Potentiation of quantal secretion by insulin-like growth factor-1 at developing motoneurons in Xenopus cell culture.
Although evidence suggests that insulin-like growth factor (IGF) plays an important role in the development and growth of the nervous system, the effect of IGF-1 in the regulation of neurotransmitter release in the peripheral nervous system remains unknown. Here we show that acute application of IGF-1, a factor widely expressed in developing myocytes, dose-dependently enhances the spontaneous acetylcholine (ACh) secretion at developing neuromuscular synapses in Xenopus cell culture using whole-cell patch clamp recording. We studied the role of endogenously released IGF-1 by examining the effect of IGF-1 antibody on the frequency of spontaneous synaptic currents (SSCs) at high-activity synapses, and found SSC frequency was markedly reduced at these high-activity synapses. The IGF-1-induced synaptic potentiation 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+ influxes through voltage-activated Ca2+ channels are not required. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate (IP3) or ryanodine receptors effectively occluded the increase of SSC frequency elicited by IGF-I. Treating cells with the phosphoinositide-3 kinase (PI3-K) inhibitors wortmannin or LY294002, and with phospholipase Cgamma (PLCgamma) inhibitor U73122, but not the inhibitor of mitogen-activated protein (MAP) kinase PD98059, abolished IGF-1-induced synaptic potentiation. Taken collectively, these results suggest that endogenously released IGF-1 from myocytes elicits Ca2+ release from IP3- and/or ryanodine-sensitive intracellular Ca2+ stores of the presynaptic nerve terminal. This is done via PI3-K and PLCgamma signalling cascades, leading to an enhancement of spontaneous transmitter release.
D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects; D002478:Cells, Cultured; D003470:Culture Media; D004594:Electrophysiology / Q000379:methods; D004625:Embryo, Nonmammalian / Q000502:physiology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D017774:Long-Term Potentiation / Q000187:drug effects / Q000502:physiology; D009469:Neuromuscular Junction / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D013116:Spinal Cord / Q000187:drug effects / Q000502:physiology*; D013569:Synapses / Q000187:drug effects / Q000502:physiology; D014981:Xenopus
D003470:Culture Media; D002104:Cadmium; D007334:Insulin-Like Growth Factor I; D002118:Calcium