Endocannabinoids Induce Lateral Long-Term Potentiation of Transmitter Release by Stimulation of Gliotransmission

Cerebral Cortex October 2015;25:3699–3712 doi:10.1093/cercor/bhu231 Advance Access publication September 26, 2014 Endocannabinoids Induce Lateral Long-Term Potentiation of Transmitter Release by Stimulation of Gliotransmission Marta Gómez-Gonzalo1 , Marta Navarrete1,5, Gertrudis Perea1, Ana Covelo2, Mario Martín-Fernández2, Ryuichi Shigemoto3, Rafael Luján4 and Alfonso Araque1,2 1 Instituto Cajal, CSIC, Madrid 28002, Spain, 2Department of Neuroscience, University of Minnesota, Minneapolis, MN 55455, USA, Division of Cerebral Structure, National Institute for Physiological Sciences, Okazaki 444-8787, Japan, 4Instituto de Investigación en Discapacidades Neurológicas (IDINE), Departamento de Ciencias Médicas, Facultad de Medicina, Universidad Castilla-La Mancha, Albacete 02006, Spain and 5Current address: Department of Neurobiology, Centro de Biología Molecular “Severo Ochoa,” (CSIC/UAM), Madrid, Spain 3 Marta Gómez-Gonzalo, Marta Navarrete, Gertrudis Perea, and Ana Covelo contributed equally to this work. Endocannabinoids (eCBs) play key roles in brain function, acting as modulatory signals in synaptic transmission and plasticity. They are recognized as retrograde messengers that mediate long-term synaptic depression (LTD), but their ability to induce long-term potentiation (LTP) is poorly known. We show that eCBs induce the long-term enhancement of transmitter release at single hippocampal synapses through stimulation of astrocytes when coincident with postsynaptic activity. This LTP requires the coordinated activity of the 3 elements of the tripartite synapse: 1) eCB-evoked astrocyte calcium signal that stimulates glutamate release; 2) postsynaptic nitric oxide production; and 3) activation of protein kinase C and presynaptic group I metabotropic glutamate receptors, whose location at presynaptic sites was confirmed by immunoelectron microscopy. Hence, while eCBs act as retrograde signals to depress homoneuronal synapses, they serve as lateral messengers to induce LTP in distant heteroneuronal synapses through stimulation of astrocytes. Therefore, eCBs can trigger LTP through stimulation of astrocyte–neuron signaling, revealing novel cellular mechanisms of eCB effects on synaptic plasticity. Keywords: astrocytes, endocannabinoid signaling, LTP, mGluRs, nitric oxide Introduction The endocannabinoid (eCB) system comprises 2 cannabinoid receptors, type 1 (CB1) and type 2 (CB2), and the endogenous ligands, eCB. eCBs retrogradely modulate synaptic transmission widely throughout the central nervous system playing relevant neuromodulatory roles in multiple physiological processes (Freund et al. 2003; Chevaleyre et al. 2006). They are released from postsynaptic neurons, activate presynaptic CB1 cannabinoid receptors, and reduce transmitter release either transiently (endocannabinoid-mediated short-term depression; eCB-STD) or persistently (endocannabinoid-mediated longterm depression; eCB-LTD) (Kreitzer and Regehr 2001a; Alger 2002; Wilson and Nicoll 2002; Freund et al. 2003; Chevaleyre et al. 2006; Heifets and Castillo 2009), and are responsible for most of the behavioral effects of cannabinoids (Maldonado et al. 2006). Recent studies in goldfish (Cachope et al. 2007), lampreys spinal cord (Song et al. 2012), and rodent hippocampal slices (Navarrete and Araque 2010) have reported that CB1R activation can transiently enhance neurotransmission. Although the role of eCB signaling in synaptic plasticity is thought to exclusively induce long-term synaptic depression © The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: (LTD), its involvement in the long-term potentiation (LTP) of synaptic transmission is still poorly known (Xu et al. 2012). Astrocytes have emerged as active elements directly involved in synaptic physiology. They respond with Ca2+ elevations to neurotransmitters released by neurons and modulate neuronal excitability and synaptic transmission by releasing gliotransmitters (Araque et al. 2001; Nedergaard et al. 2003; Volterra and Meldolesi 2005; Haydon and Carmignoto 2006; Perea et al. 2009; Parpura and Zorec 2010; Singh et al. 2014), hence being active elements of synaptic function at tripartite synapses (Araque et al. 1999; Perea et al. 2009). While eCBs released by neurons exert their effects at short distances [<20 µm (Wilson and Nicoll 2001; Piomelli 2003; Chevaleyre and Castillo 2004; Chevaleyre et al. 2006; Navarrete and Araque 2010)], the possible long-range effects of eCBs signaling on synaptic plasticity through astrocyte network stimulation remain largely unknown. The demonstration that hippocampal astrocytes express functional CB1Rs, which increase astrocyte Ca2+ levels and stimulate glutamate release (Navarrete and Araque 2008), suggests that astrocytes are directly involved in the eCB intercellular signaling as well as in its functional consequences on synaptic transmission. Indeed, a more recent study has shown that eCB-mediated neuron-to-astrocyte signaling lead to the short-term potentiation of synaptic transmission in hippocampal synapses (Navarrete and Araque 2010). Furthermore, the eCB-astrocyte signaling has been related with cortical LTD (Min and Nevian 2012) and the impairment of spatial working memory and hippocampal LTD induced by exogenous cannabinoids (Han et al. 2012). However, despite these studies revealing the role of eCB-astrocyte signaling on synaptic depression processes, the consequences and underlying molecular mechanisms of eCB-astrocyte signaling on long-lasting enhancement of synaptic activity remain largely unknown. Our previous studies have demonstrated that calcium elevations in astrocytes elicited by Ca2+ uncaging or eCBs released from neurons induce a transient synaptic potentiation of hippocampal synaptic transmission (Perea and Araque 2007; Navarrete and Araque 2010), and that pairing astrocyte Ca2+ uncaging and mild postsynaptic depolarization can induce long-term changes in synaptic efficacy. Using electrophysiological and Ca2+ imaging techniques in mouse brain slices, we have investigated whether eCBs released from neurons, acting as endogenous signals that activate astrocytes (Navarrete and Araque 2008), could induce the LTP of synaptic transmission. Address correspondence to Prof. Alfonso Araque, Department of Neuroscience, University of Minnesota, 4-110 Wallin Medical Biosciences Building, 2101 6th Street SE, Minneapolis, MN 55455, USA. Email: Materials and Methods Ethics Statement All the procedures for handling and sacrificing animals followed the European Commission guidelines (2010/63/EU). Hippocampal Slice Preparation Hippocampal slices were obtained from C57BL/6 mice (13–18 days old). In some cases, slices from CB1 receptor knockout mice and IP3R2 knockout mice, generously donated by Dr A. Zimmer and Dr J. Chen, respectively, were used (Zimmer et al. 1999; Li et al. 2005). Animals were anesthetized and decapitated. The brain was rapidly removed a (...truncated)