Foundations I Fall, 2016 Voltage-gated Ion Channels
Na+ Channel Hille, 2001 300 nM TTX Most Na+ channels are blocked by TTX in nM concentrations
Ogata and Ohishi, 2002
TTX resistant Na Channels DRG cells- nocioception Blair and Bean, 2002
TTX resistant Na Channels Blair and Bean, 2002
K currents blocked pharmacologically Hille, 2001 Na+ (and other VGCs ) only relatively selective
Sodium Channel The selectivity filter is scorpion toxins based on size of the hydrated ion. For Na channel, pronase TTX, STX Na~=Li>Tl>K>Rb>Cs Density in range of 35-500 2 channels/ µ m , except at nodes of Ranvier where it may be as high as 23,000/ 2 µ m Single channel conductance ~4-18 pS Hille, 2001
The barrier to movement of a heavily hydrated ion into a narrow pore is the energy required to dehydrate the ion. Water forms shells (usually at least 3) around ions. When an ion enters a pore it is dehydrated and the pore walls solvate the ion. If the diameter of the crystal radius of the ion plus one water molecule is equal to the pore size, the channel is permeable to that ion. Maximum channel conductance is limited by (among many other variables) the length of the pore. see Hille 3rd Ed. 2001
Voltage-gated Na+ channel (Na 1.1-1.9) v pore inactivation voltage gate sensor modulatory sites 4 transmembrane repeat domains in the alpha subunit each channel made from one alpha and 2 beta subunits (beta 1, 2, 3) similar basic structure for voltage gated Na+, Ca++ and K+ channels
Hille, 2001 The voltage-gated Na+ channel is very similar among different species and tissues
Mechanisms of Inactivation Ball and chain model
All Ca++ currents blocked What is this? Persistent Na+ Current Text Kay et al., 1998
Potassium Channels 2 Lower density than Na channels - ~7-240 channels/ µ m 2 up to 11,000/ µ m at nodes single channel conductance ~ 2.4-230 pS Highly diverse - much more so than Na+ channels
Hille, 2001 Kv3 Kv4 Kv2 Kv1 Alpha subunits of several different K+ channels were first identified in different Drosophila mutants (1987-1990)
Ogata and Ohishi, 2002 Multiple K+ genes are always coexpressed in mammalian neurons
Potassium Channels each K+ channel is comprised of 4 subunits
Song, 2002
Outward Rectifier (Delayed Rectifier, I ) K 30.00 mV -1.10 nA 1.300 nA 40 mV -70.0 mV 1.6 nA 20.0 ms blocked by mM TEA or 4-AP, or Cs+ or Ba+ ions spike repolarization limits depolarization
A-current Hille, 2001 I A activates on depolarization I A inactivates with depolarization and is inactivated at rest not a single molecular entity
Hille, 2001 The voltage dependence of both activation and inactivation is very steep and the curves only overlap over a narrow range of membrane I A potential. Thus only conducts in a narrow window of membrane potential from about -65 mV to -40 mV. “Window Current”
Hille, 2001
A-currents allow for slow repetitive firing
Kv 3.x channels Rudy and MacBain, 2001
Kv 3.x channels Kv 3.x channels activate at very depolarized membrane potentials
Kv3.X is associated with fast-spiking interneurons Rudy and MacBain, 2001
Many fast-spiking interneurons express the calcium-binding protein, parvalbumin
Kv3.1 HEK293 Cells Kv1.1 hippocampal basket cells in TTX Kv3.x is blocked by µ M 4-AP Rudy and MacBain, 2001
Inward Rectifier Text Wilson, 1993 K+ channel that opens when hyperpolarized K 1.x - 5.x ir Rectification mechanism appears to involve intracellular polyamines (spermine) plus a Mg++ block
Inward Rectifier Command steps of 10 mV from -60 mV to + 60 mV
Calcium-activated potassium conductance 3 families BK IK SK or (SK1, SK2, SK3) (non-neuronal) maxiK
BK
Calcium-activated potassium conductance BK Tetramer Sah and Faber, 2002 large conductance - 200 - 400 ps blocked by TEA in low µ M and scorpion toxins (charybdotoxin, iberiotoxin)
Calcium-activated potassium conductances β subunit shifts BK voltage dependence and increases Ca++ Ca++ and voltage sensitivity dependent - requires depolarization
Calcium-activated potassium conductance SK Sah and Faber, 2002 small conductance - 2 - 20 ps unaffected by TEA in low concentrations very sensitive to apamin (bee venom toxin) calcium sensor is external - calmodulin
I K(Ca) and the AHP 3 components of AHP fAHP, mAHP, sAHP Sah and Faber, 2002 fAHP is voltage dependent, is blocked by µ M TEA, iberiotoxin and paxilline BK channel
Shepard and Bunney, 1991 rat DA neuron
mAHP Sah and Faber, 2002 mAHP is voltage-insensitive and is blocked by apamin SK channel
rat STN Hallworth et al., 2003
sAHP Sah and Faber, 2002 voltage-insensitive, not blocked by apamin or TEA blocked by several neurotransmitters underlying channel unknown -SK with unknown β subunit? -not a true calcium-activated K+ channel?
