Cellular Neuroscience (207)
Ian Parker
Lecture #5 - Voltage-gated ion
channels
http://parkerlab.bio.uci.edu
Diversity of voltage-gated channels
• Ion permeability
Na+
Upstroke of action potential
Rapid repolarization during action potential
K+
Control of excitability / interspike interval
Electrical signal: i.e. depolarization (Ca2+ action potential)
Ca2+
Chemical signal: rise in intracellular [Ca2+], e.g. opening of
Ca2+-dependent K+ channels
• Gating properties
Voltage sensitivity :
e.g. open with strong or weak depolarization, or with hyperpolarization
Gating kinetics : e.g. open quickly or slowly,
inactivating or non-inactivating
Nomenclature and classification of voltage-gated
channels
•
Called by the ion that goes through them :e.g. sodium channel.
•
Old nomenclature was arbitrary, based on functional distinctions (e.g. A-typeand K-type
K+ channels), or on gene mutations from which channels first cloned (e.g. shaker and
shaw K+ channels)
(different to
nomenclature for ligand-gated channels, which are usually named for their ligand : e.g.
GABA receptor)
New, systematic (but boring)
nomenclature and classification
based on ‘family tree’ of sequence
homology in channel genes (e.g.
CaV1.3, KV3.1)
For illustration – you don’t have to memorize this!
All Na+ channels have similar properties, whereas
K+ channels are highly diverse
Similar inactivation kinetics of
Na+ currents from many
species/organs - Na channels
have only one, stereotyped
job; to make the action
potential go up. (why don’t C.
elegans have any Na+ channels?)
Different K+ channels show
very different inactivation
kinetics – they serve many
different purposes
Relation between single channel currents and whole-cell
current
• A. Inactivating Na channels
give transient whole-cell
current
•
Questions…..
•
What determines peak whole-cell current?
•
What determines rise-time of whole-cell
current?
•
What determines the decay rate of wholecell current?
Relation between single channel currents and
whole-cell current
•
B. A. Non-inactivating K channels give
sustained whole-cell current
•
Questions…..
•
What determines mean steady-state whole-cell
current?
•
What determines rise-time of whole cell current?
•
Why is current whole-cell current trace ‘noisier’
during depolarization? Might this tell us something?
Role of A-type (shaker) K channels in spike frequency
adaptation
Frequency adaptation sets the interval
between action potentials, and allows a
neuron to fire at different rates depending on
stimulus strength (frequency encoding – a
squid axon can’t do this)
Inter-spike interval is determined by the
recovery from inactivation, then slow reinactivation of A-type K channels
Voltage-gated Ca2+ channels and Ca2+-activated K+
channels allow neurons to generate ‘bursting’ pacemaker
activity
Mechanism of voltage-dependent activation – gating
charge movement
S4 region of channel contains highly
charged amino acids, and physically
moves in response to voltage change.
This causes opening of channel (but we
don’t yet know how).
Movement of S4 exposes residues to extracellular
solution, and generates a ‘gating current’, which can
be measured.
Mutations in S4 that reduce the # of charges reduce
the gating current and make the voltage dependence
of Na conductance (i.e. probability of channel
opening) a less steep function of voltage.
Channel inactivation – ‘ball and chain’ model for
inactivation of Shaker K+ channel
Inactivation – channel stops passing current, even with maintained depolarization. Mechanism
involves a ‘gate’ different to that controlling activation. ‘Ball and chain’ is one (but not the only)
mechanism.
Peptide ‘ball’ on flexible tether
(all parts of K channel subunit)
swings in to block channel soon
after it opens.
Test of model.
Mutation of shaker channel that deletes ball
removes activation. Can then recover inactivation by separately
expressing peptide balls, even though these are no longer
tethered to the channel