Cellular Neuroscience (207)Ian Parker

Lecture #5 - Voltage-gated ion channels

http://parkerlab.bio.uci.edu

Diversity of voltage-gated channels

• Ion permeability

Na+

K+

Ca2+

Upstroke of action potential

Rapid repolarization during action potential

Control of excitability / interspike interval

Electrical signal: i.e. depolarization (Ca2+ action potential)

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. (different to nomenclature for ligand-gated channels, which are usually named for their ligand : e.g. GABA receptor)

• 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)

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 whole-cell 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 intervalbetween action potentials, and allows aneuron 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 re-inactivation 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