Hille, 2001
Cortical Pyramidal Neuron Madison and Nicoll, 1986
I h opens slowly under hyperpolarization E between -20 and -40 mV rev mixed cation channel blocked by Cs+ and Rb+, not strongly blocked by Ba++
I h 10 mV 0.2 nA 40 ms rat substantia nigra dopaminergic neuron I h participates in rhythmic firing and rebound from strong inhibition
Voltage-gated Calcium Channels There are two main families of voltage-gated Ca++ channels They are distinguished primarily by their activation thresholds, and are termed Low Voltage Activated (LVA) and High Voltage Activated (HVA) Best characterized HVA Ca++ channel is the L-Channel L(arge) conductance L(ong-lasting) current Best characterized LVA Ca++ channel is the T-Channel T(ransient) conductance
Calcium Channels Hille, 2001
Calcium Channels fi fi fi 2+ Most Ca++ channels blocked by Cd and transition metals 2+ 2+ 2+ 2+ 3+ La >Co >Mn >Ni >Mg
Calcium Channels 2+ 2+ 2+ 2+ Ca channels are permeable to Ca , Ba or Sr calcium-dependent inactivation
2+ 2+ HVA Ca channels show Ca-dependent inactivation
Ca 1.x v Hernandez-Lopez et al., 2000 L-channels are blocked by Cd+ and dihyropyridines nifedipine, nitrendipine, diltiazem L-channel openings are prolonged by BAY K8644
Kammermeier, P. J. et al. J Neurophysiol 77: 465-475 1997 N-type calcium channel ( ) is sensitive to Ca 2.2 v low µ M concentrations of -conotoxin ω
Kammermeier, P. J. et al. J Neurophysiol 77: 465-475 1997 Spider venom toxin blocks little of the HVA in thalamic neuron but almost 100% of the HVA in a Purkinje cell The Purkinje cell HVA is called P-current and isextremely sensitive to - Agatoxin ω
Jahnsen and Llinas, 1984 It can be activated upon depolarization but only from a hyperpolarized membrane potential. It is inactivated at rest. It is Na+ -independent It is blocked by Co++and low Ca+
The LTS is a low voltage activated calcium conductance The underlying conductance is a member of the Ca 3 v family (Ca 3.1, Ca 3.2 and Ca 3.3) and is called a T- v v v current T stands for transient - note the shorter mean open time compared to L-channels
dopamine neuron in vitro LTS deinactivation is time and voltage dependent The LTS is blocked by low µ m concentrations of Ni
CsCl, 4-AP, TTX activation inactivation Kang and Kitai, 1993
relay mode bursting mode thalamocortical neuron
forebrain (LMAN) neuron in songbird Livingston et al., 1997
Rhythmic bursting mediated by I and T-currents h or ACh, NE, Glu (or anything that enhances Ih) McCormick and Pape, 1990
L- channel T-channel Kang and Kitai, 1993
What is the reversal potential for Ca++ ? ++ ] o = 2 mM [ Ca [ Ca ++ ] i = 100 nM E Ca ++ = + 124 observed (+52 mV) predicted (+124mV)
What is the reversal potential for Ca++ ? ++ ] o = 2 mM [ Ca [ Ca ++ ] i = 100 nM E Ca ++ = + 124 observed (+52 mV) predicted (+124mV)
What is the reversal potential for Ca++ ? ++ ] o = 2 mM [ Ca [ Ca ++ ] i = 100 nM E Ca ++ = + 124 the channel is also permeable to K+ with + / ρ C ++ = 1:1000 ρ K a there is so much more K inside than Ca that it exits when the channel is open and the observed reversal potential is a mixture of the two Nernst equilibrium potentials
Cl- Channels Verkman & Galietta, 2009
Huang et al, 2012 Calcium-activated chloride channel (CaCC) in hippocampal pyramidal neuron CaCC are selectively blocked by niflumic acid (NFA).
sub threshold oscillations in striatal PLTS interneurons Song and Wilson, 2016 independent of Na+ channels but requires Ca++ channels
sub threshold oscillations in striatal PLTS interneurons are blocked by NFA Song and Wilson, 2016 sub threshold oscillations in striatal PLTS interneurons are caused by CaCC
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