Original Research Article
published: 01 July 2010 doi: 10.3389/fnsyn.2010.00015
SYNAPTIC NEUROSCIENCE
Vesicular release of glutamate utilizes the proton gradient between the vesicle and synaptic cleft Jon T. Brown1†, Kate L. Weatherall 2†, Laura R. Corria 2, Thomas E. Chater1, John T. Isaac 3 and Neil V. Marrion 2* Department Anatomy, School of Medical Sciences, University of Bristol, Bristol, UK Department of Physiology and Pharmacology, School of Medical Sciences, University of Bristol, Bristol, UK 3 National Institute of Neurological Disorders and Stroke, National Institute of Health, Bethesda, MD, USA 1 2
Edited by: Reinhard Jahn, Max-Planck Institute for Biophysical Chemistry, Germany Reviewed by: Christian Rosenmund, Neurocure and Charite Universitaetsmedizin, Germany Reinhard Jahn, Max-Planck Institute for Biophysical Chemistry, Germany *Correspondence: Neil V. Marrion, Department of Physiology and Pharmacology, School of Medical Sciences, University of Bristol, University Walk, Bristol, BS8 1TD, UK. e-mail:
[email protected] Jon T. Brown and Kate L. Weatherall have contributed equally to this work. †
Glutamate is released from synaptic vesicles following formation of a fusion pore, connecting the vesicle interior with the synaptic cleft. Release is proposed to result from either full fusion of the vesicle with the terminal membrane or by ‘kiss-and-run,’ where release occurs through the fusion pore. ‘Kiss-and-run’ seems implausible as passive diffusion of glutamate through the pore is too slow to account for the rapidity of release. Vesicular accumulation of glutamate is driven by a proton gradient, resulting in the co-release of protons during exocytosis. We tested whether the proton gradient between the vesicle and cleft contributes to glutamate exocytosis. Collapse of the gradient reduced hippocampal glutamatergic transmission, an effect that was not associated with presynaptic changes in excitability, transmitter release probability, or postsynaptic sensitivity. These data indicate that approximately half of glutamate release utilizes the proton gradient between vesicle and cleft, suggesting a significant proportion of release by ‘kiss-and-run.’ Keywords: fusion pore, kiss-and-run, vesicles, synaptic release
Introduction The amino acid l-glutamate mediates the vast majority of excitatory transmission in the vertebrate central nervous system. Transmitter release from synaptic vesicles into the synaptic cleft was classically thought to occur following formation of a fusion pore that dilated to result in full vesicle fusion with the presynaptic membrane. This process is considered to be responsible for the omega-shaped profile of membranes observed in ultra-structural investigations of the release sites of presynaptic terminals (Heuser, 1989). This classical release mechanism requires that components of the vesicle membrane are retrieved after the vesicle has merged with the terminal to discharge its contents into the synaptic cleft, with vesicle retrieval and repriming taking tens of seconds (Ryan et al., 1993; Liu and Tsien, 1995). More recently, it has been proposed that vesicles can remain intact, with release occurring through a transient fusion pore, which can close to produce incomplete exocytosis of the vesicle contents. This is often referred to as ‘kiss-and-run’ exocytosis (An and Zenisek, 2004). The contribution of full fusion and ‘kiss-and-run’ exocytosis to transmitter release in the CNS is a matter of considerable debate. It has been reported that up to 70–80% of glutamate release in the hippocampus occurs by ‘kiss-and-run’ during low frequency stimulation (Aravanis et al., 2003; Richards et al., 2005; Harata et al., 2006; Zhang et al., 2007). The release of dopamine in ventral midbrain is also proposed to be largely by ‘kiss-and-run’ (Staal et al., 2004). In contrast, other studies have suggested that as few as 20% of vesicles in the CNS release their transmitter by ‘kiss-andrun’ (Stevens and Williams, 2000; He et al., 2006). It is becoming apparent that the two release mechanisms can co-exist, with it being proposed that synapses with a low release probability favor
Frontiers in Synaptic Neuroscience
‘kiss-and-run,’ while high probability terminals preferentially utilize full fusion exocytosis (Gandhi and Stevens, 2003; Harata et al., 2006). It is estimated that transmitter must exit a vesicle within 100 μs to support the rapid rate of synaptic communication observed at glutamatergic synapses (Clements, 1996; Wadiche and Jahr, 2001). This fact alone can be used to question the contribution of exocytosis via ‘kiss-and-run’ for rapid synaptic communication, because the estimated conductance (200–300 pS, Klyachko and Jackson, 2002; He et al., 2006) and open time (
