HNS06 Chemical Neurotransmission I Flashcards
Electrical synapse
Presynaptic terminal + Postsynaptic terminal
—> connected by Gap junctions
—> Ions flow through gap junction channels (from high conc to low conc)
6 members of Connexons —> gap junction of one side of terminal
Electrical synapse vs Chemical synapse
Stimulus —> Response
Electrical synapse: Micro-second delay (almost instantaneous)
Chemical synapse: ~0.5 milli-second delay
Discovery of quantal nature of neurotransmitter release
- Noticed miniature end-plate potentials in frog NMJ
- fixed size: 0.5 mV
- spontaneous in the absence of stimulation
- ↑ frequency with depolarisation
- dependent on the presence of a synapse —> ∴ must be neurotransmitter —> figured out that about 5000 ACh molecules required to produce 0.5 mV mini-end plate potential - Low extracellular Ca —> endplate potential of 0.5-2.5 mV
- sometimes no response detected (failure of eliciting response)
- minimum non-zero response is 0.5 mV
- all other synaptic potential are integral multiples of unit response (0.5 mV) - When extracellular Ca ↑
- unit response remains same
- failure of eliciting response ↓ + likelihood of higher amplitude response ↑
Quantal hypothesis
Signal transmission at synapses is mediated by the release of neurotransmitters packed in units called “quanta”
—> now called synaptic vesicles
Events during transmission at a chemical synapse
- Transmitter synthesized and stored in vesicles
- An action potential reaches presynpatic terminal
- Depolarization of presynpatic terminal causes opening of voltage-gated Ca channels
- Influx of Ca through channels (extracellular Ca (10^-3)»_space; cytosolic Ca (10^-7))
- Ca causes vesicles to fuse with pre-synpatic membrane
- Transmitter released into synaptic cleft via exocytosis
- Transmitter binds to receptor in post-synaptic membrane —> conformational change
- Opening / Closing of post-synaptic channels —> allow ions to go into post-synaptic membrane (neurotransmitter DON’T go in!!!)
- Post-synaptic current causes excitatory / inhibitory post-synaptic potential —> changes excitability of post-synaptic cell
- Retrieval of vesicular membrane from plasma membrane into cell via endocytosis
Molecular machine of NT release
Synpatobrevin, Syntaxin: SNARE protein (SNAp REceptors)
- Synaptobrevin: protein inserted on synaptic vesicles membrane
- Syntaxin: protein inserted on pre-synaptic plasma membrane
SNAP-25: linking Synaptobrevin + Syntaxin (SNAP: Soluble NSF-Attachment Protein)
Steps:
- Vesicle docks
- SNARE complexes form to pull membranes together
- Entering Ca (from opened voltage gated Ca channel) binds to synaptotagmin (therefore must have a Ca channel in close proximity)
- Ca-bound synaptotagmin catalyses membrane fusion
- NT released from vesicles
Proteins involved in NT release machinery
Fusion:
- Synaptobrevin
- Syntaxin
- SNAP-25
- Complexin (bind to synaptotagmin)
Ca-triggering:
- Synaptotagmin
Ca channel tethering (make sure Ca channel is held in close proximity to site of fusion):
- protein complex (e.g. Muc13, RIM, RIM-BP, Rab3/27)
Toxins that affect NT release
Clostridial toxins cleave SNARE protein
- Tetanus toxin (TeTX)
- cleaves synaptobrevin (V SNARE protein) - Botulinum toxins (BoTX)
- B, D, F, G: cleaves synaptobrevin
- C: cleaves syntaxin (T SNARE protein)
- A, E: cleaves SNAP-25 (T SNARE)
—> prevent vesicle fusion / docking —> NT release
Medical applications:
- Reduce dynamic wrinkles
- Face slimming
α-latrotoxin
- venom of female black widow spider
- causes discharge of synaptic vesicles without need for extracellular Ca
- binds Neurexin (mediates binding of Neurexin to synaptotagmin) —> bypass Ca requirement for triggering vesicle fusion
Presynaptic targets of neurological disorders: Congenital myasthenia syndromes
Causes:
- defect in Choline acetyltransferase (in presynpatic nerve terminals, facilitates transfer of acetate to choline)
- **mutation effects on ChAT protein
—> loss of function of ChAT (kinetic disorder) + reduced protein expression (but not absent)
—> altered affinity of ChAT for AcCoA
—> **defect in ACh resynthesis / repackaging
—> synaptic vesicles (rested muscle: small synaptic vesicles; stimulation: increased / no change in size)
—> BUT normal no. of AChRs on post-synaptic membrane, normal post-synaptic morphology
Genes:
- autosomal recessive, gene mutation often missense (occasionally 1 mutation is frame-shifting), most patients compound heterozygotes
- familial infantile myasthenia
Symptom:
- presynaptic CMS: ↓ production / release of ACh
—> episodic apnea (CMS-EA)
—> weakness of eye muscle (double vision), mouth/throat muscle (difficulty chewing/swallowing)
Treatment:
- ***AChE inhibitors: Prophylactic Pyridostigmine
Lambert-Eaton myasthenic syndrome
Presynaptic disorder:
- ***↓ number of Ca channels on active zone of presynaptic terminal due to IgG attack
Physiology:
- ↓ K stimulated Ca influx into pre-synaptic terminal
- ↓ Ca dependent quantal ACh release
Serum Antibodies:
- ***IgG against voltage-gated Ca channel (VGCC)
—> P/Q type Ca channel (85%)
—> N type Ca channel (35%)
Electrophysiology
- Small amplitude of compound muscle action potential (CMAP)
- Diagnosis: Repeated nerve stimulation: (As nerve stimulation is rapidly repeated, the acetylcholine stored in the nerve terminal is gradually depleted, and there is a slight weakening of the acetylcholine signal sent to the muscle fiber, resulting in smaller endplate potentials (EPPs). In normal muscle, although the EPPs become smaller with repetitive stimulation, they remain above the threshold needed to trigger muscle contraction. In myasthenia gravis, where many of the acetylcholine receptors are blocked, the EPP may exceed the threshold initially, but quickly falls below threshold with repetitive stimulation, resulting in the muscle fiber failing to contract. As one by one the muscle fibers fail to contract, the overall CMAP measured grows smaller and smaller, leading to the pathologic decremental response)
Microphysiology:
- Ca channel blockage: P most common, Q often, N minority
- ***↓ quantal ACh release from presynaptic nerve terminal
Treatment (Symptom alleviation):
- Plasma exchange to remove Ca channel Ab from the blood
- Use of immunosuppressant drugs
