In article <30r5sl$g71 at portal.gmu.edu>, herwin at mason1.gmu.edu (HARRY R.
> GABAergic synapses are generally assumed to be inhibitory, but it was a
> GABAergic synapse that Tom Vogl predicted would be switchable from
> inhibitory to excitatory, and that Dan Alkon eventually confirmed to be
> so. Walter Freeman indicates that no one who has studied the GABAergic
> synapse has investigated the effect of chloride concentration on its
> excitatory/inhibitory nature. Has there been any work done here? I'm
> reaching the point in my Katchalsky network modeling where I'm beginning
> to substitute up-to-date neuronal models, and this area is important.
The answer to your question is yes, but I'm not sure my example is what you
had in mind. Luhman and Prince (mid to late 80's) showed that in the early
postnatal period chloride gradients in cortical neurons are reversed
relative to the mature state (higher concentration of Cl- inside the cell).
Consequently, GABA causes depolarization until the chloride gradients
mature. Kandler and Friauf reported a similar result for glycinergic
synapses in the auditory brainstem at the '92 Neuroscience meeting, but I'm
not sure whether it's been published yet.
On general terms, as someone else pointed out, there's no reason for GABA
to be exclusively inhibitory. It just happens that in most cases GABA
receptors are coupled to chloride channels (GABA-A) or potassium channels