TITLE simple AMPA receptors
COMMENT

Simple model for glutamate AMPA receptors
=========================================
 FIRSTORDER KINETICS, FIT TO WHOLECELL RECORDINGS
Wholecell recorded postsynaptic currents mediated by AMPA/Kainate
receptors (Xiang et al., J. Neurophysiol. 71: 25522556, 1994) were used
to estimate the parameters of the present model; the fit was performed
using a simplex algorithm (see Destexhe et al., J. Computational Neurosci.
1: 195230, 1994).
 SHORT PULSES OF TRANSMITTER (0.3 ms, 0.5 mM)
The simplified model was obtained from a detailed synaptic model that
included the release of transmitter in adjacent terminals, its lateral
diffusion and uptake, and its binding on postsynaptic receptors (Destexhe
and Sejnowski, 1995). Short pulses of transmitter with firstorder
kinetics were found to be the best fast alternative to represent the more
detailed models.
 ANALYTIC EXPRESSION
The firstorder model can be solved analytically, leading to a very fast
mechanism for simulating synapses, since no differential equation must be
solved (see references below).
References
Destexhe, A., Mainen, Z.F. and Sejnowski, T.J. An efficient method for
computing synaptic conductances based on a kinetic model of receptor binding
Neural Computation 6: 1014, 1994.
Destexhe, A., Mainen, Z.F. and Sejnowski, T.J. Synthesis of models for
excitable membranes, synaptic transmission and neuromodulation using a
common kinetic formalism, Journal of Computational Neuroscience 1:
195230, 1994.

ENDCOMMENT
NEURON {
POINT_PROCESS AMPA_S
RANGE R, g
NONSPECIFIC_CURRENT i
GLOBAL Cdur, Alpha, Beta, Erev, Rinf, Rtau
}
UNITS {
(nA) = (nanoamp)
(mV) = (millivolt)
(umho) = (micromho)
(mM) = (milli/liter)
}
PARAMETER {
Cdur = 0.3 (ms) : transmitter duration (rising phase)
Alpha = 0.94 (/ms) : forward (binding) rate
Beta = 0.18 (/ms) : backward (unbinding) rate
Erev = 0 (mV) : reversal potential
}
ASSIGNED {
v (mV) : postsynaptic voltage
i (nA) : current = g*(v  Erev)
g (umho) : conductance
Rinf : steady state channels open
Rtau (ms) : time constant of channel binding
synon
}
STATE {Ron Roff}
INITIAL {
Rinf = Alpha / (Alpha + Beta)
Rtau = 1 / (Alpha + Beta)
synon = 0
}
BREAKPOINT {
SOLVE release METHOD cnexp
g = (Ron + Roff)
i = g*(v  Erev)
}
DERIVATIVE release {
Ron' = (synon*Rinf  Ron)/Rtau
Roff' = Beta*Roff
}
: following supports both saturation from single input and
: summation from multiple inputs
: if spike occurs during CDur then new off time is t + CDur
: ie. transmitter concatenates but does not summate
: Note: automatic initialization of all reference args to 0 except first
NET_RECEIVE(weight, on, nspike, r0, t0 (ms)) {
: flag is an implicit argument of NET_RECEIVE and normally 0
if (flag == 0) { : a spike, so turn on if not already in a Cdur pulse
nspike = nspike + 1
if (!on) {
r0 = r0*exp(Beta*(t  t0))
t0 = t
on = 1
synon = synon + weight
state_discontinuity(Ron, Ron + r0)
state_discontinuity(Roff, Roff  r0)
}
: come again in Cdur with flag = current value of nspike
net_send(Cdur, nspike)
}
if (flag == nspike) { : if this associated with last spike then turn off
r0 = weight*Rinf + (r0  weight*Rinf)*exp((t  t0)/Rtau)
t0 = t
synon = synon  weight
state_discontinuity(Ron, Ron  r0)
state_discontinuity(Roff, Roff + r0)
on = 0
}
}
