A rapid, nongenomic pathway facilitates the synaptic transmission induced by retinoic acid at the developing synapse

We have previously shown that retinoic acid (RA), a factor highly expressed in spinal cord, rapidly and specifically enhances the spontaneous acetylcholine release at developing neuromuscular synapses in Xenopus cell culture, using whole-cell patch-clamp recording. We have now further investigated the underlying mechanisms that are involved in RA-induced facilitation on the frequency of spontaneous synaptic currents (SSCs). Buffering the rise of intracellular Ca²⁺ with BAPTA-AM hampered the facilitation of SSC frequency induced by RA. The prompt RA-enhanced SSC frequency was not abolished when Ca²⁺ was eliminated from the culture medium or there was bath application of the pharmacological Ca²⁺ channel inhibitor Cd²⁺, indicating that Ca²⁺ influx through voltage-activated Ca²⁺ channels are not required. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] or ryanodine receptors effectively blocked the increase of SSC frequency elicited by RA. Treating cells with either wortmannin or LY294002, two structurally different inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase) and with the phospholipase C (PLC) inhibitor U73122, abolished RA-induced facilitation of synaptic transmission. Preincubation of the cultures with pharmacological inhibitors, either genistein, a broad-spectrum tyrosine kinase inhibitor, or PP2, which predominantly inhibits the Src family of nonreceptor tyrosine kinase, completely abolished RA-induced synaptic facilitation. Taken collectively, these results suggest that RA elicits Ca²⁺ release from Ins(1,4,5)P₃ and/or ryanodine-sensitive intracellular Ca²⁺ stores of the presynaptic nerve terminal. This is done via PLC/PI 3-kinase signaling cascades and Src tyrosine kinase activation, leading to an enhancement of spontaneous transmitter release.