Origin of quantal size variation and high-frequency miniature postsynaptic currents at the Caenorhabditis elegans neuromuscular junction.

In the absence of action potentials or direct nerve stimulation, neurotransmitters may be released spontaneously and sporadically. Such synaptic events may be detected as changes in the membrane potential or current of the postsynaptic cell, which are known as miniature postsynaptic potentials (mPSPs) and miniature postsynaptic currents (mPSCs), respectively. Both mPSPs and mPSCs are commonly referred as minis. Minis were first discovered as membrane potential fluctuations at the end-plate of vertebrate neuromuscular junctions (NMJs), and were called miniature end-plate potentials (mEPPs). They were thought to represent elementary units of neurotransmitter release known as quanta (Del Castillo and Katz 1954; Fatt and Katz 1950). Until recently, minis were considered to have the following properties: (1) they reflect leakage of neurotransmitters under resting conditions and do not have specific physiological functions (Zucker 2003); (2) each mini represents the full-collapse fusion of one synaptic vesicle, and the release from the vesicle is all-or-none (Katz 1971); (3) the quantal size, as represented by the amplitude of minis, is constant (Del Castillo and Katz 1954); and (4) minis result from exocytosis of the same pool of synaptic vesicles as evoked responses (Del Castillo and Katz 1954; Groemer and Klingauf 2007). However, these earlier concepts are challenged by some recent data. For example, minis have been implicated in several physiological functions, including postsynaptic receptor clustering (Saitoe et al. 2001), shift of NMDA receptor subunit compositions during development (Barria and Malinow 2002), regulation of dendritic protein synthesis (Sutton et al. 2006; Sutton et al. 2004), maintenance of dendritic spines (McKinney et al. 1999), and action potential firing (Carter and Regehr 2002; Sharma and Vijayaraghavan 2003). In addition, synaptic vesicles may be able to release neurotransmitters by establishing a transient fusion pore with the presynaptic plasma membrane without losing its vesicular identity, which is known as a kiss-and-run event (Aravanis et al. 2003; Gandhi and Stevens 2003; He et al. 2006), and minis and evoked responses appear to result from two distinct pools of synaptic vesicles (Chung et al. 2010; Fredj and Burrone 2009; Hablitz et al. 2009; Mathew et al. 2008; Sara et al. 2005). Moreover, quantal size appears to be variable at many synapses (Forti et al. 1997; Liu et al. 1999; Liu and Tsien 1995) due to a variety of pre- and postsynaptic factors, including variable neurotransmitter content among synaptic vesicles (Reimer et al. 1998; Wu et al. 2007), possibly incomplete discharge of synaptic vesicle content during kiss-and-run exocytosis (Harata et al. 2006), merging of synaptic vesicles prior to fusion with the presynaptic plasma membrane (He et al. 2009), synchronized release of multiple synaptic vesicles (Llano et al. 2000; Sharma and Vijayaraghavan 2003), and saturation of postsynaptic receptors by a single quantum of neurotransmitter (Edwards et al. 1990; Tang et al. 1994; Tong and Jahr 1994). Nevertheless, a great deal of controversy still exists with respect to some of these new findings. These uncertainties highlight the necessity and importance of determining the mechanisms regulating mini frequency and quantal size. The nematode Caenorhabditis elegans (C. elegans) is a very useful model system for studying cellular and molecular mechanisms of synaptic transmission due to several major advantages. First, most cellular and molecular mechanisms of synaptic transmission are conserved between C. elegans and mammals. Second, C. elegans has a simple nervous system with only 302 neurons. The entire neural circuit, including all the chemical synapses and gap junctions, has been described through analyses of serial section electron microscopy (Chen et al. 2006; White et al. 1986). Third, viable mutant alleles of many genes related to synaptic transmission already exist in C. elegans; and mutants of new genes may be obtained with relative ease through either forward or reverse genetics. Fourth, C. elegans is transparent, which allows convenient analyses of expression and subcellular localization patterns of genes/proteins using fluorescent proteins as reporters. Fifth, many public resources exist to support research with C. elegans, such as the Caenorhabditis Genetics Center where existing C. elegans mutants may be obtained, gene knockout facilities where targeted gene knockout may be requested at no charge, and extensive web resources on C. elegans genome (www.wormbase.org), anatomy (www.wormatlas.org) and biology (www.wormbook.org). In C. elegans, analyses of minis and evoked postsynaptic currents are performed almost exclusively with the NMJs for technical reasons. In this assay, one electrode is used to voltage-clamp a body-wall muscle cell (typically at −60 mV) to monitor postsynaptic currents while a second electrode is placed near the ventral nerve cord to stimulate axons innervating the muscle cell. mPSCs are recorded in the absence of nerve stimulation whereas evoked postsynaptic currents (ePSCs) are caused by applying a square-wave current pulse through the stimulation electrode (Richmond et al. 1999). Compared with many other synapses, the frequency of minis at the C. elegans NMJ is high (typically 50–90 Hz) (Gracheva et al. 2006; Liu et al. 2005; Richmond and Jorgensen 1999). Furthermore, minis at the C. elegans NMJ show great variation in amplitude (Figure 1). While the mean amplitude is approximately 25 pA, many events are smaller than 10 pA or larger than 50 pA, and some are as large as 100–200 pA (Liu et al. 2005). The presence of many large-amplitude minis causes a severe skew of the amplitude distribution histogram (Figure 1, B), which is in sharp contrast to the normal distribution of vertebrate mEPPs (Del Castillo and Katz 1954). What are the mechanisms for the high frequency and great quantal size variation of minis at the C. elegans NMJ? Answers to these two questions are potentially valuable for the interpretation of synaptic data from C. elegans and the understanding of fundamental mechanisms of synaptic transmission. In this review, I discuss several factors implicated in controlling the frequency and quantal size of minis at the C. elegans NMJ. Figure 1 Miniature postsynaptic currents (minis) at the C. elegans neuromuscular junction occur at a high frequency and show great variation in amplitude. A: A representative trace of minis shown at a compressed (a) or expanded (b) time scale. The trace in b is ...