UNBIASED ESTIMATES OF QUANTAL RELEASE PARAMETERS AND SPATIAL VARIATION IN THE PROBABILITY OF NEUROSECRETION

A procedure was developed for dealing with two problems that have impe ded the use of quantal parameters in studies of transmitter release. T he first, involving temporal and spatial biasing in the estimates for the number of functional release sites (nBAR) and probability of relea se (pBAR), was addressed by reducing temporal variance experimentally and calculating the bias produced by spatial variance in p (var(s)p). The second, involving inaccuracies in the use of nerve-evoked endplate potentials (EPPs), was circumvented by using only miniature EPPs (MEP Ps). Intracellular recordings were made from isolated frog cutaneous p ectoris, after decapitation and pithing of the animals, and the concen tration of K+ ([K+]) was raised to 10 mM to increase the level of tran smitter release. The number of quanta released (mBAR) by the EPP was r eplaced by the number of MEPPs in a fixed time interval (bin), and 500 sequential bins used for each quantal estimate. With the use of 50-ms bins, estimates for var(s)p were consistently negative. This was due to too large a bin (and introduction of undetected temporal variance) because the use of smaller bins (5 ms) produced positive estimates of var(s)p. Increases in m, n, and p but not var(s)p were found in respon se to increases in [K+] or [Ca2+]/[Co2+]. La3+ (20 muM) produced incre ases in m and n, which peaked after 20 min and declined toward zero. T here were also large increases in p and var(s)p, which peaked and decl ined only to initial control values. The increase in var(s)p was presu med to reflect La3+-induced release of Ca2+ from intracellular organel les. The results suggest that this approach may be used to obtain unbi ased estimates of NBAR and PBAR and that the estimates of var(s)p may be useful for studying Ca2+ release from intraterminal organelles.