The effect of internal GTPγS on GABA-release in cultured hippocampal neurons

The effects of presynaptic guanosine-5'-O-(3-thio)triphosphate (GTPγS) on GABAergic inhibitory postsynaptic currents (IPSCs) were studied in cultured hippocampal neurons using whole-cell recordings. Inclusion of GTPγS (0.5–1 mM) in the presynaptic electrode reduced both the amplitude and paired-pulse depression of IPSCs, indicating that the probability of GABA-release had been reduced. Presynaptic GTPγS increased the depression of IPSCs by the GABAB-receptor-agonist baclofen (10 µM), and the effect of baclofen was poorly reversible after washing. Stimulation of the GABAergic neuron at 80 Hz for 1 s was accompanied by tetanic depression of the IPSCs by 52±6% and was followed by post-tetanic potentiation (PTP), reaching a peak value of 71±21% and lasting about 100 s. IPSCs evoked after tetanic stimulation were depressed and PTP was absent when tetanic stimulation was applied within 3 min after starting injection of GTPγS into the presynaptic neuron. At longer times, basal release underlying a single IPSC was depressed. This affected the ratios recorded in response to tetanic stimulations such that tetanic depression was abolished, while PTP increased to 117±34%. In conclusion, GTPγS reduces the probability of GABA-release in both a use- and time-dependent manner, most likely through an inhibitory action on presynaptic Ca2+-influx through voltage-gated Ca2+ channels or an interaction with small GTP-binding proteins in the nerve terminals.