LG - Metabotropic Receptors II

Question 1

Q: What are the three main types of GABA receptors? (3)

Answer 1

• GABA_A – Ligand-gated ion channel
• GABA_A-rho (GABA_C) – Ligand-gated ion channel
• GABA_B – G-protein-coupled receptor (GPCR)

Question 2

Q: Compare the features of GABA receptor subtypes. (5)

Answer 2

GABA_A

• Subunits: α, β, γ, δ, ε, π, θ
• Agonists: Muscimol, THIP
• Antagonists: Bicuculline, Picrotoxin
• Desensitisation: Yes
• Modulators: Benzodiazepines, Barbiturates

GABA_A-rho

• Subunits: ρ₁, ρ₂, ρ₃
• Agonists: (+)-CAMP, cis-4-aminocrotonic acid, GABOB
• Antagonists: TPMPA, Picrotoxin
• Desensitisation: No
• Modulators: Zinc

GABA_B

• Subunits: GABA_B1a/b + GABA_B2
• Agonist: Baclofen
• Antagonist: Phaclofen
• Desensitisation: No
• Modulators: None known

Question 3

Q: What is the structural organisation of the GABA_B receptor? (3)

Answer 3

• Functions as an obligate heterodimer:
  • GABA_B1: Agonist binding (VFTD)
  • GABA_B2: G-protein coupling
• Can form tetramers or higher-order oligomers
• Activation requires only one agonist molecule per dimer

Question 3

Q: What structural features distinguish GABA_B1a vs GABA_B1b? (2)

Answer 3

• GABA_B1a has two N-terminal sushi domains (SDs) → axonal localisation
• GABA_B1b lacks SDs → dendritic localisation

Question 4

Q: What is the role of GABA_B receptors in the brain? (3)

Answer 4

• Modulate basal ganglia dopamine neurons important for motor control
• Maintain pacemaker activity for extracellular dopamine regulation
• Regulate neuronal excitability and neurotransmitter release

Question 5

Q: What are GABA_B receptor supercomplexes? (3)

Answer 5

• Higher-order receptor assemblies with auxiliary proteins
• Visualised using single-molecule tracking, EM, FRET
• Help regulate signalling strength and specificity

Question 6

Q: What are KCTD proteins and their function in GABA_B signalling? (4)

Answer 6

• Auxiliary proteins that bind specifically to GABA_B receptors
• Regulate signalling kinetics and amplitude
• Not potassium channels (structural resemblance only)
• Identified by Schwenk et al., 2015 (Nature Neuroscience)

Question 7

Q: How does GABA_B receptor structure differ from mGluRs? (3)

Answer 7

• Lacks cysteine-rich domains → more compact
• Requires only one agonist per dimer
• Involves smaller VFTD movement for activation

Question 8

Q: How do mGluRs and GABA_B receptors affect neuronal function? (3)

Answer 8

1) Operate over seconds to minutes

2) Regulate Ca²⁺ signalling, ion channel activity, synaptic firing

3) Implicated in disease:

• mGluR5 → Alzheimer’s
• GABA_B → Parkinson’s

Question 9

Q: How do neuropeptides differ from classic neurotransmitters? (5)

Answer 9

1) Structure:

• Classic NTs: Small molecules
• Neuropeptides: Short amino acid chains (3–50 residues)

2) Release:

• Classic NTs: Synaptic cleft only
• Neuropeptides: Synaptic and extrasynaptic

3) Synthesis:

• Classic NTs: Presynaptic terminals
• Neuropeptides: Ribosomes (as precursors)

4) Receptors:

• Classic NTs: Ionotropic & GPCRs
• Neuropeptides: Only GPCRs (Rhodopsin & Secretin families)

5) Presence in Humans:

• Classic NTs: Present
• Neuropeptides: Rhodopsin-NP & Secretin-NP not found in humans

Question 10

Q: How are neuropeptides synthesised and processed? (4 steps)

Answer 10

1) Pre-propeptide synthesis via ribosomes
2) Signal peptide removal by Signal Peptidase
3) Cleavage at R/K by Propeptide Convertases and Carboxypeptidases
4) Post-translational modifications:

• PAM: Amidates glycine residues
• TPST: Adds sulfate to tyrosines

Question 11

Q: What contributes to the functional diversity of neuropeptides? (2)

Answer 11

• Gene duplication events (e.g. CCK and Gastrin)
• One precursor can yield multiple mature peptides via processing

Question 12

Q: What techniques are used to measure drug action at metabotropic receptors? (4)

Answer 12

1) Radioligand Binding Assay

• e.g. [³H]MPEP to study mGluR5 antagonist binding
• Use Scatchard plot to determine Kd

2) IP₃ production

• Measures allosteric modulator activity in mGluR5-lacking ligand site

3) Calcium imaging

• e.g. astrocytes showing oscillatory Ca²⁺ responses

4) Membrane potential recordings

• e.g. midbrain slices to study neuronal excitability

Question 13

Q: What is the evolutionary significance of neuropeptides? (3)

Answer 13

• Pre-date classic neurotransmitters
• Existed before neurons evolved
• Monoamines (e.g. dopamine, serotonin) evolved later

Question 14

Q: What are examples of allosteric modulators for mGluR5? (3)

Answer 14

• VU-29 – potentiates mGluR5 activity at MPEP binding site
• CDPPB – similar potentiator via same site
• Effects measured via IP₃, Ca²⁺ flux, and membrane potential

Question 15

Q: What are the key summary points of this lecture? (5)

Answer 15

• GABA_B receptors are obligate heterodimers with differential localisation
• Form supercomplexes with auxiliary proteins like KCTDs
• Neuropeptides show high diversity via duplication and post-translational processing
• mGluR5 and GABA_B play roles in neurological disorders
• Allosteric modulators (PAMs, NAMs) provide valuable drug targets