The effect of internal GTP gamma S on GABA-release in cultured hippocampalneurons

The effects of presynaptic guanosine-5'-O-(3-thio)triphosphate (GTP gamma S ) on GABAergic inhibitory postsynaptic currents (IPSCs) were studied in cul tured hippocampal neurons using whole-cell recordings. Inclusion of GTP gam ma S (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 GTP gamma S increased the depression of IPSCs by the GABA, receptor-agonist baclofen (10 mu M), and the effect o f baclofen was poorly reversible after washing. Stimulation of the GABAergi c neuron at 80 Hz for 1 a was accompanied by tetanic depression of the IPSC s 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 tetani c stimulation were depressed and PTP was absent when tetanic stimulation wa s applied within 3 min after starting injection of GTP gamma S into the pre synaptic neuron. At longer times, basal release underlying a single IPSC wa s depressed. This affected the ratios recorded in response to tetanic stimu lations such that tetanic depression was abolished, while PTP increased to 117+/-34%. In conclusion, GTP gamma S reduces the probability of GABA-relea se in both a use- and time-dependent manner, most likely through an inhibit ory action on presynaptic Ca2+-influx through voltage-gated Ca2+ channels o r an interaction with small GTP-binding proteins in the nerve terminals.