Ligand Gated Channels and NeuroTransmission: Module 2.6-2.7 Flashcards
What are the three families of ligand-gated channels (ionotropics)?
- Cys-loop, Pentamers (Multiple Subunits)
- Tetramers (2 Subunits)
- Trimers
How many members are part of the cys-loop family of receptors?
4
All of them are pentrameric and ionotropic have extracellular cys-loop,
What are the cationic members of the cysloop family?
- Acetylcholine Nicotinic Receptor (nAChR)
- Serotonin (5-HT3 Receptor)
What are the structural characteristics of the Cys Loop Family?
Characteristic Loop formed by disulfide bond between 2 cysteine residues 13 amino acids apart near the N-terminal domain of the alpha subunit.
M2 is where the selectivity filter is.
What are the anionic members of the cysloop family?
- Glycine (GlyR)
- GABA (GABAa Receptor)
What type of receptor are nicotinic acetylcholine receptors; what agonist are they responsive to and what is its type?
Ionotropic
Acetylcholine
Endogenous Agonist
Endogenous = Naturally Occuring substance within the body.
nAChRs got nicotinic name due to response to nicotine
What type of agonist is nicotine?
Exogenous Agonist
Who discovered nAChr?
- 1914: muscle receptor by Henry H. Dale and Otto Loewi
- 1986: nicotinic ACh receptors were found in neuron
Explain the structure of nAChRs in muscle.
Explain the nAChR composition and distribution.
What is a-Bungarotoxin(a-BTX or a-BgT)?
Snake Venom - Neurotoxin
Competitive Irreversible Antagonist for nAchRs (muscle a1, a7, a9) at the Neromuscular junction.
Causes paralysis, respitory failure and death.
What does the IV plot for a Neuronal nAChR (2 alpha, 3 beta) look like and what causes this?
Inward Rectification
Due to Polyamine-mediated Block: Polyamine has alot of positive charges - depolarized memebrane potentials and the ring of negative cysteine residues pushes/attracts polyamine outside the cell through nAChRs however these pores are too small and cause a block.
At negative potentials polyamine is released and channel is unblocked
Why do muscle nAchR show ohmic behavior?
Receptors are much larger and polyamine can go through causing receptors to not become plugged.
When nAChRs are bound by. Ach
What ions does it let pass?
It allows the non selective flow of all cations.
Causes flow of Na, K, Ca; sometimes show permeability for a certain ion.
What experiment can be used to find nAchR reversal potential and what is it?
0 mV
Voltage Clamp of Post-Synaptic Cell
What are the adult and fetal subunits of nAChRs and what is their difference in conductance and open probability?
Adult Subunit = Epsilon - Higher Conductance and Less open Probability.
Fetal Subunit = Gamma - Less Conductance and More Open Probability.
Though to reflect different roles in synaptic transmission and synapse formation for developement .
,
What is a-BTX used to study?
Used to study nACh funciton and cell surface Expression.
What is Curare (d-tubocuranine)?
Curare is a general term for a group of plant-derived alkaloid toxins traditionally used as arrow poisons by Indigenous peoples in South America. These toxins are known for their ability to cause muscle paralysis by interfering with the transmission of nerve impulses to muscles.
Competitive Reversible Antagonist for nAChRs at the neuromuscular junction.
What neurotransmitters mediate alot of the synaptic inhibition in th CNS?
GABA = Majority
Glycine = Rest
GABA type A and Glycine Receptors are ionotropic
What ion do they select for?
Cl-
Why is synaptic inhibiton tightly regulated?
Too Much Inhibition: Loss of Consciousness and Coma
Too Little Inhibition: Siezure Activity
What are 2 significant modulators of GABA type A and what is their effect?
Benzodiazepines (Tranquilizer Valium): Increases Frequency of channel opening aswell as can increase Cl conductance.
Bariturates (sedative phenobarbital, pentobarbital): Increase the duration of channel opening.
Each bind to own specific site on the receptor.
What are some potential natural modulators of GABA type A receptor?
Metabolites of these steroid hormones: Progesterone, Corticosterone, Testosterone.
Expain how GABA responses change in the CNS as we develop.
Immature CNS neurons: Express higher levels of NKCC1(Brings Cl inside) receptors than KCC2 (Brings Cl outside) leading to a **high intracellular Cl **concentration. When GABA channels open in this case -> Cl leaves the cell causing a depolarizing effect.
Mature CNS neurons: Express higher levels of KCC2 than NKCC1 leading to low intracellular Cl. When GABA channels open -> Cl influx which causes a hyperpolarizing effect.
Nerst Equation for Cl changes.
Thought to occur during Embryonic Developement - Need depolarization to form new synapses and switches to prevent siezure activity in adults.
In Embryo GABA is Excitatory NT
What is the perforated patch configuration?
Modified version of whole-cell recording with little pores instead of big hole. Holes created by pore-forming antibiotics. Pores allow flow of small ions and prevent passing of larger ions and molecules.
Congifuration is still whole cell as you create an eletrical connection between the pipette and cytoplasm
What are some different antibiotics used in the perforated patch technique and what are their functions?
Nystatin and Amphotericin-B: Allows Cation to pass but Preserves Intracellular Ca
Gramicidin: Allows everything through but Preserves intracellular Cl
ATP is also preserved.
What are the advantages and disadvantages of the perforated patch technique?
Advantages
* Maintains integrity of Cytoplasmic Components including soluble second mesengers.
* Prevents channel “run-down”
* It reduces dialysis of cells
Disadvantages
* Higher access resistance, which increases noise and reduces signal
* Time
* The membrane perforated by the antibiotic is prone to rupture.
Why do perforated currents have a smaller amplitude and why does the reversal potential shift to 0mV in whole cell.
- Perforated currents have a smaller amplitude due to resistance at the tip of the pipette. The whole cell membrane break reduces this resistance.
- Experimental Conditions use a symmetrical solution of pipette to bath, so that after you are done the perforated reversal potential reading; you can break the membrane to whole cell and make sure the reversal potential is 0mV meaning you got an accurate perforated reading of reversal potential(this would be the naturally occuring reversal potential).
What is the main excitatory NT of the CNS?
Glutamate
What are the 3 classes of ionotropic glutamate receptors?
AMPA, NMDA and Kainate
Explain glutamate receptor structure?
Tetramer (4 sub-units), heteromultimers.
Heteromultimers are protein complexes made up of two or more different types of polypeptide chains
What ions to ionotropic glutumate receptors let through?
They are Cation Non-Selective
What are the different glutumate receptors gated or blocked by?
What are the Antagonists of the different classes of ionotropic glutumate receptors?
Explain the Mg Block of NMDA receptors.
At the resting membrane potential (−65 mV) extracellular Mg2+ binds tightly to a site in the pore of the channel, blocking ionic current. But when the membrane is depolarized (for example, by the opening of AMPA receptor channels), Mg2+ is expelled from the channel by electrostatic repulsion, allowing Na+ and Ca2+ to enter.
Important Figures.
What glutamatergic synapses are open at negative values of postsynaptic Vm?
AMPA Channels can be activated by glutumate depolarizing the cell.
NMDA receptors have a Mg block at these potentials.
What glutamatergic synapses are open at more positive values of postsynaptic membrane potential?
AMPA and NMDA
Mg detaches from NMDA
What ions are AMPA channels most permeable too?
Na and K
What ions are NMDA channels most permeable too?
Ca
NMDA channels are ___ ___and ____ ____.
NMDA channels are ligand gated and voltage dependant.
