COMMENT
Synaptic GABAergic mechanism
Reversal potential Egaba is changing according to [Cl-]i change (due to Cl- influx, which we hypothesize to be significant). Bicarbonate (HCO3) flows through the GABAR too, and therefore Egaba is also [HCO3]i/[HCO3]o -dependent
igaba = icl + ihco3 (we assume icl and ihco3 to be mutually independent)
Two state kinetic scheme synapse described by rise time tau1,
and decay time constant tau2. The normalized peak condunductance is 1.
Decay time MUST be greater than rise time.
The solution of A->G->bath with rate constants 1/tau1 and 1/tau2 is
A = a*exp(-t/tau1) and
G = a*tau2/(tau2-tau1)*(-exp(-t/tau1) + exp(-t/tau2))
where tau1 < tau2
If tau2-tau1 -> 0 then we have a alphasynapse.
and if tau1 -> 0 then we have just single exponential decay.
The factor is evaluated in the
initial block such that an event of weight 1 generates a
peak conductance of 1.
Because the solution is a sum of exponentials, the
coupled equations can be solved as a pair of independent equations
by the more efficient cnexp method.
ENDCOMMENT
TITLE GABAergic conductance with changing Cl- concentration
NEURON {
POINT_PROCESS gaba
USEION cl READ ecl WRITE icl VALENCE -1
USEION hco3 READ ehco3 WRITE ihco3 VALENCE -1
RANGE tau1, tau2, g
RANGE P, i
RANGE icl, ihco3, ehco3, e
GLOBAL total
}
UNITS {
(mA) = (milliamp)
(nA) = (nanoamp)
(mV) = (millivolt)
(uS) = (micromho)
(mM) = (milli/liter)
F = (faraday) (coulombs)
R = (k-mole) (joule/degC)
}
PARAMETER {
tau1 =.1 (ms) <1e-9,1e9>
tau2 = 80 (ms) <1e-9,1e9>
P = 0.18 : HCO3/Cl relative permeability
celsius = 31 (degC)
}
ASSIGNED {
v (mV) : postsynaptic voltage
icl (nA) : chloride current = 1/(1+P)*g*(v - ecl)
ihco3 (nA) : bicarb current = P/(1+P)*g*(v - ehco3)
i (nA) : total current generated by this mechanism
: = icl + ihco3
g (uS) : total conductance, split between bicarb (P/(1+P)*g)
: and chloride (1/(1+P)*g)
factor
total (uS)
ecl (mV) : equilibrium potential for Cl-
ehco3 (mV) : equilibrium potential for HCO3-
e (mV) : reversal potential for GABAR
}
STATE {
A (uS)
B (uS)
}
INITIAL {
LOCAL tp
total = 0
if (tau1/tau2 > .9999) {
tau1 = .9999*tau2
}
A = 0
B = 0
tp = (tau1*tau2)/(tau2 - tau1) * log(tau2/tau1)
factor = -exp(-tp/tau1) + exp(-tp/tau2)
factor = 1/factor
e = P/(1+P)*ehco3 + 1/(1+P)*ecl
}
BREAKPOINT {
SOLVE state METHOD cnexp
g = B - A
icl = 1/(1+P)*g*(v-ecl)
ihco3 = P/(1+P)*g*(v-ehco3)
i = icl + ihco3
e = P/(1+P)*ehco3 + P/(1+P)*ecl
}
DERIVATIVE state {
A' = -A/tau1
B' = -B/tau2
}
NET_RECEIVE(weight (uS)) {
A = A + weight*factor
B = B + weight*factor
total = total+weight
}