AMPA/NMDA and GABAa/GABAb conductance ratios

Christian Wilms chris at out-of-phase.de
Mon Sep 16 06:54:04 EST 2002


Bryan Price <jprice1661 at earthlink.net> wrote:

> 1) Easy question: What is an 'ns'? It's not just 1/giga-ohms, is it?
nS = nanosiemens; since 1 S = 1/Ohm ... 1nS should be 1/GigaOhm
 
> 2) Harder question: I'm trying to calculate the range of ratios between the
> AMPA and NMDA (and GABAa and GABAb) time-summed currents. I am, of course,
> applying a membrane decay convolution function to the beta functions I'm
> using to approximate the currents, with Tm ~ 7 - 50 ms. Clearly I could use
> the maximum AMPA and the minimum NMDA conductances for one limit of the
> ratios, and the opposite conductances for the other limit, as this would be
> inclusive. However, I suspect that individual synapses (containing both AMPA
> and NMDA receptors) maintain some sort of relationship between the numbers
> of each type of receptor - I don't believe that a synapse would have no AMPA
> receptors while also having a plethora of NMDA receptors, or vice-versa.

I mainly familiar with kinetic models, so what I can say to your
question isn't going to fit perfectly.

Normally a glutaminergic synapse will contain both NMDA and
AMPA-recpetors. The conductance of the NMDA-Receptor is heavyly
dependent on the existence of the AMPA-receptors. The reason for this is
the so called Mg-Block, which lowers the conductance of NMDA-Receptor to
practically zero at normal resting potential. If the post-synaptic
membrane is depolarized, the block is resolved and the conductance will
rise according to the binding of glutamate. This depolarization normally
is caused by AMPA-EPSPs.

Information on kinetic modelling of the Mg-Block can be found in
"Methods of Neuronal Modelling" by C. Koch.

In my opinion the easiest way to go would be to build the Mg-Block into
your model. Then you will have to sum up the different conductances to
get your total conductance.

I hope I'm not missing a major point here ...

bye, Chris 



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