TITLE Fast A-type potassium current (Kv4.2)

NEURON {
    SUFFIX kaf_ms
    USEION k READ ek WRITE ik
    RANGE gbar, gk, ik, q
}

UNITS {
    (S) = (siemens)
    (mV) = (millivolt)
    (mA) = (milliamp)
}

PARAMETER {
    gbar = 0.0 (S/cm2) 
    q = 1	: room temperature (unspecified)
    :q = 2	: body temperature 35 C (Du 2017)
    :q = 3	: body temperature 35 C
}

ASSIGNED {
    v (mV)
    ek (mV)
    ik (mA/cm2)
    gk (S/cm2)
    minf
    mtau (ms)
    hinf
    htau (ms)
}

STATE { m h }

BREAKPOINT {
    SOLVE states METHOD cnexp
    gk = gbar*m*m*h
    ik = gk*(v-ek)
}

DERIVATIVE states {
    rates()
    m' = (minf-m)/mtau*q
    h' = (hinf-h)/htau*q
}

INITIAL {
    rates()
    m = minf
    h = hinf
}

PROCEDURE rates() {
    UNITSOFF
    minf = 1/(1+exp((v-(-10))/(-17.7)))
    mtau = 0.9+1.1/(1+exp((v-(-30))/10))
    hinf = 1/(1+exp((v-(-75.6))/11.8))
    htau = 14
    UNITSON

    :Du 2017
    :LOCAL alpha, beta, sum
    :UNITSOFF
    :alpha = 1.5/(1+exp((v-4)/(-17)))
    :beta = 0.6/(1+exp((v-10)/9))
    :sum = alpha+beta
    :minf = alpha/sum
    :mtau = 1/sum
    :
    :alpha = 0.105/(1+exp((v-(-121))/22))
    :beta = 0.065/(1+exp((v-(-55))/(-11)))
    :sum = alpha+beta
    :hinf = alpha/sum
    :htau = 1/sum
    :UNITSON
}

COMMENT

Original data by Tkatch et al (2000) [1]. Neostriatal neurons were acutely
dissociated from young adult rats, age P28-P42.  Electrophysiological
recordings were done at unspecified temperature (room temperature 20-22 C
assumed). Potentials were not corrected for the liquid junction potential
(estimated 1-2 mV).

Activation m^1 matches experimental data [1, Fig.2C]. Activation time
constants fit tabulated data [1, Fig.2B].  Slope of inactivation function
fitted to the data [1, Fig.3B] with half inactivation potential -75.6
mV. Temperature factor q between 1.5 [3] and 3 [2] was used for body
temperature.  Conductance kinetics of m2h type is used [2], no corrections
for m^2 applied. Later modification by Du [4] is close to this model.

[1] Tkatch T, Baranauskas G, Surmeier DJ (2000) Kv4.2 mRNA abundance and
A-type K(+) current amplitude are linearly related in basal ganglia and
basal forebrain neurons. J Neurosci 20(2):579-88.

[2] Wolf JA, Moyer JT, Lazarewicz MT, Contreras D, Benoit-Marand M,
O'Donnell P, Finkel LH (2005) NMDA/AMPA ratio impacts state transitions
and entrainment to oscillations in a computational model of the nucleus
accumbens medium spiny projection neuron. J Neurosci 25(40):9080-95.

[3]  Evans RC, Morera-Herreras T, Cui Y, Du K, Sheehan T, Kotaleski JH,
Venance L, Blackwell KT (2012) The effects of NMDA subunit composition on
calcium influx and spike timing-dependent plasticity in striatal medium
spiny neurons. PLoS Comput Biol 8(4):e1002493.

[4] Du K, Wu YW, Lindroos R, Liu Y, Rózsa B, Katona G, Ding JB,
Kotaleski JH (2017) Cell-type-specific inhibition of the dendritic
plateau potential in striatal spiny projection neurons. Proc Natl Acad
Sci USA 114:E7612-E7621.

ENDCOMMENT