Brief Report Intracellular Ca21 and Ca21/Calmodulin-dependent Kinase II Mediate Acute Potentiation of Neurotransmitter Release by Neurotrophin-3 Xiang-ping He, Feng Yang, Zuo-ping Xie, and Bai Lu Unit on Synapse Development and Plasticity, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892 Abstract. Neurotrophins have been shown to acutely modulate synaptic transmission in a variety of systems, but the underlying signaling mechanisms remain un- clear. Here we provide evidence for an unusual mecha- nism that mediates synaptic potentiation at the neuro- muscular junction (NMJ) induced by neurotrophin-3 (NT3), using Xenopus nerve–muscle co-culture. Unlike brain-derived neurotrophic factor (BDNF), which re- quires Ca21 inﬂux for its acute effect, NT3 rapidly en- hances spontaneous transmitter release at the develop- ing NMJ even when Ca21 inﬂux is completely blocked, suggesting that the NT3 effect is independent of extra- cellular Ca21. Depletion of intracellular Ca21 stores, or blockade of inositol 1, 4, 5-trisphosphate (IP3) or ryan- odine receptors, prevents the NT3-induced synaptic po- tentiation. Blockade of IP3 receptors can not prevent Introduction Nerve growth factor, brain-derived neurotrophic factor (BDNF),1 neurotrophin-3 (NT3), and NT4 belong to the neurotrophin family of signaling proteins essential for the survival and differentiation of different populations of neurons. Recent studies have revealed an important but previously unrecognized role of neurotrophins in synapse Address correspondence to Bai Lu, Unit on Synapse Development and Plasticity, National Institute of Child Health and Human Development, National Institutes of Health, Building 49, Rm. 6A80, 49 Convent Dr., MSC4480, Bethesda, MD 20892-4480. Tel.: (301) 435-2970. Fax: (301) 480- 3526. E-mail: lub@codon.nih.gov Z.-P. Xie’s current address is Department of Biology, Tsinghua Univer- sity, Beijing, China. 1Abbreviations used in this paper: BDNF, brain-derived neurotrophic factor; [Ca21]i, Ca21 concentration(s); CaMKII, Ca21/calmodulin-depen- dent kinase II; CNS, central nervous system; IP3, inositol 1, 4, 5-trisphos- phate; NMJ, neuromuscular junction; NT, neurotrophin; SSC, spontane- ous synaptic current; XeC, xestospongin C. BDNF-induced potentiation, suggesting that BDNF and NT3 use different mechanisms to potentiate trans- mitter release. Inhibition of Ca21/calmodulin-depen- dent kinase II (CaMKII) completely blocks the acute effect of NT3. Furthermore, the NT3-induced potentia- tion requires a continuous activation of CaMKII, because application of the CaMKII inhibitor KN62 reverses the previously established NT3 effect. Thus, NT3 potenti- ates neurotransmitter secretion by stimulating Ca21 re- lease from intracellular stores through IP3 and/or ryan- odine receptors, leading to an activation of CaMKII. Key words: ryanodine receptors • inositol 1, 4, 5-trisphosphate receptors • acetylcholine • neuromus- cular junction • synaptic transmission transmission and plasticity (for reviews see Lu and Fig- urov, 1997; McAllister et al., 1999). In the central nervous system (CNS), the synaptic actions of neurotrophins have been studied mainly in the visual cortex and hippocampus. For example, BDNF and NT4, which activate TrkB recep- tor, but not NT3, which primarily activates TrkC receptor, have long-term modulatory effects on the formation of oc- ular dominance columns in the visual cortex (Cabelli et al., 1995, 1997; Riddle et al., 1995). In layer 4 and 6 of the vi- sual cortex, BDNF and NT3 oppose each other in regulat- ing the dendritic growth of pyramidal neurons (McAllister et al., 1995, 1997). In the hippocampus, BDNF has been shown to acutely regulate synaptic plasticity such as long- term potentiation (LTP) (Korte et al., 1995; Figurov et al., 1996; Patterson et al., 1996). In contrast, several recent re- ports have demonstrated that NT3 is not involved in the modulation of hippocampal LTP (Figurov et al., 1996; Ko- kaia et al., 1998; Ma et al., 1999). These studies suggest The Journal of Cell Biology, Volume 149, Number 4, May 15, 2000 783–791 http://www.jcb.org 783