activates protein kinase A, which phosphorylates target proteins (ionotropic receptors and V-gated channels)

Question 4

Describe the phospholipase C pathway

Answer 4

• via alpha subunit
• activates PLC, which hydrolyses PIP(2) –> DAG + IP(3)
  (a) - DAG activates protein kinase C which phosphorylates target proteins
  (b) inositol triphosphate (IP(3)) - causes release of Ca2+, which stimulates enzymes

Question 5

Describe an example of direct channel modulation

Answer 5

• done by the G protein but NOT via enzymes
  E.g. cardiac muscle
• activation of muscarinic AChRs slows heart rate via K+ channel opening (GIRK) => hyperpolarisation (= more difficult for cardiac myocyte to reach threshold and fire AP)

Question 6

Describe patch clamp recordings of direct channel modulation

Answer 6

• ACh leads to GIRK channel opening
• but ONLY when inside pipette (i.e. mAChR and GIRK adjacent)
• so, not via diffusible secondary messengers

Mechanism:
- G protein By subunit binds directly to GIRK so activation of mAChRs DECREASES heart excitability

Question 7

What is a GIRK channel?

Answer 7

G-protein-coupled, inwardly rectifying K+ channel

Question 8

What do the functional effects of metabotropic receptor activation depend on?

Answer 8

LOCATION
Presynaptic receptors - affect release of NT
Postsynaptic receptors - affect neuronal excitability and AP characteristics e.g. mAChR modulation of GIRK

Question 9

Describe the general mechanism of presynaptic modulation

Answer 9

• activation of presynaptic metabotropic receptor can lead to 2nd messenger modulation of Ca2+ and/or K+ channels

Question 10

Explain the presynaptic modulation of V-gated Ca channels

Answer 10

• in a postganglionic sympathetic neurone
• Ca current elicited by voltage stimulus
• in the presynaptic membrane, noradrenaline activates the alpha-2 adrenoceptor which inhibits Ca channels (beta-gamma signalling)
• NA means current caused by Ca channels opening is much smaller (due to inhibition by adrenoceptor) = less Ca entry = less NA released
• -> NEGATIVE FEEDBACK

Question 11

What are the roles of presynaptic metabotropic receptors?

Answer 11

Regulation of synaptic transmission

INHIBITION of release of NT via decreased Ca current (e.g. sympathetic neurone)

FACILITATION of release of NT via decreased K current (delayed rectifier) - inhibiting K channels = less K efflux = longer depolarisation phase = more Ca2+ entry

Question 12

Where are ganglia usually found in the SNS and PNS?

Answer 12

SNS = ganglia that lie close to spinal cord in a chain

PNS = ganglia tend to be closer to target tissue

Question 13

Explain the modulation of pattern of AP firing of postsynaptic receptors in autonomic ganglia

Answer 13

NT = ACh
stimulation of synapse first leads to FAST EPSP
- due to ACh binding to nAChRs (ionotropic (nicotinic)), Na channels open –> depolarisation

SLOW EPSP

• due to ACh binding to mAChs (muscarinic)
• secondary messenger activated
• G protein activation starts to activate phospholipase C –> DAG production
• DAG causes inhibition of M-type K+ channels = decreased K+ efflux i.e. decreased resting K+ conductance
• small depolarisation (EPSP) due to leak Na+ channels (NO AP)

Question 14

What is the role of the slow EPSP in autonomic neurones?

Answer 14

Shown by current clamp experiment

• originally current injected to produce AP however no synaptic stimulation
• preganglionic axon stimulated –> ACh released at synapse –> fast and slow EPSPs
• recording made during slow EPSP, current stimulus => multiple APs

Question 15

Explain the current clamp recordings of the slow EPSPs

Answer 15

NO slow EPSP

• M-type K+ channels open
• K+ efflux reduces excitability => hard to reach threshold
• so when cell is stimulated, only SINGLE AP

During slow EPSP

• M-type channels close (due to mAChR activation)
• cell is more excitable
• if cell is stimulated, multiple APs arise

THEREFORE

• repeated stimulation of synapse = increased response
• so mAChR activation increases neuronal excitability in ANS (heart is opposite as muscarinic activity decreases heart rate)