Thus, while the effect of converse look 5b on the inward currents could be explained by the direct involvement of ASICs in PSC generation as observed in the amygdala [ 5 ], the lack of an increase of the outward currents suggests that the effect of 5b is predominantly modulatory. Lack of apparent shift of estimated PSCs reversal potential (-0.75 ± 0.26 mV, n = 8) after application of 5b also supports this suggestion. (see Methods for details of estimating PSCs reversal potential). Nevertheless, to further verify this point we tried to pharmacologically isolate/enhance a fraction of synaptic current that is not mediated by GABA A -receptors (residual current) and study some of its properties.

On the other hand, direct involvement of ASICs in PSCs generation documented for lateral amygdala neurons [ 5 ], does not seemingly occur in hippocampal neurons [ 6 , 36 ]. Our results tend to agree with converse all star the latter observations. Indeed, under our experimental conditions, if a substantial fraction of synaptic current is mediated by ASICs, a decrease in the net inward current, and an increase in the net outward current would be expected once ASICs are blocked. While in the presence of ASIC antagonists we did converse all stars observe a decrease of the inward currents and a small decrease in the outward currents.

Lack of substantial direct involvement of ASICs in PSCs generation in our experiments can be also supported by comparing the possible relative contribution of ASIC current to total synaptic current. Based on the results of our experiments with application of bicuculline alone, the relative contribution of ASIC currents to total synaptic current is less than 9 % (8.29 ± 2.24). Our experiments using 5b and suramin suggest an even lower percentage. Indeed, the residual currents were nearly unaffected in presence of 5b (1 ¼M) but strongly suppressed converse off white (to 16.9 ± 4.3 % of control) by suramin at 200 ¼M concentration.

Since suramin (500 ¼M) does not affect ASIC currents in hippocampal neurons [ 24 ] the above results also indicate that relative contribution of  synaptic' ASIC current is much smaller than 9 %. At the same time, the relative magnitude of the effects produced by ASIC blockers on synaptic currents recorded in the absence of bicuculline is about 20-30 %. These results, taken together, strongly support our point that the suppressing effect of the ASICs blockers on GABAergic transmission is due, predominantly to a modulatory mechanism. Additionally, we would like to mention that absolute amplitude of proton-mediated synaptic currents in amygdala is about 7-10 pA [ 5 ].

Since the density of proton-activated currents (evoked by pH shift to 5) is ~ 75 pA/pF in amygdala and 20 pA/pF in hippocampus [ 8 ], a much reduced proton-mediated component of synaptic current in the hippocampus may be expected. In our experiments, however, the absolute value of inward synaptic current suppressed by ASIC antagonists is about 40 60 pA. As for the current mediated by ASICs in hippocampal GABAergic synapses, we believe this was undetectable due to its small absolute and relative off white converse amplitude.Functional interaction between ASICs and GABA A -receptors in isolated neurons has been recently demonstrated [ 10 , 11 ].

This suggestion is also indirectly supported by the observation that modulatory effect of GABA A -receptors activation on ASICs-currents can be observed in excised patches [ 10 ]. We assume that an interaction between ASICs and GABA A -receptors is quite likely to occur at GABAergic synapses upon acidification at the synaptic cleft. This assumption can be supported by the lack of the apparent effect of 5b on inward PSCS in the presence of bicuculline, observed in our experiments.