Intracellular Ca²⁺ and Ca²⁺/Calmodulin-Dependent Kinase II Mediate Acute Potentiation of Neurotransmitter Release by Neurotrophin-3

Neurotrophins have been shown to acutely modulate synaptic transmission in a variety of systems, but the underlying signaling mechanisms remain unclear. Here we provide evidence for an unusual mechanism that mediates synaptic potentiation at the neuromuscular junction (NMJ) induced by neurotrophin-3 (NT3), using Xenopus nerve–muscle co-culture. Unlike brain-derived neurotrophic factor (BDNF), which requires Ca²⁺ influx for its acute effect, NT3 rapidly enhances spontaneous transmitter release at the developing NMJ even when Ca²⁺ influx is completely blocked, suggesting that the NT3 effect is independent of extracellular Ca²⁺. Depletion of intracellular Ca²⁺ stores, or blockade of inositol 1, 4, 5-trisphosphate (IP3) or ryanodine receptors, prevents the NT3-induced synaptic potentiation. Blockade of IP3 receptors can not prevent BDNF-induced potentiation, suggesting that BDNF and NT3 use different mechanisms to potentiate transmitter release. Inhibition of Ca²⁺/calmodulin-dependent kinase II (CaMKII) completely blocks the acute effect of NT3. Furthermore, the NT3-induced potentiation requires a continuous activation of CaMKII, because application of the CaMKII inhibitor KN62 reverses the previously established NT3 effect. Thus, NT3 potentiates neurotransmitter secretion by stimulating Ca²⁺ release from intracellular stores through IP3 and/or ryanodine receptors, leading to an activation of CaMKII.