GABA reversal potentials for

Intracellular injections of acetate, citrate, sulphate, fluoride or ammonium ions caused no change in the reversal potential for GABA. However, intracellular injection of chloride, bromide, chlorate, bromate, or methyl sulphate caused the reversal potential for GABA to move in a positive direction. The data are summarized in tables III and IV. 

Evidence for chloride accumulation and extrusion mechanisms were examined with putative inhibitors. Intracellular injection of ammonium ions, bathing the preparation in 4-acetamido-4,-isothiocyanatostilbene-2,2 -disulphonic acid (SITS), 4,4 -diisothiocyanato-stilbene-2,2 -disulphonic acid (DIDS), acetazolamide, furosemide, NH +, Zn + or Cu + were without effect on the reversal potential for GABA. The data obtained so far suggest that chloride-regulating mechanisms present in the cockroach are not sensitive to the inhibitors which block other invertebrate (15,16) or vertebrate (17) chloride pumps. 

As illustrated in Fig. 30 the reversal potentials for GABA and 5-HT on the same cell are virtually identical. However, over the entire series of... 

Under current-clamp conditions, activation of GABA receptors produced an increase in conductance and hyperpolarization of the membrane potential. Under voltage-clamp conditions this was seen as an outward current or influx of negative charge. The reversal potential for the GABA-induced hyperpolarizations was -77+2mV (mean +s.e.m , n = 22). 

Table IV. Effects of various intracellular anion substitutions on the reversal potential for neuronal GABA response...

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). 

The action of deltamethrin on the GABA response was more difficult to determine, because low concentrations of deltamethrin caused depolarization and rapid firing of action potentials, accompanied by a decrease in membrane resistance. Deltamethrin at 0.1 nM caused a massive increase in firing frequency, and a depolarization of 30mV, after perfusion for ten minutes. Application of TTX, which specifically blocks voltage dependent sodium channels, abolished the action potentials, and also reversed the pyrethroid-induced depolarization and decrease in membrane resistance. 

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