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 inhi bitory postsynaptic currents (IPSCs) in cultured rat hippocampal neurons, w e have previously demonstrated posttetanic potentiation (PTP) of IPSCs. Tet anic stimulation of the GABAergic neuron leads to accumulation of Ca2+ in t he presynaptic terminals. This enhances the probability of GABA-vesicle rel ease for up to 1 min, which underlies PTP. In the present study. we have ex amined the effect of altering the probability of release on PTP of IPSCs. B aclofen(10 mu M), which depresses presynaptic Ca2+ entry through N- and P/Q -type voltage-dependent Ca2+ channels (VDCCs), caused a threefold greater e nhancement of PTP than did reducing [Ca2+](o) to 1.2 mM, which causes a non specific reduction in Ca2+ entry. This finding prompted us to investigate w hether presynaptic L-type VDCCs contribute to the Ca2+ accumulation in the boutons during spike activity. The L-type VDCC antagonist, nifedipine (10 m u M), had no effect on single IPSCs evoked at 0.2 Hz but reduced the PTP ev oked by a train of 40 Hz for 2 s by 60%. Another L-type VDCC antagonist, is radipine (5 mu M), similarly inhibited PTP by 65%. Both L-type VDCC blocker s also depressed IPSCs during the stimulation (i.e., they increased tetanic depression). The L-type VDCC "agonist" (-)BayK 8644 (4 mu M) had no effect on PTP evoked by a train of 40 Hz for 2 s, which probably saturated the PT P 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+ accumu lation 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. Involveme nt of L-type channels in these processes represents a novel presynaptic reg ulatory mechanism at fast CNS synapses.