Role of presynaptic L-type Ca2+ channels in GABAergic synaptic transmission in cultured hippocampal neurons

Using dual whole cell patch-clamp recordings of monosynaptic GABAergic inhibitory postsynaptic currents (IPSCs) in cultured rat hippocampal neurons, we have previously demonstrated posttetanic potentiation (PTP) of IPSCs. Tetanic stimulation of the GABAergic neuron leads to accumulation of Ca2+ in the presynaptic terminals. This enhances the probability of GABA-vesicle release for up to 1 min, which underlies PTP. In the present study, we have examined the effect of altering the probability of release on PTP of IPSCs. Baclofen (10 microM), which depresses presynaptic Ca2+ entry through N- and P/Q-type voltage-dependent Ca2+ channels (VDCCs), caused a threefold greater enhancement of PTP than did reducing [Ca2+]o to 1.2 mM, which causes a nonspecific reduction in Ca2+ entry. This finding prompted us to investigate whether presynaptic L-type VDCCs contribute to the Ca2+ accumulation in the boutons during spike activity. The L-type VDCC antagonist, nifedipine (10 microM), had no effect on single IPSCs evoked at 0.2 Hz but reduced the PTP evoked by a train of 40 Hz for 2 s by 60%. Another L-type VDCC antagonist, isradipine (5 microM), similarly inhibited PTP by 65%. Both L-type VDCC blockers also depressed IPSCs during the stimulation (i.e., they increased tetanic depression). The L-type VDCC "agonist" (-)BayK 8644 (4 microM) had no effect on PTP evoked by a train of 40 Hz for 2 s, which probably saturated the PTP process, but enhanced PTP evoked by a train of 1 s by 91%. In conclusion, the results indicate that L-type VDCCs do not participate in low-frequency synchronous transmitter release, but contribute to presynaptic Ca2+ accumulation during high-frequency activity. This helps maintain vesicle release during tetanic stimulation and also enhances the probability of transmitter release during the posttetanic period, which is manifest as PTP. Involvement of L-type channels in these processes represents a novel presynaptic regulatory mechanism at fast CNS synapses.