Facilitation of the presynaptic calcium current at an auditory synapse in rat brainstem

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Abstract
1 The presynaptic calcium current (IpCa) was recorded from the calyx of Held in rat brainstem slices using the whole‐cell patch clamp technique. 2 Tetanic activation of IpCa by 1 ms depolarizing voltage steps markedly enhanced the amplitude of IpCa. Using a paired pulse protocol, the second (test) response was facilitated with inter‐pulse intervals of less than 100 ms. The facilitation was greater at shorter intervals and was maximal (about 20 %) at intervals of 5–10 ms. 3 When the test pulse duration was extended, the facilitation was revealed as an increased rate of IpCa activation. From the current‐voltage relationship measured at 1 ms from onset, facilitation could be described by a shift in the half‐activation voltage of about −4 mV. 4 I pCa facilitation was not attenuated when guanosine‐5′‐O‐(3‐thiotriphosphate) (GTPγS) or guanosine‐5′‐O‐(2‐thiodiphosphate) (GDPβS) was included in the patch pipette, suggesting that G‐proteins are not involved in this phenomenon. 5 On reducing [Ca2+]o, the magnitude of facilitation diminished proportionally to the amplitude of IpCa. Replacement of [Ca2+]o by Ba2+ or Na+, or buffering of [Ca2+]i with EGTA or BAPTA attenuated IpCa facilitation. 6 We conclude that repetitive presynaptic activity can facilitate the presynaptic Ca2+ current through a Ca2+‐dependent mechanism. This mechanism would be complementary to the action of residual Ca2+ on the exocytotic machinery in producing activity‐dependent facilitation of synaptic responses.