dependence of the binomial parameters p and n at the mouse neuromuscular junction 1 2

26 The Ca dependence of synaptic quantal release is generally thought to be restricted to 27 probability of vesicular release. However, some studies have suggested that the number of release sites 28 (n) at the neuromuscular junction (NMJ) is also Ca dependent. In the present study, we recorded 29 endplate currents over a wide range of extracellular Ca concentrations and found the expected Ca 30 dependency of release. A graphical technique was used to estimate p (probability of release) and n using 31 standard binomial assumptions. The results suggested n was Ca dependent. The data were then 32 simulated using compound binomial statistics with variable n (Ca dependent) or fixed n (Ca 33 independent). With fixed n, successful simulation of increasing Ca required that p increase abruptly at 34 some sites from very low to high values. Successful simulation with variable n required the introduction 35 of previously silent release sites (p = 0) with high values of p. Thus, the success of both simulations 36 required abrupt, large increases of p at a subset of release sites with initially low or zero p. Estimates of 37 the time course of release obtained by deconvolving evoked endplate currents with average miniature 38 endplate currents decreased slightly as Ca increased, thus arguing against sequential release of 39 multiple quanta at higher Ca levels. Our results suggest that the apparent Ca dependence of n at the 40 NMJ can be explained by an underlying Ca dependence of a spatially variable p such that p increases 41 abruptly at a subset of sites as Ca is increased. 42 43 44 45 46 47 Ca dependence of binomial p and n 3 INTRODUCTION 48 The Ca dependence of vesicular release is a widely recognized property of synapses. However, 49 despite years of work, uncertainty remains about the mechanism(s) that govern this dependence and 50 which properties of the release process Ca controls. Early work established that the probability of 51 quantal release (p) from a given release site is controlled by the concentration of external Ca (Del 52 Castillo and Katz 1954; Katz and Miledi 1965; Dodge and Rahamimoff 1967). Based largely on 53 statistical modeling of the release process, the number of release sites (n) was thought to be fixed such 54 that only probability of release (p) was altered with changes in external Ca (for review see Korn and 55