GABA depolarization evoked

Liachenko S, Tang P, Somogyi GT, Xu Y. (1999). Concentration-dependent isoflurane effects on depolarization-evoked glutamate and GABA outflows from mouse brain slices. Br J Pharmacol. 

We have shown that the anthelmintics pyrantel, morantel and levamisole, activate acetylcholine (Ach) receptors on the muscle cell membranes of Ascaris suum and also that the avermectin 22,23-dihydroavermectin Bla (DHAVM) blocks the function of muscle cell gamma-aminobutyric acid (GABA) receptors. In experiments using micro-iontophoresis, the cholinergic anthelmintics were found to act directly on the muscle bag membrane, evoking a depolarizing response. 

Fig. 27. Intracellular recordings from a granule cell in the hippocampal slice to show the response to the iontophoretic application of GABA. (A) Voltage record made on moving film to show the way in which an intracellular injection of a depolarizing ramp of current is used to test the cell s excitability. Every alternate second a hyperpolarizing current pulse is used to measure the input resistance. As shown more clearly in the single shots in (B) and (C), the application of GABA inhibited the spike discharge evoked by the ramp, caused a substantial decrease in the input resistance, and produced a small depolarization with respect to resting membrane potential [dotted line in (B)]. (From Assaf et ai, 1981.)...

Fig. 30. Regression analysis to show the linear way in which the depolarizations (AV) evoked by both GABA and 5-HT are correlated with an associated decrease in membrane resistance Rm IRm)- The slope of the line is a measure of the difference between equilibrium potential of the putative neurotransmitter ( pni) and membrane potential (Tm). In this instance, the difference is greater than 30 mV in a depolarizing direction. The similarity of the results obtained during iontophoretic applications of GABA and 5-HT raises at least two, equally likely, possibilities. Either the action of 5-HT is mediated by the opening of ionic channels similar to those activated by GABA, or a presynaptic action of 5-HT which causes GABA release. (From Assaf et al, 1980.)...

As shown in Fig. 31 the effects of GABA and 5-HT applied consecutively to the same neuron, at a concentration of 0.1 mM, were quite different. In contrast to the action of 5-HT, the decrease in resistance evoked by GABA was very much faster and was associated with a substantial depolarization. Recovery was also very much faster. The reversal potential for 5-HT lay between - 85 and — 90 mV, and replacement of chloride in the medium by propionate did not change the effectiveness of 5-HT since a hyperpolarization evoked by the 5-HT and a low chloride medium was shorter. It should perhaps be mentioned that propronate may not be the ideal chloride replacement in studies of cortical neurons. Indeed, a number of workers have suggested that propionate will pass through the chloride channel activated during the ipsp (Dreifuss et al, 1969) or by spreading depression (Nicholson, 1979). 

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