Neurosteroid pregnenolone sulfate enhances glutamatergic synaptic transmission by facilitating presynaptic calcium currents at the calyx of Held of immature rats

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Abstract
Pregnenolone sulfate (PREGS) is an endogenous neurosteroid widely released from neurons in the brain, and is thought to play a memory-enhancing role. At excitatory synapses PREGS facilitates transmitter release, but the underlying mechanism is not known. We addressed this issue at the calyx of Held in rat brainstem slices, where direct whole-cell recordings from giant nerve terminals are feasible. PREGS potentiated nerve-evoked excitatory postsynaptic currents (EPSCs) without affecting the amplitude of miniature EPSCs, suggesting that its site of action is presynaptic. In whole-cell recordings from calyceal nerve terminals, PREGS facilitated Ca2+ currents, by accelerating their activation kinetics and shifting the half-activation voltage toward negative potentials. PREGS had no effect on presynaptic K+ currents, resting conductance or action potential waveforms. In simultaneous pre- and postsynaptic recordings, PREGS did not change the relationship between presynaptic Ca2+ influx and EPSCs, suggesting that exocytotic machinery downstream of Ca2+ influx is not involved in its effect. PREGS facilitated Ba2+ currents recorded from nerve terminals and also from HEK 293 cells expressed with recombinant N- or P/Q-type Ca2+ channels, suggesting that PREGS-induced facilitation of voltage-gated Ca2+ channels (VGCCs) is neither Ca2+ dependent nor VGCC-type specific. The PREGS-induced VGCC facilitation was blocked by the PREGS scavenger (2-hydroxypropyl)-beta-cyclodextrin applied from outside, but not from inside, of nerve terminals. We conclude that PREGS facilitates VGCCs in presynaptic terminals by acting from outside, thereby enhancing transmitter release. We propose that PREGS may directly modulate VGCCs acting on their extracellular domain.